国際化推進共同研究概要
No.1
20EA-1
タ イ ト ル： Dynamical mechanisms of stratospheric control on the tropical
troposphere and ocean研究代表者： UEYAMA Rei

所内世話人： 江口菜穂

研究概要： ここ数 10 年間で西アフリカのサヘル域で顕著な降水現象が増加している。この降水トレンドのメカニズムを知るために、客観解析データ(JRA55)と衛星データから導出した熱帯対流圏界面に到達する積雲対流活動、およびメソ
スケール規模の対流活動の指標を用いて、解析を行った。西アフリカ上
空では、対流圏は温暖化、下部成層圏は寒冷化していた。この傾向は温室効果ガスによる影響と一致するが、大西洋からの下層の水蒸気輸送による降水トレンドとは合致しない。背の高い積雲対流が活発となったサヘ
ル域の上層では、気温の鉛直勾配が減少する一方、ギニア沿岸では対流圏が温暖化し、背の低い積雲活動が活発となっていた。西アフリカ域の降水トレンドの特徴は、成層圏循環場によって駆動された背の高い積雲対流活動に起因していることが示唆された。

Dynamical Mechanisms of Stratospheric Control on the
Tropical Troposphere and Ocean
Rei Ueyama (NASA Ames Research Center)
I. Abstract
Extreme precipitation events in the Sahel region of West Africa havebecome more frequent over recent decades. We investigated the mechanism
behind this recent precipitation trend using a combination of JRA-55 reanalysis,
satellite precipitation measurements, and convection diagnostics including
observations of tropical overshooting clouds and mesoscale convective systems.
We found that the recent changes over West Africa involve a cooling of
the tropical lower stratosphere and tropopause layer, and a warming in the
troposphere. This feature is similar to that which might result from increased
greenhouse gas levels, but is distinct from the interannual variation of
precipitation associated with the modulation of water vapor transport from theAtlantic Ocean. We suggest that the decrease in the vertical temperature
gradient in the tropical tropopause region enhances extreme deep convection
over the Sahel, where penetrating convection is frequent, whereas troposphericwarming suppresses the shallower convection over the Guinea Coast. The
essential characteristic of the recent changes over West Africa is therefore thedepth of convection, rather than the total amount of surface precipitation, whichis largely driven by stratospheric forcing.
II. Introduction
West Africa is particularly susceptible to the impacts of climate change,
with significant changes in the precipitation regime likely to occur over the next
few decades (Gaetani et al., 2020). Assessing the role of the tropical tropopause
layer (TTL; around 140–70 hPa) in driving precipitation trends is a valuable step
towards improving our understanding of the present and future evolution ofWest African rainfall regimes.
Kodera et al. (2019) found that extreme deep convection in the ascending
branch of the boreal summer Hadley circulation became active over recent
decades, particularly over the African and Asian sectors. In West Africa, thisincrease in convective activity was associated with the recent recovery of rainfall
over the Sahel following the long and severe drought conditions of the 1970s and1980s. However, the recent increase in precipitation over the Sahel is not a
simple recovery to the former wet state: The characteristics of rainfall have alsochanged, becoming more intense and intermittent. The analysis of mesoscaleconvective systems (MCSs) with cold cloud top temperatures (Taylor et al.,2017) corroborates the increasing intensity of deep convection over West Africa.
Kodera et al. (2019) also note that the recent increase in tropical extreme deep
convection occurred in association with a cooling of the tropical lower
stratosphere, suggesting the possible role of TTL processes in the recent
precipitation increase over the Sahel.
In this study, we demonstrate the importance of TTL processes in driving
precipitation trends in West Africa.

III. Method/Data
We analyzed JRA-55 reanalysis data, surface precipitation measurementsfrom Global Precipitation Climatology Project (GPCP), and deep convection
diagnostics based on satellite observations of tropical overshooting clouds(COV). Our results are compared with occurrence frequencies of MCSs over the
Sahel obtained by Taylor et al. (2017).
Preliminary analyses of satellite-derived convective cloud top altitudes(Pfister et al, 2021) were also used to corroborate the tropical convective
overshooting cloud data. Our approach for calculating the convective cloud top
altitudes is based on the assumption that rainfall, properly thresholded, candefine the area where convection is occurring. These rainfall data, coupled with
the infrared satellite information, can then define both the regions where themass-transporting convective cores occur, and their altitude.
IV. Results
Precipitation during the summer monsoon season of July, August, andSeptember (JAS) from the 1980s to the present increased over the Sahel (15°W‒
20°E, 12.5°N‒17.5°N; Fig. 1c), but decreased over the Guinea Coast (15°W‒20°E,
2.5°N‒7.5°N; Fig. 1d). As such, surface precipitation does not exhibit a clear
trend when averaged over the entire West African region (15°W‒20°E, 2.5°N‒
17.5°N; (Fig. 1e).
A decreasing precipitation trend is particularly pronounced over thecoastal regions west of the Guinea Highlands and South Cameroon Plateau (Fig.
1b). Over these elevated terrains, convergence of the air from the ocean resultsin heavy precipitation (Fig. 1a). Since convection over the coastal region is
generally not deep enough to penetrate into the TTL, uplifted air diverges in theupper troposphere. In contrast, an increasing precipitation trend is foundfurther inland in regions of high equivalent potential temperature near thesurface, where extreme deep convective clouds with overshooting tops occur.
This extreme deep convection over Sahel is also evident in the large horizontal
divergence at higher levels in the TTL. These results suggest that the regional
differences in recent precipitation trends (Fig. 1b) may arise from differences inconvective activity. In particular, precipitation increased where extreme deep
convection occurs frequently, but decreased where convection is relatively
shallow. This implies the important role of the depth of convection inprecipitation changes over the last few decades.

Figure 1: (a) Climatological mean surface precipitation over West Africa
during JAS and (b) its linear trend over the period 1979–2018. (c, d, and e)
Time series of JAS mean precipitation averaged over (c) the Sahel, (d) the
Guinea Coast, and (e) West Africa. Lines and numbers indicate linear trend(mm/day/decade). Regions within West Africa are indicated by dottedlines within the brown box in (a).
Latitudinal differences between the two regions can clearly be seen in themeridional cross-section of JAS mean standardized temperature and verticalvelocity anomalies shown in Figure 2. Although cooling in the TTL occurred overa wide range of latitudes, upwelling in the troposphere was enhanced only over
the Sahel. This suggests that TTL cooling impacts only those regions where
upwelling extends from the upper troposphere to the TTL (i.e., about 200 to 140hPa), as indicated by the climatological divergence field (dotted lines). Figures
2b and 2c show the evolution of standardized COV frequencies during the period
2001–2018. The mean occurrence frequency of COV over the Sahel is 4.4 ‰,which is four times as large as that over the Guinea Coast. There is an increasing
trend superimposed on the year-to-year variability in the COV occurrence
frequency over the Sahel, which matches the evolution of MCSs with a cloud top
temperature < −70°C reported by Taylor et al. (2017). In contrast, the COV
occurrence frequency over the Guinea Coast exhibits a decreasing trend.

Figure 2: (a) Meridional cross-section of mean standardized anomalies for
JAS 2000–2018 over the Sahel (15°W–20°E). Temperature and pressure
vertical velocity are shown by color shading and contour lines,
respectively. Climatology of the horizontal divergence is shown by dottedlines. (b, c) Standardized JAS mean COV occurrence frequency from 2001 to2018 over (b) the Sahel and (c) the Guinea Coast. These two regions areindicated by the arrows along the x-axis of (a). Blue lines in (b) indicate thestandardized JAS mean MCSs with cloud top temperature below −70°C from
Taylor et al. (2017). Scale for the MCS is given on the right-hand side.
V. Discussion/Summary
The different precipitation trends seen over the Sahel and Guinea Coastcan be interpreted as a result of the differences in the depth of convective clouds
in the two regions. Penetrating deep convection over the Sahel is susceptible totemperature changes in the TTL and thus increases in response to TTL cooling.
In contrast, convective activity over the Guinea Coast is not influenced by coolingin the TTL, but rather suppressed by warming in the troposphere. Therefore, theobserved changes in TTL temperatures have led to an increasing precipitation
trend over the Sahel, while it led to a decreasing precipitation trend over the
Guinea Coast.
To better understand the details of the stratosphere‒troposphere
coupling process, we will investigate the coupling process as depicted inconvective cloud top data (Pfister et al., 2021) in a future study. Specifically, the0.25° longitude/latitude resolution, 3-hourly cloud top data will be useful for
illustrating the time evolution of deep convective activity over a given region(e.g., Africa) in response to stratospheric forcing.
In this study, we analyzed vertical velocity and divergence data from theJRA-55 reanalysis. However, vertical velocity is not an observed variable anddepends strongly on the model (i.e., the cumulus parametrization) used for thereanalysis. This is especially true in the TTL, where there is little observational
data available. Preliminary analysis of the European Centre for Medium-Range

Weather Forecasts reanalysis data (ERA Interim and ERA 5) indicates that there
is no clear increasing trend in the vertical velocity in the TTL over the Sahel.
Further detailed verification of the reanalysis data is needed.
VI. References
Gaetani, M., S. Janicot. M. Vrac, A.M. Famien, and B. Sultan, 2020: Robust
assessment of the time of emergence of precipitation change in West Africa. Sci.
Rep., 10, 7670. https://doi.org/10.1038/s41598-020-63782.
Kodera, K., N. Eguchi, R. Ueyama, Y. Kuroda, C. Kobayashi, B.M. Funatsu, and C.
Claud, 2019: Implication of tropical lower stratospheric cooling in recent trends
in tropical circulation and deep convective activity. Atmos. Chem. Phys., 19, 26552669, doi:10.5194/acp-19-2655-2019.
Pfister, L., Ueyama R., Jensen E., and Schoeberl, M., 2021: A method for obtaining
high frequency, global, IR-based convective cloud tops for studies of the tropical
tropopause layer, in preparation.
Taylor CM, D. Belušić, F. Guichard, D.J. Parker, T. Vischel, O. Bock, P.P. Harris, S.
Janicot, C. Klein, and G. Panthou, 2017: Frequency of extreme Sahelian storms
tripled since 1982 in satellite observations. Nature 544(7651): 475–478. doi:
/10.1038/nature22069.
VII. List of Publications and Selected Presentations• Kodera, K., Eguchi, N., Ueyama, R., Funatsu, B., Gaetani, M., and Taylor, C.
(2021), The impact of tropical tropopause cooling on Sahelian extreme
deep convection, J. Meteor. Soc. Japan, in review.
• Ueyama, R., E. Jensen, M. Krämer, L. Pfister, and M. Schoeberl (2020),
Impact of convectively-detrained ice crystals on the humidity of thetropical tropopause layer during boreal winter, PIRE-CIRRUS seminar.
(oral)• Ueyama, R., E. Jensen, and L. Pfister (2020), Convective impact on thestratospheric-entry water vapor through the tropical UTLS in boreal
winter and summer, American Geophysical Union Fall Meeting 2020. (oral)• Ueyama, R., E. Jensen, and L. Pfister (2020), Convective impact on thestratospheric-entry water vapor through the tropical UTLS in boreal
winter and summer, American Meteorological Society Meeting 2021. (oral)
VIII. Research meeting and discussion
Several virtual meetings with Rei Ueyama, Nawo Eguchi, and Kunihiko Kodera
Discussion topics:• Relationship between sudden stratospheric warmings and typhoons• JRA55 vs. ERA5 differences (e.g., omega trend at 150 hPa over Sahel)• Update on ongoing research projects by each member of our group• Future research topics to pursue together

IX. Additional information
Rei Ueyama has been on a reduced (75%) work schedule since March 2020 due
to COVID-19 caregiving of two young children at home. Schools in San MateoCounty, California, USA still remain closed. NASA has granted excused leave toemployees with COVID-19 caregiving responsibilities. Due to thisunprecedented circumstance, work capacity has been severely limited. All travelis still restricted so meeting in person was not possible this past year. However,
we have maintained communication via email and virtual meetings.
X. Members
Rei Ueyama
Nawo Eguchi
Kunihiko Kodera

NASA Ames Research Center
RIAM, Kyushu University
Meteorological Research Institute国際化推進共同研究概要
No.2
20EA-2タ イ ト ル： Turbulent mixing in the Kuroshio Current off Taiwan

研究代表者： JAN Sen

所内世話人： 遠藤

貴洋

研究概要：2018 年に開始された、国際化推進共同研究「Turbulent mixing in the
Kuroshio current off Taiwan」では、昨年度に引き続き九州大学応用力学研究所にて研究集会を開催する予定であったが、コロナ禍のためオンラインで実施せざるを得なかった。それにも関わらず、海外から 5 名、日本から 15 名が参加し、黒潮が海山を乗り越えることで生じる強い乱流混合の時空間変動の解明を進めていく上で、有意義な国際研究集会となった。
この共同研究の成果をもとに、国際誌へ論文 2 編が投稿された。

Report on 2020 RIAM International Joint Research Project

Turbulent mixing in the Kuroshio current off Taiwan
JAN, Sen (Institute of Oceanography, National Taiwan University)

Objective
Turbulent mixing in the ocean controls transport of heat,
freshwater, dissolved gasses, and pollutants. Turbulent
mixing is also of crucial importance for ocean biology, fromdetermining the flow field for the smallest plankton to setting
large-scale

gradientsofnutrient

availability.

Recentobservations suggest that the interaction of large-scale, lowfrequency geostrophic currents with steep topography
produces a rich sub-mesoscale and mesoscale vorticity field,
which initiates a cascade of energy down to small scales andturbulence. The Kuroshio off Taiwan is the very region wheresuch processes are highly expected, especially over the I-Lan
Ridge between Taiwan and Yonaguni Island, Japan (Figure 1). Figure 1. Bird’s-eye view of bathymetry around
This joint research project, which started in 2018, aims to

the I-Lan Ridge. The Kuroshio current flows

quantify the turbulent dissipation and associated nutrient

over the ridge to enter the East China Sea.

transport in the Kuroshio current over the I-Lan Ridge.

Research Plan
As part of this joint research project (18EA-2 and 19EA-3), we have carried out the field experiment over the
I-Lan Ridge using R/Vs Ocean Researcher I and II (OR1 and OR2) and Legend. The RIAM researchers joined theOR1 and Legend cruises to deploy their microstructure profiler, TurboMAP, and our tow-yo microstructure profiler,
VMP-250, respectively. Through the intercomparison of these microstructure data in the research workshop held inJanuary at RIAM, we quantified the turbulent dissipation and associated nutrient transport in the Kuroshio current
over the I-Lan Ridge as well as their downstream extent. In this fiscal year, we try not only to summarize our joint
research but also to share and discuss turbulent mixing processes in the Kuroshio current over various kinds ofbottom topography besides I-Lan ridge with Japanese researchers.
The members involved in this collaborative research and their roles are:•

JAN, Sen (NTU, Professor): Representative person•

YANG, Yiing Jang (NTU, Associate Professor): Analysis of the mooring data•

CHANG, Ming-Huei (NTU, Associate Professor): Analysis of the VMP-500 data•

CHEN, Jia-Lin (National Cheng Kung University, Assistant Professor): Numerical modelling using
OpenFOAM•CHENG, Yu-Hsin (NTU, Post-doc Researcher): Analysis of the VMP-500 and satellite data•

GUO, Xinyu (Ehime University, Professor): Numerical modelling using POM

•NAGAI, Takeyoshi (Tokyo University of Marine Science and Technology, Assistant Professor): Analysis of
the tow-yo microstructure profiler data•

MATSUNO, Takeshi (RIAM, Emeritus Professor): Analysis of the TurboMAP data•

SENJYU, Tomoharu (RIAM, Associate Professor): Analysis of the mooring data•

ENDOH, Takahiro (RIAM, Associate Professor): In charge of the collaborative research•

TSUTSUMI, Eisuke (The University of Tokyo, Project Assistant Professor): Numerical modelling using
MITgcmSummary of collaborative research
A simplified summary
Due to the COVID-19 pandemic, the workshop
planned to be held at RIAM had to be changed to thePlunging (HD control)

online meeting entitled “Online meeting on turbulent
mixing in the Kuroshio current over the topography”,
Wa
ter
fro
m

which was held on January 26 and 27, 2021. Associatedwith this, all the research budget provided for travel

ups
tre
am

MIXING

expenses has been returned. In addition to the members of
our joint research project listed above, we invited two
student speakers•

JIE, Gao (Ehime University, Doctoral Student)•

DURAN, Silvana (Tokyo University of Marine
Science and Technology, Master’s Student),

Both lee wave and hydraulic jump stay in the lee of the sill

Figure 2. Schematic of the small-scale features

who study the biogeochemical responses to the turbulent
mixing generated by the interaction of the Kuroshio current

resulting from the interaction between the
Kuroshio and the I-Lan Ridge (adapted from thepresentation by Dr. Ming-Huei Chang of NTU).

with an island and near-inertial internal gravity waves,respectively. In total, five overseas researchers as well as 15 Japanese researchers and students attended this online
meeting. We discussed diversity and universality in the turbulent mixing generated by the interaction of theKuroshio current with various topographic features (Figure 2).
Based on this joint research project, the following two articles have been submitted to international journals.
1.

CHANG, Ming-Huei, et al., Observations of Kuroshio flowing over a sill: small-scale processes and turbulent
mixing, submitted in January, 2021.

2.

NAGAI, Takeyoshi, et al., The Kuroshio flowing over seamounts and associated submesoscale flows drive
100-km-wide 100-1000-fold enhancement of turbulence, submitted in February, 2021.

The program of the online meeting is attached below.

Online meeting on turbulent mixing in the Kuroshio current over the
topography
Time Table in JST
January 26 (Tue.)
14:30

Opening remarks

14:35

Sen Jan (IONTU): Update of the result from direct TKE dissipation rate measurements off Taiwan withthe data collected in 2020
15:05

Ming-Huei Chang (IONTU): Observations of Kuroshio flowing over a sill: small-scale processes andturbulent mixing

15:35-45 Break
15:45

Jia-Lin Chen (NCKU): Mixing enhancement modulated by unsteady shear flow in the Kuroshio abovea system of seamounts
16:15

Eisuke Tsutsumi (AORI): An updated analysis on the observation and numerical modeling of turbulent

mixing in the I-Lan ridge

January 27 (Wed.)
10:00

Takeyoshi Nagai (TUMSAT): The Kuroshio Nutrient Stream, where the diapycnal mixing matters

10:35

Silvana Duran (TUMSAT): Elevated nutrient supply caused by an approaching Kuroshio to the southerncoast of Japan
11:05

Jie Gao (CMES): Occurrence of phytoplankton bloom as the Kuroshio passes an island

11:35-

Discussion and Closing remarks

国際化推進共同研究概要
No.3
20NU-1
タ イ ト ル： Numerical simulation of EC and EBW in QUEST研究代表者： BERTELLI Nicola

所内世話人： 出射研究概要：

浩

コロナ禍の影響で研究代表者は来日できなかったが、本共同研究課題は、遠隔で開催された別課題の「Plasma start-up and sustainment in
spherical tokamak configuration by RF」に関するワークショップでのQUEST に関する発表で議論された。
議論された QUEST に関する発表は、出射 浩：「QUEST における相対論的
ドップラーシフト効果に基づく第２高調波電子サイクロトロン加熱の速度空間での制御」である。

Numerical simulation of EC and EBW in QUESTBERTELLI Nicola(Princeton University, U.S.A)

I could not visit Research Institute for Applied Mechanics in Kyushu University due tothe Corona pandemic. This collaboration subject was discussed at the QUEST talk in the
remote workshop on “Plasma start-up and sustainment in spherical tokamak
configuration by RF” that is a different joint international research subject.
The related QUEST talk is as follows.
“Momentum-space control on relativistic Doppler-shifted resonance in second
harmonic ECH on QUEST” by H. Idei
The 28 GHz heating equipment of the launcher system with a focusing / steering mirror
and the quasi-optical polarizer system has been developed to conduct the EC plasma
ramp-up with local heating effect, or real and momentum space control. Heating andramp-up scenario were assessed following TASK/WR ray-tracing analysis (with full
relativistic-effect) and resonant momentum pitch analysis. The refractive indexes inparallel to the magnetic field N//s were scanned to control the resonant momentum space.
In the larger N// case, the energetic electrons were effectively generated, and ramped
plasma current became large. In the smaller N// case (~0.1), the bulk electrons were heated
up to 500 eV at ~1-2 x 1018m-3.

国際化推進共同研究概要
No.4
20NU-2タ イ ト ル： Plasma start-up and sustainment in spherical tokamak configuration
by RF

研究代表者： SHEVCHENKO

所内世話人： 出射研究概要：

Vladimir浩

高温プラズマ理工学研究センターで毎年開催され、これまでに８回を数えるワークショップを遠隔会議として実施した。英国の研究者が応用力学研究所・国際推進共同研究としてワ ークショップを開催している。これまでも一部の発表はリモートで実施されていたが、今回は、ワークシ
ョップ参加者が見込めないため、３日間、１４件の講演を全て遠隔会議で実施した。英国、米国からの参加があることから、大きな時差がある
中 で の 開 催 で あ っ た が 、「 Plasma start-up and sustainment in
spherical tokamak configuration by RF」 につき、活発に最新の実験
データ・解析、理論計算が議論された。ワークショップには、センターの海外連携分野の研究者も参加しており、QUEST 実験への提言、コメントも頂いている。

Plasma start-up and sustainment in spherical tokamak
configuration by RF
SHEVCHENKO Vladimir(Tokamak Energy Ltd., United Kingdom)

A workshop on “Plasma start-up and sustainment in spherical tokamak configuration
by RF” was held remotely as follows because no participants to the workshop were
expected due to Corona pandemic.
Agenda (25th to 27th Jan. in 2021)
Monday 1/25
JST 20:00 / GMT 11:00 / EST 6:00 [ + 10 min. ]Vladimir Shevchenko / Hanada
WS purpose and agenda
JST 20:10 / GMT 11:10 / EST 6:10 [ + 40 min. ]Yuichi Takase
Survey of RF research on TST-2
JST 21:00 / GMT 12:00 / EST 7:00 [ + 30 min. ]Akira Ejiri
Electron temperature and density profile measurements by Thomson scatteringsystems on TST-2 and QUESTJST 21:30 / GMT 12:30 / EST 7:30 [ + 40 min. ]Kazuaki Hanada
Historical Progress on QUEST for steady operation- What we could obtain? JST 22:10 / GMT 13:10 / EST 8:10 [ + 30 min. ]Makoto Hasegawa
Extension of Operation Region for Steady State Operation on QUEST
by Integrated Control with Hot Walls
Tuesday 1/26
JST 21:00 / GMT 12:00 / EST 7:00 [ + 40 min. ]Vladimir Shevchenko
ECRH & CD in spherical tokamaks

JST 21:40 / GMT 12:40 / EST 7:40 [ + 20 min. ]Erasmus du Toit
Developing a 0D model for studying EBW start-up in MASTJST 22:00 / GMT 13:00 / EST 8:00 [ + 30 min. ]Syunichi Shiraiwa
Recent development in Petra-M FEM framework and application
to tokamak experiments
JST 22:30 / GMT 13:30 / EST 8:30 [ + 30 min. ]Nicola BertelliHigh harmonic fast wave simulations in NSTX-Upgrade by using thePetra-M FEM framework
JST 23:00 / GMT 14:00 / EST 9:00 [ + 30 min. ]Masayuki Ono
Multi-Mode Excitation in Radio-Frequency Heating and Current Drive
Wednesday 1/27
JST 20:00 / GMT 11:00 / EST 6:00 [ + 30 min. ]Naoto Tsuji
Electron cyclotron heating assisted Ohmic start-up in the trapped
particle configuration on spherical tokamaks
JST 20:30 / GMT 11:30 / EST 6:30 [ + 30 min. ]Hiroshi Idei
Momentum-space control on relativistic Doppler-shifted resonance insecond harmonic ECH on QUESTJST 21:00 / GMT 12:00 / EST 7:00 [ + 30 min. ]Ryuya Ikezoe
Development of a magnetic fluctuation measurement system on QUESTJST 21:30 / GMT 12:30 / EST 7:30 [ + 30 min. ]Takeshi Ido
Design of a Heavy Ion Beam Probe for QUESTJST 22:00 / GMT 12:30 / EST 7:30 [ + 30 min. ]Kengo Kuroda
Initial results from high-field-side Transient CHI start-up on QUESTJST 22:30 / GMT 12:30 / EST 7:30 [ + 10 min. ]Vladimir Shevchenko / Hanada
Closing Remarks

Summary
Yuichi Takase
Survey of RF research on TST-2
RF experiments performed on TST-2 were reviewed. TST-2 started operation in 1999 on
the Hongo Campus of U. Tokyo. During initial operation, plasma current of nearly 100
kA was achieved by OH operation, and ST plasma formation with 1 kA plasma current
was achieved by 1 kW of ECH at 2.45 GHz. TST-2 was moved to Kyushu University in2003 to perform EBW experiments at 8.2 GHz with 200 kW power. X-B mode conversion
heating with up to 70% heating efficiency was demonstrated, and a steady state currentof 4 kA was maintained. A plasma ramp-up scenario using RF and vertical field ramp
was developed and applied to JT-60U where a formation of advanced tokamak with up
to 90% bootstrap current fraction was demonstrated. TST-2 was moved to the Kashiwa
Campus of U. Tokyo and resumed operation in 2005. It was shown that the formation of
ST configuration by pressure driven current is possible using lower frequency RF waves(21 MHz, 200 MHz). Electron heating by HHFW at 21 MHz was demonstrated. Current
drive by LHW at 200 MHz was developed using different methods of wave excitation,inductively coupled combline antenna, dielectric loaded waveguide antenna, andcapacitively coupled combline antenna (outboard launch and top launch). The highest
plasma current was achieved by top launch LHW, consistent with GENRAY/CQL3D
modelling. Further improvements are expected at higher toroidal field based on bothexperimental and computational results. A new type of travelling wave antenna (finline
antenna) is being developed for use at 2.45 GHz.
Akira Ejiri
Electron temperature and density profile measurements by Thomson scatteringsystems on TST-2 and QUEST
TS systems have been constructed and operated for TST-2 and QUEST. The systems canmeasure 5+5 (TST-2) / 6+6 (QUEST) spatial points with 6 wavelength channels. The
minimum measurable density is around 1x1017m-3, and the systems suitable for RF
driven low density plasma in TST-2 and QUEST.
RF induced transport model has been introduced to explain the HX spectra, central
temperature and response of RF turn off. Some features can be explained by the model,
and some cannot be.

A double pass TS scheme is under development to measure temperature anisotropy ofbulk electrons in RF driven plasmas. Theoretical work on the scheme was done to findoptimum configuration.
Increasing measurement days in QUEST is important, and some efforts were made, andsome are in progress/preparation.
Kazuaki Hanada
Historical Progress on QUEST for steady operation- What we could obtain? We could successfully promote steady state operation of tokamak with QUEST which
has all-metal plasma facing walls and a hot wall more than 10 years. It found thathydrogen transport barrier is formed between a plasma-induced deposition layer and asubstrate made of metal, which plays an essential role in fuel particle recycling. The
hydrogen barrier gives rise to presence of new time constant for steady state operation
which is related to the surface recombination coefficient. We have developed the way ofmonitoring of the value shot by shot using fuel emission just after the plasma termination.
The method can be applied to all the plasma experimental devices.
Temperature dependence of surface recombination coefficient is a key to handle particle
balance and the time constant. We have demonstrated that the higher wall temperatureprovides faster wall saturation and it leads to shorter pulse length. To overcome thedifficulty, wall temperature control is a candidate. Recently, several 6 hours discharges
could be obtained and we confirm that wall temperature control is a promising way toextend the pulse duration due to regulation of fuel particle balance.
In near future, we will try to achieve longer discharges under the high temperature
relevant to fusion reactors.
Makoto Hasegawa
Extension of Operation Region for Steady State Operation on QUEST
by Integrated Control with Hot Walls
The plasma first wall could be maintained at a temperature over 600 K. Long plasma
discharge over 6 hours could be obtained under a high temperature environment of 473
K with hot wall of APS-W and CS of SUS. The control of particle supply is sensitive totemperature changes in the first wall (HW and CS). Detailed temperature control of thefirst wall leads to an increase in operating area.

Vladimir Shevchenko
ECRH & CD in spherical tokamaks
In February-March 2020 there was a short experimental campaign P2.1 on ST40. This
campaign was conducted to re-confirm operational status of ST40 after re-commissioning.
After that several new diagnostics have been installed and commissioned. Solenoid andadditional vertical field power supplies were installed and tested. 0.6 MW 25 keV Heating
Neutral Beam (HNB) injector has been tested and installed. Some plasma heating andfuelling were observed with HNB injection into plasmas. To date plasma currents
achieved in ST40 are above 0.5 MA with the current flattop up to 100 ms. Toroidal fields
close to 2 T at the major radius of 0.4 m have been delivered on a regular basis.
Kiloelectron volt range electron and ion temperatures have been regularly achieved
during this campaign. All diagnostics were commissioned and prepared for full scale
experiments. Currently, installation of liquid nitrogen cooling for TF and Bv coils iscompleted. Next experimental campaign P2.2 is prepared to achieve first plasma in
February 2021.
Further upgrades of ST40 include installation of the 2nd HNB 1 MW 50 keV. It wascommissioned and prepared for experiments with plasma. The contracts for two dual
(140/105 GHz) frequency 1 MW gyrotrons were placed and first of them is scheduled fordelivery in Q3 2021. Capabilities of limitations of ECRH and CD at both frequencies
were discussed in relation to ST40 and the future project ST-F1.
Erasmus du Toit
Developing a 0D model for studying EBW start-up in MASTElectron Bernstein wave (EBW) start-up, including the generation of a plasma current
and formation of closed flux surfaces (CFS), was successfully demonstrated in thespherical tokamak MAST [1]. The formation of CFS is a crucial part of start-up, and,while various mechanisms have been proposed to explain their formation, no detailed
theoretical studies have previously been undertaken to study the time-evolution of theplasma current and formation of CFS. In this work, we present a simple model that
explains the experimental observations in terms of the underlying physics, by studying
the time evolution of the electron distribution function under a number of effects,
including an electron source, orbital losses, collisions, EBW heating, and plasma
induction.
Simulations show good agreement with experiments, providing explanations for several
experimentally observed effects, including the current drive mechanism and role of thevacuum magnetic field. In particular, we show that collisions are responsible for only asmall part of the current drive. The open magnetic field line configuration during startup, which leads to an asymmetric confinement of electrons [2] and is controlled by thevacuum magnetic field, is responsible for the majority of the generated plasma current.
[1] V.F. Shevchenko et al., EPJ Web of Conf. 87, 02007 (2015).
[2] T. Yoshinaga et al., Phys. Rev. Lett. 96, 125005 (2006).
Syunichi Shiraiwa
Recent development in Petra-M FEM framework and application
to tokamak experiments
We present the recent progress of Petra-M framework development. Petra-M is a finite
element (FEM) analysis platform we are developing for the integrated RF modeling. The
goal is to realize the full wave RF simulation which includes the entire tokamak plasma
from antenna to the hot core region with high physics and geometrical fidelity. Petra-M
uses the MFEM finite element library developed by LLNL for the FEM assembly andcombine it with other Open Source software including ASCR developed efficient linear
solvers and meshing algorithm. This year. significant development was made to enhance
its geometry editing capability, allowing for creating the detailed antenna structure basedon the information directly imported from the CAD software. We also extend our usageof the high order finite element basis to reduce the total degree of freedoms, which allows
for resolving the high harmonic fast waves in the entire NSTX-U device for the first time.
The verification and validation effort of Petra-M models are in progress through domestic
and international collaborations. This collaboration network covers the almost all RF
waves including ICRF, HHFW, Helicon and LH waves and major tokamak experimental
facilities worldwide, which makes the extensive test of our simulation platform possible.
Potential path to incorporate the arbitrary order finite Larmor radius (FLR) effects to FEM
wave simulation is discussed. Initial test in the 1D geometry using the O-X-B mode
conversion scenario foreseen on NSTX-U is consistent with the dispersion relation, andextension to 2D/3D is in progress.
Nicola BertelliHigh harmonic fast wave simulations in NSTX-Upgrade by using thePetra-M FEM framework
In this work we present the recent applications of the Petra-M finite-element-method(FEM) platform to NSTX-Upgrade. Petra-M code [1] is a state-of-the-art genericelectromagnetic simulation tool for modeling RF wave propagation based on MFEM
[http://mfem.org], open source scalable C++ finite element method library. This paper

shows the full 3D NSTX-U device geometry including realistic antenna geometry and 3D
scrape-off-layer (SOL) plasma in order to capture the 3D effects and the antenna-plasmainteraction in the SOL plasma and, at the same time, the core wave propagation. The
antenna geometry and the 3D NSTX-U geometry are from the NSTX-U CAD models.
A first ever HHFW full wave simulation for the full 3D NSTX-U torus has been shown[2]. A scan of the antenna phasing shows a strong interaction between FWs and the SOL
plasma for lower antenna phasing, which is consistent with previous NSTX HHFW
observations. A large electric field is found on the wall surface even far away from theantenna region. This quantity will be important, as a next step, for studying the antenna
impurity generation and RF sheath effects. The evaluation of the launching wave spectra
for three antenna phasing have been also presented. Such spectra are consistent with the
RF wave theory. A comparison between the scattering matrix (S-matrix) between vacuum
and plasma cases has been discussed. The S-matrix evaluation is the first step to be able
to quantify the HHFW antenna performance and compared it with the measurements.
Finally, we have discussed the PPPL-MIT collaboration on direct RF power levelutilizing the RF probes installed on NSTX-U.
[1] S. Shiraiwa et al., EPJ Web of Conferences 157, 03048 (2017).
[2] N. Bertelli, S. Shiraiwa, et al, AIP Conf. Proceeding 2254, 030001 (2020).
Masayuki Ono
Multi-Mode Excitation in Radio-Frequency Heating and Current Drive
While the rf actuators are potentially quite attractive for use in fusion reactor systems
particularly with the technology and economic advantages, there are quite significant rfplasma interaction related issues still require improved understanding and predictive
capabilities in order for the rf actuators to become the mainstream fusion reactor
components. The main challenges for rf actuators is that the antenna or waveguidelauncher is placed at the plasma edge where the plasma density is essentially zero. Thenthe launched waves much traverse the so-called plasma scrape-off layer where the density
increases from zero to 1019/m3 in a short distance of a few cm. Because of the large
variation in the density, the full wave equations must be solved from antenna/launcher tothe plasma core. There are also a large variation in the electron and ion temperatures. The
rf actuators are presently in three frequency ranges: ECH (electron cyclotron heating)using gyrotrons in 28 -170 GHz, LHCD (lower hybrid current drive) in 2 – 8 GHz range,
and ICRF (ion cyclotron range of frequency) in 10 – 140 MHZ ranges. More recently,
helicon experiments are being performed in the 500 MHz range. It is also recognized thatfor the same wave frequency range, there is a possibility of exciting multiple modes. For

ECH, it is well known that there are X-mode and O- mode which can be excited. The Xmode and O-mode excitation can be controlled to a large extent by controlling thepolarization and injection angle of the launched waves. In addition, EBW (electron
Bernstein wave) excitation is possible. The EBW is considered for its accessibility to thehigh-density regime above the so-called ECH cut-off density limit. For helicon regime, it
is also well known that there are both electromagnetic wave (“helicon”) and electrostatic
electron plasma wave (EPW) which is the LHCD wave. Recent wave modeling work at
PPPL showed that both helicon and LHCD waves can be excited simultaneously [1]. Our
analyses found that the edge density profile is highly important for the EPW excitation.
In particular, the density “gap” if created in front of the antenna can lead to a significant
EPW excitation even for a perfectly aligned antenna. For HHFW/ICRH, there are also
fast electromagnetic (FW) wave and electrostatic EPWs including the IBW (ion Bernsteinwave) branches. Since the EPW waves can exists at the low-density edge region of theplasma even for the ICRF frequency range, it is important to includes those electrostatic
waves to fully understand the rf coupling physics. One of the challenges of the rf coupling
problem for the electrostatic waves is the treatment of the so-called “S”-resonance [2]where the cold plasma approximation breaks down. The FLR effects would lead to the
IBW excitation at the “S” resonance. This suggests the potential importance of including
the FLR effects in assessing the viability of rf actuators. The wave equations with the
FLR effects have been developed for multiple harmonics. Once such wave equations arevalidated, it is then possible to incorporate them into the Petra-M 3-D full wave code [3]to obtain the full picture of the rf actuator wave coupling physics.
[1] Kim, E.-H., M. Ono, N. Bertelli, S. Wang, and H. K. Park (2020), AIP Conf.
Proceeding 2254, 050010 (2020); https://doi.org/10.1063/5.0013978[2] T.H. Stix, Waves in Plasmas (American Institute of Physics, New York, 1992).
[3] S. Shiraiwa, N. Bertelli, et. al., at this workshop.
Naoto Tsuji
Electron cyclotron heating assisted Ohmic start-up in the trapped
particle configuration on spherical tokamaks
TST-2 experiment showed that breakdown became slower when electron cyclotron (EC)waves were applied to the start-up in the conventional field-null configuration. Prompt
breakdown was recovered when the trapped-particle configuration (TPC) was usedinstead of the field-null configuration. A simple numerical analysis showed thatbreakdown indeed became slower at low neutral pressure and high EC power,
consistently with the experimentally observed trends.

Hiroshi Idei
Momentum-space control on relativistic Doppler-shifted resonance insecond harmonic ECH on QUESTThe 28 GHz heating equipment of the launcher system with a focusing / steering mirror
and the quasi-optical polarizer system has been developed to conduct the EC plasma
ramp-up with local heating effect, or real and momentum space control. Heating andramp-up scenario were assessed following TASK/WR ray-tracing analysis (with full
relativistic-effect) and resonant momentum pitch analysis. The refractive indexes inparallel to the magnetic field N//s were scanned to control the resonant momentum space.
In the larger N// case, the energetic electrons were effectively generated, and ramped
plasma current became large. In the smaller N// case (~0.1), the bulk electrons were heated
up to 500 eV at ~1-2 x 1018m-3.
Ryuya Ikezoe
Development of a magnetic fluctuation measurement system on QUEST
Poloidal and toroidal arrays of pick-up coils have been firstly installed inside the QUEST
vacuum vessel. Abrupt decrease in plasma current and corresponding variation ofequilibrium field were measured. Prompt loss of energetic electrons might be related.
High-frequency waves driven by energetic electrons in a whistler band has been measured
in a spherical tokamak. RF pick-up coils installed at HFS and at the fast-reciprocatingprobe head have shown various features of high-frequency wave activities.
In the future, multi-channel data acquisition system will be operated in the next campaign
to get all the pick-up coil signals at the same time. A dedicated experiment using thedeveloped fast-reciprocating RF probes and HFS RF probes is planned in next February.
Detailed analyses will be done to characterize the high-frequency waves driven byenergetic electrons on QUEST and elucidate the physics behind. Continues to develop anice wave measurement environment, control knobs for energetic electrons, anddistribution function diagnostics.
Takeshi Ido
Design of a Heavy Ion Beam Probe for QUEST
Heavy Ion Beam Probe (HIBP) has been designed for QUEST to investigate physicalmechanism of the particle and heat transport in plasmas. The injected probe beam for the
HIBP on QUEST is singly charged cesium ions (Cs+), and the required beam energy
ranges from 10 keV to 50 keV. By controlling the beam energy and incident angle of theprobe beam, measurable area covers the upper half of the QUEST plasmas. According tothe numerical calculation, the intensity of the detected beam current is large enough tomeasure micro turbulences at the central region of plasmas with the density of 1 x 1019(m-3) or less.
Kengo Kuroda
Initial results from high-field-side Transient CHI start-up on QUEST
T-CHI current start-up by using newly designed simple electrode has made important
progress. Flux evolution from the LFS and HFS T-CHI were examined. The potential for
flux evolution with a narrow footprint appears to be possible using HFS T-CHI comparedto that for LFS T-CHI. Some anticipated HFS T-CHI improvements were observed on
the modified system with in-vessel coil and cylinder electrode. On the low injector flux
configuration, camera images suggest a persisting plasma although a confined plasma
current is unclear. On the high injector flux configuration, current up to 35 kA wasgenerated in plasma that evolved only to the mid-plane (full growth into vessel should
rapidly increase the generated toroidal current for the same injector current). Optimizationof gas injection system on the modified electrode configuration are planed for next tasks.
This is necessary to make sure CHI initiation happens in the injector region while
preventing absorber arc for formation.

国際化推進共同研究概要
No.5
20NU-3タ イ ト ル： High power mm wave transmission line technology for advanced fusion
devices研究代表者： LECHTE Carsten Hanno

所内世話人： 出射研究概要：

浩ITER プラズマでの不安定性抑制のための大電力・高周波デバイスの開発を進めている。これまでに九州大学でデバイスの低電力試験を、量子科学技術研究開発機構・那珂核融合研究所で大電力試験を実施している。今年
度、高周波電界の偏波面を変えて行う実験を実施する予定であったが、海外からの研究者の参加が見込めない中、遠隔会議を重ねることで、偏波面制御デバイスの開発（ドイツ）、据付・大電力試験（那珂核融合研究所）を進め、偏波面制御でこれまで起きていたアーキング現象を大幅に軽減することに成功した。遠隔会議での議論、遠隔支援のもとで実験を実施し、共同研究を推進した。

High power mm wave transmission line technology
for advanced fusion devices
LECHTE Carsten Hanno
University of Stuttgart, Germany
The FAst DIrectional Switch (FADIS) system has been studied for ITER application
at 170 GHz as a topic under joint international research framework at Research Institutefor Applied Mechanics in Kyushu University. The system device was developed at
University of Stuttgart, Germany, and shipped to Kyushu University in 2018. Basic
performance was tested at low power level in Kyushu University, and shipped to Naka
Institute of National Institutes for Quantum and Radiological Science and Technology in
2019. High power tests were conducted in Naka Institute along the low power test resultsin Kyushu University.
The incident wave-polarization has to be controlled for proper FADIS performance.
The optimum incident wave into the FADIS is a linear-polarized wave with an azimuthal
angle in the horizontal-vertical wave-field plane. The quarter wavelength polarizer(rotator) in Kyushu University was used to control the azimuthal angle of the polarization.
The performance of the rotator was tested at the low power test facilities in Kyushu
University. Although the azimuthal angle was properly controlled with the rotator, theellipticity was also changed by setting of the azimuthal angle. The controlled wave wasan elliptically polarized-wave, and not the optimum linearly polarized-wave. The FADIS
system was installed at the transmission line of the high-power test-stand with the rotatorin Naka Institute. Although the FADIS performance was tested at the high-power teststand in Naka Institute, but we had many unwanted arcing events inside the FADIS.
To avoid the arcing events in the FADIS, a universal polarizer has been developed at
University of Stuttgart. The universal polarizer can control the azimuthal angle of thepolarization without changing of the ellipticity by rotating the device on the transmissionline axis. The optimum linearly polarized-wave can be excited with the universal
polarizer. Figure 1 shows a photo and a drawing of the universal polarizer. It was tested
at the low power level in University of Stuttgart, and shipped to Kyushu University. It
was finally shipped to Naka Institute, and installed at the transmission line of the highpower test-stand. Figure 2 illustrates setup of the universal polarizer in the transmission.
The polarizer was set with an angle of 17.65 degrees on the transmission-line axis toexcite the optimal polarization with the azimuthal angle of 37.3 degrees. We
communicated with e-mails together with to discuss how to install the polarizer andconduct the high-power test in Naka Institute, because we could not join the high-power
test in Naka Institute due to the Corona pandemic. In the discussion on the high-power
test, electropolishing of mirrors in the FADIS was proposed to avoid the arcing events.
After the electropolishing of the mirrors, the high-power test was conducted in Naka 
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RF (a.u.)
Ic (10A/div)

窓アーク (a.u.)Ia (50mA/div)

図 1 I c ∼ 30A 、 100ms ショ ッ ト
RF (a.u.)
電解研磨後(2020)

電解研磨前(2019)

窓アーク (a.u.)

図 1 I c ∼ 30A 、 100ms ショ ッ ト
電解研磨後(2020)

電解研磨前(2019)

図 2 左が今回の発光、 右が 2019 年の発光

3. 本日の運転Fig. 2 Left side is arc light with power of 341kW in 2020. Right side is arc light in 2019
Fig.3: Leftside is arc
power of 341 kW in 2020. Right side is arc
experiments
withlight
powerwithof 324kW.
周波数を 掃引し Resonant Load に入射でき る か試みる 。
light in 2019.
図 2 左が今回の発光、 右が 2019 年の発光

Afteris decreased,frequency
scanRight
was performed
in orderto find a
Fig.confirming
2 Left sidetheis arc light
light withpower of a341kW
in 2020.
side is arc lightin 2019
3. 本日の運転The
experiments
areperformed
from
7th
to
10th
September
in
2020.
Radio
Frequency
experimentsThe
withtime
powerof 324kW. of the frequency is shown in Fig. 3. As shown inresonant frequency.
dependence
周波数を 掃引し Resonant Load に入射でき る か試みる 。
(RF)to FADIS
wasstartedstablewith aafter
relatively
power
levelfrequencyof 130kW
thepower
figure,injection
the frequencychangebecomes
100ms low
except~5MHz
After confirming
the arc lightis decreased,a thefrequency
scan wasperformed
in ordertest,
to finda power(Gyrotron
beam
current;
Ib~10A).
After~100kW(Ibfield
~10A)
power
the
oscillation.
By changing
thegyrotronoscillation
magnetic(∝Bc)
slightly,
thestableresonant frequency. The time dependence of the frequency is shown in Fig. 3. As shown inincreased
upcan
to 341kW(Ib slightly.
~30A). The arc light was observed, but the intensity of the arc
frequency
be changed
the figure, the frequency change becomes stable after 100ms except ~5MHz frequency
light apparently deceased, compared with the experimental campaign in 2019 as shown
oscillation. By changing the gyrotron oscillation magnetic field (∝Bc) slightly, the stablein Fig.frequency
3. Afterthe arc light decreased, a frequency scan was performed incanconfirming
be changed slightly.

Fig. 3 Time dependence of the frequency. Ic~10A operation with Bc=86.7A
Fig. 3 Time dependence of the frequency. Ic~10A operation with Bc=86.7A

Fig.4: Time dependence of oscillated frequency in 10 A operation of Ib withgyrotron axial magnetic field Bc made at 86.7 A. The frequency becomes stable after
100 ms except for a ~ 5 MHz frequency oscillation.

order to find a resonant frequency of the FADIS. The time dependence of the frequency
is shown in Fig. 4. As shown in the figure, the frequency becomes stable after 100 ms
except for a ~ 5 MHz frequency oscillation.
By changing the gyrotron oscillation magnetic field Bc slightly, the stable frequency
can be changed slightly. The result of the frequency scan by Bc is shown Fig. 4. As shownin Fig. 4, the frequency is changed from 169.87 GHz to 169.99 GHz (~ 120 MHz). The
frequency scan was performed with both Ib ~ 10 A (100 kW) operation and Ib ~ 30 A
operation (341 kW).
Note, the frequency was measured around 250 ms for the Ib ~10 A
The result of the frequency scan by Bc is shown Fig. 4. As shown in Fig. 4, the frequency isfrom 169.87GHz to 169.99GH (~120MHz). The frequency scan was performed with
operation and 200changed
ms forthe Ib ~ 30 A operation, since the pulse width was not extended
both Ic~10A (100kW) operation and Ic~30A operation (350kW). Note, the frequency ismore than 200 msmeasured
for Ib around~ 30250ms
A operation
to avoid
the forriskof thegyrotronfor the Ic~10A operationand 200ms
the Ic~30A
operation,
since damage by RF
the pulse width was not extended more than 200ms for Ic~30A operation to avoid the risk ofreflection from thetheFADIS.
gyrotron damage by RF reflection from FADIS.
170

Ic=10A
Ic=30A

169.98Frequency [GHz]169.96169.94169.92169.9

169.88169.86
85.4

85.6

85.8

86

86.2

86.4

86.6

86.8

87

87.2

87.4

87.6

Bc [A]
Fig. 4

Measured frequency according to gyrotron magnetic field Bc.

Fig. 5: Measured frequency according to gyrotron magnetic field Bc .
During the frequency scan, the non-resonant load and the resonant load input power ismeasured as shown in Fig. 5. The power is increased as magnetic field Bc is decreased, whichis nature of gyrotron. However, as shown in Fig. 5, there was no specific change of power in
Non-resonant dummy load and resonant dummy load with any specific frequency. As a

During the frequency scan, coupled power in non-resonant and resonant dummy loads
conclusion, we could not find any evidence that we operated with resonant frequency. The
region of the frequency
scan waspossibleto thepower
gyrotron conditionand power
in the FADIS waswider
measuredas shown
innotFig.
6.dueThe
increased
as magnetic field
supply condition. So, we stopped the frequency scan as it is.
Bc decreased, which is the nature of gyrotrons. The coupling into non-resonant or resonant
dummy load should depend on the frequency as switching performance in the FADIS
operation. However, as shown in Fig. 6, there was no specific change of power in the
non-resonant and resonant dummy loads with any specific frequency. As a conclusion,
we could not find any evidence that we operated with resonant frequency in the highpower test. The wider region of the frequency scan was not possible due to the gyrotron
condition and power supply condition. Fine tuning of frequency or resonant conditions isneeded to confirm the switching FADIS operation in the high-power test.

400

Ic=10A Non-res
Ic=30A Non-res
Ic=10A Res
Ic=30A Res

350

Power [a.u.]

300
250

200

150

100

50

0
85.4

85.6

85.8

86

86.2

86.4

86.6

86.8

87

87.2

87.4

87.6

Bc [A]

Fig. 5

Measured power according to gyrotron magnetic field Bc, i.e., frequency.

Fig. 6: Measured power according to gyrotron magnetic field Bc ,i.e. frequency.

国際化推進共同研究概要
No.6
20NU-4
タ イ ト ル： Development of Core-SOL-Divertor model for simulating tokamak
plasmas with impurities
研究代表者： WISITSORASAK Apiwat

所内世話人： 糟谷

直宏

研究概要：
トカマクプラズマにおける不純物輸送について統合的な解析を可能とするために、
TASK/TI コードへ SOL ダイバータ領域の効果を組み合わせるのが本研究の課題である。
コア輸送モデルと SOL 流体モデルとの結合を図っている。本年度はコロナ禍の影響で応
力研出張が取りやめとなった。代わってオンラインでの打ち合わせによりお互いの研究
状況を理解し、今後の展望について議論した。

Development of Core-SOL-Divertor model for simulating tokamak plasma with impurities
Apiwat Wisitsorasak, Department of Physics,
King Mongkut's University of Technology Thonburi, Thailand
Impurities in tokamak plasma introduce several deleterious effects on the overall performanceof the devices. Large amount of the impurities can dilute the fuel and reduce the rate of the fusion
reactions. Furthermore, one of the most immediate effect is the radiated power loss which leads to lower
the temperature of the plasma. For examples, impurity ions, such as oxygen and carbon, which are
originating from the tokamak vessel, cool the plasma strongly near the edge. However, too much edge
cooling destabilizes the plasma and leads to plasma disruption which can severely damage the wall andother structures [1, 2]. On the other hand, metal ions from the plasma-facing components, such as
tungsten, can travel farther from the edge and cause significant radiation in in the core. This prevents
the plasma to reach high enough temperature for ignition. Hence the concentration of the impurities
should be minimized. For a tokamak with the divertor configuration, the impurities shall be pumped
away near the divertor, otherwise they will accumulate in the vessel. Besides the downside effects of
the impurities, the radiation of the plasma impurities nevertheless has some helpful consequences.
Injection of noble gases, such as argon or neon, is intentionally used to increase the radiation in theedge region of the plasma. A well-controlled amount of these seeded impurities helps to disperse theplasma power exhaust over wider surface areas and reduce the temperature in front of the plasma facing
components.
This work ultimately aims to understand the transport behavior of the underlying physicalmechanisms of the impurities by using a simulation. TASK is a major code in Japan for integrated
plasma simulation. The module TI for impurity transport in TASK has been developed by Prof.
Emeritus Atsushi Fukuyama in Kyoto University in collaboration with Prof. Naohiro Kasuya in Kyushu
University and Theory group in National Institute for Quantum and Radiological Science andTechnology (QST). It is based on a fluid description of the plasma and able to compute the profile
evolution of the densities and temperatures of all ion stages. The code is a one-dimensional (radial)code in which a method of flux averaging is used. However, the present version of the code only
considers the transport in the plasma core in which the magnetic field lines are closed. The transport in
the scrape-off layer (SOL), the region outside the plasma core, is not considered yet. Thus, this work
will eventually develop the edge transport modeling code and couple it to the impurity transport codein the core by collaboration with RIAM.
The dynamics of the plasma in the edge is complex and involves several nonlinear plasma
phenomena [2]. To simulate the plasma in this region, one may reduce the complexity of the problem
by only considering the transport along a magnetic field line. The dynamics five-point model is further
simplifying the transport and is developed to investigate the response of the plasma in the SOL anddivertor regions at five points: the stagnation point (0), upstream throats of divertors (𝑢# , 𝑢% ), anddivertor plates (𝑠# , 𝑠% ), see figure 1 [3, 4].
The five-point model simply considers that the plasma particles are transported along an open
magnetic flux tube which is divided into four regions (figure 1). By integrating the fluid equations along
the magnetic flux tube in each region, one obtains the dynamical equations of the density (𝑛), ion particle
flux (Γ), electron (𝑇*) and ion (𝑇+ ) temperatures at each point along the SOL as follows [3, 4].
𝐿-./ 𝑑𝑛1 /𝑑𝑡 = −Γ78 −Γ79 + 𝑆1 𝐿-./ ,(1)

𝐿<=> 𝑑𝑛?9,8 /𝑑𝑡 = −Γ79,8 −Γ?9,8 + 𝑆9,8 𝐿-./ ,(2)

0.5𝑚+ C𝑙E,F + (𝑅9,8 + 1)𝐿<=> K𝑑Γ79,8 /𝑑𝑡 = 𝑛1 (𝑇*1 + 𝑇+1 ) − 𝑛?9,8 C2𝑇M?9,8 + (1 + 𝑔)𝑇=?9,8 K,(3)

1.5𝐿-./ 𝑑(𝑛1 𝑇O1 )/𝑑𝑡 = −𝑄O78 − 𝑄O79 − 𝛿O 𝐽(𝜙78 − 𝜙79 ) + (𝑊O1 + 𝑊OMU1 )𝐿-./,(4)1

1.5𝐿<=> 𝑑(𝑛?9,8 𝑇O?9,8 )/𝑑𝑡 = −𝑄O79,8 − 𝑄O?9,8 − 𝛿O 𝜎9,8 𝐽(𝜙79,8 − 𝜙?9,8 ) + (𝑊O9,8 + 𝑊OMU9,8 )𝐿<=> ,(5)

where the subscript 𝑗 refers to particle species (𝑖 for ions and 𝑒 for electron) with 𝛿* = 1, 𝛿+ = 0, 𝜎# = −1and 𝜎% = 1. 𝑆1 and 𝑊1 are the particle and energy source in the radial direction. The neutral particle source
and the ionization energy due to these neutrals are given by 𝑆9,8 and 𝑊O9,8 , respectively. Γ and 𝑄 are theparticle and heat flux from the core.

Figure 1: Schematic diagram showing the geometry of the five-point model which considers the transport along the magnetic field.
The dynamic five-point model can be extended to explicitly describe impurity production andtransport in the tokamak edge region. This task can be achieved by considering the multifluid equationsof impurity species. In the edge, the temperature and density of the plasma are typically less than 200
eV and 1021 m-3, respectively [2]. Thus, the charge recombination of the impurity ions may be neglected.
Upon neglecting the inertia terms and only considering the transport along the field line, the impurity
transport equations of each charge state at the steady state can be written as
𝜕𝑛[ 𝑣[= 𝑛[^_ 𝛼[^_ − 𝑛[ 𝛼[ ,𝜕𝑧𝑛[ 𝑣 [ = −𝜏[ 𝑇+ 𝜕𝑛[ 𝑘𝑒𝑛[ 𝜏[ 𝜕𝜙𝜕𝑛[−+ 𝑛[ 𝑣+ ≡ −𝐷[+ 𝑤[ 𝑛[ ,
𝑚b 𝜕𝑧
𝑚b 𝜕𝑧
𝜕𝑧

(6)
(7)

where the subscript 𝑘 denotes the charge state of impurity (𝑘 = 1, . . . , 𝑍hij), 𝑚b is the impurity mass, 𝑣[ isthe parallel velocity, 𝛼[ = 𝑛* 〈𝜎𝑣〉[ is the ionization rate coefficient of impurity species 𝑘 [5, 6]. These
two equations can be solved in two different limits which are based on the relative magnitudes of theionization and parallel transport contribution for the density of each charge state. For the lower charge
state (𝑘 < 𝐾 < 𝑍hij), the characteristic time of the ionization process is approximately faster than theparallel transport process. This subsequently leads to the following relation:
𝑛[ =

𝛼[^_
𝑛 .
𝛼[ [^_(8)

On the opposite case, for the higher charge state (𝑘 ≥ 𝐾 ), the ionization process will occur on the timescale much longer the parallel transport. The continuity equation then becomesr𝑛[ 𝑣[ = p 𝛼[^_ 𝑛[^_ (𝑧) d𝑧,(9)s

where the integration is performed along the magnetic field line (𝑧 direction). Finally, one can explicitly
solve equations (6) and (7) for the density of the kth charge state:

2

s

w

𝑛[ (𝑧) = 𝑒 tu (s) v𝑛[,?9,8 + p 𝑒 ^tu (s ) p
1

r

sw

𝛼[^_
𝑛 (𝑧 xx ) d𝑧 xx d𝑧 x y,
𝐷[ [^_(10)

where 𝑃[ (𝑧) is an integrating factor which depends on local velocities and electric potential [5]. In
principle, once the background plasma is numerically determined by the basic five-point model,
equations (8) and (10) can be directly used to solve the impurity density of each charge at each point
along the field line.
Even though the ultimate goal of this work is to develop the calculation method for providing
the plasma boundary conditions for self-consistently simulating the impurity transport in the core, suchcalculation has not been integrated into the transport code yet. The COVID-19 pandemic has large
impact on international travel in which several commercial flights have been canceled and many
countries have also imposed restrictions for all travelers. These limited visiting RIAM during the lastyear. However, we have communicated and collaborated the project by using online tools such as Zoom
and exchanging emails.
In the future work we have planned to implement the calculation of the impurity transport into
TASK/TI code which will be a useful and effective method to simulate complex plasmas in both core,
SOL, and divertor regions. The simulation result based on this scheme will be eventually evaluated withexperimental results from QUEST, PLATO, WEST, and other tokamak, based on availability of thedata.
References:[1] Wesson, J., & Campbell, D. J. (2011). Tokamaks (Vol. 149). Oxford University Press.
[2] Stangeby, P. C. (2000). The plasma boundary of magnetic fusion devices (Vol. 224). Philadelphia,
Pennsylvania: Institute of Physics Publication.
[3] Hayashi, N., Takizuka, T., Hatayama, A., & Ogasawara, M. (1998). Onset condition ofthermoelectric instability in divertor tokamaks. Nuclear Fusion, 38(11), 1695.
[4] Hayashi, N., Takizuka, T., & Hosokawa, M. (2007). Modeling of dynamic response of SOLdivertor plasmas to an ELM crash. Journal of Nuclear Materials, 363, 1044-1049.
[5] Zagorski, R., & Romanelli, F. (1996). Modelling of impurity production and transport in the
scrape-off layer of a high-density limiter tokamak. Nuclear Fusion, 36(7), 853.
[6] Stangeby, P. C., & Elder, J. D. (1995). Impurity retention by divertors: one dimensional models.
Nuclear Fusion, 35(11), 1391.

3

国際化推進共同研究概要
No.7
20NU-5
タ イ ト ル： Design of a Permanent CHI injector coil for QUEST研究代表者： RAMAN Roger

所内世話人： 花田

和明

研究概要：
これまでの QUEST の CHI 実験において、バイアス電極とセンタースタック間に入射磁束を形成して CHI 放電を行う場合、その入射領域におけるプラズマ着火が困難であることが示された。また入射領域から磁場形成コイル(PF5-1)の距離が離れているため高いコ
イル電流を印加する必要があり、その結果電極に接続する 2 本の磁束の脚(footprint)は入射領域を超えた領域まで広がってしまった。2020 年度の実験では CHI 放電のために磁場コイルを増設する案を想定して、その評価のための簡易的な試作コイル(2x12mmの角型カプトン被膜導体を 12 巻したコイル)を真空容器内の電極近傍位置に導入した。
更に磁束の形成を入射領域のみに制限するための垂直円筒板をバイアス電極に設置した。放電結果では、入射磁束量を下げた配位において磁束は大きく拡大し、入射電流の駆動が終了した後もドーナッツ形状のプラズマが容器中心部で持続する様子が観測された。この結果は閉磁気面の形成によりプラズマが閉じ込めれていることを示唆している。本実験での結果に基づき次の CHI システムの改造が検討された。まず一つ目に本実験で導入した垂直円筒板をバイアス電極に対して下方向に設置する。これにより垂直円
筒板がプラズマ形成に影響を与えることはなくなるので ECH などとの併用が可能になる。二つ目にバイアス電極を現状位置より下げる。これにより適切な入射磁束を PF5-1のコイルにより形成することが可能になり、コイルを増設する必要がなくなる。現状増設コイルの設置には装置下部の改造が必要であり、PF5-1 コイルを使用する方が好ましい。また電極を下げることでアブソーバー放電の発生が低減すると考えられる。2021 年
度に QUEST での高温壁温度制御実験が予定されているため、高温壁の水冷管とバイアス
電極の距離を十分に離す必要がある。上記改造は 2021 年度に実施する予定である。CHI
立ち上げによる閉じ込め配位の形成が達成されれば、これと ECH の組み合わせ手法を用いた QUEST での非誘導電流駆動方式による長時間定常運転の進展が期待される。

Design of a Permanent CHI injector coil for QUEST

2 January 2020
R. Raman1, K. Kuroda2, K. Hanada2, M. Ono3, B.A. Nelson1, T. Onchi2, M. Hasegawa2, J. Rogers1, M. Nagata4, O.
Mitarai5, S. Kojima2, T.R. Jarboe1
1

University of Washington, Seattle, WA, USA

2

Kyushu University, Kyushu, Japan3

Princeton Plasma Physics Laboratory, Princeton, NJ, USA

4

University of Hyogo, Himeji, Japan

5

Institute for Advanced Fusion and Plasma Education, Japan

Introduction
Methods for starting a plasma discharge in a spherical tokamak (ST) without reliance of the center solenoid areessential for the validity of the ST concept. These methods could also simplify and reduce the cost of tokamakbased systems and make them more economical by eliminating components that are not needed during steadystate operation. Coaxial Helicity Injection (CHI) for an ST, first developed on HIT-II at the Univ. of Washington
and then further developed on NSTX, is the method for which scaling to larger devices such as reactors is wellunderstood. On QUEST, this method would be further developed using the unique all metal capability of QUEST,
which is predicted to reduce low-z impurities. In addition, CHI on QUEST will develop a new configuration thatis much more suited to an ST-FNSF.

Basic concept for CHI operation on QUESTOn HIT-II, and on NSTX, CHI is implemented bydriving current along externally produced field linesthat connect the inner and outer vacuum vessel
components in the presence of externally generated
toroidal and poloidal magnetic fields. This isqualitatively shown in Fig. 1 (a). On QUEST, as briefly
described in Fig. 1 (b), a toroidal ring electrode ismounted on top of the existing lower divertor plate,
and the electrode separated from the divertor plateusing a toroidal alumina insulator. Magnetic flux
generated by the lower divertor coils connects thiselectrode plate (the cathode) to the outer vessel (theanode). Gas is injected in the gap between theseelectrodes and a 20-30mF capacitor bank, charged up
to 2kV is discharged across these electrodes to
Figure 1: (a) Layout of the transient CHI startup systems in NSTX.
generate the CHI plasma. In experiments conducted
The blue circle is the poloidal injector flux produced by the lower
divertor coils. This connects the two lower divertor plates, which are
during 2019, in which the electrode was biased withinsulated. Gas is injected in the region below the divertor gap. On
respect
to the center stack on QUEST it was found that
NSTX typically a 5 to 15mF capacitor bank charged up to 1.7kV isdifficulty in forming a suitable injector flux using theused to produce the injector current. (b) Typical vacuum field line
configuration for CHI discharge initiation on QUEST.
existing PF coil set on QUEST resulted in difficulty ingood discharge initiation in the region between theelectrode and the center stack. Furthermore, because the PF5-1 coil is far away from the injector region, a
relatively high coil current was required, and the resulting flux footprint was also widespread and beyond themain injector region.

Permanent Coil and CHI System for QUEST
In support of a permanent coil and CHI system on QUEST, during 2020, experiments were conductedusing a temporary in-vessel coil to develop an understanding of the needs for a permamnt coil. Figure
2 shows that concept. This coil allowed improved localization of the injector flux footprint locations (to induce

reconnection in this region), This coil is composed of 12 turns wound Kapton insulated copper rectangularconductor. In a second modification, a steel cylindrical electrode was attached to the bias electrode so that theouter leg of the injector flux is limited to the radius of this cylinder.
Fig. 3 shows improved results due to thesemodifications. Shot #42640 is a discharge when theinjector flux magnitude is low, in which about
3 mWb injector flux is formed by 0.5 kA of invessel-coil current and 1.0 kA of PF5-1 coil current.
After the flux evolution, and after some of thisextended flux decays, a detached plasma seen topersists in the camera image even though the injector
current has been reduced to zero at 𝑡 = 38.75ms. The
Fig. 2 Modification of t-CHI system. Temporarily installed in-vessel- camera images are encouraging in that they suggest
the formation of closed flux plasma, because acoil and cylinder attached to the bias electrode.
doughnut shaped plasma at the vessel mid-plane,
apparently not connected to any other part of thevessel could probably only exist if currents are flowing on closed field lines. Unfortunately, the extensive absorberarching, which in some discharges produces a transient negative current signal, does not permit a Rogowski coil
to clearly detect the CHI produced toroidal current.
However, these studies provided sufficient information that wasused to develop a design for a permanent coil and CHI system for
QUEST. The concept is shown in Figure 4. The primaryadvantages are:Instead of the ‘L’ shaped region that sticks into the vessel in thetemporary system shown in Fig. 2, the ‘L’ shaped region is now
below the divertor plates, so that it does not protrude into the main
plasma region. Thus, this is compatible with normal plasma
operations on QUEST. This would support operations of CHI +ECH.
The CHI electrode is lowered. This is important for two reasons.
First, it allows the existing PF5-1 coil to be used as the main CHI
injector coils as the flux generated by this coil in the injector regionis actually now superior to the flux generated by the in-vessel coilshown in Fig. 2. Since it is quite expensive to wind a high current
coil without dismantling parts of the components below the
QUEST vessel, this would save considerable time and cost as thePF5-1 coil ratings are much more than what is needed for CHI
operations.

Fig. 3 Improved CHI discharge in shot
#42640.

The second benefit is that the electrode plates are now farther awayfrom the water lines that will feed the hot wall components during
QUEST operations during 2021 and beyond. This larger separation
distance combined with the fully enclosed CHI region (which will
now avoid the CHI gas from leaking down below the divertor
plates) should reduce absorber arcing.

These improvements are planned to be implemented during 2021. With these important changes, it is anticipatedthat routine CHI operations that generate significant amounts of closed flux, can be combined with ECH
operations to develops heating and non-inductive current drive capabilities in support of long-pulse steady-state
operations on QUEST.

Fig 4: Concept drawing of the new CHI system

Fig 5: Vacuum flux pattern in themodified CHI region resulting from theuse of the PF5-1 coil as the main injector
flux coil.

References[1] Kuroda K et al 2018 Plasma Phys. Contr. Fusion 60 115001.
[2] Raman R et al 2004 Phys. Plasmas 11 2565.
[3] Raman R et al 2011 Phys. Plasmas 18 092504__________________________________________*

We acknowledge helpful discussions with Prof. Zushi, Mr. Noda (V-Tech Limited) and Mr. Rogers (Univ. of

Washington) and with other members of the QUEST Team.

国際化推進共同研究概要
No.8
20NU-6
タ イ ト ル： Joint study of long pulse high beta discharges and related egde
turbulence transport in steady state operation (SSO) plasmas on
QUEST and EAST

研究代表者： GAO Xiang

所内世話人： 花田和明

研究概要： 今年度は遠隔でのディスカッションを行った。QUEST で開発された熱負荷計測を EAST の長時間 H-mode プラズマに適用した。IR カメラ計測では表面のわずかな堆積層が発熱することで表面温度が上がるので絶対値の計測が困難であった。一方、冷却水の経路に沿って積算された
熱負荷しか測れないため分布の計測が困難である。この両者の欠点を補いあうことで、ダイバータ部の熱負荷分布を絶対値まで含めて行った。この結果は論文として出版された。また当該学生は本研究により中国で学位を取得した。

[1] Y.K. Liu, X, Gao, K. Hanada, Y.W.Yu, H.Q.Liu,…,Y.X.Jie, J.P.Qian et al., Nuclear
Fusion 60(2020) 096019.

（様式 11・拠点）

RESEARCH REPORT

Date: March 5

Visiting scientists: (name)Xiang Gao

(position)

Professor

2021(university / institute) Institute of Plasma Physics,
Chinese Academy of Sciences

Host scientist: (name)

Kazuaki Hanada(position)

Professor(university / institute)

KyushuResearch period: (from)

University(to)Research subject: Joint study of long pulse high beta discharges andrelated egde turbulence transport in steady state operation (SSO) plasmas
on QUEST and EAST

（様式 11・拠点）

Introduction
This year, given the situation of COVID-19, the joint research was done remotely.
Steady state operation (SSO) of tokamak plasma is one of the basic requirements for
future fusion reactors. Long pulse high beta operation is one of important missions for
ITER. Joint study long pulse high beta discharges in SSO plasma research field on QUEST
and EAST is strongly supporting ITER experiment from both experience and theory. In thisyear, the collaboration research focused on the power balance in SSO high performance
discharge, which is benefit for the SSO high beta discharges of EAST and QUEST.

New results in 2020
The Experimental Advanced Superconducting Tokamak (EAST) research program
concentrates on demonstrating steady-state high-performance H-mode operations with
ITER-like tungsten divertor. Calorimetry was applied to actively water-cool the plasma
facing components (PFCs) by increasing the water temperature for power balance
investigation. Considering the energy balance of EAST long-pulse high-performance
discharges with upper single null (USN) configuration, thus far, approximately 78% of theinjected energy could be accounted for.The method of estimation of heat flux on upper
tungsten divertor target with a high time- and spatial-resolved infrared camera has beendeveloped, and the sum of its heat load was found to be significantly consistent with thatmeasured through calorimetry. The record longest steady-state H-mode plasma #73 999
was sustained for up to 101.2 s with net injected energy exceeding ~0.25 GJ in the USN
configuration. Heat load analysis of this discharge using calorimetric measurement
indicates that the modification of heat load distribution was observed and this was induced
by a slight change in the magnetic configuration. Not all temperature increments in the fivecooling water modules reached the saturated state for the 100 s level discharge, whichmeans that 100 s timescales are insufficient as compared to the thermal transport timescalein the targeted PFCs. The heat load on the tungsten divertor targets is not evenly distributedwith the ratio of ~2 in favour of the outer divertor.

（様式 11・拠点）

Figure 1 Time evolution of Tdep and Tsurf at the outer and inner strike points (a), of heat flux at theouter and inner strike points (b), of power load (c) for #73 999..

Figure 2 Time evolution of heat flux deposited on outer (a) and inner (b) targets of tungsten divertor for#73 999. The black lines show the distance between strike points and divertor corners..

（様式 11・拠点）

As shown in Figure 1, the time evolutions of temperature and heat flux at outer and inner
strike points for #73 999 are shown in figures 1(a)–(b), heat transmission coefficients forouter and inner targets are 7.5 and 10.1 kWm−2 K−1, respectively. The maximum of Tsurf on
inner target is slightly above that on outer target, and is not larger than ~450 ◦C. The
maximums of heat flux deposited on outer and inner targets are below ~3 MW m−2, whichis three tenth of the limit of heat flux on the monoblock for steady state, 10 MW m−2. Thismeans that the divertor monoblock has a capability to withstand 3.3 times of injected powerof #73999, which is about 8.3 MW if no mitigation of heat flux on divertor targets canapply. The total power load on divertors decreased by 0.56 MW from 53 s to 103 s due toslight variation of magnetic configuration. The total power on divertor targets is balancedwith exhausted power of 1.33 MW by cooling water in steady state. Figure 2 shows thetime evolution of heat flux deposited on tungsten divertor targets by above method of fastestimation based on bulk temperature. The measurement denotes that heat deposition on thewhole divertor targets was individually observed and the reduced heat flux must be
deposited on the other PFCs. The total injected energy and power during the discharge must
be equal to that lost on all PFCs. However, the power lost to PFCs also must keep normal
distribution. Especially for the achievement of steady-state plasma operation, it is easy for
PFCs to cause overheating and production of impurity, ultimately discharge termination if
power distribution is extraordinary for a long time. Therefore, the real-time and reliable
monitoring of deposited power by diagnostics of surface heat load is crucial in achieving of
long-pulse operation.

Discussions
The major motivation of this project is to realize SSO plasma, based on QUEST and
EAST device. In 2021, this subtheme will continue pursue in long pulse high beta
discharges and related pedestal structure and egde turbulence transport in SSO plasma on
QUEST and EAST. Comparsion and combination study of the results will be done on bothdevices. We will focus on the studying of limit cycle oscillations (LCO) on EAST and
QUEST. LCO had been found in many devices during L-H-L transitions. It suggests that tounderstand the underlying physics mechanism of LCO is important for obtaining steadystate high performance plasma. The LCO had been found during L-H transitions on EAST
and also had been found during L-LCO-L on QUEST. The combined study will be helpful
for understanding the underlying physics and obtaining H mode plasma on QUEST. It isbenefit for the long pulse high beta discharges of EAST and QUEST.
Further, I discussed the task of particle orbit calculating arranged by Professor
K.Hanada with student Mr. Yunfei Wang, who entered in the Kyushu university for doctor
course with Prof. Hanada in last year.

（様式 11・拠点）

Acknowledgement and comments:
Work supported by the international joint research at the Joint Usage of ResearchCenters for Applied Mechanics for 2020. I would like to thank our host, Professor K.
Hanada, who helps a lot during our remotely research and very appreciate the fruitful
discussions and comments. Also RIAM and QUEST staffs are thanked for her kindly helps
for this year’s joint research. We hope that the international joint research at the Joint Usageof Research Centers for Applied Mechanics could continue to enhance China-Japan
cooperation on fusion plasma research in the future.
My Co-Publications in 2020-2021:[1] Y.K. Liu, X, Gao, K. Hanada, Y.W.Yu, H.Q.Liu,…,Y.X.Jie, J.P.Qian et al., Nuclear
Fusion 60(2020) 096019.

(Signature)(Name in print) Xiang Gao

国際化推進共同研究概要
No.9
20NU-7
タ イ ト ル：Soft x-ray spectra in inboard poloidal field null (IPN)
configuration and relevant physical research on QUEST研究代表者： JIE Yinxian

所内世話人： 花田和明

研究概要： QUEST に設置された２次元の X 線カメラに関する共同研究を実施した。対象としては非常に高い X 線放射を伴う高βP 放電（IPN 配位）で、大半径内側に X 点が存在する。この配位は TFTR で 1991 年に報告されて
おり、QUEST では容易にこの配位が形成できることを活用して高速電子の粒子軌道の観点から議論を行った。高速電子の粒子軌道は EASTの長時間 H モード維持を阻害する hot spot 形成に関して重要である。

（様式 11・拠点）

RESEARCH REPORT

Date

Visiting scientist: (name)(position)

Yinxian Jie
Professor(university / institute) Institute of Plasma Physics,
Chinese Academy of Sciences

Host scientist: (name)(position)(university / institute)

Research period: (from)K.

HanadaProfessor
Kyushu(to)

University

March 52021

（様式 11・拠点）

Research subject: Soft x-ray spectra in inboard poloidal field null (IPN)
configuration and relevant physical research on QUESTDue to COVID-19 pandemic，this year we worked remotely. Soft x-ray spectra system
is a very important diagnotic on tokamaks. It is one of the most widely used in measuring
the palsma electron tempreture and MHD phenomenons. The whole system needs
modification to get more detail date to get the position of IPN and relevant physicalresearch on QUEST.
First, I discussed the mission with Mr. Canbin HUANG. The mission of QUEST is todevelop the scientific basis for achieving a steady state condition at sufficiently high beta(~20%), with high confinement and low collisionality. Operating Tokamak at a highpoloidal beta value is usually attactive and this makes the spherical tokamak an interesting
choice for future reactors.The maximum achievable βp, however, is limited by a so called
equilibrium limit, where an inboard poloidal magnetic field null (IPN) appears at the highfield side of the vacuum vessel.Inboard poloidal field null (IPN) configuration in a high Bp
discharging was reported first time on TFTR in 1991. In those discharge the evolution of
the poloidal field measured at the midplane on the inboard side of the TFTR vacuum vessel
was studied.As Ip was ramped down, and Bp increased, the midplane poloidal field
decreased and eventually become negative, indicating that the separatrix had crossed thecoil position and moved into the vacuum vessel. The separatrixlimited discharge wassustained until the end of the beam heating phase.

In QUEST, such an IPN configuration

is easily achieved under a high magnetic mirror ratio and high Bz/Bt values (≈10%) via
electron cyclotron (EC) heating and current drive. A soft x-ray spectra system was set up on
QUEST and got some primary date.The whole system needs modification to get more detaildate to get the position of IPN on QUEST. Then I discussed the task of particle orbit
calculating arranged by Professor K.Hanada with student Mr. Yunfei Wang.

My Co-Publications in 2020-2021:

（様式 11・拠点）[1] Y.K. Liu, X, Gao, K. Hanada, Y.W.Yu, H.Q.Liu,…,Y.X.Jie, J.P.Qian et al., Nuclear
Fusion 60(2020) 096019.

(Signature)(Name in print)

Y. JieYinxian Jie

国際化推進共同研究概要
No.10
20NU-8タ イ ト ル： Joint study of calorimetric measurement of heat load and power
balance estimation in steady state operation (SSO) plasmas on
QUEST and EAST

研究代表者： LIU Haiqing

所内世話人： 花田和明

研究概要： QUEST で開発された冷却水の水温変化で行う熱負荷計測を EAST に適用した。EAST で得られた 100 秒を超える H-mode のプラズマのパワーバラ
ンスを計測し、放電後半でダイバータへの熱負荷が減っていることを確認した。この減ったパワーは局所的な熱負荷となって hot spot を形成し、そこからの炭素放出が H mode 放電の終焉に関係していることを見出した。本研究成果は以下の論文の一部となっている。

[1] Y.K. Liu, X, Gao, K. Hanada, Y.W.Yu, H.Q.Liu,…,Y.X.Jie, J.P.Qian et al., Nuclear
Fusion 60(2020) 096019.

（様式 11・拠点）

RESEARCH REPORT

Date: March 5

Visiting scientists: (name)2021

Haiqing LIU(position)

Professor(university / institute) Institute of Plasma Physics,
Chinese Academy of Sciences

Host scientist: (name)

Kazuaki Hanada(position)(university / institute)
Research period: (from)

Professor
KyushuUniversity(to)Research subject: Joint study of calorimetric measurement of heat load andpower balance estimation in steady state operation (SSO) plasmas on QUEST
and EAST

Introduction
This year, given the situation of COVID-19, the joint research was done remotely.
Steady state operation (SSO) of magnetic fusion devices is one of the goals for fusion
research. As it is predicted that an enormous heat flux (10MW/m2) is coming to the diverter(vertical heat target) locally from the plasma in the future fusion reactor, the heat load
distrubution (power balance, particle balance) and its control should be investigated torealize future fusion power plants. In last year, EAST energy balance results obtained by（様式 11・拠点）

calorimetry in long-pulse high-performance discharges provide the foundation for the
long-pulse operation of ITER and CFETR. QUEST device, the temperature measurement
has been done to measure water cooled movable limiters and other part PFCs. In this year,
the collaboration research focused on the power balance investigation in long pulse highperformance discharge with ITER-like tungsten divertor on EAST and estimation of fuelparticle balance in SSO with hydrogen barrier model on QUEST. From 2020, EAST energybalance results obtained by calorimetry in long-pulse high-performance discharges provide
the foundation for the long-pulse operation of ITER and CFETR, and even provide
experience for blanket calorimetry to measure plasma reactivity in burning plasma
experiment. QUEST device, the temperature measurement has been done to measure watercooled movable limiters and other part PFCs. Although the strong modification of plasma
configuration was applied, much of the heat load to the outer vessel was still remained. Itmeans that the heat load is mainly supplied from energetic electrons which are generated byinjected RF electric field. The measurement of heat load and researching of power balance
in EAST and QUEST will provide crucial support for ITER and CFETR.
Recent results in 2020
During the 2017 campaign, the longest steady-state H-mode plasma was achieved (#73 999
discharge) with the USN divertor configuration in toroidal field direction in which the ion
B×∇B drift pointed away from the active X-point. It lasted 101.3 s at a plasma current of
0.4 MA, a central line averaged density of 3 × 1019 m−3, a core electron temperature of 4
keV, and a confinement enhancement factor H98,y2 of 1.1. Figure 1 shows the evolution ofplasma parameters for the discharge. The central line averaged density < ne > decreasedslightly owing to an accidental absence of density feedback controlled by the supersonicmolecular beam injection (SMBI) and radiated power signal shows a sharp spike at 53 s.
Then it goes down from 53 s to 69 s. The plasma stored energy and radiated power loss
increase gradually after 69 s, at which time a number of events occurred.
Figure 2(a) shows the waveforms for the incremental changes in the cooling watertemperature in the A, C, E modules.The maximum value of the temperature difference in
the A module reached 9.1 ◦C. It should be noted that the water temperature increment of the
A module became almost constant after 28 s, then decreased after 55 s. Then, thetimegradient of temperature increment in the E module increased as denoted in figure 2(c).
This indicates that the heat load distribution on the first wall was significantly changed. The
heat load decrement on the A module is 22.6 MJ from 55 s to 105 s, as shown in yellow, if
the small injected power reduction in the later stage of this discharge is ignored and thetime constant of the exhausted power is the same with the exponential cooling time
constant, it is 20.1 MJ after taking injected power reduction into account. The clear
reduction of power deposited on the A module was observed after the absence of density
feed back control and we investigatied where the power load increased. The visible camera
observed several hot spots on the outer plate of the lower dome and the temperature
gradient measured by the thermocouples inserted in the guard limiter of 2.45 GHz and 4.6

（様式 11・拠点）

GHz LHW systems increased, which suggested that a part of power was delivered from the
A module PFCs to the E module PFCs and to the guard limiters of the LHW injection
systems. The time-integrated heat load exhausted by one part of the E module viewed byvisible camera diagnostic, is 30.9 MJ. The ratio of heat load on the E module to total
injected energy is about 13.5% when no hot spots appeared on the E module. According to
the consideration, the deposited power caused by hot spots can be estimated to be 14.3 MJ,
which corresponds to more than the half of the decrement heat load on the A module (20.1
MJ). The power deposited by the hot spots is 0.29 MW, averaged during 50 s, which will
make a great threat to the safety of lower divertors with power exhaust capability of lower
graphite divertor, 2 MWm−2. The power balance modification may be a reason of thelimited plasma pulse duration. If an in-situ monitoring of power balance is available,
maintenance of H-mode will be necessary.
A method to quickly estimate the heat flux on tungsten divertor has been developed thatanalyzes the targets heat flux for #73999. There is a good agreement between heat load
measured by the infrared camera and that calculated by calorimetry,and the target power
load is not even for this discharge.The detailed understanding of power load in-out
asymmetry for long pulse discharges might be an interesting topic, and more work would
be needed to know the power loss share of each loss channel.

Figure 1 Time evolution of the (a) plasma current Ip, (b) loop voltage and central line averaged density <ne >, (c) plasma stored energy Wp, (d) intensity of the Dα emissions, (e) RF heating power (Pecrh, Plhw,
Picrf) and total auxiliary heating power Pinjected, and (f ) the radiated power loss Prad.

（様式 11・拠点）

Figure 2 (a) Time evolution of cooling water temperature increment in the A, C, E modules. (b) Timeevolution of heat load exhausted by cooling water in the A, C, E modules. (c) Time gradient of watertemperature of the A, C and E modules (red line, blue line, and green line, respectively). In this campaign,measurement of heat load on the D module was not available for thelong-pulse (>20 s) discharge period
owing to the damages to the temperature sensor on the D module. The B module was also inadequate formonitoring the exhausted heat load for this target campaign.

Discussions
In 2020, we recommended a new student, Mr. Yunfei Wang, to enter in doctor course in
Kyushu university and continue to study the power balance (particle balance) estimationand energetic electrons loss in steady state operation (SSO) plasmas on QUEST and EAST.
I discussed the task of particle orbit calculating arranged by Professor K.Hanada withstudent Mr. Yunfei Wang during his interview.
Energetic electrons are the main source of heat load which are generated by injected RF
electric field. ECW (Electron cyclotron Wave) is the main source of energetic electrons on
QUEST. LHW (Lower Hybrid Wave) is an efficient way to heating and drive the plasma
current, and is the main source of energetic electrons on EAST. A significant fraction of
LHW power applied to EAST can be lost to the scrape-off layer (SOL) and deposited in hot
spots on the divertor rather than in the core plasma. This topic has been researched on many
devices such as JET and Tore-Supra. The result show that the LHW power flows to these
hot spots along magnetic field lines passing through the SOL in front of the antenna,
implying that the LHW power couples across the entire width of the SOL rather thanmostly at the antenna face. The energetic electrons tracing code combined with the EFIT
equilibrium code is a good simulation framework for the research both on QUEST and
EAST. Electron orbit mapping provided by the code combination can be used to explain theexperiment observations. This is a very interesting issue for the coming year’s joint
research.

Acknowledgement and comments:（様式 11・拠点）

Work supported by the international joint research at the Joint Usage of ResearchCenters for Applied Mechanics for 2020. I would like to thank our host, Professor K.
Hanada, who helps a lot during our remotely research and very appreciate the fruitful
discussions and comments. Also RIAM and QUEST staffs are thanked for her kindly helps
for this year’s joint research. We hope that the international joint research at the Joint Usageof Research Centers for Applied Mechanics could continue to enhance China-Japan
cooperation on fusion plasma research in the future.
My Co-Publications in 2020-2021:[1] Y.K. Liu, X, Gao, K. Hanada, Y.W.Yu, H.Q.Liu,…,Y.X.Jie, J.P.Qian et al., Nuclear
Fusion 60(2020) 096019.

(Signature)(Name in print) Haiqing Liu

国際化推進共同研究概要
No.11
20NU-9タ イ ト ル： Improved EFIT code of the plasma equilibrium reconstruction for
physical study on QUEST研究代表者： QIAN Jinping

所内世話人： 花田和明

研究概要： QUEST 用に導入された EFIT を用いた共同研究を実施した。EAST のダイバータ部の磁気面を詳細に解析することで、ダイバータ部の熱負荷分布の同定に貢献した。また、QUEST で行われた 28GHz の ECCD 実験で、電流が立ち上がっていく過程での磁気面再構成を行い、βP 値からプ
ラズマ圧力を導出し、結果として高速電子の持つ圧力と RF 駆動電流
から高速電子の密度と温度を算出した。この結果からプラズマ立ち上げの過程が明確になった。今研究は論文投稿を予定するとともに、
QUEST の博士後期課程学生の学位論文に貢献した。

（様式 11・拠点）

RESEARCH REPORT

Date

Visiting scientist: (name)(position)

Jinping Qian
Professor(university / institute) Institute of Plasma Physics,
Chinese Academy of Sciences

Host scientist: (name)(position)(university / institute)

Research period: (from)K.

HanadaProfessor
Kyushu(to)

University

March 52021

（様式 11・拠点）

Research subject: Develop and improve EFIT code of the plasma equilibrium
reconstruction for SSO operation and advanced physical study on QUESTDue to COVID-19 pandemic，this year we worked remotely. As a start, I help one
student familiar with QUEST device and teach him how to run EFIT, which includes
checking the connection of PF coils & power supply, update EFIT and EFUND,
benchmarking all QUEST FLUX data. We discussed how to optimize the fitting weights offlux loop data to make QUEST EFIT robust. Meanwhile, we are developing how to set up
and compile EFIT for different tokamaks. More training for physics understanding of EFIT
and how to add more constraints in the equilibrium reconstruction will be ongoing in thisyear.
In addition, we investigated the q profile optimization in EAST high βP discharges.
Using pure RF heating and current drive, EAST high βP plasmas have been extended andmaintained to long pulse duration with the integrated control. In the experiments, highdensity operation results in more off-axis lower hybrid current drive (LHCD) that further
broadens the current density profile, leading to an improved confinement (H98y2≤1.3)) withthe fBS up to 45%. With this ~45% fBS, the NCS q profile was still not observed in EAST
high βP discharges.
The recent integrated modeling prediction we chose high plasma density for enhancing
the electron-ion coupling, together with ion heating by Neutral Beam and Ion Cyclotron
Resonant Frequency . The efforts of optimization of heating and current drive (H&CD)
scheme includes 2.1 MW ECH, 2.0 MW Lower Hybrid Wave (LHW), 2MW co-Ip NB and
2.0 MW ICRF. The existing larger radius ITB and higher βP allow higher bootstrap current
fraction allows the formation of negative centra shear q profile and large radius ITB, where
the fBS increases from 45% to ~ 63%. Note that this result submitted to physics of plasma.

（様式 11・拠点）

Figure 1 Predicted reversed q profiles with ITB for EAST high betap with optimized
heating and current drive scheme

My Co-Publications in 2020-2021:[1] Y.K. Liu, X, Gao, K. Hanada, Y.W.Yu, H.Q.Liu,…,Y.X.Jie, J.P.Qian et al., Nuclear
Fusion 60(2020) 096019.

(Signature)(Name in print)

J. Qian
Jinping Qian

国際化推進共同研究概要
No.12
20NU-10
タ イ ト ル： Development of mm-wave fast switch system to stabilize neoclassicaltearing mode in tokamaks

研究代表者： BROEKENS Kristiaan Albert

所内世話人： 出射研究概要：

浩

本共同研究課題は、研究代表者がコロナ禍の影響で来日できずキャン
セルされた。

Development of mm-wave fast switch system
to stabilize neoclassical tearing mode in tokamaks
BROEKENS Kristiaan Albert(TNO Optomechatronics, The Netherlands)

This collaboration program was cancelled because I could not visit Japan due to thecorona pandemic.

国際化推進共同研究概要
No.13
20NU-11
タ イ ト ル ： Identification of chemical compositions of nano-sized oxide
particles in high performance ODS ferritic steels for applicationto accident tolerant fuel cladding

研究代表者： DOU Peng

所内世話人： 渡邉

英雄

研究概要：
酸化物分散強化材料は、ナノサイズの酸化物を鉄合金に分散させて分布させることにより、特に高温重照射環境に耐える材料として期待されている。本材料は、特に核融合炉や次世代原子炉の構造材料として期待されているが、微細粒子の照射下での安定性についての情報が不可欠である。共同研究では、核融合炉原型炉に向けて応力研設置のイ
オン照射装置や収差補正電子顕微鏡にて、ナノサイズ粒子の照射環境下での健全性についての研究を開始している。

Identification of chemical compositions of nano-sized oxide particles in highperformance ODS ferritic steels for application to accident tolerant fuel cladding
Peng Dou(Chongqing University)

Super ODS steels have been developed for application to advanced nuclear systems such as
fusion DEMO and Gen-IV reactors. Recently, thess materials was considered as a candidate
material for so called accident tolerant fuel cladding materials because of their high performance
including high-temperature strength, corrosion resistance and radiation tolerance, which are also
the critical properties for fusion blanket structural materials.
One of the remained issues for the super ODS steels is the identification of the nano-sized oxide
particles dispersed in the matrix of the steels. In this collaborative research, we investigate thechemical compositions of the very fine (less than 5 nm) oxide particles in a ODS steel added witha small amount of zirconium which is considered to play a significant role in the appearance ofmuch higher materials performance. Since the oxide particles are so fine that it is almost
impossible to identify them without using the HR-TEM/HAADEF/EDS apparatus in your institute.
The utilization of your TEM is essential for an accomplishment of this research.

Nano/meso structure control for high-temperature strengthening of FeCrAl ODS
ferritic steel--Optimization of recrystallization treatment-I．Project basis and research content
1. Project basis
1.1 Heat resistant advanced ferritic steels
Recent fast economy evolution in China demands highly efficient energy transfer inpower generation systems and massive-transportation systems such as nuclear and nonnuclear power reactors, jumbo air crafts and huge tankers and so on. High energy
efficiency needs a high-temperature operation of the energy systems [1], which indicatesthat the structural material should have an excellent high-temperature strength. Heat
resistant ferritic steels have been used as structural materials for a variety of energy
systems because they have a good cost performance and good enough materials properties
at elevated temperatures.
Recently, oxide dispersion strengthened (ODS) ferritic steels have been developed
for applications to advanced nuclear power reactors which require a rather hightemperature operation and resultantly better heat resistant steels [2-5]. The ODS advanced
ferritic steels are strengthened by dispersion of ultra-fine oxide particles in the matrix ofmartensite or ferrite phase and possess much higher performance than the previous nonODS ferritic steels. The amount of Cr is ranging between 9 to 20 wt% which determines
the structure of the matrix of the steels. The ODS steels with 9-12wt%Cr have a complex
martensitic (9%) and ferritic phase (12%) consisting of dislocations in very high density,
dislocation cell structure, packets and lath structure and carbides [2, 3], while the steelswith Cr more than 13 wt.% have a simple ferritic phase with some dislocations [4, 5].
The comparison between martensitic steels and ferritic steels reveals that both thesteels have both merits and demerits according to the strengthening mechanisms ofmaterials. The complex microstructure of martensite structure is considered to be betterfor strengthening than the simple microstructure of ferrite structure, because a variety oflattice defects may additively contribute to the strengthening. However, if we consider
the thermal stability of the phases, the martensite phase is not stable at temperatures above
850 C because of the transformation of martensite toaustenite, as shown in Fig. 1. Here, the structural
9 wt%
material selection for a practical use depends on the
12 wt%
15 wt%
operation temperature of the energy system. For
example, an electricity power plant can be operated
below 700 C, while the turbine blades of jet engines
are imposed an extremely high heat environment above
850 C, where extremely high thermal stability isinevitable for the materials at temperatures up to 1200
C, which gives a limit of the application of martensitic
steels to the turbine blades. Therefore, generally
speaking, the ODS ferritic steels essentially have awider range of operation temperature of the energy Fig.1 Fe-Cr phase diagramsystems because of no transformation of the steel phase showing  and  phasesin the operation temperature range up to 1500 C [4, 5]. depending on the Cr content.

However, it should be noted that a practical use of materials needs shaping and/or
welding processes which are often accompanied by recrystallization process. It can besaid that recrystallization process is a critical for final material performance because thisprocess is usually given as a final processing of materials production. In ferritic steels,recrystallization causes the grain growth resultantly softening, while offering ductility,
which is the first trade-off issue in this research (Trade-off A). For applications to hightemperature materials, too much ductility for shaping is not necessary but rather higher
strength is required. But small grain size may sometimes result in grain boundary sliding
at elevated temperatures, which is the second trade-off issue in this research (Trade-off
B). Therefore, an adequate recrystallization treatment is essential for the practicalapplications of the ODS ferritic steels as structural materials of advanced nuclear as well
as non-nuclear energy systems operated at elevated temperatures.
1.2 Factors controlling high-temperature strengthening
Materials science and engineering has given us several strengthening
mechanisms on the bases of dislocation theory [6], which includes solid solution
hardening, precipitation hardening, grain boundary hardening, cold work hardening andso on [6]. For ODS ferritic steels, grain boundary strengthening as well as oxide particles
dispersion strengthening are the main strengthening mechanisms, while in the ODS
martensitic steels, hardening by high density dislocations and carbides are added as theother potential hardeners.
Considering the thermal stability of these three factors, dislocations, grainboundaries and carbides in the ordinary steels (non-ODS steels), dislocations are lessstable than the others showing “recovery” process with dislocation climb at temperatures
above 450 C through the vacancy migration [7]. Further rising temperature up to 700 C
causes “recrystallization” process which is accompanied by a remarkable grain boundary
migration and coarsening carbides [8]. Above 850 C, martensite phase transforms to
gamma (austenite) phase.
In the ODS ferritic steels, however, the above processes happen at completely
different temperature regions because of the presence of oxide particles which hinder the
motion of dislocations and grain boundaries [9, 10]. Therefore, the high-temperaturestrength of the ODS steels are offered by oxide particles in origin, which suppress therecovery and recrystallization, and consequently,
Orowanthe reduction of tensile strength at elevatedHall-Petch
Providingtrapping sites toProvidingtrapping
Obstacles
to sites to
Obstacles toRefinement of grains
Refinement of grains
defects
defectsmechanism
dislocation
motiondislocation motion
relation
temperatures is retarded in the ODS steels
incomparison to the non-ODS steels [10].
As for the strengthening mechanism atelevated temperature, there are two important
microstructures in the ODS ferritic steels, oxide
particles and grain boundaries, of which thecontribution to the strengthening is represented
by Orowan mechanism [11, 12] and Hall-Petch
relation [13, 14], as shown in Fig. 2.
Orowan mechanism indicates that finely
dispersed particle, namely, finer particles in a Fig.2 Strengthening mechanism
higher number density, contribute more to at high temperatures: Orowan
strengthening. Hall-Petch relation also indicates mechanism for oxide particles
and Hall-Petch relation for grainthat finer grains with larger grain boundary area boundaryies.

contribute more. With increasing processing temperature, however, oxide particles
become coarser with lower number density and the grain growth happens to reduce grainboundary area. Thus, the processing temperatures can be as low as possible but a lower
processing temperature reduces ductility or fracture toughness of the materials, asmentioned as Trade-off A. This gives us an issue for optimization of processing oftemperature and period. As for creep properties, the rapture time is often reduced by
reduction of grain size, which is well-known as grain boundary sliding induced creep
deformation (Trade-off B).
Now, two questions arise. One is “Which is more contributing to strengthening”;
oxide particles or grain boundaries? This issue is rather critical to optimize therecrystallization processing conditions such as temperature, period and the level of cold
working. Another is the assessment of an adequate grain size being effective to both hightemperature tensile stress and creep rapture time.
In the most of ODS steels, since the size of oxide particles is in the range from a few
nm to 20 nm (nano-scale) and the average grain size is about 1 m (meso-scale) [15],“nano/meso structure control” is necessary to obtain high-temperature strength.
2．Research contents
2.1 Objectives
The objective of this research is to optimize the recrystallization treatment
conditions such as temperature, period and cold working level to strengthen ODS steels
at elevated temperatures with an enough ductility at RT in terms of nano/meso structure
control. For this purpose, the contribution of each factor, oxide particles and grainboundaries, to high-temperature strengthening, both tensile and creep properties, as wellas tensile ductility at RT are assessed for an FeCrAl-ODS ferritic steel.
2.2 Key scientific problems
The reason why the contribution assessment for high-temperature strengthening
is necessary can be seen in Fig.3, where four cases of nano/meso structure conditions areshown, Case-1: finely dispersed oxide particles and fine grains, Case-2: finely dispersedoxide particles and coarse grains, Case-3: coarsely dispersed oxide particles and finegrains, Case-4: coarsely dispersed oxide particles and coarse grains. We learned frommaterials science and engineering [11-14] that the Case-1 is the best for strengthening
and the worst is the Case-4, while ductility is often the best in the Case-4 and the worst
in the Case-1. In order to obtain both the high-temperature strength and ductility, theassessment is necessary to evaluate the contribution of each microstructure, oxide particle
dispersion morphologies and grain size to distinguish the total balance in the effectivenesscomparing between Case-2 and Case-3.

Case-1Case-2Case-3Case-4

Fig.3 Four cases of nano/meso structure conditions, Case-1: finely dispersed oxide particles
and fine grains, Case-2: finely dispersed oxide particles and coarse grains, Case-3:

coarsely dispersed oxide particles and fine grains, Case-4: coarsely dispersedoxide particles and coarse grains.

Creep is another important property for high-temperature structural materials. Creeprupture time depends on not only strength but also grain size when the grain size is fine.
In contrast to tensile stress at a high strain rate, creep rate sometimes becomes faster withdecreasing grain size, which is interpreted in terms of grain boundary sliding at elevatedtemperatures [15]. Thus, so-called inverse Hall-Petch relation becomes an issue for the
steels with fine grain size [16] as mentioned before. Since most of ODS steels has a fineaverage grain size, it is considered that obtaining both the high-temperature strength intensile property and long rapture time in creep property is one of the keys of thedevelopment of ODS ferritic steels.
Thus, the target of this research is to control both of these nano structure and meso
structure to meet the requirements of high-temperature tensile strength, creep rapture timeand ductility at RT, namely, the control of both the particles dispersion morphologies suchas the size and number density of the oxide particles and grain size of the ODS steel. The
difficulty to control nano/meso structures is due to the close relationship between thedispersion morphologies of oxide particles and grain size. It is hard to control
simultaneously the oxide particles dispersion morphology and grain size, becausechanging dispersion morphology of the particles mostly results in the change in the grainsize, depending on the heat treatment temperature, period and cold work level [10].
3．Research plan
3.1 Outline of research
A 15wt%Cr-5wt%Al ODS ferritic steel is used for this research. The oxide
particle dispersion morphologies and grain size of the steel are altered by changing
thermal heat treatment conditions such as temperature and period. Some specimens werecold rolled with different cold work levels before the heat treatment. After the heat
treatment, these nano/meso structures are observed by TEM, SEM and XRD before
mechanical tests. The tensile properties are measured at 700 C and RT for the specimens
with a variety of oxide particle size and number density as well as with different grain
size to investigate the dependence of the yield stress and ductility on the size and number
density of oxide particles and grain size. Creep rupture time is measured for specimens
with different nano/meso structures. Based on the temperature dependence of Orowan
stress (strengthening factor; ) and Hall-Petch relation (constant; k), the conceptual
design direction for recrystallization conditions is determined to show which is better
between Case-2 and Case-3. Finally, considering the way of solution of Trade-off issues
A and B, the recrystallization conditions adequate for high-temperature strengthening
with enough ductility at RT are proposed and confirmed by applying the proposed
conditions to the practical recrystallization process.
3.2 Technical pathways
The first pathway is the search for appropriate annealing temperature andperiod as well as amount of pre-strain for alteration of oxide particle dispersion
morphologies and grain size. Since the thermal stability is different from each otherbetween oxide particles and grain boundaries, there should be an appropriate temperature
range for changing effectively each feature of particle and grain boundary. It is consideredthat the grain size of the ODS steel can be changed by the annealing at a temperature withalmost no change in the oxide particle dispersion morphologies. However, it will be very
difficult to change the dispersion morphologies of oxide particles without changing grainsize, because the dispersion morphology changes require a rather higher annealing
temperature at which the grain size is also changed easily [9, 10].

The solution of above issue will be given by computer simulation study, where
the effect of dispersion morphology change without grain size change is obtained under
the assumption that the Orowan stress is independent of grain size which is considered tobe very likely according to the dislocation theory [6].
Next issue is an assessment of an appropriate grain size according to both thetensile property preferring fine grains and creep property preferring coarse grains. It is
expected that Case-2 will be the most adequate combination of oxide particle dispersion
morphology and grain size according to both tensile and creep properties, which should
be proved. It is noted that the Case-2 should be assessed on the availability of tensile
ductility at RT.
3.3 Evaluation methods
Nano/meso structures are controlled by changing heat treatment conditions,temperature, period and cold work level, namely, amount of pre-strain. The
microstructures are observed by TEM, SEM, XRD as well as APT if necessary, anddefined in terms of the size and number density of oxide particles and grain size. The
grain size is measured on the surface plane of which the normal is parallel to the extrusion
direction so that the grain shape appears to be equiaxed. Tensile tests are carried out at
700 C and RT, and creep tests at 700 C. Particle size and number density dependenceof tensile properties including yield stress, ultimate stress, uniform elongation and total
elongation are investigated. Based on the microstructure observation and tensile test
results of the specimens with different nano/meso structures, Orowan stress and HallPetch relation is derived to assess the contribution of oxide particles and grain boundaries
to the strengthening of the ODS steel. A computer simulation study helps to understandthe contribution difference between the oxide particles and grain boundaries. The effectof grain size on the creep rapture time is discussed. Finally, uniform elongation as anindicator of ductility is discussed along with the strength at high-temperatures to show
the validity and effectiveness of the nano/meso structure control to optimize
recrystallization conditions.
3.4 Feasibility assessment
According to the dislocation theory, the strengthening by oxide particles andgrain boundaries are given by Orowan equation and Hall-Petch relation, respectively, as
follows:
1) Orowan equation-1∆OR1=Gb√𝑁𝑑(1)
where  is the strength factor, G is the shear modulus of the matrix, b is the Burgers
vector in the matrix. N and d are the number density and average diameter of the oxide
particles, respectively. This formula has been used for obstacles induced by radiation andsmaller than a few tens nm in diameter [11].
2) Orowan equation-2
f 1/2

r

∆σOR2= 0.1Gb r ln( b )(2)where G is shera modulus, b is the Burgers vector in the matrix. f and r are the volume
fraction and average radius of the oxide particles, respectively. This formula was used forlarger obstacles than several 100 nm in diameter [12].
3) Hall-Petch relationYS =0 + 𝑘/√𝑑(3)
∆ σHP= 𝑘/√𝑑(4)

where YS is the yield stress, 0 is the frictional stress resisting the glide of dislocation, k
is a constant, d is the average grain diameter [13, 14].
In the above equations,  and k may depend on deformation temperature. The
temperature dependence of  and k will give an important insight into the optimizationof recrystallization conditions, namely, optimized nano/meso structures shown in Fig. 3,
where Case-2 or Case-3 will be an appropriate structure recognized as the conceptual
design direction. It is expected that the temperature range where k depends significantly
on deformation temperature is much lower than the temperature range where  depends
on it [9, 10]. This indicates that each of the strengthening mechanism works effectively
depending on deformation temperature, resultantly, the nano/meso structure control to
obtain high-temperature strength with keeping some ductility is full of feasibility.
4．Features and innovations of the research
The importance of this research is to determine the conceptual design direction of
nano/meso structure control to obtain high-temperature strength of ODS ferritic steels
which absolutely needs recrystallization process for fabrication of plates, tubes, and etc.
The recrystallization of ODS steels are well known to show two steps of recrystallization
behavior, primary and secondary recrystallization [17, 18]. The primary recrystallization
is an equiaxed grain growth and secondary one is accompanied by a partial abrupt grain
growth. Since the dispersion morphology change of oxide particles triggers the grain
growth, the change in grain size may be accompanied by the change in oxide particle
morphology more or less. It can be said that making clear which factor is the key ofstrengthening, oxide particle morphology or grain size, is more effective to determine therecrystallization conditions. The solution of this issue provide us a concept of design
microstructure to consider the high-temperature strength of structural materials.
An example showing that nano/meso structure control is a sort of innovation is
described in Fig. 4, indicating the contribution of strengthening factors at two deformation
temperatures for the ODS steels with fine or coarse grains. From the view of engineering
aspect, ductility and fracture toughness is required to structural materials at RT, andrecrystallization is imposed on them to offer ductility of the materials. The Hall-Petch
relation indicates that grain growth reduces the strength at RT as shown in the figure.
Since the research on deformation temperature dependence of Hall-Petch relation is solimited for ODS steels, it is unclear how the Hall-Petch relation is effective at 700 C inthis materials. If it is not so effective that coarsening results in no significant softening as
shown in the figure, where the difference in ∆σHP at 700 C is smaller than at RT,
recrystallization conditions, are easily determined to be those keeping oxide particles
distribution morphology to be stable while large grain growth is acceptable like Case-2
in Fig. 3. The materials innovation in this research is due to declaration of more effective
strengthening factor at high-temperatures, which provide us conceptual design ofrecrystallization methodology required for ODS advanced ferritic steels for applications
to large energy systems.

国際化推進共同研究概要
No.14
20RE-1
タ イ ト ル：Initialization of collaboration in the theoretical modeling ofgrowth of III-nitride in molecular beam epitaxy

研究代表者：MUZIOL Grzegorz

所内世話人： 寒川

義裕

研究概要：
III 族窒化物を用いた LED（Light Emitting Diode）や LD（Laser Diode）では量子井戸（Quantum Well: QW）と呼ばれる活性層（発光層）が用いられる。量子井戸は電子正孔対の閉じ込めに使用されるため、通常は数 nm の膜厚に設定されているが、本研究では、～30 nm までの範囲で膜厚を変更し、膜厚と光学特性の相関を調査した。実験結果から、長波長、特に緑色の波長領域デバイスでは、厚膜の QW が有効であることが示された。

Initialization of collaboration in the theoretical modeling of growth of III-nitride inmolecular beam epitaxy
Grzegorz Muziol(Institute of High Pressure Physics,
Polish Academy of Sciences)
The III-nitride semiconductor family is known to have extremely high spontaneous andpiezoelectric polarization. In particular, optoelectronic devices can suffer from separation ofcarriers and low overlap of electron and hole wave functions in the quantum wells (QWs). The
overlap strongly drops if the thickness of the QW is increased. However, from the point of viewof internal quantum efficiency, it would be beneficial to use wide QWs. Wide QWs should havea lower carrier density and thus reduce the detrimental impact of the non-radiative Auger process,which is known to cause the efficiency “droop” in III-nitride light emitting diodes. In this research,
we investigated the relationship between QW thickness and optical properties.
In this report, we discuss that wide InGaN QWs can have highly efficient transitions throughthe excited states [1] while the transition between ground states is extremely low, as expected dueto the built-in field. This highly efficient transition is not present without excitation. It emerges
only after screening of the polarization fields.
Photoluminescence (PL) study reveals the complex nature of the transitions in wide QWs. For
a low excitation power, the PL intensity drops with the QW width. This could be explained by adecrease of the wave function overlap between carriers. Indeed, this is expected from the band
structure of the QWs. However, under a higher excitation power the trend is opposite – the PL
intensity increases with the QW width. This is a consequence of a change in the nature of thetransitions. In case of the wide QWs, the wave function overlap between carriers is extremely low
and they cannot recombine. Thus, under excitation, their density increases up to a point in which
the built-in electric field gets screened. Then, even though the overlap between ground states<e1|h1> is still small, a new transition path emerges – through excited states. The wave function
overlap of the excited states is actually higher than in the case of ground states in a standard thin
QW.
A time-resolved photoluminescence study was carried out in order to investigate the dynamics
of the carrier recombination. For the thin QWs (thickness of 1 to 5 nm) we observe a staggering,
yet expected due to separation of carriers, increase of the decay time. However, when the QW
width is further increased the decay time drops significantly. Together with the higher PL intensity,
we interpret this as a sign of an increase in the wave function overlap of the wide QWs.
In this research, a comprehensive theoretical and experimental study was performed to show

the interplay of transitions between ground and excited states as the thickness of the QW ischanged.
Importantly, it was found that the wide QWs can be used in long wavelength devices.
Surprisingly, the higher the indium content of the QWs the higher the difference in transition
efficiency between ground and excited states. This makes the use of wide QWs advantageous
especially in the green spectral regime, in which a drop of efficiency is observed.
An application of the wide QWs to LEDs and LDs was investigated. An enhancement of the
optical gain in LDs is measured by means of Hakki-Paoli technique for wide InGaN QWs,
resulting in a lower threshold current of the devices [2]. The experimental results will be comparedwith theoretical predictions.
We discuss an anomalous photocurrent generated in p-i-n diodes with wide QWs [3].
Illumination of the photodiode with light, with photon energy which excites only the QW, results
in a forward photocurrent. Under a high-intensity illumination the photocurrent switches to thestandard reverse direction. This observation is explained by separation of excited carries by thebuilt-in electric field under the low-intensity illumination and full screening of the built-in fieldin the high-intensity regime.
References[1] G. Muziol, H. Turski, M. Siekacz, K. Szkudlarek, L. Janicki, M. Baranowski, S. Zolud, R.
Kudrawiec, T. Suski, and C. Skierbiszewski, ACS Photonics 6, 1963 (2019).
[2] G. Muziol, M. Hajdel, M. Siekacz, K. Szkudlarek, S. Stanczyk, H. Turski, and C.
Skierbiszewski, Appl. Phys. Express 12, 072003 (2019).
[3] A. Bercha, W. Trzeciakowski, G. Muziol, M. Siekacz, and C. Skierbiszewski, Opt. Express
28, 4717 (2020)

国際化推進共同研究概要
No.15
20RE-2タ イ ト ル： Analysis of Multi-Rotor System Wind Turbines

研究代表者：ISMAIEL Amr Mohamed Metwally

所内世話人： 吉田

茂雄

研究概要：
経済性に優れた将来の超大型の風車のコンセプトとして，1 本のタワー上に複数の風車を設置するマルチロータシステム（MRS）が有望視されている．ここで，支持構造の荷重計算と最適化の手法がいまだ確立されていない．本研究では，MRS の連成解析ツー
ルを開発している．今年度は，シングルロータで検証を行い，2～4 ロータの MRS について解析を実施した．検討の結果，タワーの応答において固有振動数が重要であること
や，4 ロータ MRS の技術的実現性を明らかにした．

RIAM International Joint Research
General Information
Grant No. 15 20RE-2
Title: Analysis of Multi-Rotor System Wind Turbines
Name of the research representative: Assist. Prof. ISMAIEL, Amr Mohamed Metwally
Affiliation: Future University in Egypt (FUE)
Research subject field: Renewable Energy
Name of RIAM attendant: Prof. Shigeo YOSHIDA

Abstract:
Multi-Rotor System (MRS) wind turbines can be a good alternative to large-scale wind turbines
in terms of structural and logistic advantages. An in-house tool was developed in order to analyze
the support structure of MRS wind turbines. The tool was verified by comparing the results of asingle-rotor wind turbine to an equivalent analysis using the software tool FAST. Then, threedifferent configurations of MRS were studied: including two, three, and four rotors. The tower
dynamics were calculated for the main tower in each configuration, as well as the side-booms
supporting the two rotors in the twin-rotor configuration. The analyses have shown that the natural
frequencies are of big importance and are dominant over the loading regarding the tower dynamics.
It has also shown that by comparing the tower dynamics of three and four-rotor configurations, thefour-rotor configuration is more technically feasible. In the future, some of the assumptions whichwere used in developing the code will be removed, in order to get more realistic results and be able
to analyze the wind turbine over all ranges of wind speeds.

Research outcomes:
This research is an extension to a PhD thesis made by the research representative. In the PhD
phase, aeroelastic analysis has been made for a twin-rotor configuration and compared to a single
rotor which has been verified numerically [1].
As an extension, analysis for the side booms holding the rotors on the T-shaped tower has been
conducted. Deflections of the side-boom tip in the fore-aft (Out of plane) and in the side-side (Inplane) has been calculated for different turbulent wind fields of classes A, B, and C. Figure 1 shows
the deflections for wind turbulence class A. Different diameters for the side booms were studied
to determine the relation between the boom size and its structural behavior.
It was found that in the in-plane deflections are dominated by the weight of the rotors, regardlessof the turbulence of the wind field, the deflection kept almost a constant value. While the out-ofplane deflections were dominated by the random behavior of the aerodynamic loads in the
turbulent wind field and induced high, unfavorable vibrations. It was also found that the bending
stiffness of the side-booms is directly proportional to its diameter and has a second-order relation
with the mean deflection value [2].

(a) Fore-aft direction(b) Side-side direction
Figure 1. Deflections of the side booms tip for turbulence class A

Furthermore, analyses for three and four rotors have been made for comparison. Tower-top foreaft and yawing deflections have been calculated for turbulent classes A, B, and C. Tables 1 and 2
show the statistic analysis for the results of the fore-aft deflections for three and four-rotor
configurations.
Table 1. Statistical analysis for tower-top deflection in turbulent cases – Three-rotor
TurbulenceStandard
Dominant
Mean ValueClass
Deviation
Frequencies
A
0.343
0.057
0.32 Hz and 2.24 Hz
B
0.343
0.044
0.32 Hz and 2.24 Hz
C
0.344
0.036
0.32 Hz and 2.24 Hz
Table 2. Statistical analysis for tower-top deflection in turbulent cases – Four-rotor
TurbulenceStandard
Dominant
Mean ValueClass
Deviation
Frequencies
A
0.463
0.082
0.32 Hz and 2.22 Hz
B
0.462
0.056
0.32 Hz and 2.22 Hz
C
0.463
0.049
0.32 Hz and 2.22 Hz

As compared to the results of the twin-rotor configuration, both new configurations have shown asignificant increase in the tower-top deflection, which is due to the increased height of tower andincreased weight and aerodynamic loads. On the other hand, the yawing deflection has decreased,
due to the larger and stiffer main tower.
The results of this work have also emphasized that the tower natural frequencies are dominant overthe loads in regards of the tower dynamic responses, and hence the importance of the natural
frequencies.
Comparing the dynamic responses of the three-rotor and the four-rotor configurations, they bothshow comparable results. The tower-top deflection of the four-rotor configuration has shown avery slight increase in its value compared to the three-rotor configuration. While keeping in mind
that the produced power of the four-rotor configuration is about 30% more, this slight increase in
the deflection value can be ignored. Accordingly, the four-rotor configuration can be consideredmore technically feasible than the three-rotor configuration based only on the tower deflections.

Future Approach
In order to have a better judgment on the feasibility of the three-rotor compared to four-rotor windturbines, more analyses should be conducted. Pitch control should be added and then compare
different wind loads in the high-power region. Power output and cost of energy should also betaken into consideration for comparison.

References[1] A. Ismaiel and S. Yoshida, "Aeroelastic Analysis of a Co-Planar Twin Rotor Wind Turbine,"
Energies, vol. 12, p. 1881, 2019.
[2] A. Ismaiel and S. Yoshida, "Aeroelastic Analysis for Side-Booms of a Coplanar Twin-Rotor Wind
Turbine," International Journal of Aerospace Engineering, vol. 13, no. 4, pp. 135-140, 2020.

国際化推進共同研究概要
No.16
20RE-3タ イ ト ル：Joint study on the next generation of high-reliable andcontrollable power semiconductor device technologies

研究代表者：EKKANATH MADATHIL Sankara Narayanan

所内世話人：

齋藤 渉

研究概要： 代表的なパワーデバイスである IGBT において、ターンオフスイッチング
時に発生するダイナミックアバランシェは、ターンオフ損失を制限するだけではなく、ノイズ（dV/dt）の制御性や長期信頼性を悪化させる。TCAD シミュレーションと実測結果を元にダイナミックアバランシェの発生原因が電界集中であることを明らかにした。加えて、Clustered IGBT 構造により電界集中を抑制することで、大電流動作においてもダイナミックアバランシェ
フリーのターンオフスイッチングを実現した。

Joint study on the next generation of highreliable and controllable power semiconductor
device technologies
Sankara Narayanan Ekkanath Madathil, University of Sheffield, Sheffield,
1

Abstract—It is well known that Dynamic Avalanche (DA)phenomenon poses fundamental limits on the power density, turnoff power loss, dV/dt controllability and long-term reliability of
MOS-bipolar devices. Therefore, overcoming this phenomenon isessential to improve energy efficiency and ensure their safe
operation. In this work, detailed analysis of 1.2 kV MOS-Bipolar
devices are undertaken through both calibrated TCAD
simulations and experiments to show the fundamental cause of DA
and the impact of current density, supply voltage as well as 3D
scaling rules on the DA performance. Furthermore, the dynamic
avalanche performance of a 1.2 kV NPT Trench Clustered IGBT
is evaluated for high current density and low power loss operations.
The results indicate that this device configuration is free of DA andcan be used for ultra-high current density operation in an energy
efficient manner.
Index Terms—IGBT, Clustered IGBT, dynamic avalanche,
dV/dt controllability, high current density operation, energy
efficiency, power density.

I. INTRODUCTIONTRENCH Insulated Gate Bipolar Transistor (TIGBT) is a
key component in various power electronics applications
today, such as Electric Vehicle (EV), motor drives andtransportations. Recent development of TIGBTs is focused onincreasing power densities and switching frequencies with theaims to compete with Wide Band Gap (WBG) power devices
and achieve design optimization and cost reduction for power
conversion systems [1]. Several novel technologies have been
implemented to continuously improve the switching loss (Eoff)and on-state voltage drop (Vce(sat)) trade-off through emitter side
Injection Enhancement (IE) effect [2-4]. The improvements inthe Eoff-Vce(sat) trade-off have resulted in not only low loss
operations but also increases in current densities and improved
cost performance of TIGBT modules. Low Eoff (high dV/dt) can
reduce the system size, because passive component can be
shrunk with high frequency operation. However, it is found thathigh current density and high dV/dt during switching can
induce Dynamic Avalanche (DA) within the TIGBTs, which
poses fundamental limits on the power density, turn-off power
loss, dV/dt controllability as well as long-term reliability of the
IGBT modules [5-7]. Overcoming this phenomenon is crucial
to increase energy efficiency and ensure safe operation in IGBT

applications. Tremendous efforts have been devoted tosuppressing DA and eliminating associated reliability concerns.
Using p-layers to protect the trench bottoms can suppress but
not eliminate DA in the TIGBTs [7], and the holes evacuation
is not enhanced. Moreover, an asymmetric gate oxide approachwith a designed variable thickness to realize stable long-term
operation in TIGBTs and to reduce the switching delay and gatecharge without sacrificing the electrical performance has beenreported [8, 9]. However, this design cannot suppress DA andno effective designs have been proposed to eliminate DA so far.
In previous work, an in-depth analysis of the TIGBT
switching behavior focusing on DA was presented throughcalibrated 3D TCAD models to show, for the first time, thatremoval of the high electric field concentration beneath thetrench gates was the most important solution to manage the DA
in TIGBTs. Moreover, for the first time, a DA free design withhigh current density operation capability was demonstrated in a
Trench Clustered IGBT (TCIGBT), through in both simulations
and experiments [10]. In this paper, the operation of this device
is studied in detail to explain the reason for its DA free behavior.
In addition, the influence of current density and supply voltage
on the DA performance of TIGBTs is experimentallyinvestigated. Finally, the impact of 3-D scaling rules of TIGBTs[3, 4] on the DA behavior is evaluated in detail.
II. DYNAMIC AVALANCHE IN TIGBTS
A. Schematic of DA in TIGBTs
Fig. 1 shows the schematic of DA in the turn-off transient of
trench gated IGBTs. During on-state, the carrier density (p ≈ n)is typically in the range of 1016 to 1017 cm-3 due to conductivity
modulation, which is at least two or three orders of magnitude
higher than the background doping concentration (ND). Whenthe device turns off, an increase in the potential drop occurs
within a small space charge region of the device with a large

Fig. 3. Switch-off characteristics of TIGBT at various Rg.

Fig. 1. Schematic of DA during turn-off of TIGBT.

Fig. 2. Test circuit configuration.

part of stored carriers still present. The electric field distribution
within the device can be expressed as𝑑𝐸 𝑞= (𝑁 + 𝑝 − 𝑛).
𝑑𝑥 𝜀 𝐷(1)

As depicted in Fig. 1, the stored excess holes evacuate throughthe P-base region, resulting in a peak electric field (Emax) which
is much higher than the off-state electric field strength. Aselectric field crowds beneath trench gates, the Emax appears at
trench bottom rather than at the P-base/N-drift junction. If theresulting Emax exceeds the concentration dependent criticalelectric field (Ecr), DA will be triggered even when the collector
voltage is well below the off-state breakdown voltage. More
excessive carriers are thus generated to result in additional Eoffand lower dV/dt. Moreover, the excessive carriers generated
due to Impact Ionization (I.I.) can have enough energy to be
injected into the trench oxide to affect the gate stability andcause associated reliability concerns.
B. Influence of DA on the TIGBTs Electrical Performance
To analyze the DA in silicon TIGBTs, the 3D Sentaurus
Device [11] is utilized to simulate the switching behavior, witha circuit configuration for mix-mode simulation as specified inFig. 2. The dependence of switch-off characteristics of a 1.2 kV
TIGBT in Field-Stop (FS) technology on gate resistance (Rg) isshown in Fig. 3. In practice, smaller Rg should induce larger
dV/dt during turn-off, the relationship between Rg and dV/dt
can be expressed as

Fig. 4. Comparison of (a) Turn-off curves and (b) I.I. rates and Emax of a
TIGBT at Rg = 0.1 Ω and Rg = 50 Ω.

𝐼𝑔 =𝑉𝑡ℎ + 𝐼𝑐 ⁄𝑔𝑚
𝑅𝑔

𝐼𝑔
𝑑𝑉𝐶𝐸
=
𝑑𝑡
𝐶𝐺𝐶

(2)

(3)

where Ig is the gate current, Vth is the threshold voltage, gm is thetransconductance of the MOSFET structure, Ic is the collector
current, and CGC is the miller capacitance. However, the DA
decreases the dV/dt, which results in decrease in surge voltage
even with small Rg conditions, as shown in Fig. 3. This clearly
indicates that DA occurs in the cases of Rg < 20 Ω. Fig. 4(a)compares the turn off curves while Fig. 4(b) compares the
maximum electric fields (Emax) and maximum I.I. rates in thecases of Rg = 0.1 Ω and Rg = 50 Ω, respectively. In the case of
Rg = 0.1 Ω, due to faster increase in collector voltage (higher
dV/dt), the stored excessive holes do not have enough time tobe evacuated from the device and flow along the trench bottom,
leading to a peak electric field strength which exceeds thecritical value, as shown in Fig. 4(b). The critical electric fieldstrength is calculated with (4) and (5),Fig. 5. Dependence of Eoff on Rg with AG model and without AG model.

Fig. 7. Simulated potential distributions as a function of collector voltage
during turn-off of the TIGBT and TCIGBT.

Fig. 6. 3-D cross-sectional view of TCIGBT.

𝐸𝑐𝑟 (𝑆𝑖𝑙𝑖𝑐𝑜𝑛) = 4010 × 𝑄𝑒𝑓𝑓 1/8 [12](4)𝑄𝑒𝑓𝑓 ≈ 𝑁𝐷 + 𝑝(5)

where Qeff is the space charge at trench corner. As a result, DA
occurs and generates more excessive charge to lower the dV/dt.
In contrast, under large Rg conditions, the time to reach supply
voltage takes longer, during which most stored charges are
removed and DA does not materialize. However, this comes atthe expense of increased switching loss and longer turn-off
delay time. Moreover, Fig. 5 shows the simulated Eoff with andwithout Avalanche Generation (AG) model. The saturation
trend of Eoff with AG model at small Rg conditions is due to DA.
In summary, DA can be triggered by high current density
operation, high dV/dt condition, and current filamentation [13].
This phenomenon poses fundamental limits on operating
current density, switching frequency, dV/dt controllability, andleads to reliability issues due to hot carrier effect. Therefore,
eliminating DA is essential for the development of TIGBTs.
III. DYNAMIC AVALANCHE FREE DESIGN: TCIGBT
Fig. 6 shows the 3D cross-sectional view of the TCIGBT.
The TCIGBT features a MOS-gated thyristor structure, which
consists of P-anode, N-drift, P-well and N-well. Its turn-on
mechanism has been explained in [14]. In the on-state, the Nwell and P-well are conductivity modulated and the device
undergoes self-clamping. During turn-off, due to self-clamping,TIGBTTCIGBT
Fig. 8. Comparison of (a) electric field distributions, (b) I.I. rate
distributions and (c) hole densities when Vce raises to 600V (Rg = 0.1 Ω).

the potential of the N-well layer, which acts as the body of the
PMOS is held at a fixed collector potential of less than 20 V,
which is the self-clamping voltage of the TCIGBT, as shown in
Fig. 7. When the gate voltage decreases below its thresholdvoltage, because of the increase in body potential, holes areformed along the sidewall of trench gates to connect the P-welllayer with P-base region through PMOS action, as shown in Fig.
8(c). Therefore, whether the gate potential goes negative or notduring turn-off has no impact on the turn-off behavior. Such aunique design, not available in TIGBTs, provides a direct
evacuation path for excess holes to be collected within emitterregion. The Eoff is thus significantly reduced compared to the
TIGBT, as shown in Fig. 5. Moreover, as the collector voltage
is supported by the P-well/n-drift junction, the trench gates areprotected from high electric field so that there is no electric fieldFig. 10. Impact of current density on E-field and I.I. rate during turn-off.

Fig. 9. Experimental switch-off curves of TCIGBT at various Rg at (a) Jc =140 A/cm2 and (b) Jc = 300 A/cm2.
Fig. 11. Experimental results of the I-V curves of TIGBT and TCIGBT.

crowding in TCIGBT, as shown in Fig. 8(a). Thus, it provides
a fundamental solution for electric field management at trench
regions to prevent occurrence of DA, as shown in Fig. 5.
Figs. 8(a), (b) and (c) show a comparison of the electric field
distributions, I.I. rates and hole densities at the time point of Vce
increases to 600 V between TIGBT and TCIGBT under Rg =0.1 Ω and identical Vce(sat) conditions, respectively. As can beseen, the trench gates of TCIGBT are protected from highelectric field concentrations during turn-off. In comparison, the
TIGBT shows a strong electric field crowding in excess of the
Ecr under the trench bottom and leads to a high I.I. rate.
Absence of DA in TCIGBT is clear from the experimental
results of the switching waveforms of 1.2 kV TCIGBTs
measured as a function of Rg, as shown in Figs. 9(a) and (b).
These devices show a Vce(sat) of 1.8 V at 140 A/cm2 at Room
Temperature (R.T.) and can support 1.6 kV and are short circuit
proof [15]. Although the demonstrated devices were made in
Non-Punch-Through (NPT) technology, moving to a thinner FS
technology has no impact on the DA, as discussed later. Fig.
9(b) shows that TCIGBT does not show DA even at Jc = 300
A/cm2. This confirms that TCIGBT can be operated at highcurrent density without DA and associated reliability concerns
and with very low power losses.
IV. IMPACT OF CURRENT DENSITY ON DA PERFORMANCE
The continuous increase of power density is crucial for thedevelopment of IGBTs to achieve low cost and design

optimization for power electronic systems. Higher power
density requires higher operating current density as well as low
power loss per chip area. However, Fig. 10 shows that DA issignificantly enhanced at high current density operations, which
poses a limit on the operating current density of TIGBTs. In
order to clarify the influence of current density on the DA
performance, a 1.2 kV 25 A TIGBT device in FS technology[16] was investigated in detail and compared with the measured
results of a 1.2 kV NPT TCIGBT. Fig. 11 shows the comparisonof the measured IV characteristics at 25 ºC and 125 ºC. Despite
the fact that the FS TIGBT (device thickness = 115 µm) features
a much thinner device thickness than the NPT TCIGBT (device
thickness = 200 µm), the TCIGBT shows much lower on-state
losses in comparison to that of TIGBT at both rated current
density (Jc = 140 A/cm2) and high current densities due tothyristor conduction. The other characteristics of NPTTCIGBT and FS-TIGBT have been reported in [15]. Fig. 12shows the measured Eoff of the TIGBT as a function of Rg atvarious operating current densities. The Eoff increases moresignificantly at small Rg in the case of high current density
operations due to enhanced DA. In contrast, the self-clamping
feature ensures TCIGBTs to remain DA free performance athigh current density operations, as shown in Fig. 13. The Eoffshows linear decreases as Rg reduces at both R.T. and 125 ºC.
Therefore, TCIGBTs are well suited for operating at highcurrent densities without DA effects.

Fig. 12. Impact of current density on the Eoff of TIGBT.

Fig. 15. Impact of supply voltage on the Eoff of TIGBT.

Fig. 13. Impact of current density on the Eoff of TCIGBT.

Fig. 16. Impact of supply voltage on the Eoff of TCIGBT.

Fig. 17. 3-D scaling rules of TIGBT [4]. Structural parameters are from [4].

Fig. 14. Impact of supply voltage on E-field and I.I. rate during turn-off.

V. IMPACT OF SUPPLY VOLTAGE ON DA PERFORMANCEAs the collector voltage has a direct impact on the electricfield strength within the TIGBT during turn-off, the DA
phenomenon is enhanced as supply voltage increases, as shownin Fig. 14. The measured Eoff of the TIGBT as a function of Rg
at various supply voltages is shown in Fig. 15. Note that theminimum Eoff at Vce = 800 V appears at a larger Rg in comparison
to the case of Vce = 600 V, which confirms that higher supply
voltage enhances the DA performance of TIGBTs. In contrast,
the supply voltage has no impact on the DA free performanceof TCIGBT, as shown in Fig. 16.

Fig. 18. 3-D scaling rules of TCIGBT. Structural parameters of cathodecells are from [17].

VI. IMPACT OF 3D SCALING RULES ON DA PERFORMANCETo understand the impact of the 3D scaling rules on the DA
performance of the devices, scaled TIGBTs [4] as well as scaled
TCIGBTs [17] in FS technologies are considered, as shown inFig. 19. Calibration of simulated 3-D TIGBT models with experimental
data in [4].

Fig. 22. Comparison of (a) electric field distributions, (b) I.I. rates and (c)hole densities when Vce raises to 600 V (Rg = 20 Ω).

Fig. 20. Comparison of I-V curves between k3-TIGBT and k3-TCIGBT.

Fig. 21. Impact of Eoff versus Rg with scaling in TIGBTs and TCIGBTs
under same Vce(sat) condition.

Fig. 17 and Fig. 18, respectively. The structures were evaluatedthrough 3D modelling with models calibrated against measured
data, as shown in Fig. 19. Note that the device thickness in all
structures are identical as in [4] in order to compare theelectrical characteristics. Fig. 20 compares the I-V curves of k3TIGBT and k3-TCIGBT under identical threshold voltage andsame P-anode conditions. The k3-TCIGBT yields a low Vce(sat)of 1.67 V even at Jc = 500 A/cm2 at R.T., which is 23 % lower
than that of k3-TIGBT. Furthermore, the non-saturated I-V
behavior of narrow mesa TIGBTs is effectively suppressed in

k3-TCIGBT due to enhanced self-clamping feature [17]. The
Eoff dependence on Rg between scaled TIGBTs and TCIGBTs
were compared under same Vce(sat) conditions, as shown in Fig.
21, which is achieved by adjusting P-anode concentration. The
low switching losses of both devices decrease as a function of
scaling rule. Moreover, the lower losses of k3-TCIGBT is clear
as shown in Fig. 21. In TIGBT, 3D-scaling rule does not
suppress DA, because enhanced IE effect influences arestronger than relaxation of electric field concentration as shownin Fig. 22. However, as can be observed from Fig. 21, k3TIGBT does not show any obvious increase in Eoff, which is due
to fast hole evacuation by shallow trench. In any case, care must
be taken to address reliability concerns with thinner gate oxides
in TIGBTs.
The impact of the on-state carrier profile on DA in k3-TIGBT
is analyzed by changing the resistance Rpf between the p-float
region and the emitter, as shown in Fig. 23(a). The carrier
concentration at the emitter-side can be increased with high Rpf
due to the IE effect. The rate of change of voltage (dV/dt) is
increased with decrease in Rpf (Fig. 23(b)). Both I.I. rates andEmax values are enhanced by the positive charge of excess holes
around the trench gate bottom (Fig. 23(c)). Most importantly,
these results shown in Fig. 23 demonstrate that diversion ofholes away from the trench bottom alone does not suppress DA.
In Figs. 24(a), (b) and (c) are shown some analysis of DA due
to the increase in carrier concentration at the collector-side.
Although the turn-off time can be increased by increasing Panode concentration (Fig. 24(b)), the collector-side carrier
concentration has no influence on the DA performance of k3TIGBT (Fig. 24(c)).

Fig. 23. Influence of Rpf on (a) on-state hole density and excess hole density when Vce raises to 600 V, (b) switch-off characteristics, and (c) maximum
electric field and maximum I.I. rate when Vce raises to 600 V in the case of k3-TIGBT. (Vg = +/- 5 V)

Fig. 24. Influence of anode injection efficiency on (a) on-state hole density and excess hole density when Vce raises to 600 V, (b) switch-off characteristics,and (c) maximum electric field and maximum I.I. rate when Vce raises to 600 V in the case of k3-TIGBT. (Vg = +/-5 V)

VII. CONCLUSIONS
The 1.2 kV Trench IGBT (TIGBT) switching behavior
focusing on the DA was analyzed through calibrated 3D TCAD
models. Management of the electric field concentration beneathtrench gates is the most critical way to minimize the DA. In
addition, a DA free turn-off operation is demonstrated in a
Trench Clustered IGBT (TCIGBT), through in both simulations
and experiments. As a MOS controlled thyristor device,
TCIGBT can be operated with very low power losses at highcurrent densities without DA and associated reliability concerns.
This is because of the its PMOS action, which eliminates
electric field crowding at trench bottom during turn-off
transients. Moreover, experimental results confirm that DA isenhanced at high current densities and high supply voltages andresults in significant increase in Eoff in the TIGBTs. In
comparison, TCIGBTs remain DA free performance at highcurrent density operations and high supply voltage conditions.
Low turn-off energy loss can be easily achieved by reducing
gate resistance. Finally, the impact of device scaling design on
the DA has also been analyzed with calibrated models.

[4]

[5]

[6]

[7]

[8]

[9]

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[2]

[3]

T. Laska, "Progress in Si IGBT Technology – as an ongoing
Competition with WBG Power Devices," in IEDM Tech. Dig., Dec.
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C. Jaeger, A. Philippou, A. V. Ilei, J. G. Laven, and A. Härtl, "A new
sub-micron trench cell concept in ultrathin wafer technology for next
generation 1200 V IGBTs," in Proc. 29th Int. Symp. Power
Semiconductor Devices and IC's (ISPSD), May 2017, pp. 69-72, doi:
10.23919/ISPSD.2017.7988895.
K. Kakushima, T. Hoshii, K. Tsutsui, A. Nakajima, S. Nishizawa, H.
Wakabayashi, I. Muneta, K. Sato, T. Matsudai, W. Saito, T. Saraya, K.
Itou, M. Fukui, S. Suzuki, M. Kobayashi, T. Takakura, T. Hiramoto, A.
Ogura, Y. Numasawa, I. Omura, H. Ohashi, and H. Iwai, "Experimental
verification of a 3D scaling principle for low Vce(sat) IGBT," in IEDM

[10]

[11]
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[13]

Tech. Dig., Dec. 2016, pp..6.1-10.6.4, doi:
10.1109/IEDM.2016.7838390.
T. Saraya, K. Itou, T. Takakura, M. Fukui, S. Suzuki, K. Takeuchi, M.
Tsukuda, Y. Numasawa, K. Satoh, T. Matsudai, W. Saito, K.
Kakushima, T. Hoshii, K. Furukawa, M. Watanabe, N. Shigyo, K.
Tsutsui, H. Iwai, A. Ogura, S. Nishizawa, I. Omura, H. Ohashi, and T.
Hiramoto, "Demonstration of 1200V Scaled IGBTs Driven by 5V Gate
Voltage with Superiorly Low Switching Loss," in IEDM Tech. Dig.,Dec. 2018, pp. 8.4.1-8.4.4, doi: 10.1109/IEDM.2018.8614491.
T. Ogura, H. Ninomiya, K. Sugiyama, and T. Inoue, "Turn-off
switching analysis considering dynamic avalanche effect for low turnoff loss high-voltage IGBTs," IEEE Trans. Electron Devices, vol. 51,
no. 4, pp. 629-635, Apr. 2004, doi: 10.1109/TED.2004.825109.
J. Lutz and R. Baburske, "Dynamic avalanche in bipolar powerdevices," Microelectronics Reliability, vol. 52, no. 3, pp. 475-481, Mar.
2012, doi: 10.1016/j.microrel.2011.10.018.
S. Machida, K. Ito, and Y. Yamashita, "Approaching the limit ofswitching loss reduction in Si-IGBTs," in Proc. 26th Int. Symp. Power
Semiconductor Devices and IC's (ISPSD), Jun. 2014, pp. 107-110, doi:
10.1109/ISPSD.2014.6855987.
C. Sandow, P. Brandt, H. Felsl, F. Niedernostheide, F. Pfirsch, H.
Schulze, A. Stegner, F. Umbach, F. Santos, and W. Wagner, "IGBTwith superior long-term switching behavior by asymmetric trenchoxide," in Proc. 30th Int. Symp. Power Semiconductor Devices and IC's
(ISPSD), May 2018, pp. 24-27, doi: 10.1109/ISPSD.2018.8393593.
L. Ngwendson, I. Deviny, C. Zhu, I. Saddiqui, J. Hutchings, C. Kong,
Y. Wang, and H. Luo, "New Locos Trench Oxide IGBT Enables 25%
Higher Current Density in 4.5kV/1500A Module," in Proc. 31th Int.
Symp. Power Semiconductor Devices and IC's (ISPSD), May 2019, pp.
323-326, doi: 10.1109/ISPSD.2019.8757651.
P. Luo, E. M. S. Narayanan, S. Nishizawa, and W. Saito, "Dynamic
Avalanche Free Design in 1.2kV Si-IGBTs for Ultra High Current
Density Operation," in IEDM Tech. Dig., Dec. 2019, pp. 12.3.1-12.3.4,
doi: 10.1109/IEDM19573.2019.8993596.
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B. J. Baliga, Fundamentals of power semiconductor devices: New
York, NY, USA, Springer, 2016.
Y. Shiba, I. Omura, and M. Tsukuda, "IGBT avalanche current
filamentaion ratio: Precise simulations on mesh and structure effect," inProc. 28th Int. Symp. Power Semiconductor Devices and IC's (ISPSD),
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[14] O. Spulber, M. Sweet, K. Vershinin, C. K. Ngw, L. Ngwendson, J. V.
S. C. Bose, M. M. D. Souza, and E. M. S. Narayanan, "A novel trenchclustered insulated gate bipolar transistor (TCIGBT)," IEEE Electron
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10.1109/APEC.2015.7104510.
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Narayanan, "Numerical Analysis of 3-D Scaling Rules on a 1.2-kV
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国際化推進共同研究概要
No.17
20RE-4
タ イ ト ル： Renewable Energy Dynamics
研究代表者： MANICKAM Srinivasan

所内世話人： 柿本浩一

研究概要： 本研究では、シリコン太陽電池を安価に製造するために必要な結晶育成炉の設計のために、大規模数値解析を行った。その結果、ルツボ底部の温度分布の最適化が非常に重要であることが分かった。特に温度分布制御の時系列変化を最適化することが重要であることが分かった。現
在最適化の途中であり、今後も継続的に共同研究を行っていく予定である。

Dislocation Analysis of Si grown by Directional Solidification Method
MANICKAM Srinivasan
SSL, Chennai, India
Abstract:
This paper reports the relationship between oxygen concentration and dislocation
multiplication in silicon crystals during directional solidification using numerical analysis. This
analysis involved oxygen diffusion from the bulk to dislocation cores during crystal growth andannealing processes in a furnace based on the Alexander–Haasen–Sumino model. The calculated
results showed that the dislocation density increased during cooling process, rather than crystal growth,when the effect of oxygen diffusion to dislocation cores was ignored. Meanwhile, the dislocation
density increased during both crystal growth and cooling processes when the effect of interstitialoxygen diffusion was considered.
Introduction:
Dislocation is one of the most harmful defects in silicon crystals, causing current leakage inphotovoltaic cells and large scale integrated circuits. M’Hamdi et al. reported that the dislocation
density increased during the cooling process, rather than during crystal growth. The effect of interstitialoxygen on dislocation multiplication was not discussed. Imai et al. reported that oxygen concentration
affected the dislocation velocity as a function of shear stress in silicon crystals. The crack was
introduced into silicon crystals grown by the floating zone, Czochralski (CZ)-grown, and magnetic
field-applied CZ methods. They also reported the measured dislocation velocity under applied stressat elevated temperature having different oxygen concentrations.
In this study, we focus on the relationship between oxygen concentration and dislocation
multiplication in silicon crystals. We studied the effects of interstitial oxygen on the dislocation
multiplication in silicon crystals having inhomogeneous distribution of oxygen concentrations during
both crystal growth and cooling processes using numerical analysis. We also took into account oxygen
diffusion. The model included the effect of the dislocation density which was equivalent to the distance
between dislocations when the diffusion of interstitial oxygen atoms was considered.
Calculation method:
We applied the viscoelastic model for stress analysis using a two-dimensional axisymmetricmodel. A three-dimensional analysis was carried out to calculate the dislocation density in silicon
crystals using the numerical method reported elsewhere. We took into account the activated mechanics
of slip systems in different growth directions, immobilization of mobile dislocations, and the jog

formation between different slip systems. The silicon crystals have 12 slip directions. The resolved
shear stress in each slip direction can be calculated by the tensor transformation technique using stresscomponents obtained from a three-dimensional analysis of the stress.
Results:
The result shows that the dislocation density increased during crystal growth and thenremained constant at 1300 K in the case without considering the oxygen diffusion, shown by a dashedline, whereas the density became constant at 1500 K in the case with the oxygen diffusion. M’Hamdi
reported that the density mainly increased during crystal cooling process, whereas the density increase
occurred during both crystal growth and cooling processes by considering oxygen diffusion todislocation cores. The oxygen concentration in dislocation cores increased remarkably at 1500 K, due
to the oxygen diffusion from the bulk to dislocation cores at elevated temperature.
Conclusion:
This paper reports the relationship between oxygen concentration and dislocation
multiplication in silicon crystals during a directional solidification method using numerical analysis.
The analysis was based on the HAS model, and involved oxygen diffusion from the bulk to dislocation
cores during the crystal growth, and the annealing process in a furnace. When the dislocation density
was larger than 1.0 × 105 cm−2, interstitial oxygen concentration in bulk decreased, due to the diffusion
process of interstitial oxygen atoms between dislocations.

国際化推進共同研究概要
No.18
20RE-5
タ イ ト ル：

Predictions of Viscous Damping of Floating Structure Based on Computational Fluid
Dynamics Method

研究代表者：

GAO, JUNLIANG

所内世話人：

劉 盈溢

研究概要：

Floating wind turbines (FWT) introduce new challenges in evaluating the dynamic motion
and the power generation performance. At present, there are three branches of methods topredict the performance of a floating wind turbine, i.e., physical experiment, numericalevaluation based on potential flow theory, and numerical simulation based on computational
fluid dynamics. Physical experiments are often limited by the experimental site and the hugecost. Although the evaluation efficiency of the existing model (such as the FAST program,etc.) is very high, it often requires an artificial damping term for the Morison Model, whichneeds to be determined by physical experiments. Instead, we are seeking for a substitution
method based on computational fluid dynamics (CFD) solvers, to do an equivalent work thatan experiment does (e.g., evaluate the viscous damping in all six DOFs of FWT motions
from the free decay test performed by a CFD method), hence to substantially reduce the costrequired by a model test or on-site experiment.

Introduction
Offshore wind turbine offers a promising solution to harnessing abundant wind resources. With thedevelopment of technology, the offshore wind turbine industry is gradually moving from shallow water tointermediate water and deepwater. As one of the three mainstream concepts of the floating foundation to keep
stability, the semi-submersible type has a smaller surge response than the TLP (Tension Leg Platform) typeand a smaller pitch response than the spar type (Goupee et al., 2014). The installation cost of the mooring
system of a semi-submersible foundation is also relatively lower than the other types of floating foundations(Liu et al., 2016). Researches have been undertaken on various aspects of the semi-submersible concept,
involving model testing (Kim et al., 2017), numerical modeling and conceptual study (Liu et al., 2018).
At present, there are three branches of methods to predict the performance of a floating wind turbine (FWT),
i.e., physical experiment, numerical evaluation based on potential flow theory, and numerical simulation basedon computational fluid dynamics. Physical experiments are often limited by the experimental site and the hugecost. Although the evaluation efficiency of the existing model (FAST, etc.) is very high, it often requires theartificial damping term for the Morison Model, which needs to be determined by physical experiments. In thecurrent study, to substantially reduce the cost required by a model test or on-site experiment, an open-sourced
CFD software OpenFOAM is adopted to do an equivalent work that an experiment does.
Methodology
Governing equations
The multiphase flow solvers, waveDyMFoam, use the Navier–Stokes equations to describe the motion of
the fluid continuum. These equations can be expressed as:  ( U )=0
t

(1)

U
  ( UU )   ( U )   g  p  ft(2)

where U is the flow velocity vector, ρ is the density of the fluid, μ is the dynamic viscosity, g is theacceleration due to gravity, p is the pressure of the fluid, and f is the surface tension.
To track the shape and position of the free surface, the volume of fluid (VOF) method has been employedin OpenFOAM. In grid cells, the volume fraction used in the VOF method is defined as follows:0,  0    1,
1,


The

velocity

field

can

be

obtainedin airon the surface
in water

using

the

weighted(3)
averages

using

the

equationU   U water  1   U air . According to this equation of the velocity field, the transport equation of the VOF

field can be expressed as:  ( U )   [ 1   U r ]  0t(4)where U water and U air are the velocities of the corresponding water and air, respectively. Ur  U water  Uair
indicates the relative velocity between air and water.
The spatial variation of any fluid property  (e.g., the fluid density ρ and dynamic viscosity μ) can be
expressed as weighting using  : =water  (1   )air(5)

where the subscripts “water” and “air” denote the corresponding fluid property of water and air, respectively.

Body motion equations
In this study, the motion of the floating body is restricted to one degree of freedom, allowing solely theheave motion (z-direction). The vertical position of the floating box is solved using Newton's second law at
the current time step n+1:
F n1  man1(6)

where Fn+1 is the total vertical force, and an+1 is the body’s vertical acceleration. Once the acceleration an+1 is
known, the vertical velocity vn+1 and vertical position zn+1 at the current time step n+1 are calculated using an
integration strategy:v n 1  v n 1  (1   )a n T   a n 1T(7)z n 1  z n  (1   )v n T   v n 1T

(8)

where n is the previous time step, T is the time step, and  is a blending parameter. For   0 , the forward
Euler method, which is explicit in time arises, and for   1 , the backward Euler method, which is fully
implicit in time, are employed.

Mesh motion
The mesh motion of the computational domain is calculated by solving the cell-center Laplace smoothing
equation (Jasak and Tukovic, 2006): • (u)  0(9)where  is the diffusion field and u is the point velocity for modifying the point position of the mesh:xn1  xn  u  T

(10)where x n and xn1 are the point positions before and after mesh motion, respectively, and T is the time step.
Using the variable diffusion field  , the deformation of each grid point is scaled from the total body
displacement to no deformation. Further,  is a function of the distance r between the center of the cell andnearest selected boundary, where r  (ri , ro ) . ri and ro are the inner and outer distances of the scaling,respectively.
Results
A numerical example of the Kyushu University FWT is performed applying the above method. Figure 1
shows the layout of the mooring system and the definition of the coordinate system. For comparison, natural
periods of the FWT system calculated by the potential flow method are also listed in Table 1.

Figure 1. The layout of the mooring system of the semi-submersible FWT. The origin of the coordinate system
for the subsequent analysis locates at the planar center of the triangular platform at MSL (Mean Sea Level).
Table 1. Natural frequencies of the FWT system calculated by the potential flow method.
Mode

Natural frequencies (rad/s)
without mooringwith mooring

-

0.07

Surge
Sway

-

0.07

Heave

0.36

0.36

Roll

0.31

0.32

Pitch

0.32

0.32

Yaw

-

0.09

Figure 2 shows a heave free decay test performed by CFD in OpenFOAM (Gao et al., 2019, 2021).
Based on this figure, the heave natural period can be calculated as 17.44 s, which is very much close to the

Heave displacement

heave natural period calculated from the potential flow method, i.e., 2π/0.36=17.45 s.

1.0
0.8
0.6
0.4
0.2
0.0
-0.2 0.0
-0.4

20.0

40.0

60.0

80.0

Time t (s)Figure 2. Free decay test simulated by CFD in OpenFOAM.

References
Goupee A., Koo B., Kimball R., Lambrakos K., Dagher H., 2014. Experimental comparison of three floating wind turbine concepts.
Journal of Offshore Mechanics and Arctic Engineering 136(2): 020906.
Jasak, H., & Tukovic, Z. (2006). Automatic mesh motion for the unstructured finite volume method. Transactions of FAMENA, 30(2),1-20.
Kim, H. C., Kim, K. H., Kim, M. H., Hong, K., 2017. Global performance of a KRISO semi-submersible multiunit floating offshore
wind turbine: Numerical simulation vs. model test. International Journal of Offshore and Polar Engineering, 27(01), 70-81.
Liu, Y., Li, S., Yi, Q., Chen, D., 2016. Developments in semi-submersible floating foundations supporting wind turbines: A
comprehensive review. Renewable & Sustainable Energy Reviews 60: 433–449.
Liu, Y., Yoshida, S., Yamamoto, H., Toyofuku, A., He, G., Yang, S. 2018. Response Characteristics of the DeepCwind Floating Wind
Turbine Moored by a Single-Point Mooring System. Applied Sciences, 8(11), 648 2306.
Gao, J., Zang, J., Chen, L., Chen, Q., Ding, H., & Liu, Y. (2019). On hydrodynamic characteristics of gap resonance between two fixed
bodies in close proximity. Ocean Engineering, 173, 28-44.
Gao, J., He, Z., Huang, X., Liu, Q., Zang, J., & Wang, G. (2021). Effects of free heave motion on wave resonance inside a narrow gap
between two boxes under wave actions. Ocean Engineering, 224, 108753.

国際化推進共同研究概要
No.19
20RE-7
タ イ ト ル： Downwind Turbine Technologies, Model Development and Verification

研究代表者： HÖNING Leo

所内世話人： 吉田

茂雄

研究概要：
経済性に優れた将来の超大型風車，ならびに，導入地点を大幅に拡張する浮体式洋上風車に有望な風車のコンセプトとして，タワーの風下側にロータを配置したダウンウィ
ンドロータが有望視されている．本ワークショップでは，風力エネルギーに関する国際共同研究プラットフォームである IEA Wind Task 40 を母体として，ダウンウィンド風車に関する技術的な課題，ならびに，得失の評価を実施している．各々の関連のテーマに関して， 2020/05/19-20，ならびに，2020/10/21-22 にワークショップを開催し，当該分野の世界最先端の技術についての情報交換，ならびに，今後の普及や標準化にあたっての討議を行った．

20RE-7 Downwind Turbine Technologies, Model Development and Verification
Applicant: Leo, Hoening, Fraunhofer IWES
RIAM Attendant: Shigeo Yoshida

1. OUTLINE OF THE WORKSHOP
1.1 Plenary Meeting, Online(1) Date: 2020/05/19(tue)-20(wed)(2) Place: Online (GoToMeeting)(3) Participants:(IWES) Leo Hoening, Bernhard Stoevesandt(TUM) Carlo Sucameli(X1 Wind) Rocio Torres(NREL) Nick Johnson, Brian Smith, Pietro Bortolotti(Hitachi) Shigehisa Funabashi, Soichiro Kiyoki,
Shinya Ohara, Nobuo Namura, Takeshi Takimoto(WEIT) Masataka Owada, Yoshitaka Totsuka(KU RIAM) Shigeo Yoshida
1.2 Workshop, Online(1) Date: 2020/10/21(wed)-22(thu)(2) Place: Online (GoToMeeting)(3) Participants:(IWES) Leo Hoening, Bernhard Stoevesandt, Johannes
Theron(TUM) Carlo Sucameli, Helena Canet, Thorsten Lutz
(X1 Wind) Rocio Torres(NREL) Nick Johnson, Pietro Bortolotti, Tyler Stehly(BWC) Sandy Butterfield(UTD) Todd Griffith(UVA) Eric Loth
(AIST) Tetsuya Kogaki, Shigemitsu Aoki
(Hitachi) Soichiro Kiyoki, Shinya Ohara, Nobuo
Namura, Takeshi Takimoto, Shinya Ohara(WEIT) Masataka Owada, Yoshitaka Totsuka(UTokyo) Atsushi Yamaguchi(RCCM) Noriki Iwanaga(KU RIAM) Shigeo Yoshida
1.3 Outlines of the Task
WP1) Model Development& Verification
WP1-1) 2MW Baseline Turbine Model
WP1-2) Tower Shadow
WP1-3) Nacelle-Rotor Interaction
WP1-4) Stability & Control
WP1-5) Complex Terrain
WP2) Design and LCOE Assessment
WP2-1) Blade Optimization for DTs
WP2-2) Scalability Benefits for DTs
WP3 Recommended Practice

WP3-1) Standards Evaluation for DTs
WP3-2) RP for DTs

2. REPORTS
WP1-1) Baseline Model, UTokyo
Aeroelastic models of a 2MW baseline downwind
turbine model were defined. Power coefficient to tip
speed ratio is shown in Fig 1. The tower was designed
to avoid resonace in the opration conditions as shownin Fig 2.
The Bladed and FAST models were delivered to themembers to proceed the verification study in eachresearch subject.

Fig 1 Power coefficient to tip speed ratio

Fig 2 Steady characteristics of aeroelastic models
WP1-1) CFD Model Definition, KU
The CAD model of a 2 MW downwind turbine wasprovide by Hitachi. The CFD mesh which is applicabe
for ANSYS Fluent was defined for the model. The
rotor is rotatable using the sliding mesh around therotor axis. Furthermore, the blades are also rotatablearound the pitch axes of the 3 blades.

Fig 4 CFD model outline(a) CFD domain

Fig 5 CFD model outline(b) Side view(c) Mesh around the bladeFig 3 CFD model outline
Table 1 Dimensions of the CFD Model
Rotor Diameter
Hub Height
Domain Diameter
Domain Hight
Rotor Part Diameter
Rotor Part Thickness
Blade Part Length
Blade Part Diameter

80 m
78 m
300 m
240 m
88 m
21 m
40.3 m5m

WP1-2) CFD Result, IWES
- Model: Hitachi 2MW-80m downwind- Wind speed: 8.6 m/s (steady)- Pitch angle: 1.6 deg- CFD solver: OpenFOAM- 1/3 domain (steady), full WT (unsteady)- Number of cells: 16million per bladeFig 6 CFD result: blade root edgewise bending (top)and flapwisebending (bottom)

WP1-2) Tower Shadow, KU
The measurement data of a 2 MW commercial
downwind turbine was provided by Hitachi. The data
was analyzed.
(1) Data outlines- Operation data: wind speed, wind direction, nacelledirection, rotor speed, pitch angle, rotor azimuth
angle- Load data: blade root flapwise/edgewise moment,
tower top/base fore-aft/side-side moment(2) Conditions- Power generation (optimal: 8.6 m/s, rated: 16.6 m/s)
- Idling (19.9 m/s)

Fig 9 Blade root bending moments of the 2 MW
ptotype downwind turbine as the reference(3) Results
Tower shadow loads of are less remarkable ascompared with those of the the prototype. Influencesof wind shaer, turbulence, etc. will be studides in the
future.

WP1-2) Tower Shadow, KU
Some research results on tower shadow modeling of
downwind turbines in Blade-Element and Momentum(BEM) method, which was not considered in theformer methods were reported.
(1) Load Equivalent Model [1]The load equivalent model tower shadow model was
introduced. The wind speed profile of the tower wakeis defined based on the load fluctuation calculated by
the CFD. It showed good agreement with themeasurement.
BEM(Load Equivalent)
BEM(Isolated Tower)
1.2
CFD(Rotor-Tower)

1.2

1

[-]
XNA,AV

/F

0.6

XNA

/M
XNA

0.8

0.8
0.6

F

M

Fig 7 Field measurement data of a 2MW downwind
turbine

XNA,AV

[-]

1

0.4

0.4

0.2

0.2

0
160

180
Azimuth Angle[deg]

200

0
160

180
200
Azimuth Angle[deg]Fig 10 Rotor thrust and torque of a downwind turbine
stiff model around the tower shadow at 13 m/s

Fig 8 Blade root bending moments of the 2 MWcommercial downwind turbine

1.5

x 10

Fig 13 Variable loads of the downwind turbine tower
at 100% rotor radius: (T) 13 m/s, (B) 25 m/s

6

BEM(Load Equivalent)
BEM(Isolated Tower)
Field Test

1(2) Tower Average Load [3]The average tower load model was also introduced
based on the momentum theory, which consists withvelocity and pressure gradient terms.

MYMS[Nm]

0.5
0
-0.5
-1

CdTV  CdTV  CdTP-1.5

 CdT 0 1  T 2 

-2



0

60

120

180

240

300

360

Azimuth[deg]Fig 11 Mainshaft bending a 2 MW prototype
downwind turbine at 13 m/s 2  dd

T

T

T

The model was validated by the wind tunnel test. It
shows good agreement with the wind tunnel test data
as shown in Fig 14.

(1) Tower Variable Load [2]The variable load model of downwind turbine tower,
which was not considered in the previous model, isformulated as below, using lifting-line theory.
CdT 

 DT 
2 

2U 0 

U 0

du
dv
dw 
 r
w

dxT
dxT
dxT 

It was verified by the CFD of rotor-tower-nacelleconfigurations at rated and cut-out operating
conditions. It shows fairly nice agreement with the
CFD in particular out-board section and at low thrust
conditions as shown in Fig 13. However, there still besome more room for improvement in inboard sections.
Blade Element

Fig 14 Rotor thrust to tower drag: (T) 13m/s, (B) 25
m/s

Tower Section



dr

U0
e ZB

xTB

dzT

 xTB eTBduFig 12 Outlines of the modelFig 15 Tower section drag to the rotor thrust and theclearance between the rotor and the tower: (T) 50 %R,(B) 80 %R(3) Dynamic Blade Load [4]The dynamic blade load model was reported. Thismodel was developed based on the former study ofMunduate [5]. Two points were modified from thereference; 1) application of Moriarty’s tower wake

model [6] and 2) wake entrance condition. Fig 16
shows the analysis and experiment results on a 1.0 m
diameter model. Where, UG indicates University
Glasgow’s former model and KU does present model.
The present model was successfully shown theincrease in load before the wake entrance was modeled
better than the previous work.

are shown in Fig 22. The results are consistent with themeasurement.

Fig 18 Passive yaw (free yaw)

The scale effects of the model were investigated. Fig
17 is the simulation results for the similar models. The
top, middle and bottom subplots are analysis resultswith x1 model (1 m rotor diameter), x3 (3 m), and x10(10m), respectively. Here the tip speeds are set to be
identical. As shown in these figures, the variation of
the lift coefficients are decreasing as the scale getting
larger.

Fig 19 Typical yaw system
10minutes Averaged Yaw Misalignment[deg]Fig 16 Blade section (75% rotor radius) lift coefficient
to rotor azimuth, UG model
10
5
0
-5
-10
-15
-20
-25

Measurement(13-15hour)
Measurement(13.5-14.5hour)
Simulation(13.5-14.5hour)
35404510minutes Averaged Wind Speed[m/s]50

Fig 20 Yaw misalignment to wind speed

Fig 17 Blade section lift coefficient to rotor azimuth:scale (T) x1, (M) x3, (B) x10, KU model
WP1-4) Stability, Hitachi [7][8]The extreme loads of a 5.2 MW DT in parked
condition in Typhpoon #21 in 2017 were simulated.
Yaw angles by the measurement and the analysis areshown in Fig 20. Blade root flapwise bending moment

10 minutes Statistics of Normalized MYS[-]

1
0.8
0.6
0.4
0.2
0
-0.2

Measurement Max.
Measurement Mean
Measurement Min.
Simulation Max.
Simulation Mean
Simulation Min.

-0.4
-0.6
-0.8
-1

40
42
44
46
10 minutes Averaged Wind Speed[m/s]Fig 21 Blade root flapwise bending to wind directionBlade Flapwise Bending Moment
Normalized by the Load of Active Yaw Control[-]

Here, the cost of the production and transportationof the prebent blades are still not considered in the costmodel of the prebent blades. Therefore, LCoE of the
upwind turbines are estimated a little optimistic.

1

0.8

0.6

0.4

0.2

0

Each Seed & Each Blade
Typical value-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Long-period Change Rate of Wind Direction [deg/s]

Fig 22 Blade root flapwise bending to wind directionchange rate

Fig 24 LCoE to rotor diameterFig 23 Recommendation for analysis of passive yaw
idling in storm

Fig 25 Design parameters to the rotor diameter

WP2-2) Scalability Benefits, Hitachi[9]1. Outlines
Fatigue calculation and tower shadow model wereintroduced in this research. Natural frequencies inflap-wise and edgewise are the output of the
optimization. Design variables are not optimized forpre-bent blades as present cost is not considered. Less
than 5m is recommended due to the manufacturing andtransportation. The tower shadow model is too
pessimistic. Therefore, the advantage of downwind
turbines is underestimation.
WISDEM, the system engineering code developed
by NREL, was modified to consider the tower shadow
effect of downwind turbines.
2. Conditions- Wind class: 1A- Downwind: prebent 0 m- Upwind-1: prebend max 6 m- Upwind-2: prebend max 20 m
3. Results
Downwind turbine shows lower LCoE than upwind
turbines.

Fig 26 Optimal blade shapes

WP2-2) Largest Wind Turbine, UVA[10]Morphing blades, which adapt the alignment of theblade in acoordance with the thrust and the centrifugal
forces, are promising concept for super large windturbines.
The studies in SUMR show promising results, suchas 27% RNA mass reduction and 24% LCoE reduction,bu the fatigue load reduction.

Fig 27 Typical force to wind speed

bo

CONR

SUMR

Fig 28 Concept of the SUMR bladeFig 29 von Mises stress of CONR (top) and SUMR
(bottom)Fig 30 Average moment (left) and its DEL (right) towind speed

Table 2 Dimensions of the CFD Model
CONR
SUMR
3-Blades2-BladesRotor Mass
100%
73%
Power Output
100%
98%
LCoE
100%
76%

3. REFERNCES[1] Yoshida, S., Load Equivalent Tower Shadow
Modeling for Downwind Turbines, European Wind
Energy Conference, 2007.
[2] Yoshida, S., Combined Blade-Element Momentum- Lifting Line Model for Variable Loads on
Downwind Turbine Towers, energies, 2018.
[3] Yoshida, S., et al., Rotor Thrust Effects on Mean
Load of Downwind Turbine Towers, #40 Wind
Energy Utilization Symposium, 2018.
[4] Yoshida, S., et al., Unsteady Tower Shadow Modelfor Downwind Turbines, #41 Wind Energy
Utilization Symposium, 2019.
[5] Munduate, X., Coton, F.N., Galbraith, R.A.M., An
Investigation of the Aerodynamic Responses of aWind Turbine Blade to Tower Shadow, J. Solar
Energy Engineering, 126, 1034-1040, 2004.
[6] Moriarty, P.J. and Hansen, A.C., AeroDyn Theory
Manual, NREL/EL-500-36881, National Renewable
Energy Laboratory, 2005.
[7] Kiyoki, S., et al., Evaluation of wind loads by apassive yaw control at the extreme wind speed
condition and its verification by measurement,
GRE2018, 2018.
[8] Okuno, A., et al., alidation of Extreme Loads ofWind Turbine by Measurement Data and Simulation,
JWEA Wind Energy Utilization Symposium 2018.
[9] Namura, N., Shinozaki, Y., Design Optimization of
10MW Downwind Turbines with Flexible Blades
and Comparison with Upwind Turbines, J.Phys.:Conf. Ser. 1618 042021, 2020.
[10] Noyes, C., et al., Analytic analysis of loadalignment for coning extreme‐scale rotors, Wind
Energy, 2020:23:357-369, 2020.

国際化推進共同研究概要
No.20
20RE-8タ イ ト ル： A Machine Learning based Software-In-the-Loop method to Predict
dynamic responses of floating wind turbine

研究代表者： HU Zhiqiang

所内世話人： 胡 長洪
研究概要：
浮体式洋上風力発電システム用のセミサブ型浮体の運動特性解析に機械学習を応用す
る国際化推進共同研究について今年度は予定通り実施した。特に、Software-In-theLoop-Simulation 手法に対して検討を行い、有望な成果が得られた。R3 年１月に応研で開催されたオンライン国際研究集会「The 2nd International Symposium on Novel
Computational

andExperimental

MethodsforComplicated

Fluid-StructureInteractions」に参加し、「A Software-in-the-Loop Combined Reinforcement Learning
Method for Dynamic Response Analysis of Floating Wind Turbines」のタイトルで共同利用成果の発表を行い、参加者との研究交流を行った。

Report for 2020 RIAM International Joint Research Project[20RE-8]
A Machine Learning based Software-In-the-Loop method to Predict
dynamic responses of floating wind turbine
Zhiqiang Hu
Marine, Offshore and Subsea Technology, School of Engineering, Newcastle University, UK

During year of 2020, although the pandemic brought some negative influences on theinternational collaborations, but the collaborative works between RIAM Kyushu University and
Newcastle University have been carrying out steadily. The outcomes are listed as below.
1) Attendance of The 2nd International Symposium on Novel Computational and Experimental
Methods for Complicated Fluid-Structure Interactions, at RIAM Kyushu University in Jan
2021 and was held online.
2) During the collaborative research in year 2020, a new AI technology-based method, named
SADA, was proposed for the prediction of dynamic responses of FOWTs. Currently, it is
SADA version 1.0. The methodology of SADA starts with the selection of Key Disciplinary
Parameters (KDPs). The AI module in SADA was built by a coupled aero-hydro-servoelastic in-house program DARwind with the machine learning algorithms deep
deterministic policy gradient (DDPG). This latest research outcome has been presented in
the RIAM conference in Jan 2021.
3) Based on the research in year 2020, a collaborative conference paper entitled ‘Software-inthe-Loop combined Machine learning for Dynamic Responses Analysis of Floating
Offshore Wind Turbines’ OMAE2021-65524 has been submitted to OMAE Conference
2021. Prof Changhong Hu is the co-author of this paper. At this moment, the paper is under
the revision status. If it can be accepted, it will be presented by Mr. Peng Chen, a PhD
student of Zhiqiang Hu at Newcastle University in June 2021.
1. RIAM international symposium
The 2nd International Symposium on Novel Computational and Experimental Methods forComplicated Fluid-Structure Interactions was successfully held online on 29th Jan 2021.
Professor Zhiqiang Hu attended this symposium and gave an presentation with topic on‘Software-in-the-Loop combined Reinforcement Learning Method for Dynamic Response
Analysis of Floating Wind Turbines’. This presentation introduced the collaborative researchoutcomes of Zhiqiang Hu’s research team at Newcastle University and Professor Changhong
Hu’s research team at Kyushu University during year 2020. This presentation introduced thein-depth knowledge, structure of SADA and challenges need to be overcome in the future. Afterthe presentation, Zhiqiang Hu made fruitful discussions with other scholars online. After the
symposium, professor Changhong Hu and Zhiqiang also made a detailed discussion andconfirmed to continue the collaborative research in the field of AI-based knowledge andfloating wind turbines.

2. Collaborative research on AI technology for floating wind turbine
In the year of 2020, a collaborative research on AI-based technology for dynamic responses
analysis of floating wind turbine was conducted very well and fruitful research outcomes havebeen achieved. The SADA method is newly proposed for the prediction of dynamic responses
of FOWTs with a smart application of artificial intelligence technology. Another new skill, theselection of Key Disciplinary Parameters (KDPs) is also proposed as the first key skill in SADA.
The AI module in SADA was built by a coupled aero-hydro-servo-elastic in-house program
DARwind with the machine learning algorithms deep deterministic policy gradient (DDPG).
Basin experimental data was used in SADA for the AI training. The SADA method was used
to analyze the dynamic responses of Hywind 5MW wind turbine and the results showed verysatisfactory.
In addition, thanks for the support and collaboration in 2020, a collaborative conference paper
submitted to the 40th International Conference on Ocean, Offshore and Arctic Engineering(OMAE2021). The paper is titled ‘Software-in-the-Loop combined Machine learning for
Dynamic Responses Analysis of Floating Offshore Wind Turbines’. This paper is under revision
status. If it can be accepted for publication, the first author will give a presentation online in the
OMAE2021 conference.
In year 2021, the collaborative research will be conducted to a further step. Currently, there arestill some challenges in the anticipation of using SADA method for a higher accuracy. Some
investigations on statistical criterion application in error estimation will be conducted as apriority. The aerodynamic influence will also be investigated to a further detailed extend. The
AI technology application in offshore renewable energy is also the strong side of Professor
Changhong Hu’s research team. So, it is expected the novel method currently proposed will
have a potential opportunity to be used in some other offshore renewable energy facilities, suchas ocean wave energy converter.

国際化推進共同研究概要
No.21
20RE-9タ イ ト ル： Novel computational and experimental approaches to the analysis
of complicated fluid-structure interactions

研究代表者： WAN Decheng

所内世話人： 胡 長洪
研究概要：
本国際化推進共同研究について今年度は 2 年目で，共同研究・研究集会とも予定通り実施した。共同研究について、海洋工学分野での流体・構造連成解析問題に関する CFD 手法開発に関する検討が行われ、関連の研究成果は４編の国際学会論文に纏められ採択された。R3 年 1 月 29 日にオンライン国際研究集会「The 2nd International Symposiumon Novel Computational and Experimental Methods for Complicated Fluid-StructureInteractions」が開催され、外国から 22 名、日本から 25 名の参加者があり、流体・構造連成解析問題に関する数値解析方法及び水槽実験方法の高精度化・高効率化に関して有意義な国際研究集会となった。

Report for 2020 RIAM International Joint Research Project[20RE-9]Novel computational and experimental approaches to the analysis ofcomplicated fluid-structure interactions
Decheng Wan
School of Naval Architecture, Ocean and Civil Engineering, Shanghai Jiao Tong University, China

1. Purpose
Marine renewable energy devices are usually installed in a sea area where severe environmental conditions
have to be considered. On the other hand, cost control is strictly required for those devices in order to passeconomic evaluations. Therefore for successful design of those devices, accurate numerical methods as wellas efficient experimental methods are required to evaluate the hydro- and aerodynamic performance of thesedevices. This joint research project focuses on one of the most important fundamental fluid mechanics
problems in marine renewable energy development, the fluid-structure interaction problem. Researchers,
especially younger researchers, including but not limited to SJTU and RIAM, are invited to exchange theirknowhow on development of computational and experimental tools for the purpose.

2. Research Plan
This joint research project is planned for two years. FY2020 is the second year of the project. Besides
continuing comparison and discussion of the existing CFD codes which are developed in SJTU and RIAM,
emphasis of this year’s joint research will be on discussion of the future CFD and experimental methods foranalysis of complicated fluid-structure interaction problems in ocean engineering and marine renewable energy
developments. In the end of FY2020, as the main event of this international joint research project, International
RIAM Symposium on Novel Computational and Experimental Methods for Complicated Fluid-StructureInteractions will been carried out. Researchers involved in this joint research project will present and discuss
their research progresses.
The members involved in this collaborative research are shown in the following table.
Researcher’s Name

Name of University
or Institute

Present Status or Grade(graduate students)

Decheng Wan

SJTU

Professor

Representative person (CFD )Jianhua Wang

SJTU

Assistant Professor

Co-researcher (CFD)

Cheng Liu

SJTU

Associate Professor

Co-researcher (CFD)

Weiwen Zhao

SJTU

Assistant Professor

Co-researcher (CFD)

Liushuai Cao

SJTU

Assistant Professor

Co-researcher (CFD)

Xiaosong Zhang

SJTU

PhD student

Co-researcher (CFD)

Xiaobin Yang

SJTU

PhD student

Co-researcher (CFD)

Jinqi Li

SJTU

PhD student

Co-researcher (CFD)

Seiya Watanabe

RIAM

Assistant professor

Co-researcher (CFD)

Researcher role

Mohamed M. Kamra

RIAM

Research Fellow

Co-researcher (CFD)

Hongzhong Zhu

RIAM

Research Fellow

Co-researcher (Experiment)

Changhong Hu

RIAM

Professor

RIAM Attendant

3. Summary of Collaboration Research
In 2020, due to the international travel restriction, research discussions have been made online. Main topics
that have been studied include (1) Numerical simulation of interaction between internal solitary waves anduniform bottom slope by MPS method; (2) RANS simulation and LES of tip vortex cavitation around hydrofoil;(3) development of a numerical model for simulation of vortex-induced vibration (VIV) of a long flexible riser
under offshore platform horizontal motion; and (4) Multi-objective hull form optimization analysis of atrimaran’s total drag at different speeds by CFD simulations. The research results have been presented at the2020 ISOPE conference and the 2nd International Symposium on Novel Computational and Experimental
Methods for Complicated Fluid-Structure Interactions.
International conference papers related to this joint research project in 2020 are listed as follows.
1) Xinze Duan, Jianhua Wang, Decheng Wan, CFD Investigations of Wake Flow Interactions in a Wind
Farm with 14 Wind Turbines, International Journal of Offshore and Polar Engineering, 2020, 30(3):257–265;
2) Yang Huang, Decheng Wan, Investigation of Interference Effects between Wind Turbine and Spar-Type
Floating Platform under Combined Wind-Wave Excitation, Sustainability, 2020, 12(1), 246;
3) Xiao Wen, Decheng Wan, Changhong Hu, Numerical Study of Interaction between Internal Solitary
Waves and Uniform Slope by MPS Method, Proceeding the Thirtieth (2020) International Ocean andPolar Engineering Conference Shanghai, China, October 11-16, 2020, pp.1986-1993
4) Shang Liang, Decheng Wan, Changhong Hu, CFD Study of Tip Vortex Cavitation around Hydrofoil,
Proceeding the Thirtieth (2020) International Ocean and Polar Engineering Conference Shanghai, China,
October 11-16, 2020, pp.2345-2351
5) Yuqi Zhang, Decheng Wan, Changhong Hu, Numerical Study of Vortex-Induced Vibration of a Flexible
Riser under Offshore Platform Horizontal Motion, Proceeding the Thirtieth (2020) International Ocean
and Polar Engineering Conference Shanghai, China, October 11-16, 2020, pp.2555-2562
6) Xinwang Liu, Decheng Wan, Changhong Hu, Multi-Objective Hull Form Optimization of Trimaran’s
Total Drag at Different Speeds, Proceeding the Thirtieth (2020) International Ocean and Polar
Engineering Conference Shanghai, China, October 11-16, 2020, pp.3850-3857

As a main event of this international joint research project, ‘The 2nd International Symposium on Novel
Computational and Experimental Methods for Complicated Fluid-Structure Interactions’ was held on
January 29, 2021. Due to the global pandemic situation of COVID-19, the symposium has been held online.
On the symposium, overseas and domestic scholars have been invited to present their recent researches on
their recent research results on development of computational and experimental methods for complicatedfluid-structure interactions. The program of the symposium is as follows.

The 2nd International Symposium on Novel Computational and Experimental Methods
for Complicated Fluid-Structure Interactions
Date: January 29, 2021
Place: Zoom Meeting Room (From 9:00 Japan time)

Organized by Research Institute for Applied Mechanics, Kyushu University

TIME TABLE
9:30 - 9:35

Opening Address by Changhong Hu

Session 1

Next Generation CFD Development

9:35 - 10:30

Feng Xiao (Tokyo Institute of Technology)
Invited Lecture
Design New Fidelity Numerical Schemes by BVD Principle

10:30 - 10:55

Jabir Salami, Mohamed Kamra, Changhong Hu (Kyushu University)
Development of Flux Reconstruction Method for Multiphase Flows

10:55 - 11:20

Zheng Li, Cheng Liu, Decheng Wan (Shanghai Jiao Tong University, China)

11:20 - 11:45

High-Fidelity Numerical Simulation of Hydraulic Jump Phenomenon of a SurfacePiercing NACA Foil
Yunong Xiong (Tokyo Institute of Technology)
Unify VOF and Level Set using THINC Scaling Method

11:45 - 12:10

Xizeng Zhao (Zhejiang University, China)
Development of Three-Dimensional Wave Tank based on VPM-THINC/QQ Method
and its Application

12:10 - 13:30

Lunch breakSession 2

Computational Fluid Structure Interaction

13:30 - 13:55

Seiya Watanabe (RIAM, Kyushu University)
A Cumulant Lattice Boltzmann Method for Free Surface Impact Pressure Prediction

13:55 - 14:20

Yudai Yokoyama and Takahito Iida (Osaka University)
Verification of Enhanced MPS solver in Wedge Slamming for Development of Fluid-IceShip Interaction Problem

14:20 - 14:45

Xiaosong Zhang, Jianhua Wang, Decheng Wan (Shanghai Jiao Tong University, China)
A hybrid VOF/E-L Method for Simulating Complex Multiscale Bubbly Flows

14:45 - 15:10

Tomoaki Hirakawa (Akita University)
A Boundary Element Method for Interaction of Wave and Arbitrary Shape Bodies

15:10 - 15:20

Break

Session 3

Ocean Renewable Energy Technologies

15:20 - 15:45

Yusaku Kyozuka, Daisaku Sakaguchi (Nagasaki University)
Demonstration Experiments of a Floating/Submersible Tidal Current Power System in NaruStrait in Goto, Nagasaki

15:45 – 16:10

Ali Alkhabbaz, Byung Ha Kim, Ho Seong Yang, Young-Ho LEE (Korea Maritime & Ocean
University, Korea)
Concept Design Experiences of 8MW Floating Offshore Wind Platform at Korea WesternSea

16:10 - 16:35

Yang lin, Ma qingwei, Liao kangping (Harbin Engineering University, China)
Aero-Elastic Modeling of FOWT using Classic Rotational Beam Theory and Finite Element
Method

16:35 - 17:00

Hongzong Zhu (RIAM, Kyushu University)
A Unified Seakeeping and Maneuvering Analysis of Multiple Linked Towing System with
Triangular Bodies

17:00 - 17:25

Zhiqiang Hu (Newcastle University, UK)
Software-in-the-Loop Combined Reinforcement Learning Method for Dynamic Response
Analysis of Floating Wind Turbines

17:25 - 17:30

Closing Address by Decheng Wan

国際化推進共同研究概要
No.22
20RE-10
タ イ ト ル： Flexible energy harvesting system

研究代表者： JEON Insu

所内世話人： 東藤
研究概要：

貢

本 研 究 で は 、 天 然 高 分 子 の セ ル ロ ー ス と 合 成 高 分 子 の poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate)を原料として新しい製造方法を用いて新規ゲル材料を作製した。作製したゲルはすぐれた弾性率、強度、電気伝導性を示し、さらに-50～35℃の範囲でゲル状態を維持することが可能であった。このゲルは次世代の柔軟性を示す構造体用材料としての使用可能性が示唆された。

Flexible energy harvesting system
Chonam National University, Prof. Insu Jeon

Summary
In this research, we have developed a novel class of gels through a new fabrication route. The gels were synthesized
by integrating cellulose, a natural biopolymer, with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), an
exceptionally conductive and biocompatible synthetic polymer, in tripropylene glycol, a green organic solvent withextremely high-boiling and low-freezing temperature. The prepared gels exhibited properties with the highest
Young’s modulus, tensile strength, and electrical conductivity of ~500 MPa, ~55 MPa, and ~200 S m−1, respectively.
Moreover, the gels maintained their properties for 14 d over a wide temperature range (−50 to 35 oC), covering almost
the entire atmospheric temperature range on Earth. Owing to these remarkable properties, the developed gels areprominent candidates for many applications in next-generation flexible devices.

MethodFirst, isotropic water-equilibrated (Cellulose/PEDOT:PSS)(x/y) hydrogels were prepared (the detailed procedure is
described in the Supporting Information). The hydrogels were subsequently immersed in TriPG (tripropylene glycol)
solvent for ~2 d for solvent exchange with several TriPG solvent replacements. Lastly, the gels were stabilized in air
at ~25 oC for 1 d to obtain the desired isotropic (Cellulose/PEDOT:PSS) TriPG gels, which were subjected tosubsequent characterization.
Results
To fabricate a TriPG gel with optimal composition, we prepared a series of (Cellulose/PEDOT:PSS)(x/y) TriPG gels(where x and y are the initial cellulose and PEDOT:PSS concentrations (wt%), respectively) using the steps illustrated
in Figure 1, and analyzed their properties utilizing tensile tests, and electrical conductivity measurements.

Figure 1. Fabrication process of (Cellulose/PEDOT:PSS)(x/y) TriPG gels.

When the PEDOT:PSS concentration was increased from 0 to 1 wt% at a set cellulose concentration of 1.5 wt%, theYoung’s modulus, tensile strength, and work of extension of the TriPG gels increased gradually (Figure 2). However,when the cellulose concentration was increased from 0.5 to 1.5 wt% and the PEDOT:PSS concentration was set to 1
wt%, the mechanical properties of both the TriPG gels increased gradually. These changes in mechanical propertiesof the gels were in line with the changes in the final solvent content of the TriPG gels. Our composition variation
study indicated that TriPG can serve as a good solvent for integrating both cellulose and PEDOT:PSS in gel materials,
despite their highly contrasting chemical structures. Therefore, the mechanical properties of the TriPG gels increased
proportionally with the concentrations of cellulose and PEDOT:PSS. Because cellulose is electrically non-conductive
and PEDOT:PSS is highly conductive, the electrical conductivity of the TriPG gels and corresponding hydrogels
strongly depended on the final PEDOT:PSS concentration and relative cellulose and PEDOT:PSS concentration in
the gels.

Figure 2. (a) Typical tensile stress–strain curves of the isotropic (Cellulose/PEDOT:PSS) TriPG gels prepared byvarying the PEDOT:PSS concentration in the range of 0–1 wt% at a set cellulose concentration of 1.5 wt%. (b)Young’s modulus and tensile strength, (c) work of extension, and (d) solvent content of prepared(Cellulose/PEDOT:PSS)(x/y) TriPG gels.

Conclusion
We successfully developed a novel class of gels by integrating cellulose and PEDOT:PSS, which are bio-friendly
high-value polymers, in TriPG, a green organic solvent. Despite the significant differences in structure and propertiesof the polymers, the novel fabrication method proposed in this study allowed us to fabricate a hybrid gel withexcellent mechanical and electrical properties, and extremely high stability over a wide temperature range.
Considering these remarkable properties and facile synthesis, the developed gels are expected to have high potential
for numerous applications in next-generation flexible devices.