RF-only ST plasma confinement, sustainment,
and interactions with wall materials
Vladimir Shevchenko

A program of the Workshop which was held on 30-31 January 2020 and was as following:
8th Workshop Agenda, RIAM 2020
30 January AM
10:00 ‐10:10
Vladimir Shevchenko / Kazuaki Hanada
WS purpose and agenda

10:00 - 10:40
Vladimir Shevchenko
ST40: Recent results and ECRH & HTS plans for future

10:40 - 11:20
Kazuaki Hanada
Recent results for steady state operation and plasma current start-up on QUEST（TBD）

11:20 - 12:00
Modification of plasma-facing surface in QUEST due to PWI and development ofadvanced W wall panel for ultra-long pulse operation
Naoaki Yoshida

31 January PM
12:00 - 13:00
Lunch Time

13:00 - 13:40
Recent results from TST-2
Yuichi Takase

13:40 - 14:10
Peng Yi
Thomson scattering system in TST-2

14:10 - 14:40
James Rice
Detection of Fast Electrons in SOL Plasma Using a Langmuir Probe Diagnostic

14:40 - 14:55
Yongtae Ko
Measurement of LH waves in TST-2

14:55 - 15:15
Coffee Break

15:15 - 15:55
Hitoshi Tanaka
Overview of Recent EBW Experiment in LATE

15:55 - 16:35
Kengo Kuroda
CHI Experiments in QUEST

16:40 - Group Photo & QUEST Machine Tour
31 January AM
9:30 - 10:10

X. Gao
Non-inductive current drive experiments on EAST tokamak

10:10 - 10:50
Masayuki Ono
Update on ECH /ECCD modeling

10:50 - 11:30

Nicola Bertelli
3D full wave simulations in NSTX-U plasmas with the recent developed Petra-M code

11:30 - 12:10
Sadayoshi Murakami
MHD equilibrium reproduction by visible light computed tomography in QUEST

12:10 – 14:00
Lunch

31 January PM (Drafting of proposals for experiments, diagnosis, and analysis)
14:00 ‐ All Suggested focus and output for this joint drafting session_____________________________________________________________________________The presentation summaries are as following:Vladimir Shevchenko
ST40: Recent results and ECRH & HTS plans for future
In September 2019 there was a short experimental campaign on ST40 after relocation of themachine into the new building. 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 and additional vertical field power supplies were
installed and tested. Diagnostic Neutral Beam (DNB) injector has been delivered and partly
tested.
In Autumn 2019 installation of the second part of TF power supply was completed and 1 MW
25 keV Heating Neutral Beam (HNB) injector has been tested and installed. These
installations were followed by commissioning on ST40 in December 2019 and January 2020.
Both beams demonstrated good performance in hydrogen gas. An improved plasma
performance has been demonstrated in this second 2019 campaign taking advantage of theplasma sustainment using solenoid and upgraded plasma diagnostics. Some plasma heating
and fuelling 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 inexcess of 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 is progress. Next
experimental campaign is planned after completion of the cooling system.
Further upgrades of ST40 include installation of the 2nd HNB 1 MW 50 keV in Summer
followed by commissioning and plasma experiments in Autumn 2020. A dual (140/105 GHz)
frequency 1 MW gyrotron is scheduled for delivery and commissioning in Q1 2021.
Another significant activity at Tokamak Energy (TE) is a high temperature superconductor(HTS) magnet development. We believe that HTS technology gives an opportunity to build acompact tokamak-reactor which would allow achievement of plasma parameters close toignition. A special magnet winding and testing facilities have been built at TE. HTS tapes
from several suppliers were tested and compared against requirements for the tokamakreactor. Using the best tape, a test magnet was developed demonstrating a world record
field of 24 T. It was shown that HTS based magnets are resilient to a thermal quench andcan withstand internal damages without causing catastrophic degradation. The next step is a
development of a full scale HTS toroidal magnetic system DEMO-4 with the target magnetic
field of 10 T at a major radius of 0.25 m. This milestone is to be achieved by the end of 2020.

Kazuaki Hanada
Recent results for steady state operation and plasma current start-up on QUEST• The QUEST project is focusing on plasma start-up with RFCD and steady state operation.

• The 28GHz RF injection system has the capability to regulate both the wave polarization
and N//. The system has also the capability to focus RF beam down to 5 cm in radius.
• More than 100kA could be obtained in N//=0.78 X-mode with a little OH provided from thepoloidal field increment. According to the RF absorption, the RF is likely to absorb toenergetic electrons. Which is useful to drive plasma current.
• Low N//=0.1, X-mode injection could successfully heat up bulk plasma up to 0.5 keV of Te.
Single pass absorption of achieved plasma parameters is expected to be approximately
40%, therefore the effective bulk heating could be obtained with a sophisticated neutral
control to avoid production of energetic electron.
• High field launch of RF has been tried to effectively excite EBW in the plasma. A new
tentative waveguide and antenna was installed for the experiment. The designed value ofmode conversion to EBW is 93%. The rest RF is absorbed at the fundamental ECR.
• Higher density and better absorption of RF in HFS launch than those in the LFS launch
could be obtained. PDI signal was measured with a movable probe and larger PDI in the
HFS launch was measured. This supports to higher mode conversion to EBW.
• To get evidence of EBWCD, TF and PF direction modification has been executed. The
plasma reproducibility is insufficient to compare the value of plasma current. The flux loop
signals denote the inversion of plasma current direction in the mid-plane, bur need to more
confirmation.
• Plasma Breakdown has been investigated with 2nd harmonic ECW. The threshold power
was much higher than fundamental ECW predicted by numerical calculation. The connection
length dependence was different from fundamental ECW.
• The hot wall plays essential roles in reducing the amount of wall-stored H and facilitating H
recycling. Only top hot wall has the capability to water cool down since 2018SS campaign.
• The clear reduction of Ha after water cooling of the top hot wall. This indicates the recycling
could be control by wall temperature.
• The cooling down from 473K of only top hot wall with water can extend the plasma duration• During long duration discharges, neutral compression could be achieved behind the bottom
divertor plate. The reason is still unclear, because there are many candidates to make thisto-bottom asymmetry. It is a future work.

Naoaki Yoshida
Modification of plasma-facing surface in QUEST due to PWI and development ofadvanced W wall panel for ultra-long pulse operation
Impurity deposition on the plasma facing surface in QUEST at preset is aggregation of
nanocrystals of carbide and metallic oxide and little amount of amorphous carbon. TDS
results indicate that its contribution to the retention and thermal desorption of H underplasma discharge is little. In contrast, large amount of H can be retained in hot wall liner
APS-W and its thermal desorption continues up to around 1000K. It can be considered that
active desorption of H from the APS-W liner causes the termination of long pulse plasma
discharges.
For ultra-long pule discharges at higher temperatures (≤773K) we have already started
development of advanced W liners, in which retention of H at high temperatures is low
enough.

Yuichi Takase

Recent results from TST-2
The capacitively-coupled combline (CCC) antennas for exciting the traveling LHW for Ip
start-up and ramp-up were developed. Outboard launch, top-launch, and “bottom-launch”(top-launch with reversed Bt) were compared. The driven current was higher for top-launchthan outboard-launch as expected, but the highest driven current was achieved by “bottomlaunch”, contrary to initial expectation. Thomson scattering measurements show peaked ne
profile and hollow Te profile with the highest temperature in the inboard edge region, andequilibrium reconstruction using an extended MHD model shows a hollow j profile. Hard Xray spectra show higher flux and effective temperature for top-launch compared to outboardlaunch, especially in the inboard edge region, consistent with Te and j profiles. A large orbit
loss fraction was inferred from the RF power modulation experiment. A combined ray-tracing
and Fokker-Planck analysis indicate n|| upshift and strong damping around r/a ~ 0.5 for toplaunch. For “bottom-launch”, n|| downshifts first, but upshifts after reflection. Once electrons
are accelerated to high energies, these electrons can absorb the downshifted n|| LHW andvery high energy electrons can be generated. The result of this analysis can explain theexperimental results.
An EC-assisted inductive Ip start-up scenarios being developed for JT-60SA on TST-2. The
trapped-particle configuration (TPC) with a weak Bv with positive decay index enlarged theoperating window for reliable start-up compared to the usual field-null configuration (FNC).
Application of EC power extended both the low pressure limit for breakdown and the highpressure limit for burn-through. The TPC was particularly beneficial at low pre-fill pressureand high EC power.
AC Ohmic coil operation is a reliable pre-ionization method. A small inboard coil can beused for pre-ionization. The breakdown voltage is nearly the same as the regular Ohmic coil.
Positive and negative triangularities can be obtained using the small inboard coil. Recent
results from TST-2

Peng Yi
Thomson scattering system in TST-2
It is useful to find the optimum operational conditions for fusion reactors if the electron
temperature and density profile measured by the Thomson scattering measurements, aswell as their scaling could be obtained.
In the study, the relation of the electron temperature (Te) and pressure (P) to the electron
density (ne), RF power (Pw), toroidal magnetic field (Bt), and plasma current (Ip), for the
scenario of D+CW+Top (deuterium, clockwise toroidal field, and top antenna), is obtained byseparate fitting for the parameters in the presumed formula. In addition, the comparisons ofdifferent scenarios, such as hydrogen (H) and D, top and out antenna, CW and counterclockwise (CCW), have been conducted to investigate the effects of them on the electron
temperature. Compared with the D+CW+Top scenario, the results show the H fuel gas gives
rise to higher electron temperature, CCW toroidal field would produce lower electron
temperature, and the out antenna results in rough same electron temperature within error
bars. On the basis of temperature relaxation time calculation for the TST-2 and theestimation of confinement time, as well as the heating power for bulk electrons and theequivalent heating power calculated by stored energy in the plasmas and confinement time,
it is feasible to explain qualitatively the measured results of electron temperature bycollisional heating.
In order to observe the temperature anisotropy in the plasmas, the double-optical-passconfiguration for Thomson scattering system in TST-2 is also investigated. The effects ofdisplacement of the focal points on the fiber’s collection efficiency are estimated, as well as
the deviation of the returned beam from the center of laser source, caused by the tilt angle ofthe distant mirror, is analyzed since the returned beam will pose the laser source injeopardy. Also, the theoretical calculations of the displacement of returned beam are verified
by corresponding measured results, and that indicates the possibility of blocking the returned
beam by putting an aperture in front of the lens or the laser source. The optimization of the
double-optical-pass configuration will be implemented in the next step.

James Rice
Detection of Fast Electrons in SOL Plasma Using a Langmuir Probe Diagnostic
Electron Energy Distribution Function (EEDF) analysis has been used to confirm thepresence of fast electron populations in SOL plasma in TST-2. A full profile of TST-2’s SOL
plasma is desired for ray-tracing and plasma simulations. To this end, a new Langmuir probe
has been installed as a prototype for a full profile of SOL parameters. This prototype has
been used to refine the measurement methods used.
Recent results from power modulation experiments show strong alteration to Te and Vflduring RF power. The positive increase to Vfl indicates RF sheath rectification is not thedominant alteration. It is thought that the non-thermal electron energy distribution in TST-2
causes an overestimation of Te. To confirm this, EEDF analysis has been used to confirm
the presence of fast electrons in SOL plasma. Preliminary results show low-temperature bulk
electrons and a higher energy component at lower density. Further research is required tobetter classify this fast electron component and identify its effect on measured plasma
parameters.

Yongtae Ko
Measurement of LH waves in TST-2
14-ch RF magnetic probe has been developed to detect LH waves (200 MHz). Parametricdecay instability corresponding to ion cyclotron quasi-mode were observed in TST-2 LH
driven plasma experiments. The interest is what the differences among launching mode,
outboard-launch, top (CW) launch and top (CCW) launch. In the experimental results, large
peak of PDI associated with ICQM was observed on sideband of the pump-wave frequency
in hydrogen plasma with outboard and top (CW) launch case, while disappeared indeuterium plasma with top (CW and CCW) launch case. The fraction of side band power(100 – 199.9 MHz) was dominant (over 100%) in inboard region with the outboard launch
and also dominant (over 200 %) in bottom-outboard region with top (CW) launch. However,with top (CCW) case, fraction was suppressed under 80 %, but direct relationship between
current drive efficiency and PDI suppression is not realized.

Hitoshi Tanaka
Overview of Recent EBW Experiment in LATE
Microwave at 2.45 GHz is used to start-up and form ST plasmas. In order to excite the EBW
via O-X-B mode conversion, three polarizations of microwave such as O-mode-like lefthanded elliptically polarized ones and X-mode-like ones are used and compared. Results ofeffect on bulk electron heating are qualitatively related to the mode conversion rate
calculated by the linear theory with the cold resonance absorption model in a slab model.
HIBP measurement have been carried out during the intermittent plasma ejection events
which usually appear when the density exceeds several times of the plasma cut off density
and the central safety factor becomes about 8. Fast space potential variations are observed
during the event, which indicate the local escape of electrons and its recovery.
An electron beam is injected from a cold cathode installed at the bottom port into a ST
plasma which is produced non-inductively by EBW. Investigation of synergistic effects ofelectron beam injection and EBW current drive has begun.

Kengo Kuroda
CHI Experiments in QUESTCHI current start-up by using simple electrode has been tested in QUEST. Initial result, inwhich the injector flux forms between electrode and outer wall shows that the reliable gas
breakdown is achieved under appropriate conditions and plasma evolves with increasing
toroidal current which is dependent on the PF configuration, but keeping a “narrow
footprint” is difficult and current multiplication I_tor/I_inj is low, ~1.
In the discharges, in which the injector flux forms between electrode and inner wall highcurrent multiplication, 5~10 and long duration 10~30ms are achieved and plasma evolves tofill vessel with keeping “narrow footprint”.
We plan to improve and further test the second configuration by installing a new (temporary)
injector flux coil that is much closer to CHI electrodes.
After verifying good closed flux surface formation a more permanent CHI coil design will beconsidered to support CHI + ECH studies.

X. GaoNon-inductive current drive experiments on EAST tokamak
Overview of experimental progress on EAST is presented. Steady-state fully non-inductive
scenarios (such as long pulse H mode, high beta-p plasma, and high beta-N plasma et al.) isdemonstrated with extension of fusion performance. Regular and low loop voltage start-ups
without resistors have been achieved on EAST tokamak in 2019.

Masayuki Ono
Update on ECH /ECCD modelling
The QUEST ECH solenoid-free start-up experiment utilizing the 28 GHz gyrotron at 2nd
harmonic frequency has demonstrated remarkable efficiency and achieved record start-up
current values [1]. The experiment provides rich opportunities to understand and optimize
ECH-based tokamak/ST current start-up and ramp-up concept. Another potentially
noteworthy aspect of the QUEST 28 GHz experiment is its very high frequency to toroidal
magnetic field ratio, which is 28 GHz/0.25T or 112GHz/1T. The higher frequency enables
higher density limit and for reactors with several Tesla toroidal field, this start-up scenario
can largely avoid the usual density limit often encountered by ECCD. Conversely this higher
harmonic scenario would enable utilization of ECH at lower magnetic field as in the case ofmany ST experiments. This scenario maybe also attractive for the ECH assisted start up forthe initial phase of ITER where the toroidal magnetic field maybe relatively low ~ 2 T. To
better understand the QUEST experimental results, we initiated a modelling effort at PPPL.
Improved modelling should also help develop better predictive capability for future ST andtokamak-based reactors. An ST/tokamak start-up modelling is a highly coupled non-linear
problem as the magnetic field topology evolves dramatically from an open vacuum field
configuration to a closed configuration. The plasma temperature evolves from a very cold
collisional regime to a very hot collision-less regime. For this task, we developed a gridbased start-up code where plasma parameters, generated plasma currents, and resulting
poloidal magnetic fields are evolved from the vacuum fields. The grad-B drift driven current
together with the processional currents can then create a closed flux surface configurationand then the bootstrap current in a closed configuration can further enhance the plasma
current. The ECH heating efficiency increases with plasma current since the confinement isincreased and resulting electron temperature rise would further increase the ECH absorption
and plasma currents. Once the plasma temperature becomes sufficiently high ~ 1 keV, asingle-pass absorption can rise sufficiently to transition to the ECCD phase. An important
point to note is that two-component distribution (hot minority and colder bulk) is highly
advantageous for hot electrons to be generated for efficient ECCD as observed in the
QUEST start-up experiment. An update of the modelling is given including somebenchmarking of RT-4 with Genray at higher temperature range of ~30 keV and also
including the effects of particle loss to better simulate the experimental situation.

Nicola Bertelli
3D full wave simulations in NSTX-U plasmas with the recent developed Petra-M code
In this work we present the recently developed tool Petra-M and its current applications.
Petra-M code [1] is a state-of-the-art generic electromagnetic simulation tool for modeling RF
wave propagation based on MFEM [http://mfem.org], open source scalable C++ finite
element method library. Commonly, RF simulation has been often limited to a relatively small
volume in front of the antenna, and it involves physics simplification from the actual
experimental situation such as stratifying antenna strap structure, flat antenna model and/or
treating the antenna front volume as vacuum. This paper, instead, shows the full 3D NSTX-U

device geometry including realistic antenna geometry in order to capture the 3D effects andthe antenna-plasma interaction in the SOL plasma and, at the same time, the core wave
propagation. A scan of the antenna phasing shows a strong interaction between FWs andthe SOL plasma for lower antenna phasing, which is consistent with previous NSTX HHFW
observations. Furthermore, the effect of the 3D wave field on the fast ion population from
NBI beams in NSTX-U is quantified by using the 3D field obtained from the Petra-M
simulations in the SPIRAL full-orbit following particle code [2]. Then we show the 3D fullwave simulations of a new HHFW 4-straps antenna recently installed by Tri-Alpha Energy on
LAPD with an initial comparison with the experimental data. Finally, initial Petra-M
simulations on the JET ITER-like antenna, ICRH/LH in WEST machine, and helicon in
KSTAR are discussed.
[1] S. Shiraiwa et al., EPJ Web of Conferences 157, 03048 (2017). [2] G. J. Kramer et al.,Plasma Phys. Control. Fusion 55, 025013 (2013).

Sadayoshi Murakami
MHD equilibrium reproduction by visible light computed tomography in QUESTWe propose a method for reconstructing image of a poloidal plasma cross section fromvisible light information in the tangential direction of tokamak. We realize a robust
reconstruction method using Laplacian eigenfunction expansion & L1 regularization. The
MHD equilibrium is calculated from reconstructed image applying TASK/EQ code. Rotating
toroidally the obtained outer flux surface, we obtain a similar image with the measured
visible light.