Plasma and Fusion Research: Regular Articles

Volume 11, 2402106 (2016)

Comparison of Ion Internal Transport Barrier Formation between
Hydrogen and Helium Dominated Plasmas∗)
Kenichi NAGAOKA1,2) , Hiromi TAKAHASHI1,2) , Kenji TANAKA1) , Masaki OSAKABE1,2) ,Sadayoshi MURAKAMI3) , Shogo MAETA3) , Masayuki YOKOYAMA1) , Keisuke FUJII4) ,Haruhisa NAKANO1,2) , Hiroshi YAMADA1,2) , Yasuhiko TAKEIRI1,2) , Katsumi IDA1,2) ,Mikiro YOSHINUMA1,2) and the LHD Experiment Group1)

National Institute for Fusion Science, Toki 509-5292, JapanSOKENDAI (The Graduate University for Advanced Studies), Toki 509-5292, Japan3)
Department of Nuclear Engineering, Kyoto University, Kyoto 606-8501, Japan
4)
Department of Mechanical Engineering and Science, Kyoto University, Kyoto 615-8540, Japan
2)(Received 9 December 2015 / Accepted 22 June 2016)
Ion internal transport barrier (ITB) was formed in both hydrogen discharge and helium dominated discharge.
The central ion temperature was investigated as a function of the hydrogen density ratio (nH /(nH +nHe )). The
central ion temperature increases with the decrease of the hydrogen density ratio, while the electron temperaturedoes not change significantly. The experimentally observed ion temperature was not reproduced by the predictionof TASK-3D modeling. The dominant activity in the density fluctuation measured by the phase contrast imaging
is consistent with ion temperature gradient mode and they are almost identical between the hydrogen dominatedand helium dominated plasmas.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: ion ITB, helical plasma, isotope eﬀect, deuterium experiment
DOI: 10.1585/pfr.11.2402106

1. IntroductionThe deuterium experiment is planned in the Large Helical Device (LHD) project. The physics mechanism of theisotope eﬀect is one of the most important subjects in thedeuterium experiment [1], where the isotope eﬀect means
the diﬀerences of confinement characteristics between ahydrogen discharge and a deuterium discharge.
In tokamak plasmas, it is widely recognized that theheating power threshold of the L-mode to H-mode transition depends on the ion species. In the deuterium discharges in a tokamak, the power threshold is smaller thanthat in the hydrogen discharges [2, 3]. The transport improvement in the core region was also identified in JT60U [3]. However, the physics mechanism is not yet understood.
A gyrokinetic simulation of the ion temperature gradient (ITG) mode in the LHD configuration shows the reduction of zonal flow damping rate in a deuterium plasma. As
a result, the enhancement of zonal flow and the suppression of ion heat transport are predicted in the deuterium
discharges in LHD [4].
The transport characteristics and their ion species dependence have been investigated using hydrogen and helium dominated plasmas in LHD. In this paper, the depenauthor’s e-mail: nagaoka@nifs.ac.jp∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).

dence of the heat transport characteristics of ion internaltransport barrier (ion ITB) discharges upon the density ratio of hydrogen to helium is presented. In the next section,
the ion ITB plasma and the experimental observations arepresented. The density fluctuation characteristics are alsodiscussed. Finally, important points for research on isotope
eﬀect in deuterium experiment in LHD are discussed.

2. Ion ITB Discharges
The experimental stage is the Large Helical Device(LHD) at the National Institute for Fusion Science (NIFS)[5]. The coil systems are composed of super conductors.
The magnetic field strength is up to 3T. The major and theaveraged mirror radii are 3.75 m and 0.6 m, respectively.
The toroidal and poloidal mode numbers are n = 10 and m= 2, respectively.
In order to replace the gas species, intense dischargecleaning was carried out before the experiment. Thelong pulse helium discharges with the duration time of 60
sec were repeated with ion cyclotron range of frequency(ICRF) heating [6–8]. The hydrogen recycling was reduced significantly.
Next the ion ITB discharge with helium gas puﬀing was performed with neutral beam injection (NBI)
heated plasmas. The hydrogen ion ratio (nH /(nH +nHe )) was
scanned from 0.34 to 0.78 on shot by shot basis by changing the gas puﬀ from helium to hydrogen. Figure 1 shows

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Science and Nuclear Fusion ResearchPlasma and Fusion Research: Regular Articles

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Fig. 1 (a) Time evolution of heating power of negative NBI, positive NBI and ECH. The port-through powers of negative-ion-based
NBIs (tangential injection) and the positive-ion-based NBIs (perpendicular injection) are 10.1 - 12.3 MW and 11.6 - 11.7 MW,
respectively. (b) Time evolutions of the line averaged density and (c) hydrogen ratio (nH /(nH + nHe )) of four shots. The helium gas
was puﬀed at t = 3.5 - 3.55 and 3.6 - 3.75 sec for 128670, and t = 3.5 - 3.54 and 3.6 - 3.75 sec for 128665. The hydrogen gas waspuﬀed at t = 3.5 - 3.9 sec for 128708. No gas puﬀ was operated for 128717. Radial profiles of (d) electron temperature, (e) iontemperature and (d) electron density of four shots with the time of t = 4.7 sec.

four discharges and we compare plasmas with the sameline averaged density of 1.3 × 1019 m−3 and with diﬀerent hydrogen ion ratio. Figures 1 (d-f) show the typicalprofiles of electron temperature, ion temperature, and electron density. The one perpendicular NBI (BL4) was modulated with ON/OFF = 80 msec / 20 msec for the calibrationof charge-exchange spectroscopy (CXS) diagnostic and theion temperatures are estimated by the line spectrum of carbon impurities [9]. The hydrogen ratio was experimentally
evaluated by line emissions of hydrogen and helium [10].
The total hydrogen beam current injected into the plasma is
about 350A, and the particle fueling rate for the hydrogen
beam is the order of 1021 s−1 . The minimum hydrogen ratio (nH /(nH +nHe )) with helium gas puﬃng was 0.34 in thecondition of low hydrogen recycling wall. The wall condition with very low hydrogen recycling was sustained inonly a few discharges and the wall condition changes shotby shot during the experiment. The electron density near
the edge (reﬀ /a99 > 0.7) slightly changes depending on thewall condition and gas puﬀ condition, however the electron density in the core (reﬀ /a99 < 0.5) is almost identicalamong the four shots compared in Fig. 1. Therefore, theclear change of particle transport was not identified in theexperiment.

The electron temperature profiles are identical among
the four plasmas presented in Fig. 1. On the other hand, thecentral ion temperature of around 6 keV was observed withhydrogen ion ratio of 0.34, and the central ion temperature
decreased with the increase of hydrogen content. In orderto evaluate the heat transport in the ion ITB discharges,
the experimental observation was compared with the prediction with the empirical transport model. The thermal
diﬀusivity in the transport model is given byTB
χi,e = χNeoi,e + χi,e ,where the subscript “Neo” and “TB” indicate the neoclassical transport and turbulent transport components, respectively. The turbulent diﬀusivity is modeled based on gyroBohm scaling with temperature gradient eﬀect,a ∂TTB,χi = χiGB ·T ∂ρwhere χiGB , a, T , ρ are gyroBohm thermal diﬀusivity, minor radius, temperature and normalized minor radius, respectively [1]. Figure 2 shows the comparison of experimental observations and the model prediction calculatedwith TASK-3D transport code [12, 13]. In the model calculation, the population of hydrogen ions, helium ions and2402106-2

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carbon impurity ions are taken into account, and the transport coeﬃcients are assumed to be the same among diﬀerent ion species. The model prediction in the hydrogen ion

Fig. 2 Ion and electron temperature at the magnetic axis (reﬀ ∼0) as a function of hydrogen ion ratio (nH /(nH + nHe )).
Solid lines show the experimental observation, and thedashed lines show the prediction of the empirical model.

dominated regime is consistent with the experimental observation. However, the ion temperature in the helium ion
dominated regime is higher than the model prediction. In
order to evaluate the NBI heating power more accurately,
GNET code was utilized. The full orbits of fast ions during the energy slow-down are calculated with multi-ion
bulk plasma in GNET [14], while an analytical formulaof Fokker-Plank equation with prompt finite orbit eﬀect
is utilized in TASK-3D code [13]. The total ion particle
number changes with the hydrogen ratio, however the ion
heating power normalized by the particle number evaluatedwith GNET code is almost identical and is (0.14 ± 0.01)
× 10−19 MW/particle for four cases due to the decrease ofcritical energy (Ecr ∼ (Z/A)2/3 ) for helium dominated plasmas. Therefore it was identified that the ion heat transportchanges with the ion species. On the other hand, the electron temperature does not depend on the ion species content and agrees with the model prediction. Theoretical research for developing predicable model with transport improvement with Zeﬀ or Aeﬀ dependence is in progress [15],which is beyond the scope of this paper.
In order to discuss anomalous transport, the density
fluctuation properties measured with the phase contract
imaging (PCI) diagnostic [16] were compared between hy-Fig. 3 Radial profiles of the density fluctuation measured with PCI. (Upper: a/b-1) Amplitudes of density fluctuation directed electron
diamagnetic drift, ion diamagnetic drift and total of them, (Middle: a/b-2) the distribution of the perpendicular wave number
dependence, and (Bottom: a/b-3) the distribution of the perpendicular velocity to the magnetic field in the laboratory frame. The
density fluctuation with frequency range of 20 - 500 kHz are plotted. The color contour represents the intensity in the logarithmic
scale. The profiles of poloidal rotation velocity evaluated by the CXS are also plotted in the (a/b-3).

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drogen ion dominated and helium ion dominated plasmas.
Figure 3 shows radial profiles of (upper) the amplitudeof the density fluctuation, (middle) intensity of the wave
number of the density fluctuation, and (bottom) intensityof phase velocity of the density fluctuation in the laboratory frame. The density fluctuation with a frequency of
20 - 500 kHz are analyzed and plotted in Fig. 3. The density fluctuation rotating in the direction of ion-diamagnetic
drift with the wave number of 0.1 - 0.4 mm−1 was observedin the both plasmas with hydrogen ratio of 0.34 and 0.78.
Although the mode identification is diﬃcult from only thePCI observation, ion temperature gradient (ITG) mode wasidentified to be dominant mode in the high ion temperature discharges in LHD from the comparison between
the experimental observation and gyrokinetic calculation[17–19]. The dominant mode observed in this experimentis very similar to that obtained in the previous experiment,
thus the mode seems to be the ITG mode. The global characteristics of the density fluctuation observed with PCI,
such as, the amplitude profile, peak level, wave number
profile, rotation, etc. are almost identical between the twoplasmas, while the ion heat transport is diﬀerent between
them. The observed wave number spectra seem to be reasonable, because the characteristics scale of ITG is determined by the ion Larmor radius (ρi ∼ (A1/2 /Z), where Aand Z are mass number and atomic number, respectively),
which is identical between hydrogen plasma (A = 1 andZ = 1) and helium plasma (A = 4 and Z = 2). However,
the observation of fluctuation amplitude seems to be inconsistent with the fact that the ion heat transport is diﬀerent
between helium dominated and hydrogen dominated plasmas. Based on the mixing length theory, the thermal diﬀusivity is given by χi ∼ γ/k2 , where γ and k are the growthrate and wave number perpendicular to the magnetic field,and the fluctuation measurements suggest that the growthrate changes depending on ion species. In other words, thephase relation between two fluctuating quantities such as
density and temperature changes and the induced turbulent
heat flux changes as well.

3. Discussion and Summary
The transport of the plasma with ion ITB is compared
between hydrogen dominated plasma and helium dominated plasma. The experimental observation clearly showsthat the ion temperature depends on the density ratio ofhydrogen ions, and the ion temperature is higher in thelow hydrogen density ratio. On the other hand, no difference in particle transport and electron heat transport
were identified in the present experiment. The comparison with the developed empirical transport model basedon the gyroBohm scaling with temperature gradient factor indicates the improvement depending on the hydrogenion ratio in the ion heat transport. The density fluctuation
measured with PCI indicates that the density fluctuation
rotating in the ion-diamagnetic drift direction is dominatedin the plasmas and the global characteristics of the mode
do not change depending on the ion species. The importance of evaluation of growth rate is highlighted, while thephysics mechanism of the ion transport improvement is astill open issue.
The similar behavior was observed when small
amount of impurity is injected into the plasma, such as carbon pellet injection, nitrogen gas puﬃng, etc. In the caseof carbon pellet injection, the ion heat transport is clearly
reduced with the carbon density of 1 % of bulk ion density and eﬀective charge number Zeﬀ of around 2. The detailed transport analysis shows that the ion thermal diﬀusivity depends on the carbon density in the plasma [20].
These experimental results indicate the need to investigate
the dependence of ion thermal diﬀusivity on Zeﬀ and/orAeﬀ . The further upgrade of the analytical tool TASK-3D
code is ongoing [21], and the theoretical modeling, which
includes Zeﬀ and/or Aeﬀ dependence for heat transport improvement, is discussed in another paper [15].
In this paper, the transport characteristics of diﬀerent
ion species were experimentally compared in the condition that the ion Larmor radii were kept a constant, whichis considered as an important reference experiment forstudying the physics mechanism of isotope eﬀect, which isplanned in coming deuterium plasma experiment in LHD.

Acknowledgements
The authors thank Profs. H. Sugama, M. Nunami,
M. Nakata for fruitful discussions and LHD project members for their contribution to the operation of LHD.
This research was supported by NIFS13KLPR017 andNIFS14KKGR003.
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