23P126-B
X-ray measurements during non-inductive plasma current start-up experiments
using lower hybrid waves on the TST-2 spherical tokamak
TST-2球状トカマクにおける低域混成波を用いた非誘導プラズマ電流立ち上げ時のX線計測
Takuma Wakatsuki, Akira Ejiri, Hidetoshi Kakuda, Yuichi Takase, Takanori Ambo,
Hirokazu Furui, Takahiro Hashimoto, Junichi Hiratsuka, Hiroshi Kasahara1,
Kunihiko Kato, Ryuhei Kumazawa1, Kenji Saito1, Takuya Sakamoto, Testuo Seki1,Fujio Shimpo1, Ryota Shino, Takahiro Shinya, Masateru Sonehara, Osamu Watanabe,Takashi Yamaguchi
若月琢馬, 江尻晶, 角田英俊, 高瀬雄一, 安保貴憲, 古井宏和, 橋本貴博, 平塚淳一,
笠原寛史1, 加藤邦彦, 熊沢隆平1, 齋藤健二1, 坂本拓也, 関哲夫1, 新保富士夫1, 篠遼太,新屋貴浩, 曽根原正晃, 渡邉理, 山口隆史
The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa 277-8561, Japan東京大学 〒277-8561 柏市柏の葉5-1-51National Institute for Fusion Science, 322-6, Shimoishi, Toki, Gifu, 509-5292, Japan1
核融合科学研究所 〒509-5292 土岐市下石町322-6
Non-inductive plasma current start-up experiments using RF power in the lower hybrid frequency range isbeing conducted on the TST-2 spherical tokamak. Plasma currents of up to 15 kA have been achieved. The
effect of direct current drive can be seen by comparing the cases with co-drive and counter drive. X-rays
with various energies were measured to investigate the interaction between the wave and electrons. Soft
X-ray measurements imply that fast electrons driven by lower hybrid wave (LHW) have some contribution
to the plasma current. However, the RF amplitude modulation experiments showed that the confinement
time of these fast electrons seems to be very short (less than 0.1 ms) compared to the collisional slowing
down time. Hard X-ray measurements showed that the effective temperature of fast electrons in
co-direction for current drive was higher than that in counter direction. This fact confirmed the existence of
RF driven fast electrons.

1. Introduction
Lower hybrid wave (LHW) has demonstrated
high efficiency for driving current inconventional tokamaks [1]. If the plasma current
can be ramped-up by LHW to a sufficient levelthat is needed for further heating (e.g. NBI), thecentral solenoid can be eliminated. However,waves in spherical tokamak plasmas behave very
differently from those in conventional tokamak
plasmas because of very high dielectric constants(~ pe2/ce2), which arises from the high density
at low magnetic field ST plasmas. In addition,
deterioration of the lower hybrid current drive(LHCD) efficiency is known to occur at highdensities [2][3]. Therefore, it is very important todemonstrate the feasibility of a plasma start-up
scenario using the LHW in ST.
LHW is thought to be absorbed by high energy
electrons through Landau damping. Therefore,
X-ray measurements were performed to measure
bremeetrahlung of high energy electrons. The
main interest is to confirm the existence of RFdriven fast electrons and its contribution to theplasma current.
2. TST-2 spherical tokamak
TST-2 spherical tokamak is a small size sphericaltokamak placed at the University of Tokyo. The
major radius R0 = 0.35 m, the minor radius a = 0.23m, and the toroidal field BT = 0.1 T. The plasma
current ramp-up to 15 kA was successfully
achieved at TST-2 by LHCD with a slow verticalfield ramp-up.
LHW is excited in TST-2 by the antenna
so-called “combline antenna”. This antenna isoriginally designed for excitation of travelling fastwave at 200 MHz. However, the wave excitation
simulation using a versatile finite element method
solver package COMSOL predicts that the slow
wave (which is LHW) can be excited in thecondition of this experiment. This is because thefast wave is cutoff for the density we consider. The
parallel refractive index of excited LHW n|| = 7.2 atthe edge. The injected power is up to 150 kW, butinjected power to the plasma is less than 70 kW andthe remaining is transmitted to output port.
As shown in Fig. 1, the plasma current is nearly
proportional to the applied vertical field. The
direction of plasma current inverts with the reversalof vertical field direction. The negative value means
the plasma current flows in counter direction
against LHCD. In counter drive case, the magnitudeof plasma current cannot exceed 4 kA. This factreflects the effect of direct current drive by LHW is
important in this experiment especially highercurrent case.
Fig. 2. The time evolution of HX spectrum (rightfigure) and correspond discharge waveform. The
black and red symbols represent energy spectrumof HX in co-direction and counter direction,respectively. The lines in 3rd figure show theeffective temperature of fast electrons.

the population of high energy electrons as Eq. 1.
nHnn
  H  H  S 0 e it ,t H  loss

(1)

where nH is the density of high energy electrons,

Fig. 1. The plasma current dependency on vertical His the slowing down timeloss is typical time
field strength. Negative value denotes counter drivecase by LHCD. The data which divert fromproportionality in co-drive case require higher
filling pressure than others.

3. X-ray measurements
3.1 Soft X-ray measurements
The dependence of the perpendicular (i.e., along
the major radius) soft X-ray (SX) emissions on theplasma current was investigated. The higher energy
SX in energy range 1-10 keV significantly
increased as the plasma current increased. This factsuggests that fast electrons produced by LHW havesome contributions to the plasma current.
Hard X-ray (HX) emission is measured by NaI
scintillator in two tangential viewing chords. One
chord is oriented to detect HX emitted fromco-driven fast electrons and the other chord isoriented to detect those from counter driven
electrons. Build up of HX emission was observed as
the plasma current increased. HX from co-driven
electrons is always stronger than that from the other.
At the current flat top, a slight difference in
effective temperature of fast electrons can be seen.
The RF amplitude modulation (AM) experiments
were performed in order to investigate fast electronconfinement. We varied AM frequency from 0.2
kHz to 3 kHz. The phase of lower energy SX(whose energy was in the range 10-70 eV) started todelay at 0.5 kHz while the phase delay of higher
energy SX (whose energy is in the range 1-10 keV)
was not observed. We consider a simple model for

scale of loss term, S0 is magnitude of RF source
term and  is AM frequency. Solving this equation,
the relative phase of high energy electron density
can be written asH  Arc tan( )(2)where,

  H loss.
 H   loss

(3)

If H ≪ loss'falls on the same orderofHhowever, the slowing down time of highenergy electrons is longer than that of low energy
electrons, thus higher energy SX should start todelay at lower frequency than lower energy SX.
Therefore, the rapid loss term should be present andits timescale is less than 0.1 ms. This rapid loss offast electrons can be attributed to the orbit loss. Theorbit drift size of 10 keV fast electrons isapproximately 30 mm, thus observed fast electrons
may reside mainly near the edges.
Acknowledgments
This work is supported by JSPS Grant-in-Aidfor Scientific Research Nos. 21226021,21246137.
References[1] S. Bernabei, et al., Physical Review Letters 49,1255–1258 (1982).
[2] Y. Takase, et al., Physics of Fluids 28, 983 (1985).
[3] M. Mayberry, et al., Physical review letters 55,
829–832 (1985).