1. Overview of TST-2 Experimenta
R0 = 0.38 m
a = 0.25 m
A = R0/a = 1.5
Bt = 0.3 T
Ip = 0.1 MA
Overview of TST-2 Experiment
Y. Takase for the TST-2 Group
The University of Tokyo, Kashiwa Japan
The Second A3 Foresight Workshop on Spherical Torus
Room 105, Liu-Qing Building
Tsinghua University, Beijing, China
6-8 January 2014

2. Motivation and Goal of Research
Economically competitive tokamak reactor may be realized at low aspect ratio (A = R0/a) by eliminating the central solenoid (CS) CS
𝑃 fusion ∝ 𝑝 2 ∝ 𝛽 t 2 𝐵 t 4
no CS
higher Bt
lower A  higher bt
S. Nishio, et al., in Proc. 20th IAEA Fusion Energy Conf., FT/P7-35 (Vilamoura , 2004).

3. Formation of Advanced Tokamak Plasma without CS was Achieved on JT-60U
Is plasma current (Ip) ramp-up by LHW possible in ST?
 Demonstrate on TST-2
Noninductive
ramp-up (LH)
Transition to
self-driven phase
Start-up and initial ramp-up
advanced tokamak
S. Shiraiwa, et al., Phys. Rev. Lett. 92 (2004)

4. Three Antennas used on TST-2
Combline Antenna
Grill Antenna
ECC Antenna
traveling
wave
traveling wave
traveling wave
excites traveling FW
Ip driven by SW (LHW)
requires mode conversion
from FW to SW
excites traveling SW
excites traveling SW
sharper k spectrum
& higher directivity

5. n|| Dependence of Ip and HX Spectrum (Grill Antenna)
[kA]
Highest plasma current was obtained for
1.5 < n|| < 4.5
Count rate of high energy photons was lower when
n|| > 7.5
n||
T. Wakatsuki

6. Calculated Electric Field (Ez)
Ez (Without Plasma)
T. Inada

7. Ramp-up to 12 kA by LHW (ECC Antenna)Ip Bv Bt
FWD
REF
trans
ECH
nel
LCFS
limiter
#2 #4 #6 #9
AXUV Ha
TST-2 Shot
T. Shinya

8. Scaling of Ip with Bv and Bt
I p ∝B v
I p max∝B t
co-drive
counter-drive
Ip increases with Bv .
Upper bound of Ip increases with Bt .
T. Shinya

9. Scaling of Ip with PRF and ne
1 A/W
Higher Ip is obtained with ECCA for the same PRF .
Higher ne is obtained with ECCA.
T. Shinya

10. Comparison of the Current Drive Figure of Merit
IP [kA]
ηCD vs IP
ηCD [1016 A/m2/W ]
𝜂 𝐶𝐷 = 𝐼 𝑝 𝑛 𝑒 𝑅 𝑃 𝑅𝐹
𝐼 𝑝 : plasma current
𝑛 𝑒 : average electron density
𝑅: major radius
𝑃 𝑅𝐹 : RF power
Higher hCD is obtained with ECCA (mainly because of higher ne).
An order of magnitude improvement in hCD may be possible by suppression of edge wave power loss and fast electron loss
 operation at higher Bt, ne, Ip , PRF, Te should help.
T. Wakatsuki

11. Hard X-ray Spectra for Co/Ctr Current Drive Directions (Combline Antenna)
HX view
ctr Ip co
wave
HX view
antenna
Photon flux is an order of magnitude higher in the co direction.
Photon temperature is higher in the co direction (60 keV vs. 40 keV).
Consistent with acceleration of electrons by a uni-directional RF wave.
T. Wakatsuki

12. Hard X-ray Spectra for Co/Ctr Current Drive Directions (ECC Antenna)
SBD Be&SBD P.P
𝐼 𝑝
… SBD PP
__ SBD Be 17 2
1.5
1.0
0.5
[kA]
[j/m^2]
[1/m^3]
Analysis section
[ms] Co
Ch1
Ctr
Ch2
𝐼 𝑝
[a.u.]
■:Ch1,Co
▲:Ch2,Ctr
𝑇 𝑒,𝐶𝑜 =48±6[keV]
𝑃 𝐵
𝑇 𝑒,𝐶𝑜𝑢𝑛𝑡𝑒𝑟 =46±11[keV]
Similar co and counter “temperatures”,
but co flux is larger than counter flux. E
[keV]
K. Imamura

13. Radial Profile of the Floating Potential
(Combline Antenna)
・Floating potential (where Iprobe = 0) is sensitive to the presence of high energy electrons.
R= 585 mm
R=700 mm
R=125 mm
antenna
probe
wave
Limiter
LCFS 5
9 mm
5.6 mm 6 7
5.7 mm 3 1
・ Vf has a large gradient near the limiter radius.
H. Kakuda

14. Ion Temperature, Toroidal Flow and Poloidal Flow (Grill Antenna, Ip ~ 6kA)
Right sightline
Left sightline Ip Bt
Lower sightline
Upper sightline
Emission intensity is small in the poloidal direction and was comparable to the case of ECH.
Measurement was not possible amount of light is insufficient in the 35-60msec.
In a measurable period of time, as well as when the ECH, poloidal flow was larger than the toroidal flow
S. Tsuda

15. One-Fluid Equilibrium (EFIT)
Reconstructed equilibrium
Ip = 11 kA
bp = 0.9
current density profile peaked on the outboard side
Visible light image
A. Ejiri

16. Two-Fluid Equilibrium (Initial Result)
TST-2 Shot
Plasma pressure max ~40 Pa (?)
Peaked toroidal current distribution (?)
What is plasma sound speed?
Large Er shear  ion orbit compression
Electron: largely satisfies pe = -JB
Ion (outboard): roughly equal pi, centrifugal, and electrostatic forces balanced by -JB
M. Peng & A. Ishida

17. Floating Potential Measurement at 200 MHz : ES Probe with Embedded High Impedance Resistor１ 2 5
Chip resistor 100 kΩ 3 4
Absolute Value of Impedance
[Ω]
105
Black solid line :
Electrostatic Probe with
a 100 kΩ Chip Resistor
104 １ 2
Green solid line :
Ordinary Langmuir Probe
Electrode 1, 2, 3 ・・・　With 100 kΩ
Electrode 4　　　 ・・・ Magnetic Probe
Electrode 5　・・・　Ordinary Langmuir 　　　　　 Probe 5 3 4
103
Red broken line : Sheath impedance
H. Kakuda
0.1 1 10
100
1000
[MHz]

18. Phase Difference and Wavenumber Measurement
Electrode Separation
14.2 mm １ 2 3
H. Kakuda

19. Frequency Spectra Measured by RF Magnetic Probes (Combline Antenna)
pump
toroidal
poloidal
lower
sideband
upper
sideband
Combline antenna excites the FW, but LHW generated by PDI?
Pump wave (f = 200 MHz ± 1 kHz) has FW polarization (|Bt| > |Bp|).
PDI sidebands have SW (LHW) polarization (|Bt| < |Bp|).
Pump wave weakens when sidebands intensify.
T. Shinya

20. RF Magnetic Probe Array for k Measurement (Grill Antenna, ~ 1kA)
radial
translation a b c d e
rotation
30 mm
𝐵 p ( = 0°)
𝐵 t ( = 90°)
Array can be rotated about its axis
to distinguish RF 𝐵 polarization
to measure wavevector components
Dominant k components excited by the antenna are kt  50 m-1, kp  10 m-1.
Measured kt ≲ 10 m-1 is much smaller (higher kt absorbed?)
𝐵 p (SW)
𝐵 t (FW)
|kt|  |k|||
< 10 m-1
~ 10 m-1
|kp|
< 5 m-1
|kR|
~ 35 m-1
|k|
k|| = 10 m-1 corresponds to n|| = 2.4
T. Shinya

21. TST-2 Double-Pass Thomson Scattering System
Brewster Window
Optical isolator
YAG laser
TST-2
Nd:YAG laser
1064nm
Laser energy: 1.6 [J]
Repetition rate: 10 [Hz]
Pulse width: 10 [ns]
[1st pass]
[2nd pass]
Scattering Vector
P|| P⊥
Incident
Laser B
Spherical
Mirror
Observer
Plasma B
~6m
Spherical Mirror
Polychromator
Beam Dump
Lens
Nd:YAG Laser
TST-2
Laser beam is focused in the plasma by a lens (f = 2000mm).
Optical isolator is used to prevent laser damage by reflected light.
J. Hiratsuka

22. Typical ne and Te profiles (OH Plasma)
Thomson scattering
Timing of TS measurement
Interferometer
neL time slice
ne profile
Te profile
neL [1018 m-2]
ne [1019m-3]
Comparison with interferometer
Thomson: 5.7 × 1018 [m-2]
Interferometer: 4.9 × [m-2]
Te [eV]
reasonable agreement
Major Radius [mm]
J. Hiratsuka

23. Electron Temperature Anisotropy (OH Plasma)
R=220mm (plasma edge)
R=389mm (plasma center)
Assuming Maxwellian velocity distribution, current densities are:
plasma edge： 300kA/m2　plasma center： 500kA/m2　
(plasma current / plasma cross section ~ 300kA/m2)
J. Hiratsuka

24. Time evolution of electron temperature
Center
Edge
Evolution of Te anisotropy
TST-2
Center of Plasma
→ Cooling
Edge of Plasma
→ Heating
Central and edge Te
approach each other
K. Nakamura

25. Multi-Pass Thomson Scattering
TST-2
f = 2500 mm
Mirror #1
f = 2000 mm
Mirror #2
Pockels Cell
Polarizer
Half-wave plate
YAG Laser
Brewster Window
Laser pulse is confined between concave mirrors (red line)
H. Togashi

26. Probe System for Turbulence Study
zonal flow
Global mode
Micro-turbulence
Nonlinear energy transfer direction
location
tip１
tip2
tip3
Jup
Jdown
coil2
30mm
Plasma flow, magnetic field, potential, and density fluctuations are measured simultaneously.
coil1
this is the composite probe system used in the experiment.
the size is 30 mm in diameter at the front, and it has 7 electrodes to measure floating potential or ion saturation current, probe tip is made of Molybdenum, and encased in boron nitride.
each electrode is 1mm in diameter 5mm in length
the two electrodes located in the shadowed area can be used to measure plasma flow as a mach probe. It also has a 3 axis pickup coil to measure 3 dimensional magnetic fluctuation simultaneously.
this probe system is inserted at the midplane of the device.
this probe tip can be rotated with rotary stage. ant it is inserted at the midplane of the device.
次に、実験装置の説明をさせて頂きます。
プラズマフロー、磁気揺動、浮遊電位、イオン飽和電流を同時に計測するために以下の装置を制作いたしました。
一つのプローブヘッドに静電プローブ、マッハプローブ、磁気プローブが一体となっており、プラズマフロー、磁気揺動、浮遊電位、イオン飽和電流を同時に計測する事ができるようになっております。
各種信号が正確に計測出来ていることを確かめるため、各種信号の角度分布を計測致しました。
また先端部は回転可能になっており、各値の角度分布を計測することが出来るようになっております。
各種信号が混入せずに方向性を持っていることを調べるため、各種信号の角度分布を計測致しました。
プローブ先端はボロンナイトライドのセラミックで覆われており、電極はモリブデンでできております。セラミックカバーの直径は30mm、先端部に5mmの段差が付き、電極は約2mm突き出ております。
この段差の影になるような位置の電極のイオン飽和電流を計測することで、マッハプローブとして使用し、プラズマフローの計測を行いました。
----- 会議メモ (2013/02/26 10:25) -----
目をかいて
コイル、トロイダル磁場の方向、BT,Bp,Ipの方向を対象として書く
右上かどこかにピックアップコイルが入っていることを示す
3-axis pickup coil
Bt , Ip
M. Sonehara

27. Poloidal Flow and Stresses
Poloidal flow and radial derivative of stress (Te=Ti is assumed).
Profiles of poloidal flow and stress derivative are similar.
M. Sonehara

28. Rogowski Probe Langmuir probe Rogowski coil for measuring
3-D pick up coil
Rogowski coil for measuring
the local current was developed.　
1-D pickup coil
H. Furui

29. Successful Measurement of Local Current (OH Plasma)
Plasma current
Local plasma current density
Poloidal magnetic field generated
by the plasma current
・ Bp is estimated to be under 20 mT
at the location of the Rogowski coil.
・ Output voltage of the Rogowski coil
due to Bp is estimated to be under
100 μV.
Local current measurement was
performed successfully with small pickup of magnetic fields.
H. Furui

30. Conclusions
ST plasma initiation and Ip ramp-up by waves in the LH frequency range were demonstrated on TST-2.
Inductively-coupled combline antenna (FW launch), dielectric-loaded waveguide array (“grill”) antenna (SW launch), and electrostatically-coupled combline antenna (SW launch) were used.
Spontaneous formation of the tokamak configuration with closed flux surfaces was observed when the toroidal current in the open field line configuration exceeded a critical level (~ 1 kA in TST-2).
Similar Ip was obtained with different antennas for similar Bv and PRF, but hCD is higher with the ECC antenna because of higher ne.
At low Ip ( < 2 kA in TST-2), Ip is dominantly pressure-driven, and is proportional to Bv. In this regime, Ip is independent of the wave type. At higher Ip (> 5 kA in TST-2), Ip becomes mainly wave-driven. In this regime, control of the current density profile by externally excited waves should become possible.

31. Conclusions Various diagnostics and RF launchers are being developed.
Electrostatically-coupled combline antenna (LHW launch).
Hard X-ray spectroscopy and imaging.
UV spectroscopy for ion temperature and flow measurements.
Electrostatic probes for wave and turbulence measurements.
Magnetic probes for wave and current density measurements.
Double-pass Thomson scattering for Te anisotropy measurement.
Two-fluid equilibrium.

32. Need for Power Supply Upgrade
In order to demonstrate ramp-up to higher Ip, power supply upgrade is needed to sustain higher Bt (~ 0.3 T) for longer time (> 0.1 s). 32

33. Near-Future Plans Coil Power Supply Upgrade
Sustained high field (Bt = 0.3 T for > 0.1 s) for further Ip ramp-up.
Wave/Turbulence Diagnostics
Electrostatic probe array
Reflectometer / interferometer-polarimeter
Plasma diagnostics
Multi-pass Thomson scattering
EBW emission
Current profile measurement by Rogowski probe / magnetic probe
Ion flow measurement