Overview of TST-2 Experiment a 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 277-8561 Japan The Second A3 Foresight Workshop on Spherical Torus Room 105, Liu-Qing Building Tsinghua University, Beijing, China 6-8 January 2014

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).

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 2002.06.21 advanced tokamak S. Shiraiwa, et al., Phys. Rev. Lett. 92 (2004) 035001.

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

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

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

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 98362 T. Shinya

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

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

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

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

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

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

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

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

Two-Fluid Equilibrium (Initial Result) TST-2 Shot 75467 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

Floating Potential Measurement at 200 MHz : ES Probe with Embedded High Impedance Resistor １ 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]

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

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

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

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

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 × 1018 [m-2] Te [eV] reasonable agreement Major Radius [mm] J. Hiratsuka

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

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

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

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

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

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

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

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.

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.

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

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