1. High Harmonic Fast Wave Experiments on TST-2 Y. Takase, A. Ejiri, S. Kainaga, H. Kasahara 1), R. Kumazawa 1), T. Masuda, H. Nuga, T. Oosako, M. Sasaki, Y. Shimada, F. Shimpo 1), J. Sugiyama, N. Sumitomo, H. Tojo, Y. Torii, N. Tsujii, J. Tsujimura, T. Yamada 2) 12th International Workshop on Spherical Torus 2006 Chengdu 11-13 October 2006 University of Tokyo, Kashiwa, 277-8561 Japan 1) National Institute for Fusion Science, Toki, 509-5292 Japan 2) Kyushu University, Kasuga, 816-8580 Japan

2. TST-2 spherical tokamak and RF system HHFW experiment –Electron heating experiment –Wave diagnostics RF magnetic probes Reflectometry –Wave measurements parametric decay scattering –TORIC full-wave analysis EC start-up experiment Plans –200MHz experiments on TST-2 –RF sustainment of high  plasmas in UTST Outline

3. TST-2 Spherical Tokamak ECH: 2.45GHz (< 5 kW) HHFW: 21MHz (< 200 kW x 2) ECH HHFW R / a = 0.38 / 0.25 m (A = 1.5) B t = 0.3 T / I p = 0.1 MA

4. 21 MHz Matching/Transmission System

5. RF power 400 kW Frequency f = 13, 21, 30 MHz (  /  H ~ 7 at B T = 0.2 T, f = 21 MHz) Toroidal wavenumber k  = n  /R 0 = 11, 16, 26 m -1 (n  = 4.3, 6, 10) varied by changing the strap spacing Faraday shield angle ~ 30° current straps (0,  ) Mo limiters Faraday shield Variable k  Two-Strap Antenna

6. Single-pass Absorption Calculation Single-pass absorption is greater for double-strap excitation Single-pass absorption –increases with n e –increases with T e –decreases with B t (increases with  e ) double-strap

7. single-pass absorption = = 0.18 ELD + TTD ELD + TTD + CROSS ELD ELD + CROSS Imag ( k ⊥ ) B t = 0.15 T n e = 1.0×10 19 m -3 T e = 100 eV n  = 10 Single-Pass Absorption Improves with  e

8. Analysis of HHFW heating scenarios used on TST-2 is being carried out using the TORIC full-wave code. B t = 0.2 T, f = 21 MHz, n = 10, n e0 = 2  10 19 m -3, T e0 = 0.2 keV TORIC Full-Wave Calculations Electron absorption: 100%

9. Soft X-ray increased, but density and radiated power did not change  electron heating Strongest response near plasma center t (ms) IpIp nelnel P rad SX (> 200 eV) 360 kW RF Electron Heating by HHFW Low field side High field side ~ R 0 180kW 360kW no HHFW R=0.19m R=0.26m R=0.38m R=0.43m R=0.54m Center PS noise

10. Increases in stored energy and visible-SX emission are greater for double-strap excitation –Consistent with single-pass absorption calculation Single-Strap vs. Double-Strap Excitation double-strapsingle-strap no RF with RF Edge emission visible-SX emission (A.U.) P NET = 120 kW

11. RF magnetic probes –Sensitive to electromagnetic component –Plasma edge only Reflectometry –Sensitive to electrostatic component –Can probe the plasma interior Both parametric decay instability (PDI) and frequency broadening due to scattering by density fluctuations were observed. –These processes can alter the wavenumber spectrum, and affect both wave propagation and absorption. Wave diagnostics on TST-2

12. φ ＝ -60 ° φ ＝ -30 ° φ ＝ -55 °φ ＝ -65 ° φ ＝ -115 ° φ ＝ -120 ° φ ＝ 155 ° φ ＝ 150 ° φ ＝ 145 ° φ ＝ 65 ° φ ＝ 60 ° φ ＝ 55 ° φ ＝ 30 ° φ＝0°φ＝0° Top view S.S. enclosure Slit Core (insulator) 1-turn loop Semi-rigid cable 2cm Direction of B field to be measured RF Magnetic Probes at 14 Toroidal Locations toroidal direction BzBz BB φ＝9°φ＝9° φ ＝ -9 ° φ ＝ -125 ° BB

13. RF 21MHz e i  t One of three sources is used Frequency sweepable VCO for profile measurements Fixed Gunn Osc. (25.85 or 27.44 GHz) for RF measurements E p x B t Ae i  t Ae i  t+i  I cos(  p +  t+  RF ) sin(  p +  t+  RF ) VCO 6-10GHz X4 Q LO RF coaxial waveguide scalar horn Digitizer or Oscilloscope <250MHz sampling F.G. X5 X10 Gunn 25.85 or 27.44 GHz D.C. -3dB 5-20mW DC-500MHz 24-40GHz 100mW TST-2 Reflectometer System

14. Window  200 TF Coil TST-2 V.V. Mirror Horn Antennas Microwave Reflectometer

15. Most probable decay process: High Harmonic Fast Wave (HHFW)  Ion Bernstein Wave (IBW) + Ion-Cyclotron Quasi-Mode (ICQM) Magnetic field dependence  H at outboard edge Threshold power ~ 20 kW High power Low power Parametric Decay: FW  IBW + ICQM Parametric Decay Instability (PDI) PDI ① ② probe ② RF probe

16. 1.8 MHz HHFW 250 kW Time [  s] Reflectometer 25.85 GHz (cosine) Reflectometer 25.85 GHz (sine) RF Probe (dB/dt) Antenna Limiter, P12 Frequency [MHz] Comparison of RF Probe and Reflectometer Spectra QM IBW

17. Comparison of RF Probe and Reflectometer Spectra Reflectometer (cos) Reflectometer (sin) RF Magnetic Probes f (MHz) FW ? IBW QM IBW noise (Al reflector) 251020 0 -20 -40 -60 -80 0 P (dB) 0 -20 -40 -60 -80 P (dB) 0 -20 -40 -60 -80 P (dB) 15 5

18. PDI becomes stronger as the plasma outer boundary approaches the antenna Rout Dependence on Plasma Position antenna limiter

19. ① ② ③ ④ ①② ③④ Outboard vs. Inboard Comparison Inboard spectrum similar to outboard, but weaker RF probes

20. φ ＝ 21 ° straps φ ＝ 39 ° R=125 R=630mm R=700mm Z = 0mm Z = -150mm port10 ① ② ③ ④ ①② ③④ Broadened spectrum is only weakly dependent on vertical position midplane B  midplane B z Z = 150mm Inboard Side Spectra RF probes

21. Vacuum Plasma ①① ②② Frequency broadening of the pump wave by the plasma is observed. Possible processes: Parametric decay Scattering by density fluctuations Frequency Broadening ① ②

22.  f (HHFW) Pump wave power Lower sideband power ① ② ③ ④  SX / SX Density Dependence Varies with Probe Position 10dB  f (HHFW) PDI is generally reduced at high density Only weak effect on heating Preliminary nelnelnelnel

23. A k || variable antenna was installed in TST-2, and the RF power capability was increased to 400kW. –Dependence on k || spectrum (same spectral shape but different k || ) will be studied. –Single-pass absorption is expected to change from 10% to 35% when n e0 = 3.0  10 19 m −3 and B t = 0.3T. In electron heating experiments, soft X-ray emission increased with RF power. –Stored energy increase was larger for double-strap excitation. –More direct measurement of T e is necessary (TS in preparation). Analysis of HHFW scenarios used on TST-2 is being carried out using TORIC. Summary (HHFW Heatng)

24. PDI and frequency broadening due to scattering were observed by RF magnetic probes. –The strength of PDI increased as the outer boundary of the plasma approached the antenna. –Density dependence varies with RF probe location. –Parametric decay became weaker at high densities where single-pass absorption is predicted to become stronger. –The effect of parametric decay on plasma heating is not clear. Initial results of RF wave detection inside the plasma by microwave reflectometry were obtained. –PDI spectrum clearly observed –Differentiation of ES and EM components may be made. Summary (RF Measurements)

25. EBW Heating on TST-2@K (2003)  (dW/dt) indicates P abs /P in > 50% when  n e in front of antenna is steep enough Thursday: S. Shiraiwa, et al., “Study of EBW Heating on TST-2” < 200 kW @ 8.2 GHz

26. Typical EC Start-up Discharge (a) B t decreases gradually. (R ECH decreases gradually.) (b) I PF is kept constant. (c) P EC is kept constant. (d) I p increases with time, but disappears when the  =  e layer moves out of the vacuum vessel. (up to 0.5 kA produced by 4 kW) (e) n e is almost constant near the cutoff density. inboard limiter Previously achieved: 1kA/1kW (2.45GHz) 4kA/100kW (8.2GHz)

27. Dependence on power and resonance position I p depends on the  =  e resonance position (R ECH ). I p increases with P EC, whereas n e saturates around the cutoff density 7  10 16 m -3 [NL = (5-6)  10 16 m -3 ]. time inboard limiter

28. Dependence on PF Strength and Decay Index PF2+PF5 PF1 PF2PF3 1m -1m 0.1m 0.7m Mirror ratio I p maximizes at a certain field strength. Highest I p is achieved with PF2 (medium decay index).

29. Effect of HHFW Injection 3 cases are compared: ● ECH only ● ECH + HHFW ● ECH turned off during HHFW I p responds quickly to HHFW n e and P rad increase during HHFW After ECH turn-off, I p decays and HHFW reflection increases.

30. Single Particle Orbit Analysis Phase space for confined orbits is large for low A devices. Orbit-averaged toroidal precession is co-directed for all confined orbits. (c) ctr trapped(b) co trapped(a) circulating 0.10.8 m +1 Confined region in phase space (inside outermost blue boundary) (b) (a)(c) V || /V 0 V  /V 0 0 1 -0.401.2 Electron orbits starting from R = 0.38m / Z = 0m Velocity is normalized by V 0 =R s  p (  p is the cyclotron frequency corresponding to B p ) A. Ejiri, et al., to be published.

31. Driven Current Based on Single Particle Analysis Under a low T e (or high B Z ) approximation (V Te << V 0 ), j is given by This current has the same parameter dependence and the same order of magnitude as pressure driven currents The generated field is where  R is the thickness of the EC deposition layer

32. Comparison with Experiment B z dependence agrees qualitatively with experiment. Dependence on PF curvature is different. Predicted current for PF3 is negligibly small. Assuming correspondence of I pmax for PF2 Further assuming estimate for driven I p is

33. Conclusions (EC Start-up) I p of up to 0.5 kA was obtained by ECH start-up (2.45 GHz / 4 kW). I p increases with the decrease in B t. Highest I p was achieved with PF configuration with medium index and appropriate field strength. I p and n e increase with P EC, but n e saturates around the cutoff density. I p increased by up to 0.4 kA by addition of HHFW power. Analytic expressions for generated current were derived based on single particle orbits: – Generated current has a similar form to pressure driven currents. – Self-field generation becomes siginificant at high  p. The proposed model and experimental results are not inconsistent, but further theoretical and experimental studies are needed.

34. Preparation in Progress for 200 MHz Experiments EE B t = 0.3 T, f = 200 MHz, n = 10, n e = 2  10 18 m -3, T e = 0.3 keV Full-wave calculation by TASK/WM TST-2 200 MHz transmitters (from JFT-2M) Combline antenna (GA)

35. UTST HHFW (~ 20 MHz) LHFW (~ 200 MHz) TS-3 / TS-4 TST-2 UTST A New Ultra-High  ST Based on Plasma Merging Merging scenario Tokyo Formation of ultra-high  ST plasma using plasma merging Sustainment using innovative RF methods Friday: Y. Ono, et al., “Initial operation of UTST High-Beta Spherical Tokamak Merging Device”