Plasma Start-up, Sustainment, and Heating by RF Waves in TST-2

1. Plasma Start-up, Sustainment, and Heating by RF Waves in TST-2 Y. Takase, A. Ejiri, Y. Nagashima, O. Watanabe, Y. Adachi, B. An, H. Hayashi, S. Kainaga, H. Kobayashi, H. Kurashina, H. Matsuzawa, T. Oosako, J. Sugiyama, H. Tojo, K. Yamada, T. Yamada, T. Yamaguchi, T. Masuda, Y. Ono, M. Sasaki The University of Tokyo Joint Meeting of the 4th IAEA Technical Meeting on Spherical Tori and the 14th International Workshop on Spherical Torus 7-10 October 2008Frascati, Italy

2. TST-2 Spherical Tokamak Nominal parameters: R = 0.38 m a = 0.25 m Bt = 0.2 T Ip = 0.1 MA HHFW 21 MHz 400 kW ECH 2.45 GHz 5 kW

3. Part I. Noninductive Ip Start-up and Sustainment • Ip start-up by ECW (2.45 GHz) • Three phases of Ip start-up • Dynamics of closed flux surface formation • MHD activity and Ip collapse • Ip sustainment by RF (21 MHz) power alone

4. 0.1 z[m] 0 -0.1 0.4 -0.4 0 0 y[m] x[m] 0.4 -0.4 Three Phases of Ip Start-up by ECH Phase 1 2 3 will use dIp/dt (17ms～22ms) 0.6 0.6 Open Field Lines Closed Flux Surfaces Current Jump 0 0 -0.6 -0.6 0 0.5 0 0.5 0.4 z[m] 0 -0.4 -0.4 0.4 0 0 y[m] x[m] 0.4 -0.4

5. Sustainment by RF Power RF RF only EC sust. RF sust. • RF (21MHz) power can induce a current jump. • Plasma current can be sustained by RF power alone. • Antenna excites waves with a broad spectrum of toroidal mode numbers, up to |n| ~ 20. But only |n| = 0, 1 can propagate to the core. • Ion heating is not expected due to high harmonic number (> 10). • Ion (H/D, C, O) heating was not observed. • Electron heating is expected to be weak due to low bT. • Soft X-rays (~ 2 keV) were observed at high RF power (~ 30 kW). 80 ms RF only

6. Initial Current Ramp-up Rate and Ip-Wk Trajectory Dependence on various parameters are summarized by a scaling law J. Sugiyama et al., Plasma Fusion Res., 3, 026 (2008). Single-particle orbit theory predicts A. Ejiri and Y. Takase, Nucl. Fusion, 47, 403 (2007). closed: after jump RF EC open: before jump is confirmed by equilibrium analysis

7. Equilibrium Reconstruction mag. axis pressure pol. field pol. flux “Truncated equilibrium” was introduced to include finite pressure and current in the open field line region. A. Ejiri et. al., Nucl. Fusion 46, 709 (2006). Truncated equilibrium can reproduce magnetic measurements (~80 channels), and can be used to analyze all three phases. F term pol. flux tor. current force bal. jt LCFS p term Truncated boundary saddle loops pickup coils flux loops pol. angle z pol. angle

8. Dynamics of EC Induced Current Jump EC induced current jump occurs when Ip exceeds a critical value Ip,crit where Ip after current jump is given by Equilibrium reconstruction reveals slow and soft formation of initial closed flux surfaces. While Ip increases rapidly, Wk and Rjmax increase slowly. Just before and after closed flux surface formation

9. Conditions for RF Induced Current Jump and Sustainment RF induced current jump occurs when the injected RF power exceeds a threshold, which is different for H and D. O. Watanabe,et al., Plasma Fusion Res. 3, 049 (2008) . Injection timing should be just before a current jump. Current jump does not occur by RF power alone, and Ip stays at a low level < 0.3 kA. Only low n||waves can propagate to the plasma core, and formation of high energy electronsis expected. Soft X-ray energy spectrum indicates the presence of high energy (2-3 keV) electrons. However, Ip can be sustained even when soft X-rays are not observed.

10. Comparison of Equilibria during Sustainment RF sustained, Ip = 0.6 kA EC sustained, Ip = 0.6 kA EC sustained, Ip = 1.3 kA #53197 90ms #53773 50ms #53783 50ms Inboard limiter Inboard limiter Outboard limiter Inboard limiter LCFS Outboard limiter Outboard limiter LCFS LCFS Truncated boundary Truncated boundary Truncated boundary jj LCFS LCFS jj jj LCFS y y y

11. Ip Collapses are Often Observed during RF Sustainment 4 discharges with almost the same operational conditions Power spectra of inboard BZ Ip Inboard Bz ECH alone Outboard Bz RF sustainment w/o collapse Low and high frequency components are observed for collapsed discharges

12. Expansion of Open Field Line Region is Observed before MHD Activity Phenomelogy of Ip collapse Slow fluctuations EC RF Inward shift and expansion of open field line region Rapid growth of high frequency fluctuations Collapsed discharges are different from the beginning of RF pulse. Ip collapse

13. Summary • Sustainment of ST plasma by low frequency RF power was demonstrated. • Equilibrium analysis revealed detailed information during each phase of discharge. • Initial current formation phase is characterized by a slow increase in Ip, proportional to the stored energy. • During the current jump phase, initial closed flux surfaces are formed gradually, and changes in Wk and Rjmax are small.  soft dynamics • Sustained ST plasma has high bp>1 and high q0>30 • MHD instability often terminates the RF sustained plasma, but no such phenomenon is observed for the EC sustained plasma.

14. Part II. HHFW Heating and Parametric Decay • Electron heating • Parametric Decay Instability (PDI) • Parameter dependences • Newly discovered sub-harmonic decay branch

15. Introduction • A degradation of heating efficiency is observed during high-harmonic fast wave (HHFW) heating of spherical tokamak plasmas when parametric decay instability (PDI) is observed. Understanding and suppression of PDI is necessary to make HHFW a reliable heating and current drive tool in high b plasmas. • In TST-2, wave measurements were made using a radially movable electrostatic probe (ion saturation current and floating potential), RF magnetic probes distributed both toroidally and poloidally, microwave reflectometry, and fast optical diagnostic.

16. Typical Discharge Heated by HHFW (Inboard Shifted Plasma) • Te = 140  170 eV over 0.4 ms after RF turn-on (PRF = 200 kW) • PDI becomes stronger and Te decreases slowly after 0.4 ms (causality?)

17. RF Diagnostics Reflectometer f = -75 HHFW Antenna RF magnetic probes inner wall probes ES probe f = 165 front surface of S.S. enclosure at R = 635mm

18. Microwave Reflectometer Gunn 25.85 or 27.44 GHz Mirror VCO 6-10GHz waveguide coaxial scalar horn ~500mm 24-40GHz 100mW X4 Ep x Bt D.C.-3dB cutoff surface 5-20mW X5 cos(fp+wt+fRF) I X10 LO RF DC-100MHz DC-500MHz Q F.G. sin(fp+wt+fRF) Aeiwt Aeiwt+ip Digitizer (25MHz) or Oscilloscope (~250MHz) Second Mirror RF 21MHz eiwt Launching horn Receiving horn

19. PDI Spectra Measured by Reflectometer H plasma Reflectometer

20. Time Evolution and Power Correlation Reflectometer Df=1MHz HHFW HHFW Sideband power varies quadratically with the pump wave power

21. Electrostatic Probe Digitizer channels Ch. 1: mag. probe at f = 155 Ch. 2-4: ES probe at f = 165 2: Ff1 3: Iis 4: Ff2

22. Spectral Analysis of RF Data time resolution for : dtNm/2 = 0.49 ms sampling rate: dt (2 ns) data window for FFT: N (10000) overlapping of data window: N/2 (5000) points for smoothing along time : m(49)

23. Time Evolution and Power Correlation Bf and Iis H plasma correlation becomes higher and phase shift becomes definite during second half of RF pulse

24. Time Evolution and Power Correlation Ff1 and Ff2 correlation is nearly one and phase shift is almost zero for the pump wave

25. Time Evolution and Power Correlation Iis and Ff1 correlation is intermediate and phase shift is non-zero

26. Newly Discovered Sub-Harmonic Decay Modes Two additional peaks were discovered between f0 and f0 – fcH in H plasmas (note that there is a dip at f0 – fcD) • These modes may involve molecular ions or partially ionized impurity ions. deuterium f3 Df increases with Bf f2 hydrogen f1 f0 Df

27. Radial Fall-off is Steeper for Iis than Ff2 R (mm) R (mm) outboard limiter

28. Phase Difference Between Neighboring RF Probes sampling rate: dt (2 ns) data window for FFT: N (500) no window overlapping no smoothing time resolution = 1 ms • = - 65 @ RF • = - 55 t (ms) t (ms) t (ms) t (ms) Df= - 0.5 corresponds to |n| = 18 Phase shift is not constant throughout the RF pulse.

29. Bf Dependence of Frequency Spectrum at Different Locations D plasma

30. Summary of RF Magnetic Probe Bf Scan • RF probes on the outboard side have similar signal levels. • RF probe on the inboard side has much smaller signal levels compared to the outboard side in low Bf discharges, but comparable in high Bfdischarges. • The vertical (poloidal) polarization is much weaker than the horizontal (toroidal) polarization. • The frequency difference between the pump wave and the lower sideband wave increases with the magnetic field. • The lower sideband becomes weaker, and the lower sideband peak becomes unresolved at low magnetic field.

31. Summary of PDI Observations • The frequency spectrum exhibits peaks at ion-cyclotron harmonic sidebands f0 ± nfci and low-frequency ion-cyclotron harmonics nfci, consistent with the HHFW pump wave decaying into the HHFW or ion Bernstein wave (IBW) sideband and the ion-cyclotron quasi-mode (ICQM). • PDI becomes stronger at lower densities, and much weaker when the plasma is far away from the antenna. • The lower sideband power was found to increase quadratically with the local pump wave power. • The lower sideband power relative to the local pump wave power was larger for reflectometer compared to either electrostatic or magnetic probes. • The radial decay of the pump wave amplitude in the SOL was much faster for Iis than for Ff.

32. Conclusions • Simultaneous measurements were made with electrostatic probes, RF magnetic probes, and microwave reflectometry. • Observed PDI is consistent with decay of HHFW into HHFW/IBW sidebands and ICQM. • New PDI into sub-harmonic modes were observed. • Causality between PDI and degradation of heating efficiency is suggestive, but not conclusive.