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 2008 Frascati, Italy

TST-2 Spherical Tokamak Nominal parameters: R = 0.38 m a = 0.25 m B t = 0.2 T I p = 0.1 MA HHFW 21 MHz 400 kW ECH 2.45 GHz 5 kW

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

Three Phases of I p Start-up by ECH 3Phase 12 Open Field Lines Current Jump Closed Flux Surfaces z[m] x[m] y[m] will use dI p /dt (17ms ～ 22ms) z[m] x[m] y[m]

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  T. Soft X-rays (~ 2 keV) were observed at high RF power (~ 30 kW). RF only RF RF sust. EC sust. 80 ms RF only Sustainment by RF Power

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

mag. axis jtjt F term p term Truncated boundary LCFS pol. fluxtor. currentforce bal. flux loops saddle loopspickup coils pressurepol. fieldpol. flux pol. angle z “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. Equilibrium Reconstruction

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

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 I p stays at a low level < 0.3 kA. Only low n || waves can propagate to the plasma core, and formation of high energy electrons is expected. Soft X-ray energy spectrum indicates the presence of high energy (2-3 keV) electrons. However, I p can be sustained even when soft X-rays are not observed.

Comparison of Equilibria during Sustainment Truncated boundary LCFS  jj Inboard limiter LCFS Outboard limiter # ms Truncated boundary LCFS  jj Inboard limiter LCFS Outboard limiter # ms Truncated boundary LCFS  jj Inboard limiter LCFS Outboard limiter # ms RF sustained, I p = 0.6 kAEC sustained, I p = 0.6 kAEC sustained, I p = 1.3 kA

I p Collapses are Often Observed during RF Sustainment Power spectra of inboard B Z RF sustainment w/o collapse ECH alone Inboard B z Outboard B z IpIp 4 discharges with almost the same operational conditions Low and high frequency components are observed for collapsed discharges

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

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 I p, proportional to the stored energy. During the current jump phase, initial closed flux surfaces are formed gradually, and changes in W k and R jmax are small.  soft dynamics Sustained ST plasma has high  p >1 and high q 0 >30 MHD instability often terminates the RF sustained plasma, but no such phenomenon is observed for the EC sustained plasma.

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

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  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.

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

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

I cos(  p +  t+  RF ) sin(  p +  t+  RF ) VCO 6-10GHz X4 5-20mW DC-500MHz Q LO RF coaxial scalar horn RF 21MHz e i  t E p x B t Ae i  t Ae i  t+i  Digitizer (25MHz) or Oscilloscope (~250MHz) F.G. X5 X10 Gunn or GHz DC-100MHz waveguide D.C.-3dB cutoff surface Mirror Second Mirror ~500mm Launching horn Receiving horn 24-40GHz 100mW Microwave Reflectometer

PDI Spectra Measured by Reflectometer Reflectometer H plasma

 f=1MHz HHFW Time Evolution and Power Correlation Reflectometer Sideband power varies quadratically with the pump wave power

Electrostatic Probe Digitizer channels Ch. 1: mag. probe at  = 155  Ch. 2-4: ES probe at  = 165  2:  f1 3: I is 4:  f2

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) time resolution for : dtNm/2 = 0.49 ms Spectral Analysis of RF Data

Time Evolution and Power Correlation B  and I is correlation becomes higher and phase shift becomes definite during second half of RF pulse H plasma

Time Evolution and Power Correlation  f1 and  f2 correlation is nearly one and phase shift is almost zero for the pump wave

Time Evolution and Power Correlation  is and  f1 correlation is intermediate and phase shift is non-zero

Newly Discovered Sub-Harmonic Decay Modes f3f3 f2f2 f1f1 f0f0 ff deuterium hydrogen  f increases with B  Two additional peaks were discovered between f 0 and f 0 – f cH in H plasmas (note that there is a dip at f 0 – f cD ) –These modes may involve molecular ions or partially ionized impurity ions.

R (mm) Radial Fall-off is Steeper for  is than  f2 outboard limiter

Phase Difference Between Neighboring RF Probes t (ms) RF  = - 65   = - 55   = corresponds to |n| = 18 Phase shift is not constant throughout the RF sampling rate: dt (2 ns) data window for FFT: N (500) no window overlapping no smoothing time resolution = 1  s

B  Dependence of Frequency Spectrum at Different Locations D plasma

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 B  discharges, but comparable in high B  discharges. 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. Summary of RF Magnetic Probe B  Scan

Summary of PDI Observations The frequency spectrum exhibits peaks at ion-cyclotron harmonic sidebands f 0 ± nf ci and low-frequency ion- cyclotron harmonics nf ci, 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 I is than for  f.

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.