S. Kainaga

Parametric decay instability during high harmonic fast wave heating experiments on the TST-2 spherical tokamak

A degradation of heating efficiency was observed during high harmonic fast wave (HHFW) heating of spherical tokamak plasmas when parametric decay instability (PDI) occurred. Suppression of PDI is necessary to make HHFW a reliable heating and current drive tool in high ? plasmas. In order to understand PDI, measurements were made using a radially movable electrostatic probe (ion saturation current and floating potential), arrays of RF magnetic probes distributed both toroidally and poloidally, microwave reflectometry and fast optical diagnostics in TST-2. The frequency spectrum usually exhibits ion-cyclotron harmonic sidebands f0 ? nfci and low-frequency ion-cyclotron quasi-modes (ICQMs) nfci. 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 with either electrostatic or magnetic probes. The radial decay of the pump wave amplitude in the SOL was much faster for the ion saturation current than for the floating potential. These results are consistent with the HHFW pump wave decaying into the HHFW or ion Bernstein wave (IBW) sideband and the low-frequency (ICQM). Two additional peaks were discovered between the fundamental lower sideband and the pump wave in hydrogen plasmas. The frequency differences of these peaks from the pump wave increase with the magnetic field. These decay modes may involve molecular ions or partially ionized impurity ions.

Detection of a new parametric decay instability branch in TST-2 during high harmonic fast wave heating.

Parametric decay instability (PDI) is often observed in the TST-2 spherical tokamak during high harmonic fast wave heating by rf pickup probes. The frequency spectrum exhibits lower and upper sideband peaks in addition to the pump wave at f(0)=21 MHz. Two types of PDI are observed. One is the well-known decay into the ion-cyclotron quasimode (nf(ci)) and the ion Bernstein wave (f(0)-nf(ci)). The other is a newly found decay with the sideband frequency between f(0) and f(0)-f(ci). The frequency difference between this sideband and the pump increases in proportion to B(t). Moreover, high-speed visible light measuring systems with photomultiplier tubes or hybrid photodetectors viewing the plasma core detected oscillation of light emission at around f(0).

Features of a new PDI branch observed in TST-2 during high harmonic fast wave injection

TST-2 is a spherical tokamak (ST) device, and High Harmonic Fast Wave (HHFW) injection experiments were performed to heat electrons in the spherical tokamak. In TST-2, electron heating was observed in some discharges, but when Parametric Decay Instability (PDI) occurred the injected wave energy is transferred to two daughter modes. PDI during HHFW injection in the NSTX ST device creates modes, which are presumably an Ion Bernstein wave (IBW) and a quasi mode at ion cyclotron frequency (ICQM). A similar decay was observed for low toroidal magnetic field discharges in TST-2. For higher toroidal magnetic field discharges, we found another type of decay, in which another peak in the frequency domain was detected between a pump wave (i.e., HHFW) frequency and the IBW frequency. Experiments are performed to identify this new peak. RF pickup probes are used to measure the magnetic field oscillations outside the closed magnetic surface region. Major results are as follows. The frequency of the decay waves coupled with ICQM increases with the toroidal magnetic field as expected. When Bt varies from 0.13 to 0.18 T, the frequency difference of the decay wave and the pump wave increases from 1.5 to 1.9 MHz in proportion to Bt- The new unidentified wave shows a similar dependence, and its frequency difference varies from 0.54 to 0.70 MHz in proportion to Bt- The width of this new peak is broader than that of IBWs peak. These results suggest that the new peak corresponds to the decay wave coupled with Alfven mode, because the fact that the frequency of Alfven mode is in proportional to the magnetic field, is consistent with the experimental results.

ECH and HHFW Start-Up Experiments on the TST-2 Spherical Tokamak

Plasma start-up experiments without using inductive field have been performed on the TST-2 spherical tokamak device. A low power ECH (2.45GHz/4kW) and a medium power High Harmonic Fast Wave (HHFW) (21MHz/up to 50kW) were used. Comparisons of various operational parameters were carried out to find the optimum conditions for the toroidal and the poloidal field strengths, poloidal field configurations, ECH power and HHFW power. Using only ECH, currents up to about 0.4 kA were generated. Current increments by up to 0.4 kA were observed when we apply HHFW to ECH plasmas.