Byung Il An

Non-inductive plasma current start-up by EC and RF power in the TST-2 spherical tokamak

Non-inductive plasma current start-up by EC and RF power was carried out on the TST-2 device. Low frequency RF (21 MHz) sustainment was demonstrated, and the obtained high βp spherical tokamak configuration has similar equilibrium values as the EC (2.45 GHz) sustained plasma. Equilibrium analysis revealed detailed information on three discharge phases: (i) in the initial current formation phase, the plasma current increases with the stored energy, and the current is in the same order as that predicted by theory. (ii) In the current jump phase, the current density profile, which is peaked near the outboard boundary, is not deformed but increases slowly and the initial closed flux surface appears when the current reaches a maximum. (iii) In the current sustained phase, equilibrium is characterized by the hollowness of the current density profile, and it determines the fraction of the current inside the last closed flux surface to the total current. Both EC and RF injections show a similar equilibrium. While MHD instabilities often terminate the RF sustained plasma, no such phenomenon was observed in the EC sustained plasma.

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.

Equilibrium Analysis of EC‐Sustained and RF‐Sustained ST Plasmas

Plasma current start‐up and formation of the ST configuration without the use of the central solenoid is a critical issue in ST research. In the TST‐2 spherical tokamak (R = 0.38 m, a = 0.25 m), sustainment of an ECRF (2.45 GHz) produced ST plasma by low frequency (21 MHz) RF power alone was demonstrated. Since direct RF current drive can be ruled out, this result implies that the ST configuration is sustained entirely by pressure‐driven currents. The Grad‐Shafranov equilibrium was generalized to take into account the open field line region with finite plasma current and pressure (truncated equilibrium). In addition to the precessional current of trapped particles, Pfirsch‐Schluter current flowing along the open field line (and partially returning through the vacuum vessel) contributes to the toroidal plasma current. Three phases of plasma start‐up are analyzed: (i) the current formation phase, (ii) the current jump phase, and (iii) the current sustainment phase. In the current formation phase, the plasma...

High Harmonic Fast Wave experiments on TST‐2 and UTST

HHFW experiments on TST‐2 and UTST were performed. In the TST‐2 HHFW experiment, the pickup probe mounted on the inboard wall detected the same PDI lower sideband peak as probes on the outboard side. Since the frequency difference from the pump wave corresponds to the ion cyclotron frequency on the low field side, the lower sideband wave is believed to be generated at the plasma edge on the low field side, and propagated to the high field side. This component may be the FW. In the UTST HHFW experiment, direct measurements of the RF fields in the plasma were made successfully.