Yuuki Adachi

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.

The response of microwave reflectometry under generalized configuration

The response of microwave reflectometry for various configurations is studied theoretically. Based on the Kirchhoff integral (i.e. physical optics), analytic formulae for the measured electric field are obtained. Phase and power sensitivities for weak fluctuations are derived. It was found that the effective spot size, which is a combination of the real spot size and the Fresnel zone, of the launching and the receiving beams is an important parameter, determining the sensitivities. Fringe jump and phase runaway phenomena are attributed to two parameters: one is proportional to the normalized wavenumber of the fluctuations, and the other is proportional to the plasma shift (i.e. misalignment). In the case of frozen turbulence, a complex power spectrum of the measured field becomes a universal quantity, because of easy correction of the configuration effect. Using this feature, an ideal complex amplitude signal (i.e. free from diffraction effects) can be reconstructed.

Plasma current start-up experiments without the central solenoid in the TST-2 spherical tokamak

Several techniques for initiating the plasma current without the use of the central solenoid are being developed in TST-2. While TST-2 was temporarily located at Kyushu University, two types of start-up scenarios were demonstrated. (1) A plasma current of 4 kA was generated and sustained for 0.28 s by either electron cyclotron wave or electron Bernstein wave, without induction. (2) A plasma current of 10 kA was obtained transiently by induction using only outboard poloidal field coils. In the second scenario, it is important to supply sufficient power for ionization (100 kW of EC power was sufficient in this case), since the vertical field during start-up is not adequate to maintain plasma equilibrium. In addition, electron heating experiments using the X–B mode conversion scenario were performed, and a heating efficiency of 60% was observed at a 100 kW RF power level. TST-2 is now located at the Kashiwa Campus of the University of Tokyo. Significant upgrades were made in both magnetic coil power supplies and RF systems, and plasma experiments have restarted. RF power of up to 400 kW is available in the high-harmonic fast wave frequency range around 20 MHz. Four 200 MHz transmitters are now being prepared for plasma current start-up experiments using RF power in the lower-hybrid frequency range. Preparations are in progress for a new plasma merging experiment (UTST) aimed at the formation and sustainment of ultra-high β ST plasmas.

Identification of the magneto-thermal plasma response for plasma state control in advanced tokamaks

The paper deals with the basic elements of an integrated model-based control strategy for extrapolating present-day advanced tokamak scenarios to steady state operation. Taking advantage of the large ratio between the time scales involved in the magnetic and thermal diffusion processes, the model identification procedure makes use of a multiple time scale approximation. The methodology is generic and can be applied to any device, with different sets of heating and current drive actuators, controlled variables and/or parameter profiles. It has been applied to experimental data from JET and JT-60U, and satisfactory models have been obtained. A profile controller can then be articulated around two composite feedback loops operating on the resistive and confinement time scales, respectively. The controller aims to use the combination of H&CD systems in an optimal way to regulate the evolution of the magneto-thermal state of the plasma. First experimental results obtained with three H&CD actuators to control the safety factor profile on JET are displayed. Simultaneous real-time control of the q-profile and toroidal velocity profile on JT-60U, using four groups of neutral beam injectors, has been simulated and typical results are presented.

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).

Poloidal mode analysis of magnetic probe data in a spherical tokamak configuration.

A method to determine the poloidal mode number m in a spherical tokamak based on magnetic probe data was developed. Perturbed magnetic fields at Mirnov coils are calculated for distributed helical filamentary currents on rational surfaces assuming the maximum current amplitude, m and n (toroidal mode number), and the toroidal location of the filaments. These free parameters were determined from the best fit to the measured signals. The residual error was reduced by a factor of 2 by introducing helical filaments instead of toroidal filaments. Using this method, m/n=2/1 and 3/2 modes were identified in Mega-Ampere Spherical Tokamak discharges, and the time evolution of the tearing modes was derived.

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.