Transient process of a spherical tokamak plasma startup by electron cyclotron waves

Abstract

The results of non-inductive startup in the SUNIST spherical tokamak (R/a: 0.3 m/0.23 m; B T0 : 0.15 T) by a 2.45 GHz microwave through electron cyclotron resonance heating (ECRH) are presented. Two discharge regimes with different transient characters, which determine the plasma current, are observed. The transient processes of discharges are experimentally investigated by scanning the radial resonance position, vertical field and the microwave power. Analysis of the microwave reflection and visible light emission prompt a process dominated by the combination of ionization, loss along the open field line and the gradient B drift. The discharges are modeled in one dimension. The simulation results qualitatively agree with the experiments. Both experiments and simulations suggest that the discharge character has less dependency on the experimental parameters except gas filling pressure confirming that the control of filling pressure is of great importance for startup a spherical tokamak by ECRH using low frequency microwaves.

Open field equilibrium current and cross-field passing electrons as an initiator of a closed flux surface in EC-heated toroidal plasmas

“…Equations ( 8) and ( 9) indicate that the terms including j φ or B φ are the same order leading terms as the pressure gradient term, while all the terms in equation ( 5) are secondary since [j × B] φ is secondary compared with the terms in equations ( 8) and ( 9). Once we have information on the pressure profile for a given toroidal current profile we can calculate the diamagnetic and paramagnetic currents using equation (10). Then some information on plasma flow may be obtained using equation (5), which is, however, outside the scope of this paper.…”

“…A toroidal current was generated in a number of experiments when a weak vertical field, B V , was superposed in electron cyclotron (EC) heated toroidal plasmas [1][2][3][4][5][6][7][8][9][10][11][12]. The current increased as EC power was increased, and often, a closed flux surface was initiated via a rapid current increase (current jump) in small low aspect ratio devices [4,5,7,9,10]. Once a closed flux surface was formed, even current ramp-up by the EC current drive (ECCD) was possible [13].…”

Generation of initial closed flux surfaces by ECH at a conventional aspect ratio of R/a ∼ 3: experiments on the LATE device and JT-60U tokamak

“…Electron cyclotron heating and current drive (ECH/ECCD) is an attractive candidate for noninductive current start-up since the microwave beam can be injected from a simple small launcher remote from the plasma. While initial closed flux surfaces were shown to be generated by ECH in a number of low aspect ratio devices, including CDX-U [2], LATE [3], TST-2 [4], MAST [5], CPD [6] and SUNIST [7], there was no successful experiment at the conventional aspect ratio except for one in DIII-D [2,8]. In this paper we report experiments and analyses on the formation of initial closed flux surfaces by ECH up to R/a ∼ 3 on the LATE device and JT-60U tokamak, aiming to apply this scenario in ITER and DEMO.…”

Study of Electron Orbits for Formation of Toroidal Closed Flux Surface by ECH

“…Recently, a closed flux surface was found to be formed via current jump by ECH under a weak vertical field B V in small and low-aspect-ratio torus devices, namely LATE [2] , CPD [3] and SUNIST [4] . Furthermore, the plasma current was ramped up by EC current drive (ECCD) after the formation of closed flux surface in LATE [5,6] .…”