Electron cyclotron/upper hybrid resonant pre-ionization in the ISX-B tokamak

Abstract

Pre-ionization experiments have been performed on a tokamak by injecting about 80 kW of microwave power at 35 GHz for up to 15 ms. Microwave absorption occurs at the electron cyclotron and upper hybrid resonance frequencies as predicted by theory. Pre-ionization causes substantial (40%) reductions in loop voltage during the initial phase of the tokamak shot. Flux (volt-second) savings with pre-ionization are about 30% in the first 2 ms or about 2% of the total flux expenditure in a tokamak shot. The plasma current begins 200 μs earlier and rises 1.4 times more rapidly in the pre-ionized case. Electron densities of 5 × 1012 cm−3 can be sustained throughout the microwave pulse with only a toroidal magnetic field during microwave injection. The bulk electron temperature in the pre-ionized plasma is about 10 eV although there are indications of higher electron temperatures (50 eV) in the upper hybrid resonance layer. Although questions exist concerning the quiescent behaviour of the pre-ionized plasma, the observed parameters are shown to be consistent with a theory which employs classical models of energy and particle balance. During the early stages of Ohmic heating, the pre-ionization is effective in decreasing the peak of the radiated power.

Current circulation and equilibrium in toroidal electron cyclotron resonance (ECR) plasmas in the LATE device

“…By injection of microwaves at the ECR frequency, breakdown easily takes place at the ECR layer, after which the discharge is maintained by the deposition of the microwave power at the ECR layer and/or the upper hybrid resonance (UHR) layer [1,2]. This is a standard method of pre-ionization and heating to assist the start-up of plasma current in tokamaks [3]. This technique is intended to be used for the International Thermonuclear Experimental Reactor tokamak [4].…”

Electron cyclotron resonance breakdown studies in a linear plasma system

“…Electron cyclotron resonance (ECR) assisted pre-ionization is a common technique used in tokamaks [1][2][3][4][5][6][7][8][9][10][11][12], stellerators [13,14] and other plasma devices such as tokapole [15] for start-up and to assist plasma formation in the initial stage of the discharge. In ohmic discharge, high loop voltage is required to initiate the discharge when plasma density, temperature and conductivity are low and the line radiation from impurities are the dominating losses.…”

Inductive current startup in large tokamaks with expanding minor radius and rf assist

“…Experimental studies by Anisimov et al [27] indicated the feasibility of UHR absorption by a plasma in a purely toroidal magnetic field, but only for short pulse lengths (<1 as). Recently, longer pulse (-15 ms) preionization experiments on the much larger ISX-B tokamak [28,35] have also confirmed UHR absorption and heating using time-resolved density measurements, temperature diagnostics and high-speed photography to examine the plasma characteristics during preheating. To provide the physics basis for the UHR preheating process examined in this study, this section briefly reviews the criterion for wave-plasma interactions at the UHR.…”