Technological Issues of Ion Cyclotron Heating of Fusion Plasmas

Abstract

With the recent promising results of plasma heating using electromagnetic waves (EM waves) in the ion cyclotron range of frequency (ICRF) on the Princeton Large Torus (PLT) tokamak the feasibility of employing ICRF heating to a reactor-like magnetic confinement device is increasing. The high power ICRF experiments funded on JET (Joint European Torus in England) and JT-60 (in Japan) will have rf source power in the range of 10-30 MW. The time scale for the duration of the rf pulse will range from seconds up to steady-state. The development of new rf components that can transmit and launch such high power, long pulse length, EM waves in a plasma environment is a major technological task. In general, the technology issues may be divided into two categories. The first category concerns the region where the plasma comes in contact with the wave launchers. The problems here are dominated by plasma-material interaction, heat deposition by the plasma onto the wave launcher, and erosion of the launcher material. It is necessary to minimize the heat deposition from the plasma, the losses of the rf wave energy in the structure, and to prevent sputtering of the antenna components. A solution involves a combined design using special materials and optimal shaping of the Faraday shield (the electrostatic shields which can be used both for an EM wave polarization adjustment and as a particle shield for the launcher). Recent studies by PPPL and McDonnell Douglas Corp. on the Faraday shield designs will be discussed.1 The second important area where technology development will be necessary is the transmission of high power rf waves through a gas/vacuum interface region. In the past, the vacuum feedthrough has been the bottle neck which prevented high power operation of the PLT antennae. However, with major improvements of the feedthrough design, the power level attainable has risen from 200 kW per antenna to over 1.25 MW per antenna. In the present experiment, the power limit of the vacuum/gas interface has not been reached. As the pulse length of the rf is increased, modification (or cooling) of the rf components will be essential. According to the recent studies of vacuum feedthroughs2 by PPPL and the Grumman Corp., it is expected that with proper cooling the present components will be able to withstand steady-state operation.