High-voltage test of feedthroughs for a high-power ICRF antenna

A feedthrough is one of the most important parts of an ion cyclotron range of frequencies antenna for plasma heating in nuclear fusion devices. It must transmit RF power while keeping the antenna in high vacuum, and support antenna elements against electromagnetic forces and thermal stress. The standoff voltage, a key property for high-power transmission, is experimentally studied for six different types of feedthroughs. The Princeton Plasma Physics Laboratory (PPPL), Massachusetts Institute of Technology (MIT), Japan Atomic Energy Research Institute and Oak Ridge National Laboratory types show a standoff voltage of 40-52 kV with the pulse length 1 s, which indicates the possibility of power injection 1-1.6 MW in the present 50-Ω systems. If a 30-Ω feedthrough designed for the International Thermonuclear Experimental Reactor antenna, has a similar standoff voltage, 3-4 MW can be injected. Here, the antenna loading resistance is assumed as 3 Ω. In particular, the PPPL and MIT types show higher standoff voltages, which is inferred to be due to the design of the base flange covering the ceramic end. Multipactoring is observed in the voltage range less than 0.9-1.6 kV as predicted, but it is not a crucial problem. Therefore, a key point in designing a high-voltage standoff feedthrough is the optimization of the base flange and the ceramic ends