Protection Against High-Energy Breakdowns in Neutral Beam Systems for Future Fusion Reactors

In this paper, the capacitances of the two-stage bushing that is installed in the 1-MV test bed for neutral beam injectors in the future fusion reactors are calculated by modeling the bushing in a 3-D electrostatic code. One of the modeling results is the stored energy in the bushing, which corresponds to 1/2 CV
²
. After evaluating the bushing capacitance, a MATLAB/Simulink model is presented to model the possible breakdown in the bushing and between the electrodes in the 1-MV test bed. When a breakdown occurs between anode and cathode, an overcurrent is drawing that can cause damage in the electrode surfaces while an overvoltage takes place when the breakdown happens between the bushing stages. Such damage has been observed experimentally in the past. This paper also presents some of the possible protection against the breakdown in the 1-MV test bed to dissipate the stored energy in order to protect the electrode surfaces and the bushing. The suggested protections presented in this paper are damping resistors, surge arrester individually, and both of them. Also, a coil can be added to the DR either in series or in parallel.