Simulation of high-frequency modes and their effect on insulator breakdown in the pulse line ion accelerator

The pulse line ion accelerator (PLIA) produces a traveling electromagnetic (EM) wave by applying a voltage pulse to one end of a helix that accelerates and axially confines a heavy-ion beam pulse. An anomalous flashover phenomenon has been observed on the vacuum-insulator surface that limits the amplitude of the accelerating field. It has been suspected that a small component of high-frequency modes in the input pulse may be the cause of the breakdown. Simulation using MAFIA (MAxwellʹs equations by Finite Integration Algorithm) was conducted to investigate the fields on the insulator surface. A scaling law was proposed to reduce substantially the computational time in simulation. It is based on the hypothesis that the pattern of EM field for a given wavelength is independent of the wire spacing as long as the wavelength is much longer than the inter-wire spacing and the termination resistors are adjusted to maintain impedance matching. On the basis of these numerical simulations, we conclude that high-frequency modes, even at very low amplitudes, may indeed lead to the observed insulator flashover.