Electron flow in positive-polarity multigap inductive accelerators

We study the electron flow in multi-gap inductive accelerators, such as Hermes III operating in positive polarity by numerical simulation and modeling. The objective of this work is to determine the operating principles such that an optimally efficient design of the Hermes-type machine can be achieved for intense ion beam generation. We employ a 2-D fully electromagnetic particle in cell code, MASK†, to represent the electrons emitted in the accelerating gaps and their dynamics. Because the electrons emitted in different gaps have different energies and canonical momenta, the simple theory of magnetic insulation[l] has to be extended to such multi-component electron flows. In order to understand the effects of load impedance on the electron flows in the multi-gap accelerator and on the coupling of power to the load, MASK has been used to simulate an accelerator with a small number of gaps for various load impedances. For load impedances below the self-limited impedance ZSL of the last segment of the accelerator, the electron flow in both segments is well insulated. The overall current efficiency is over 90% and is insensitive to the load impedance ZL for ZL < ZSL· Beyond ZSL, the current efficiency decreases rapidly with increasing load impedance. Because of this rapid decrease in efficiency, the power delivered to the load also falls off rapidly as load impedance is increased. To better understand these results, a simple theoretical model for multi-component electron flows has been developed to predict the distribution of electron flows. The model will be compared with existing multi-component models[2] and simulations. For both high and low impedance loads the electron flow from the leading edge of each cathode is unsteady, producing an intermittent train of diamagnetic electron vortices. These vortices are not circular but elongated with an aspect ratio near one half. The center of ea- h vortex E × B drifts along with the local electron flow. In each vortex, electrons E × B drift about the vortex center along the potential contours of the vortex´s self-electric field in a reference frame moving with the vortex center. These vortices were compared to a self-consistent solution of a cylindrically symmetric fluid model which showed good agreement with data observed from the runs.