Particle-in-Cell Simulations of ION Dynamics in a Pinched-Beam Diode1

Abstract

A pinched-beam diode consists of a thin annular cathode which emits electrons that are accelerated toward a planar anode. Once the energy deposited by the electrons on the anode is sufficiently large, a plasma forms on the anode and the electrons are strongly pinched toward the axis of the diode. An intense ion beam with a current of 100’s kA is also produced. Previous simulations of the pinched-beam diode focused on the electron dynamics. [1] In this paper, we focus on the properties of the ion beam formed in the diode. Specifically, we focus on the electric and magnetic field distributions in the diode and how these charge distributions influence the uniformity of the ion beam. There are a number of factors that can affect the overall quality of the ion beam in a pinched beam diode including spotty emission from the anode and the distribution of ion and electron charge in the diode gap. In this paper, we will present results from simulations that neglect spotty emission and focus on the role that the electron and ion charge distribution in the diode play on the overall ion beam quality. The simulations show the formation of “hot spots” in the electron flow that attract the ion flow. This causes non-uniformity in the ion flow as it passes through the electron flow. As the ion beam exits the diode region it picks up electrons either from the electron flow in the diode itself or from field emitted electrons that come from conducting surfaces located behind the cathode tips. The geometry behind the cathode tips influences how much of the neutralizing electron flow is pulled from the high-energy diode flow and how much comes from the wall emission behind the tips. Simulations results will be presented to address the effect that this geometry has on the electron hot spots and ion beam uniformity.