Radio-frequency biasing of ion acceleration grids

Abstract

The extraction and acceleration of ions from a plasma source is often performed electrostatically using a set of direct-current (DC) biased grids. To ensure that equal currents of positive and negative charges are ejected, and to compensate the downstream ion beam space charge, a separate electron-emitting neutraliser is required. If instead the grids are biased with a radio-frequency (RF) voltage, then both electrons and ions can be extracted from the same source and a neutraliser is no longer needed. Using a combination of theory and particle-in-cell simulations, we demonstrate the fundamental physics of RF biasing and identify important electron and ion temporal dynamics. By including a capacitor in the external circuit, a DC self-bias voltage forms across the grids due to initial particle charging. This self-bias voltage develops self-consistently and ensures equal ion and electron currents are ejected from the plasma source, while also leading to ion acceleration. A potential well is observed to form downstream of the last grid which leads to electron trapping and the formation of a quasi-neutral plume. Because of their larger mass, ions are continuously extracted from the plasma source, while electrons are only extracted in pulses synchronised with the RF cycle. As long as the RF period is less than the ion transit time between the grids, ions can be efficiently extracted with no direct grid impingement and otherwise similar behaviour to DC biased grids.