Improving the neutralization of a pulsed ion beam by electron cooling and accumulation: A kinetic study

Abstract

Direct electron injection, such as through electron-emitting filaments, is usually difficult to neutralize ion beams to a very high degree. In this paper, the possibility of a pulsed ion beam achieving very high neutralization through the continuous accumulation of cold electrons is investigated using a two-dimensional particle-in-cell code. Three schemes of electron injection, namely, single-point injection, periodic point-source injection, and periodic line-source injection, are numerically studied and compared. The simulations show that even if an excess of electrons are injected, the single-point electron source is difficult to neutralize the ion beam pulse to exceed 90%, consistent with existing experiments. It is found that the spontaneous cooling mechanism of neutralizing electrons is able to improve the neutralization of the ion beam to a certain extent, but it requires a lot of time. By using a smaller injection current, the latter two injection schemes not only effectively suppress solitary waves, but more importantly, they continuously provide cold electrons that can accumulate inside the ion beam, thereby significantly improving the neutralization of the ion beam in a short period of time. The results show that periodic line-emission sources can neutralize the ion beam to over 99%, but periodic point-emission sources exhibit relatively poor neutralization performance due to their higher virtual-cathode potential. The research results can provide a reference for the design of neutralizing sources in applications that pursue very high neutralization of ion beam pulses, such as heavy ion fusion accelerators.