Two Modes of Transportation of a High Current Ion Beam with Ballistic Focusing

Abstract

Ion sources are used to modify surface layers of different materials and manufactured items. The minimization of radiation damage of treated surfaces sets the trend to decrease the ion beam energy (< 1.5 keV). The dynamic ion beam compensation and the ion beam plasma generation should be considered to control the transport of the metallic low-energy ion beam with ballistic focusing. The ion beam transport with ballistic focusing in the equipotential drift space is studied with numerical simulation by the 2.5D axial symmetric version of the KARAT electromagnetic PiC code. It is shown that the collector current changes to the pulsed mode when the injected ion energy $W < W_{c}$, where $W_{c}$ is critical energy that depends on the gas concentration and the injected ion current. The pulsed mode is the result of the virtual anode (VA) formation and its compensation by secondary electrons. In hemispherical drift space with curvature radius of 7.5 cm, the critical energy $W_{c}=2 \text{keV}$ when the transported ion current $I_{b}=1\ \mathrm{A}$ and the gas concentration $n_{g}=10^{13}\ \text{cm}^{-3}$. The oscillation frequency of the collector current depends on energy, the system geometry and the gas concentration. The oscillating mode of the collector current when decreasing the energy ($W < W_{c}$) of the transported ions is a result of the decreased role of secondary electrons in compensating the ion beam space charge. This leads to alternating formations: the VA formation when compensation of the space charge of the beam ions compensated. Plasma in the beam transport area is formed. A critical factor that impacts the ion beam transport mode is the electron heating under the increased plasma instability. All these processes are considered in the proposed PiC simulation. The time required to the ion-beam plasma formation and the period of the collector current pulses decrease as the transported ion energy $W$ and the gas concentration increase.