Charged particle dynamics in a ‘high-pressure’ laser ion source

Abstract

Charged particle sources require beam transport techniques specific to the application for optimum operation. The complexity of techniques increases as the degree of ionization and kinetic energy of charged particles increases. The Dublin City University laser ion source (DCU-LIS) utilizes a short field region (L = 48 mm) to maximize the average charge state and peak current extracted, thus ion extraction occurs at ‘high pressure’. The presence of large space-charge forces, high average plasma plume temperature and the expansion dynamics of laser-generated plasmas results in significant divergence of the ion bunch upon injection into the drift tube. To facilitate efficient beam transport, and to maximize system throughput, we employ a rather unique electrostatic transport system, termed as ‘continuous einzel array’ (CEA). Ion electrodynamics in such a system exhibit a number of distinct features which modify the system performance and alter the expected distribution of kinetic energies (KE), the times of flight and ion bunch diameters. System scalability in regard to beam kinetic energy is also important. In this paper the superior performance of the LIS equipped with a CEA is compared with a traditional einzel lens electrostatic beam transport system based on the usual 3-element and also a 5-element lens system.