Perveance and ion bunch structure from a 'compact, high-pressure' laser ion source

The Dublin City University (DCU) laser ion source (LIS) is a 'compact high-pressure' laser ion source utilizing a table top Q-switched laser. The DCU-LIS combines high laser fluence (F>4 kJ cm-2), high laser intensity (I>1011 W cm-2) with a short field free region (L=48 mm) and high source potential (Vext>40 kV) in order to offset recombination losses within the plasma and maximize the proportion of highly charged ions which are extracted from the plasma plume. Such a configuration also provides high peak currents (Ip>3 mA), high current densities (J>5 mA cm-2), and high charge states (Cu6+) in the extracted ion-bunch train. However, to obtain and utilize these parameter values in a high pressure LIS requires characterization and control of a number of processes related to ion dynamics and space charge effects on the extracted ions at the plasma plume-anode-extraction gap interface. Relevant issues include electric field distortion, Debye shielding, beam divergence, overfocusing, and perveance (P) in addition to current density profiles for the extracted ion beam. In this paper we focus on these issues and their impact on charge particle extraction and acceleration with a view to elucidating the parameter regimes within which the DCU-LIS performance envelope is optimal.