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 $(F>4kJcm−2)$⁠, high laser intensity $(I>1011Wcm−2)$ with a short field free region $(L=48mm)$ and high source potential $(Vext>40kV)$ 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 $(Ip>3mA)$⁠, high current densities $(J>5mAcm−2)$⁠, 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.