Characterization of a high-pressure laser ion source with dc and pulsed extraction

Abstract

The Dublin City University (DCU) laser ion source (LIS) is a compact, high-throughput system which utilizes a very short field-free region (L = 4.8 cm) so ion extraction at the anode occurs at ‘high pressure’. Detailed studies of the system performance are reported here for a range of laser power densities (Ip ∼ (0.35–2.4) × 1011 W cm−2) and extraction voltages (dc 5–17 kV, pulsed 15–40 kV). Extracted ion beam currents were measured with the aid of a Faraday cup and ranged from ∼3.2 mA (Cu2+) to 0.1 mA (Cu6+). Pronounced turning points in the current profiles were observed for both dc and pulsed extraction, related to so-called ‘plasma bulge’ at the anode which in turn has its origins in the high plasma density at extraction. The kinetic energy of Cu2+ ions was quantified for a range of extraction bias values and used to determine the extraction efficiency. A planar probe, employed to measure the radial profile of the ion beam, yielded minimum beam diameters of ∼6–8 mm for Cu+ and Cu2+. These data were used to determine the collected current density J, which peaked at 1.92 mA cm−2. Using a gated intensified charge coupled device camera, the internal structure of the extracted ion bunches was imaged and ion number densities estimated. The dependence of other key beam parameters on focusing optics, laser energy and extraction voltage was also investigated.