The DCU laser ion source

Abstract

Laser ion sources are used to generate and deliver highly charged ions of various masses and energies. We present details on the design and basic parameters of the DCU laser ion source ͑LIS͒. The theoretical aspects of a high voltage ͑HV͒ linear LIS are presented and the main issues surrounding laser-plasma formation, ion extraction and modeling of beam transport in relation to the operation of a LIS are detailed. A range of laser power densities ͑I ϳ 10 8 -10 11 W cm −2 ͒ and fluences ͑F = 0.1-3.9 kJ cm −2 ͒ from a Q-switched ruby laser ͑full-width half-maximum pulse duration ϳ35 ns, = 694 nm͒ were used to generate a copper plasma. In "basic operating mode," laser generated plasma ions are electrostatically accelerated using a dc HV bias ͑5-18 kV͒. A traditional einzel electrostatic lens system is utilized to transport and collimate the extracted ion beam for detection via a Faraday cup. Peak currents of up to I ϳ 600 A for Cu + to Cu 3+ ions were recorded. The maximum collected charge reached 94 pC ͑Cu 2+ ͒. Hydrodynamic simulations and ion probe diagnostics were used to study the plasma plume within the extraction gap. The system measured performance and electrodynamic simulations indicated that the use of a short field-free ͑L =48 mm͒ region results in rapid expansion of the injected ion beam in the drift tube. This severely limits the efficiency of the electrostatic lens system and consequently the sources performance. Simulations of ion beam dynamics in a "continuous einzel array" were performed and experimentally verified to counter the strong space-charge force present in the ion beam which results from plasma extraction close to the target surface. Ion beam acceleration and injection thus occur at "high pressure." In "enhanced operating mode," peak currents of 3.26 mA ͑Cu 2+ ͒ were recorded. The collected currents of more highly charged ions ͑Cu 4+ -Cu 6+ ͒ increased considerably in this mode of operation.

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

“…The DCU -LIS [48][49][50] employs a range of extraction voltages both DC (5-18 kV) and pulsed (15-50 kV). Laser intensities used to generate plasma ions range from ~10 9 -10 11 Wcm -2 via a Q -switched Ruby laser (τ ~35 ns, λ = 694 nm).…”

“…In this paper, we expand upon the performance of the DCU laser ion source (LIS) [48]. Specifically, we present experimental data and detailed ion electrodynamic simulations on a beam transport system that we term a 'continuous einzel array (CEA)'.…”

“…Such a condition requires continuous refocusing of the injected ion bunches for optimum system throughput. Experimentally, traditional three element einzel units have been found to be unsatisfactory when applied to a high -pressure LIS [48]. It becomes necessary to test experimentally the concept of a multiple element electrostatic system [47].…”

“…Experimental [48][49][50] studies on our system indicated that the electrostatic optical system must be biased to ~ 45 -60% of V ext for efficient ion transportation and capture. The system is designed to be a compact LIS.…”

“…A detailed description of the electrostatic lens system design and specifications is given elsewhere [48]. The initial electrostatic transport system employed in the DCU -LIS consisted of three einzel lens units (each unit was composed of three hollow cylinders, a central element which was positively biased and two grounded cylinders, one located on either side of the central ring).…”