Laser Ablation Ion Source for the KEK Digital Accelerator

Abstract

A laser ablation ion source (LAIS) that provides a fully ionized carbon ion beam is under development in a collaboration between KEK and BNL. It turned out that the laser pulse width dependence of C 6+ current and energy is very large. Modified Langmuir-Child law formula was evaluated to examine C 6+ ion beams delivered from LAIS with a large initial velocity. It is compared with IGUN simulations. A example of extraction region design is given with beam properties such as beam emittance. Recent experimental results on carbon targets are reported here. It turned out that the generated ion current and charge spectrum are strongly dependent of the laser pulse width. In addition, it is noted that a drift energy of carbon ions is order of 10 keV. This fact affects on designing of the extraction region downstream from the LAIS. In order to refine the design of the extraction region including the orifice, which divides the plasma region and extraction region, and the extraction electrode, a space-charge limited current (SPLC) in a case with an initial velocity has been reconsidered. The analytic result in an idealized planar diode is compared with IGUN code simulations. A large difference in the SPLC density in magnitude between them has been found, although the extraction voltage dependence or initial energy dependence can be in agreement with each other. It is not concluded whether or not this difference can be attributed to the geometrical difference between an infinite parallel plate model and realistic 3D model. Assumed the initial beam energy and current density, the extraction region has been designed, resorting to the IGUN code.