Drift distance survey in direct plasma injection scheme for high current beam production.
In a laser ion source, plasma drift distance is one of the most important design parameters. Ion current density and beam pulse width are defined by plasma drift distance between a laser target and beam extraction position. In direct plasma injection scheme, which uses a laser ion source and a radio frequency quadrupole linac, we can apply relatively higher electric field at beam extraction due to the unique shape of a positively biased electrode. However, when we aim at very high current acceleration such as several tens of milliamperes, we observed mismatched beam extraction conditions. We tested three different ion current at ion extraction region by changing plasma drift distance to study better extraction condition. In this experiment, C(6+) beam was accelerated. We confirmed that matching condition can be improved by controlling plasma drift distance.
High current carbon beam production with direct plasma injection scheme
We have been studying a new heavy-ion production technique called “direct plasma injection scheme,” DPIS, since 2000. A new radio frequency quadrupole (RFQ) designed especially for the DPIS was commissioned in 2004 and very intense carbon beam was successfully obtained, reaching more than 60mA accelerated current from the RFQ. Most of the contents of the accelerated beam was carbon 4+ as verified by beam analysis.
Nd–YAG laser ion source for direct injection scheme
The feasibility of a “direct injection scheme” using Nd–yttritium–aluminum–garnet laser has been studied experimentally. The relationship between charge distribution and laser power density was measured. The obtained yield rate of C6+ was 46.8% at the peak current pulse with 1.6×1011 W/cm2 laser intensity. The current dependence of the distance from the target at this intensity was measured. The peak current of C6+ will be 60 mA with φ4.8 mm extraction aperture. It was found from the input beam condition simulation by the code PTEQ–HI that high current (from 12 to 28.6 mA) can be accelerated by the TIT–RFQ.