Two-beam interdigital-H-type radio frequency quadrupole linac with direct plasma injection for high intensity heavy ion acceleration

Abstract

We developed a two-beam interdigital-H-type radio frequency quadrupole (IH-RFQ) linac as a prototype of a multibeam IH-RFQ for high intensity heavy ion acceleration in the low energy region. This linac has two sets of RFQ electrodes within an IH-type resonant frequency cavity that is a power-efficient structure for low energy beam acceleration. The linac can accelerate two beams in parallel in one cavity with a reduction in the coulomb repulsive force (the space charge effect) between the accelerated heavy ion particles. The resonance frequency and the $Q$ factor of the linac were found to be 47 MHz and 5900, respectively. We also developed a two-beam laser ion source with a direct plasma injection scheme as an injection system for the two-beam IH-RFQ linac and built a system to demonstrate the use of the two-beam IH-RFQ linac. Using this linac system, we were able to accelerate carbon ions from 5 to $60\text{ }\text{ }\mathrm{keV}/\mathrm{u}$ and generate an output beam current of about 108 mA ($2\ifmmode\times\else\texttimes\fi{}54\text{ }\text{ }\mathrm{mA}/\mathrm{channel}$). A coherency between the two beams, derived from the imbalance of the beam loading, was observed in the acceleration test with carbon ions.