Abstract
High current and low emittance are principal requirements for heavy-ion injection into a linac driver for inertial fusion energy. An electrostatic quadrupole injector is capable of providing these high charge density and low emittance beams. We have modified the existing 2-MV injector to reduce beam emittance and to double the pulse length. We characterize the beam delivered by the modified injector to the High Current Transport Experiment and the effects of finite rise time of the extraction voltage pulse in the diode on the beam head. We demonstrate techniques for mitigating aberrations and reducing beam emittance growth in the injector.
Highlights
Heavy-ion fusion accelerator (‘‘driver’’) design architecture generally envisions multiple parallel beam channels with each channel transporting a beam of line charge density of about 0:25 C=m
Simple scaling relations [4] show that the transportable current density in an electrostatic aperture column (ESAC) decreases with beam energy as Vbÿe1am=2, but in an electrostatic quadrupole (ESQ) accelerator column, the transportable current density increases with beam energy as Vb1e=a2m, where Vbeam is the beam ion kinetic energy in electron volts
Requirements for the acceleration of beams with high charge density and low emittance have led to renewed interest in the ESQ accelerator [19]
Summary
Heavy-ion fusion accelerator (‘‘driver’’) design architecture generally envisions multiple parallel beam channels with each channel transporting a beam of line charge density of about 0:25 C=m. The 2-MV injector is an ESQ-based accelerator that was developed in 1993 for the Induction Linac Systems Experiment (ILSE)/Elise project [5], and represents the injector for a single heavy-ion fusion driver beam It delivers a K beam at energy up to 2 MeV and a beam current up to 0:8 A. When the ratio of transverse quadrupole potential Vq is a significant fraction of Vbeam, the resulting S-shaped kinematic distortions across the beam radius lead to emittance growth This effect has been mitigated in the injector by reducing the beam envelope through the ESQ section, and by increasing the energy of the beam entering the ESQ section as much as practical without risking breakdown in the diode section. The critical issues are discussed in a comparison of experimental results for beam current, density and transverse phase-space distributions with the predictions of particle-in-cell (PIC) simulations
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