The role of space charge in the performance of the bunching system for the ATLAS positive ion injector

The bunching system of the ATLAS positive ion injector consists of a four-frequency harmonic buncher, a beam-tail removing chopper, and a 24.25 MHz spiral resonator sine-wave rebuncher. The system is designed to efficiently create beam pulses of approximately 0.25 nsec FWHM for injection into and acceleration by the ATLAS superconducting linac. Studies of the effect of space charge on the performance of the system have been undertaken and compared to simulations as part of the design process for a new bunching system to be developed for a second ion source. Results of measurements and modeling studies indicate that the present system suffers significant bunching performance deterioration at beam currents as low as 5 eμA for ²³⁸U²⁶⁺ at a velocity of β=0.0085. The low beam current tolerance of the present system is in reasonable agreement with computer simulations. Studies of two alternatives to the present bunching system are discussed and their limitations are explored