A versatile klystron bunching system for a 5.5 MV Van de Graaff accelerator

Abstract

A special version of the ORTEC Klystron bunching system has been installed on the SUNI 5.5 MV Van de Graaff accelerator. The design considerations are outlined. The realization of the full capabilities of this system has been made possible by two modifications: (a) the incorporation of a cross-field analyser as an integral part of the pulser unit and (b) the installation of a split buncher tube. With the duoplasmatron ion source, continuous current operation was made virtually impossible because of overloading of the accelerator tube by defocussed particles of different masses. The cross-field analyser selects a single mass (or change) particle for acceleration and thus eliminates this problem. The use of the cross-field analyser is also required for optimum utilization of the pulsed beam. The incorporation of the analyser as part of the pulser is described. A buncher tube of fixed length with a bunching voltage of fixed frequency is only optimum for one particle mass. A modified buncher tube is shown to give superior performance for mass 1 to mass 3 beams for which pulses of 1 to 1.5 ns width (fwhm) respectively, have been obtained. Using the original buncher tube the best result achieved with mass 3 beams was 2.6 ns. Effective bunching factors greater than 20 have been achieved with an average current of about 5 μA on the target at a pulse rate of 2 MHz. The energy spread in the bunched proton beam is about 4 keV. The SUNI system is designed to switch readily from continuous to pulsemode operation with protons, deuterons or helium ions.