Design of the MEBT Rebunchers for the SPIRAL2 Driver

The SPIRAL2 project uses room temperature RFQ and rebunchers and a superconducting linac to accelerate high intensity beams of protons, deuterons and heavier ions. All cavities work at 88 MHz, the beta after the RFQ is 0.04 and 3 rebunchers are located in the MEBT line, which accepts ions with A/q up to 6. The paper describes the RF design and the technological solutions proposed for an original 3-gap cavity, characterised by very large beam holes (60mm) and providing up to 120 kV of effective voltage. INTRODUCTION The SPIRAL2 [1] driver presents a quite long medium energy beam transport line (Figure 1) to insert a second beam line from a future RFQ for heavier ions: q/a=1/6, a single bunch selector and the corresponding beam dump. The line is seven and a half meters long and is equipped with three rebunchers to keep the beam longitudinal phase dimension. Room requirements for all the devices are very tight and the cavities have to be compact on the beam axis direction. Moreover, the beam transverse section can be quite large in the line, then the beam aperture in the cavity drift tubes is much longer than the tube length and the gap electric fields interact with each other. The Transit Time Factor (TTF) is consequently quite low and voltages higher than usual have to be applied on the electrodes to obtain the required effective voltage. REBUNCHER REQUIREMENTS The first and the last cavities will work at an effective voltage of 120 kV with the heaviest ions while the second one will work at 60 kV CW only. For the RFQ initial commissioning, only the first cavity will be installed before the diagnostic test bench and pulsed voltages up to 190 kV will be used for emittance measurements. The line beta is 0.04 which is a reasonable figure with respect to the injector working frequency of 88,0525 MHz. RF DESIGN To keep the longitudinal length of the cavity as small as possible and to handle reasonable values of RF power and electric field, a 3-gap structure has been chosen. The double quarter wave resonator of Figure 2 has been preferred to the more usual split ring, to have right stems with more homogeneous loss, easier cooling opportunities and better alignment guarantees. The central part of the tank has a square section to host the beam ports and the tuner (trimmer) while the rest of the tank is cylindrical. The stems are conical to progressively increase the diameter from the drift tube end (where it couldn’t be bigger) to the short circuit. The drift tubes are spaced in order to obtain the required beta value and to limit the maximum electric field. Electric field on the beam axis (@1 joule of stored energy) TTF 3 gaps (beta 0.04) 0 0,1 0,2 0,3 0,4 0,5 0,02 0,04 0,06 0,08 0,1 0,12 0,14