CALCULATIONS OF THE BEAM TRANSMISSION AND QUALITY IN THE

The highly advanced plan of the RIKEN AVF cyclotron (Japan) is under way. The intensity of the 14 N 5+ ion beam more than 10 p A is required to obtain a sufficient yield of secondary particles. The computer model of the AVF electromagnetic field has been prepared and successfully checked against the measurements. The focus of the present study is on the 2 nd RF harmonic regime. The intermediate goal of the upgrade is improvement of the transmission efficiency and the beam quality of the regime. Measured Hyper ECR output emittances were 64 and 135 mm mrad, from which the emittance of 100 mm mrad was assumed for both transverse oscillations in the simulation. The detailed account of the transmission efficiency and incremental losses are given. The optimization of the starting beam parameters for the existing electrode structure was considered first. The goal is to obtain a sufficiently small axial angle at the exit from the inflector to decrease axial losses in the initial turns. To this end, a new regime was formulated. The ECRIS voltage and the dee voltage were decreased by 15 % to 9kV and 40 kV, respectively. The modification is needed to ensure the beam passage through the puller with the crossing of the 1 st acceleration gap at the positive RF phase that gives the electrical focusing there. Accordingly, the buncher voltage should be changed to 125 V. As a result, the RF phase range of the bunched beam became ±12° and energy spread ±0.8keV with the tails of ~ 25% of the beam intensity. The magnetic field variation by ~50 % of the maximal contribution of the 1 st trim coil is needed to ensure acceleration at the top of the dee voltage. The RF frequency 16.2 MHz was assumed. The particle losses in the range from the inflector ground to the ESD mouth became ~ 35% instead of previous ~ 60%. The obtained regime was tested experimentally at the AVF cyclotron before proceeding to a more radical modification of the central region as the next step.