Dynamical Properties of the Electromagnetic Field Of

The compact isochronous cyclotron is considered as a source of 1.75 MeV protons for detection of explosives using the gamma–ray resonant absorption technique. Dynamical properties of the so-called Customs Cyclotron magnetic and acceleration fields were estimated analytically and digitally for the set of ion closed equilibrium orbits and by computer simulation of the beam acceleration process. The acceleration of the injected bunched beam was attempted first. Axial beam profile shows that no axial losses are visible with axial aperture in the low intensity limit. But the final beam quality does not completely meet the requirements. The results of the space-charge dominated beam acceleration revealed the axial losses. The transmission at ~5 mA injected beam intensity is less than ~30% making ~1.5 mA in the output beam only. Calculation of the 30 mA CW beam motion through the cyclotron gave a ~6 mA beam accepted in the acceleration regime. Several methods to improve the quality of the output beam were proposed. INTRODUCTION Requirements to the output beam [1] are summarized in Table 1. Table 1: Required output beam parameters Parameter Value Comments Type of emerging particles Proton Peak beam current 40mA Macro pulse width Variable Pulse repetition rate Variable Duty factor 25% Average beam current > 10 mA > 6.2·10 pps Mean beam energy 1.747 MeV Energy spread < 2 keV Beam spot at the target 10 mm Divergence at target 3 mrad The compact cyclotron, shown in Fig. 1, was selected to fulfill the requirements. Dynamical properties of the cyclotron magnetic [2] and acceleration fields were estimated analytically and digitally for the set of ion closed equilibrium orbits (EO) and by computer simulation of the beam acceleration process. At the initial stage of the study, an analytical approximation of the spatial electrical field in the Deeanti-Dee structure was assumed. The 3D electrical field simulation was provided for the further calculation (see Fig. 2). Given the above mentioned marginal requirement for the beam intensity and quality, H ̄ ions were selected for acceleration in the cyclotron aiming at the high efficiency extraction either by stripping at the 1st stage or by ESD (Electrostatic Deflector) attempting to meet the output beam specifications. Figure 1: Cyclotron with the Upper Part of the Magnet Removed. Figure 2: Central Region Acceleration Field Distribution. EQUILIBRIUM ORBIT PROPERTIES Main Acceleration Radial Region Assuming 60 kV dee voltage amplitude with two ~45° dee structure and hrf=4 acceleration mode harmonic, one can estimate the maximal energy gain per turn ~ 240 keV. Injecting H ̄ ions with energy ~ 30 keV, the energy of the beam after passing the 1st acceleration gap would be ~ 90 keV. Fig. 3 shows several EOs starting from the EO with the energy = 67 keV (somewhere inside the 1st acceleration gap) with the step 240 keV and the final orbit with energy = 1.747 MeV. So, Fig. 3 depicts an accelerator turn structure in the case of maximal energy gain per turn. 135 Proceedings of RuPAC XIX, Dubna 2004