Flat-top acceleration system for the variable-energy multiparticle AVF cyclotron

Abstract

A flat-top acceleration system appropriate to minimization of energy spread in an ion beam was investigated for the JAERI AVF cyclotron. A combination of the fundamental- and the fifth-harmonic voltages to obtain a homogeneous energy gain distribution of accelerated particles is ideally suited to a variable-energy multiparticle cyclotron using acceleration harmonics of 1, 2, and 3. The flat topping of the energy gain distribution using the fifth harmonics has the advantages of minimizing amplifier power, reducing power dissipation in a resonator, and increasing the energy gain per turn. The flat-top acceleration system was designed to reduce the energy spread to 0.02%, which fulfills a beam focusing condition for production of a microbeam with a beam spot diameter of 1 μm. Tolerable fluctuations of acceleration voltages, required to achieve the energy spread of 0.02%, were 2.0×10−4 for the fundamental voltage and 1.0×10−3 for the fifth-harmonic voltage. Both fundamental- and fifth-harmonic phases were required to be stabilized within 0.2 rf degrees. The tolerance of the magnetic excitation was 1.9×10−5. In order to enhance compactness of the flat-topping cavity and to make a substantial saving of the amplifier power, optimum geometric parameters of the flat-topping cavity were determined by a cold model test and a calculation using the MAFIA code. A full range of the fifth-harmonic frequency, 55–110 MHz, was covered by the flat-top system, which enables us to apply the flat-top acceleration to a wide range of energy.