Method of computing magnet coil current settings for the NRL isochronous cyclotron

Abstract

In order for the modern isochronous cyclotron to accelerate ions of different mass and charge state to a variable final energy, the magnetic field in the accelerating region must be precisely tailored. A deviation of 15 G in 18 kG may lead to a loss of beam. The approximate magnetic field of the NRL cyclotron for isochronous accelaration is achieved by iron pole pieces arranged in a 3-sector, hill-valley configuration. This basic field may deviate from that necessary for isochronous acceleration by two or more kG at some radii. Compensation for this field error is achieved by various current-carrying coils wound in the magnet gap. This paper discusses the extent of the magnetic field measurements and the computer program developed at NRL for computing operational parameters (knob-settings) for all current- carrying coils. Innovations introduced in the calculations include minimizing the first harmonic introduced by the extraction elements and imperfections of the iron. In addition, allowance is made for the lack of linear independence of the trim coils as well as for the perturbation of the flutter field due to the trim coils. The influence of the extraction elements on the accelerating region and the function of each element for beam extraction is also discussed. Measurements on the internal and external beam verify the calculations.