Beam Spread with the Princeton FM Cyclotron

Abstract

Two methods have been employed on the Princeton 18‐meV FM cyclotron to increase the duty cycle of the proton beam. In the first method, the dee oscillator was cut off just before normal beam extraction and turned on again about 50 μsec later. A moderate beam spread is produced. In the second method, an auxiliary accelerating electrode, in the form of a ``C'' subtending an azimuthal angle of 70° and encompassing the outer 20% of the radial orbits, was added. It is placed opposite the single dee in the region near the deflector and exit channel. In order to produce a spread beam, the regular beam is allowed to enter the region of the C without rf drive on the C. The remaining acceleration necessary for extraction is accomplished by driving the C with an rf program and turning off the dee oscillator. Several frequency‐modulation programs were tried; noise, sinusoidal, sawtooth, and combinations of these. Sawtooth modulation produced best beam spread. The duty factor (as measured by the ratio of pile‐up to single counts from elastically scattered protons using a sodium iodide‐scintillation counter) was increased by a factor of about 4 to 5 in the first method and 12 to 15 in the second method. The average current was reduced by a factor of three. However pulsed operation of the dee oscillator makes it possible to use a higher peak dee voltage for the same average power, thereby reducing the loss of current due to stochastic extraction.