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Experimental and theoretical studies of the behaviour of an H-ion beam during injection and acceleration in the TRIUMF central region model cyclotron Root, Laurence Wilbur

Abstract

A comparison is made between the experimental and theoretical behaviour of the H" beam in the TRIUMF central region cyclotron. The axial injection process and the first six accelerated turns are studied in detail. In order to optimize the cyclotron performance the phase space emittance of the beam at the injection line exit must be matched to the central region acceptances. To this end,a theoretical study was made of the ion optical properties of the injection elements: the magnet bore, the spiral electrostatic inflector, the electrostatic deflector and the first radio-frequency accelerating gap. In many cases these results were confirmed by experimental observations. It was also shown theoretically that by a suitable choice of the accelerating gap, under optimum conditions, 10% of the injected beam can be directed within the radial acceptance and 30% within the vertical acceptances. The effects of a chopper and buncher in the injection line were also measured. A minimum pulse length of approximately 2.5 nsec was obtained with a bunching factor of 3-0. To accelerate a beam to full radius, vertical steering had at first to be provided by means of asymmetrically-powered trim coils and electrostatic deflection plates for each turn. The steering required is known to be consistent with the effects of magnetic field asymmetries and dee misalignments measured later. The size and shape of the vertical beam envelopes were found to be consistent with theory. The vertical tune vz was estimated to be 0.17 ± 0.03 for 20 deg phase ions. This agreed with the predicted value of 0.17- The transition phase which separates the vertically-focused and defocused phases was estimated to be -3 ± 3 deg, while the predicted value was 0 deg. The radial beam diagnostic techniques used for determining proper centring and isochronous operating conditions are discussed. With these techniques it was possible to centre a 30 deg phase interval to within 0.15 in., which was the approximate uncertainty in our measurements. A simplified treatment of radia 1 - longitudinal coupling is given and used to explain qualitatively the behaviour of a small emittance beam. The effects of space charge on the first six accelerated turns are calculated. For a beam occupying a phase width of 30 deg, these effects are predicted to be negligible for average accelerated currents below 100 uA. The experimental observations made on high-current beams are described.; prior to the shutdown of the cyclotron beams of up to 1^0 uA average current were accelerated.

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