C. J. Kost

COMA - A Linear Motion Code for Cyclotrons

COMA, a computer program written in FORTRAN IV, accurately calculates the relative motion of up to 5000 particles over the complete energy range of the TRIUMF cyclotron at a cost about 1% of our numerical integration codes. In addition it can simulate the action of diagnostic probes and slits. COMA uses linear transformation matrices, obtained from an equilibrium orbit code, that include coupling between radial and longitudinal motion. The energy gain and vertical focusing forces at the dee gap are given by an improved analytical model.

Improvements to the Beam Properties of the TRIUMF Cyclotron

The behaviour of the internal H- and external proton beams has been considerably improved during the past year. Better steering near the centre has resulted in the internal vertical emittance being reduced to hTr mm-mrad, while the external beam emittances are now 37¿ mm-mrad vertically and 3¿ mm-mrad horizontally, for 90% of the beam. Digitization of probe data together with computer-aided trim coil tuning has enabled the beam to be centred vertically to within ±6 mm; this has been important in simplifying the simultaneous extraction of two beams at independently variable energies (183 to 520 MeV) and intensities (split-ratios from 1/1 to 1/5000). Beam losses in the cyclotron are <20%; direct evidence is presented for gas and electromagnetic stripping, and also for a loss of a few per cent by resonant processes. New techniques have been developed to measure the phase, and have enabled the phase excursions (¿sin¿) to be reduced from ± 0.7 to <± 0.2 below 400 MeV. At high energies the phase excursions reach ±0.4, as anticipated from the magnetic field survey. However, a method is proposed by which separated turns could still be achieved and the energy spread reduced to 0.1 MeV, just as in a perfectly isochronous field.

Optimization of the Phase Acceptance of the Triumf Cyclotron

The horizontal and vertical beam behaviour in the TRIUMF cyclotron has been calculated numerically up to 20 MeV. Effects limiting the cyclotron phase acceptance for an extracted beam with good emittance and energy resolution are discussed, as wel1 as ways of overcoming these effects. Two effects which are critical because of their strong phase dependence are vertical electric focusing at the dee gaps and coupling between the radial and longitudinal motions. With only the RF fundamental present it is shown that the second of these effects can be reduced considerably by the use of local bumps in either the average magnetic field or its second harmonic component, or both. The addition of a third harmonic RF component suitably phase shifted from the fundamental results in the phase dependence of both effects being considerably reduced over a wide phase range. For an initial beam emittance of 0.5π in.‐mrad and an extracted energy resolution of ±600 keV the net phase acceptance is expected to be ∼30 deg with fu...

A Ring Cyclotron Kaon Factory

A two stage isochronous ring cyclotron is proposed for accelerating a 100 ¿A proton beam to 8.5 GeV. The first stage of 15 sectors and 10 m radius would take a 450 MeV beam from TRIUMF to 3 GeV, the acceleration being completed by a second stage of 30 sectors and 20 m radius. Superconducting magnets would be used, the weight of steel being estimated to be 2000 m tons for the first stage and 1800 m tons for the second. Numerical orbit tracking through simulated magnetic fields has confirmed that the focusing properties of the design are satisfactory and has emphasized the importance of using small pole-gaps to prevent fringing field effects weakening the edge focusing. Steel is provided outside the coils on the focusing edge to help keep it hard and increase the flutter. The accelerating system consists of SIN-style cavities, with flat-topping provided by operating some at the second harmonic (first stage) or third harmonic (second stage). The phase compression effect is also utilized to allow higher fundamental frequencies to be used on successive stages.

Properties of the TRIUMF Cyclotron Beam

8% of the 300 keV d. c. beam from the ion source can be transmitted to 500 MeV in the TRIUMF cyclotron, without using the buncher. The beam losses are entirely accounted for by the 40° phase acceptance at injection, 20% gas stripping and 6% Lorentz stripping; there are no significant losses due to orbit dynamic problems during 1500 turns of acceleration. The phase history, like \( v_z^2, \) is in good agreement with predictions based on the magnetic field survey. The effect of the harmonic coils and injection parameters on beam quality has been investigated; they can be used, with a chopper, to reduce the energy resolution of the extracted beam to 0.9 MeV FWHM and the emittance for 90% of the beam to 4π mm. mrad horizontally and 11π mm. mrad vertically.

Magnet Sector Design for a 15-GeV Superconducting Cyclotron

Pole shape designs are presented which maintain focused and isochronous orbits in the two stages of a superconducting ring cyclotron kaon factory (CANUCK). The first stage of 15 sectors takes a 100 ¿A proton beam extracted from TRIUMF to 3.5 GeV. The second stage of 42 sectors continues the acceleration to 15 GeV. The orbit properties have been determined using a median plane magnetic field computed from the current distribution in the coils and in current sheets simulating the saturated steel. The design process and the effects of various factors, including softness of the edges, cross-section of the coils and negative field gullies, will be described.

Effects of Axial Misalignment of the Dees and Their Correction

Axial misalignment of the dees near the centre of a cyclotron can lead to the build‐up of large coherent axial oscillations. Numerical computations made for the TRIUMF cyclotron are shown to be in agreement with an analytic treatment of the problem. General formulae are given, applicable to all cyclotrons. In the case of TRIUMF the tolerance allowed on the vertical positions of the dee lips in order to maintain beam quality is ±0.25 mm, quite tight considering the size of the cyclotron.Various methods of reducing the effects of dee misalignment are considered. For TRIUMF a system of specially shaped electrostatic deflecting plates acting over the first few turns has been proposed; these could relax the misalignment tolerance by a factor four. Other advantages and disadvantages of this scheme are discussed, including its effects on the radial motion.

Measurements and Corrections to the Beam Properties in the TRIUMF Cyclotron

The recent commissioning of two centring probes and four pairs of internal slits has for the first time made possible systematic measurements and improvements to the central orbits, and internal selection of beam emittance and phase acceptance. The beam signals are digitized and computer-processed to allow rapid analysis and correction of the centring by means of steering electrodes and harmonic trimming coils. Radial-longitudinal coupling effects agree with theory and are used to optimize the injection conditions for the wide phase acceptance. With the internal slits it has been possible to improve the incoherent radial betatron oscillation amplitude and to observe isolated turns to 220 MeV and turn structure out to 500 MeV. Fluctuations in the dee voltage (~ ±0.1%) and the magnetic field (± 2 x 10r-6) give rise to fluctuations in the energy and intensity of highly selected beams. Improved stability has been achieved by regulation of both RF voltage and frequency with beam-derived signals.

Beam Developments at TRIUMF

The cyclotron is routinely operated with three simultaneously extracted beams and more are planned. A double beam has been sent down one line with sufficient separation for magnetic splitting. The extracted beam intensity can now be continuously varied from 103 to 1015 protons/s (170 ¿A). The highest intensities were obtained by the addition of a second harmonic buncher in the injection beam line. Direct measurements of the electromagnetic H- stripping losses agree with those expected. Beam-defining slits have been used to reduce the energy spread (¿E/E) of the extracted beam to 10-3 at all energies between 200 and 500 MeV. Under certain circumstances the medium resolution spectrometer has measured a resolution a factor of two better. Depolarizing resonances have been located and one of them corrected. A number of new experimental facilities have been commissioned recently. These include a low intensity polarized proton line operating to a polarized target, a high luminosity, low energy ¿-¿ channel, and a clean cloud or surface muon channel using a velocity separator. The influence of the beam size and target shape on the characteristics of secondary particles has been examined.

Improved Beam Quality at TRIUMF

Improved stability, together with the use of beam-defining apertures, have enabled us to extract separated turns at 200 MeV. This implies an extracted energy spread of 0.25 MeV FWHM and a time width of 0.3 ns FWHM; 0.6 MeV and 0.6 ns, which includes instrumental resolution, have been measured. Mechanical stiffening of the RF resonators has recently reduced the dee voltage fluctuations to ±0.015% while the magnetic field fluctuations are ±2 × 10-6. The machine set-up procedure for separated turns will be described. The compensation of the residual magnetic first harmonic has also been improved and should eventually permit lower loss in high-current operation.