P. W. Schmor

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.

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.

The preliminary laser control system for the TRIUMF optically pumped polarized ion source

A system to provide detailed information on control parameters for the lasers used in the optically pumped polarized ion source at TRIUMF has been constructed. The authors describe the overall structure of the system and give details of the instrumentation used. Test and control programs to operate some of the devices and implement a slow spin-flipping operation have been developed and run on the workstation. During initial tests laser beam positions were found to be more stable than expected, and the requirement for controlling mirror motors and beam position detectors has been dropped from the final system. Laser frequency drift was less in the temperature-controlled laser room than in initial tests. >

Progress Towards Higher Intensities and Improved Beam Stability at TRIUMF

The TRIUMF accelerator routinely delivers up to 120 ¿A of 500 MeV protons. For tests, 150 ¿A have been extracted in a cw mode, 225 ¿A equivalent in a 10% dutycycle pulsed mode. Longitudinal space-charge effects are observed at these higher currents. A lead target, used as a beam dump and thermal neutron source, is being upgraded to allow extracted currents up to 375 ¿A cw. The reliability and performance of the cyclotron has significantly increased as the result of several recent developments. Improvements to the main magnet power supply (18,000 A) have resulted in a magnetic field stability better than ±0.8 ppm for periods of 2 h. The effect on beam phase, instability and separated turn operation is presented. A Lamb-shift polarized H- source provides up to 300 nA extracted. An ECR proton source has been tested as a replacement for the duoplasmatron on the polarized source. A gain in current of order 5 is expected. To satisfy the long-term needs, work has begun on developing an intense optically pumped, polarized source with the aim of increasing the current by a factor of 100.

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.

Achievement and control of the 100 μA beam at triumf

TRIUMF has recently achieved its design goal of a 100 μA, 500 MeV proton beam to the meson production target. Beam losses are particularly critical along the 40 m long 300 keV electrostatic injection line where beam heating can cause metallization of the insulators. Activation criteria limit the spills at higher energies although the possibility of thermal damage cannot be excluded. A number of special devices have been built to control beam losses and simplify high current operation. These include a variable duty-cycle electronic pulser in the ion source terminal, halo monitors and nonintercepting beam transformers in the injection line, secondary emission spill monitors in the cyclotron and target protect monitors, capacitive and radiation monitors along the external beam line. The procedure followed in setting up high current beams and the special systems designed to maintain acceptable losses will be described.

High Intensity Sources of Polarized Protons

The capabilities and future possibilities of intense polarized proton sources are summarized. The best Lamb-shift sources have achieved ~4 ¿A of H¿-. Atomic sources have achieved 6 ¿A H¿- dc (30 ¿A pulsed) and 125 ¿A H¿+ dc (200 ¿A pulsed). Planned developments are described which should increase the atomic source output current to the multi-milliampere range for both H¿+ and H¿- Optical pumping techniques have succeeded in producing 25 ¿A of H¿- and 200 ¿A of H¿+. Polarized ion sources using optical pumping which may eventually lead to amperes of H¿- have been proposed.

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.

The Variable Energy Polarized Proton Beam at TRIUMF

A polarized H- current of 1 ¿A has been achieved within a normalized emittance of 0.3¿ mm-mrad from a Lamb-shift type source. 80% of the beam is routinely transported at 300 keV along the 45 m long electrostatic injection line to the cyclotron, where 200 nA can be accelerated to 500 MeV and extracted. A Wien filter, located near the entrance of the injection line, is used to compensate for the precession resulting from the cyclotrons fringe field and to align the spin vertically. Depolarization along the line has been calculated to be less than a few per cent. The polarization, enhanced with a diabatic zero crossing in the source, is approximately 80%; however, a slight loss in polarization has been measured in the cyclotron between 200 and 500 MeV.