G. Dutto

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...

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.

Initial Operating Experience with the TRIUMF 300 keV H- Injection System

The present TRIUMF injection system consists of a 12 keV Ehlers type unpolarized H- ion source located in a 288 keV accelerator terminal, and connected to the cyclotron by a 35 m long injection line which contains two 90 deg electrostatic bends, three periodic quadrupole sections, and elements to pulse, bunch and chop the beam. The stray magnetic field from the cyclotron has been compensated in the source and along the beam line with passive shielding and ferrite permanent dipoles. The beam enters the cyclotron axially and is bent into the median plane by means of a spiral inflector. This system is now being commissioned and currents of 50 ¿A dc and 250 ¿A peak in a pulsed mode have been obtained with 90% transmission. Currents of 500 ¿A dc and a bunched beam of 1.5 mA peak have been obtained previously in the prototype injection system, and similar performance can be expected with the present system. A polarized H- source is under construction.

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.

ISAC-1: radioactive ion beams facility at TRIUMF

This paper describes the ISAC-1 radioactive ion beam facility proposed at TRIUMF. A novel approach for the target/ion source station will allow an incident proton beam intensity of at least 10 /spl mu/A at 500 MeV. This should give high luminosity for the production of nuclei far from stability with a very large isotopic range. After mass separation the beams can be sent to two different experimental areas. One uses the 60 keV energy beam for experiments such as the neutral atoms trap, parity violation, etc. The second one, mainly dedicated to nuclear astrophysics, will use the 0.2 to 1.5 MeV/u post-accelerated beam. Singly charged ion beams, with A/spl les/30 delivered from the on line mass separator, with an energy of 2 keV/u, will be accelerated in a two stage linac consisting of an RFQ and a post-stripper drift-tube linac up to 1.5 MeV/u. CW operation mode is required to preserve beam intensity. As a consequence of the low charge to mass ratio of the ions a low operating frequency for the RFQ is required to achieve adequate transverse focusing. The main features of this accelerator are: 35 MHz RFQ, stripping at 150 keV/u, and beam energy continuously variable from 0.2 keV/u to 1.5 MeV/u.

The H- High Intensity Beam Extraction System for TRIUMF

The TRIUMF cyclotron is being upgraded for extraction of a 100 ¿A H- beam at an energy of 450 MeV for injection into a post-accelerator. The system includes RF cavities operating at the 20th harmonic of the ion orbit frequency to boost the energy gain per turn to 1 MeV, an RF radial deflector at one-half of the RF fundamental frequency for turn dilution, electrostatic deflectors with a prestripper, and iron-free magnetic channels. The principal requirements for these additional devices are outlined, as well as the space and radiation constraints they must satisfy. The layout of the extraction system with respect to the beam trajectories and the mechanical and electrical design concepts of the devices are presented and discussed.

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.