G. H. Mackenzie

Evaluation of a prototype Isotope Separator Accelerator surface ionization source

A prototype surface ionization source coupled with a fixed-geometry extraction electrode system was commissioned on the Isotope Separator Accelerator (ISAC) ion source test stand at TRIUMF. The suitability of the ion source and extraction system for use in the ISAC facility was determined by a series of emittance measurements of the extracted beams. The test stand optics were successfully commissioned using the prototype ion source; emittance measurements of the mass-separated beams demonstrated that second- and third-order beam aberrations (introduced by the magnetic dipole mass separation) could be corrected by the use of multipole electrostatic optics elements. An upper limit of the root-mean-square-energy spread (2 eV) was deduced from the emittance measurements. Emittance measurements were performed at beam energies of 10–50 keV, as well as for ion masses ranging from Li+ to Rb+, to demonstrate the feasibility of the prototype for a variety of beam energies and masses.

AN ION SOURCE TEST STAND FOR THE ISAC FACILITY AT TRIUMF

Abstract The properties of ion sources and the beams extracted from them are critical to the successful design of the ISAC facility at TRIUMF. To better understand these properties, an off line 60 kV test stand has been constructed to test and evaluate targets and ion sources intended for use at the future ISAC facility. The test stand also presents the opportunity to evaluate diagnostics, beam monitoring and system control techniques required for ISAC. The test stand vacuum chamber mimics the ISAC target module. Ion beams are produced using a fixed geometry multielectrode extraction column and transported through 5m of beamline using electrostatic elements. Mass analysis is achieved using a 45° magnetic dipole midway along the flight path. Diagnostic elements positioned throughout the beamline are used to determine beam properties.

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...

An ISOL/post-accelerator facility for nuclear astrophysics at TRIUMF

Abstract A facility to perform measurements of nuclear reaction rates in which one of the reactants is a radioactive species is described. The value of these reactions to the area of nuclear astrophysics is discussed in detail and calculations of expected yields for selected examples are given. This proposed facility is composed of an on-line isotope separator (ISOL) front-end coupled to a booster post-accelerator stage to raise the energy of a radioactive ion beam to sufficient energies (up to 1.5 MeV/u) to perform these studies. The advantages of this approach are presented along with a discussion of the feasibility of not only obtaining the necessary radioactive beam intensities of the important isotopes, but also of achieving the acceleration necessary. Details of one feasible accelerator system are presented.

Bunch shape measurements using fast Faraday cups and an oscilloscope operated by LabVIEW over Ethernet

A LabVIEW program running on a Sun SPARCstation 5 controls a Tektronix TDS820 6 GHz sampling oscilloscope via an Ethernet to GPIB adapter. A PC based X Window terminal continuously displays the trace, refreshed at 7 Hz. Functions such as gain, time base and application specific controls are selected by menus, buttons and dialog boxes. We have used the system to observe signals from fast (>1 GHz) Faraday cups (FFC) in the radio frequency quadrupole (RFQ) section of our Radioactive Beam Facility (ISAC). The setup utilizes the accelerator control display terminals while allowing the oscilloscope to be closer to the FFCs. ISAC uses the RFQ to accelerate singly charged ions to 0.15 MeV/u where they are stripped prior to acceleration in a drift tube linac (DTL). The time structure of the beam at the RFQ entrance was measured on a FFC placed 4 m downstream of the RFQ pre-buncher. Additional 50 Ω coaxial cone FFCs were installed downstream of the RFQ near the RFQ exit, just ahead of the stripping foil and at a do...

Efficient Capture in an Accumulator Ring of 20,000 Turns of Beam Injected from TRIUMF

For the TRIUMF KAON Factory a 450 MeV Accumulator ring is required to match the cw 100 ¿A beam from the isochronous cyclotron to the first acceleration stage, a 50 Hz 3 GeV Booster Synchrotron. H- ions would be charge exchange injected into the Accumulator through a 250 ¿ g cm-2 carbon foil. Approximately 2 A of protons would be accumulated over the 20 ms Booster period and transferred to the Booster in one turn. The first protons injected circulate for about 2×104 turns before extraction. If a substantial fraction of the circulating protons traverse the foil each turn, both the lifetime of the foil and the amount of beam lost would be unacceptable. The proposed acceptances of the Accumulator are 100 ¿ ¿m horizontally, 30 ¿ ¿m vertically and 7.6×10-2 eV-s longitudinally. These are much larger than the TRIUMF emittances of 2 ¿ mmmrad in each transverse plane and 10-3 eV-s longitudinally. It is shown that with simultaneous stacking in the transverse and longitudinal acceptances it is possible to reduce the number of foil traversals per proton to an acceptable number ~ 100.

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.

Impact of the cyclotron RF booster on the 500 MeV proton beam production

The TRIUMF cyclotron routinely accelerates ∼220 μA of H− ions, extracting protons simultaneously to four external beam lines. The radioactive beam facility ISAC, now operating at 10–20 μA ∼500 MeV protons, will soon require up to 100 μA at 500 MeV. The CHAOS experiment on the π+, π− secondary beam line also requires a high intensity beam (∼140 μA, 500 MeV) but with a short (2 ns) bunch length. High current operation with 2 ns beam has been facilitated by the 4th harmonic auxiliary acceleration cavity [4]. The 2 ns beam structure is now achieved by phase compression as the energy gain per turn increases near extraction. The paper focuses on improvements in the reliability of this cavity and its rf coupler. The higher energy gain per turn also reduces H− stripping losses (by about 33%) in the high energy region, hence increases the allowed beam intensity for a given beam activation. The total current will soon be increased to about 300 μA to allow for ISAC requirements.

Beam characteristics using stable isotopes from a multicusp source for the TRIUMF ISAC facility

A multicusp source for positive ion beams has been designed and constructed in collaboration with the Ion Beam Technology Department of LBNL for the TRIUMF ISAC project. This type of source has demonstrated a high yield of singly charged ions, a low energy spread, a good emittance, and is compact and simple. Several stages of tests and measurements using nonradioactive beams to characterize the source performance are being carried out both at LBNL and at TRIUMF prior to the final phase of radioactive target-source system tests. Results of these nonradioactive tests and certain problems encountered are reported and discussed in this article.

Initial operating experience with the auxiliary accelerating cavity for the TRIUMF cyclotron

A 92-MHz auxiliary accelerating cavity has been installed in the TRIUMF cyclotron. The cavity operates at the fourth harmonic of the main RF frequency with a planned peak voltage of 150 kV. At full power it will almost double the present energy gain per turn in the 400-500 MeV range, reducing by 25% the stripping loss of the H/sup -/ beam. Low-current beam tests have been conducted at voltages of up to 90 kV and a maximum voltage of 145 kV has been attained. The cavity has also been used to flattop the integrated energy gain per turn. A description of the cavity design and a summary of the operating experience are given.<<ETX>>