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A square-wave-driven radiofrequency quadrupole cooler and buncher for TITAN

University of British Columbia

The TITAN (TRIUMF'S Ion Trap for Atomic and Nuclear science) project at TRIUMF will use the unique combination of an Electron Beam Ion Trap (EBIT) charge state breeder and a precision Penning trap to carry out high accuracy mass measurements (δm/m ≈ 1 x 10⁻⁸) on short-lived (t₁/₂≈ 50 ms) isotopes. In order to obtain fast and efficient charge state breeding it is necessary to inject a cooled and bunched beam into the EBIT. For radioactive isotopes this can be achieved by employing a segmented, gas-filled, RadioFrequency Quadrupole (RFQ). Such a device has been developed for use in the TITAN experiment. Previous RFQ coolers have been driven sinusoidally, however this device will be driven with a large amplitude (V[sub pp] = 400 V) high frequency (f = 1 MHz) square-wave. This makes a broadband, linear, cooling trap possible for the first time. Using matrix methods the motion of ions in a square-wave-driven trap were studied and the acceptance and the space charge limit of the trap calculated as a function of the operating parameter, q. A simple viscous drag model for the cooling process was then used to prove the cooling concept. A Monte Carlo routine was developed and used to simulate the cooling of cesium ions in a helium buffer gas. At an operating parameter of q = 0.4 a cooling time of 600/μs was found at a gas pressure of 2.5 x 10⁻² mbar. The final temperature, T[sub f], of the ion cloud was found as a function of q (for q < 0.4, T[sub f] < 400 K)- Ion optics for the injection and the extraction of ions from the trap were designed and simulated. Transverse rms emittances on the order of 3 π mm mrad and longitudinal rms emittances on the order of 2 eV μs for the extracted beam were demonstrated at an extraction energy of 2.5 keV. Simulations of a previously designed and manufactured stable ion source system were carried out to try and match the emittance of the beam to the acceptance of the RFQ. Through a combination of simulation and experimental results it was shown that with minor modification the system would give a relatively good approximation to the actual radioactive ion beam produced at TRIUMF. Further simulations showed that an acceptance of 99.8% of the modified beam by the RFQ will be possible.

Text

2009-12-11

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http://hdl.handle.net/2429/16468

Physics

University of British Columbia

UBCV

DSpace