A low-frequency RFQ for a low-charge-state injector for ATLAS

A design for a split-coaxial, normally-conducting, 12 MHz RFQ structure is being developed to accelerate singly charged ions of mass 132 and heavier to a velocity v/c = 0.008, suitable for injection into the ATLAS superconducting heavy-ion linac. Numerical studies have shown that a transverse (normalized) acceptance of 0.25 π mm-mrad can be achieved while maintaining a longitudinal emittance as small as a few keV-nsec. A novel feature is the use of drift-tubes at the entrance and exit of the RFQ which make use of the voltage offsets intrinsic to the split-coaxial structure to increase the voltage gain by about 30%. A half-scale model of the RFQ has been built and tested. The model, with no provision for cooling, was not operable cw but was pulsed to vane-vane voltages as high as 59 kV for periods of several milliseconds. The achieved level, limited by arcing in an rf feedthrough and so not a fundamental limit for the structure, corresponds to 1.2 times the (frequency and gap dependent) Kilpatrick limit. Assuming the model results scale, a 2 meter long 12 MHz RFQ, with 8 mm minimum aperture radius, will operate at 100 kV intervane voltage with an RF input of slightly less than 25 kW. Design and construction status of a full-scale prototype is discussed