A separated function, variable energy, drift tube linac (DTL) operating in cw mode at 105 MHz is being built for the ISAC radioactive beam facility at TRIUMF. Included in the design are three buncher cavities installed in the first, second and third drift spaces between the five IH tanks. The main requirements of the bunchers are: cw mode, high effective voltage (up to 0.32 MV), large velocity acceptance (1.8-4.1%). The first DTL buncher has been developed at INR and is being tested at TRIUMF. It is a triple gap split-ring rf structure operating at 105 MHz. Signal level measurements give a Q-value of 4300 which is 74% of MAFIA simulation predictions and a resonant frequency within 0.6% of the calculated value. With cooling water flow of 20 l/min the mechanical vibrations were measured to be of the order of 1 µm. Although the design parameters are 56 kV gap voltage and 8 kW nominal power, we demonstrated 85 kV and 16 kW respectively with stable operation.

A set of triple gap split-ring 106.08 MHz bunchers (bunch-rotator for MEBT and three DTL bunchers) was designed, fabricated and tested for the ISAC project at TRIUMF. These four devices operate at low β (1.8%, 2.3%, 2.7% and 3.3%) and deliver the peak effective voltages in cw mode of 0.07, 0.19, 0.26 and 0.32 MV respectively. The shape of drift tubes was optimized to have maximum transit time factor with the fixed value of electric field at the surface (1.5 value of Kilpatrick's criterion). 3D simulations were done to adjust optimized drift tubes with split-rings to have operating frequency and to unify dimensions of the cavities. The bunchers were fabricated in INR and successfully tested at full power in TRIUMF. Some features of the design and results of the tests are reported in this paper.

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