  The high energy beam transport (HEBT) line for the ISAC radioactive beam
facility at TRIUMF requires an 11.8 MHz buncher. The main requirements of the
buncher are to operate in cw mode with a velocity acceptance of 2.2% and an
effective voltage of 100 kV, which for a three gap buncher gives a drift tube
voltage of 30 kV. A lumped element circuit is more suitable than a distributed
rf structure for this low frequency of operation. The resonant frequency of
11.8 MHz is obtained by an inductive coil in parallel with the capacitance of
the drift tube. The coil is housed in a dust free box at atmospheric pressure
whereas the drift tube is placed in a vacuum chamber and an rf feedthrough
connects them. Two design of this feedthrough, one using disk and one using
tubular ceramics, operating at 30 kV rf, are described in this paper. MAFIA and
SUPERFISH codes are used to simulate the fields in the feedthroughs,
particularly around the ceramic metal interfaces. Test results of the prototype
feedthroughs are presented and the choice of the proposed final solution is
outlined.
