Fast beam chopper at SARAF accelerator via RF deflector before RFQ

We describe design and simulations of a fast beam chopper for the SARAF accelerator based on an RF deflector preceding the RFQ. The SARAF 176 MHz RFQ, takes a DC proton or deuteron beam and accelerates and bunches the beam to 1.5 MeV/u and bunches of 0.3 ns width (FWHM) every 5.5 ns respectively. The deflector acts on the DC beam and sweeps away all but one of the pre-accelerated (pre)-bunches prior to the actual bunching and acceleration in the RFQ. Simulations were performed for a fast beam chopper, where several deflection voltage pulsing schemes have been investigated. The simulations show effective chopping with alternating positive and negative HV applied to the deflector with a fast HV switch, where the beam is transmitted to the RFQ during the cross-over of the rise(fall) of the HV switching. The simulations show that we can obtain efficient deflection of unwanted bunches, with 60% transmission efficiency for the desired bunch. The present design is for a chopper that will provide 0.3 ns bunches with a repetition rate of ∼ 10⁵bunches/sec. Plans for a fast chopper with higher repetition rates of ∼ 10⁶ Hz are discussed.