Investigating the Feasibility of a Travelling-wave Chopper for the Clean Separation of 10 MHz Bunches at HIE-ISOLDE

The feasibility of cleanly separating the main 10.128MHz bunches from the 101.28MHz satellite bunches with a travelling-wave type chopper at HIE-ISOLDE was investigated using a simple model comprising a chain of synchronised capacitors pulsed at high-voltage. Even with a relatively large transverse aperture of 30mm it appears feasible to remove the satellite bunches spaced at 75mm without significantly perturbing the main bunch. We estimate that for a chopping voltage of 1.2 kV a string of 20 capacitors is required to impart the required deflection of 4 mrad to beams with A=q = 4:5 and the mechanical length of the system can be kept under 0.5 m. The deflection imparted on the main pulse is . 1% of that received by the discarded satellite bunches and the transverse emittance growth of the beam is small if the rise/fall times are kept below 5 ns. The HIE-ISOLDE specification is similar to the specification of the meander strip-line chopper developed at CERN for Linac4 and the application of this technology at HIE-ISOLDE could be of interest.