Design of a chopper line for the CERN SPL

The SPL (Superconducting Proton Linac), a 2.2 GeV linac for high-intensity applications under study at CERN, requires a fast chopping at low energy of the H . beam. The most stringent demands on the chopper come from the operation of a Neutrino Factory, which requires 44 MHz bunch frequency in the accumulator ring and in the muon bunch rotation. This imposes a chopper structure with fast rise and fall times, below 2 ns, to remove 3 consecutive 352 MHz bunches out of every 8. An improved design of the standard travelling-wave chopper structure has been analysed and tested on a prototype. Additional effort has gone into the design of a pulse generator or power amplifier capable of providing the required rise and fall times. Since short rise times and high chopper voltages are conflicting requirements, the maximum voltage has been limited to 500 V per plate. A prototype driver has been built and tested. A very compact beam line design is proposed, which is still compatible with the low chopper voltage. The line houses the chopper structure and the dump, provides the separation between chopped and unchopped beam, and matches both from the RFQ and to a DTL. Effects of space charge and of varying beam parameters are analysed. In particular, the influence of the beam energy at the chopper on the line components is discussed in detail. A diagnostic line designed to perform the measurements necessary to validate this set-up is also described