Investigations of a Novel Bunch Compression Technique for Hadron Accelerators

Sammanfattning

Bunch compression is necessary for many accelerator applications, one of which being the creation of muons for the proposed muon collider. The design of a proton driver to deliver a short and very intense proton pulse to a target for the creation of muons is being developed as part of the International Muon Collider Collaboration. Some challenges with the proposed baseline design, notably the ambitious requirements for the radiofrequency system, motivate the investigation of alternative compression methods. Such a method is developed and investigated in this thesis. The fundamental idea of this method is that a series of chirped bunch trains can be compressed in the same compressor ring as in the baseline design, removing the need for an accumulator ring and radiofrequency cavities in the compressor ring. To study this novel scheme, the chirp creation was first developed using off-frequency cavities at the end of the linac. A toy model was also developed to investigate the parameters for the compression, giving a basic set of parameters which was then improved upon. The scheme was implemented in the particle-in-cell simulation code PyORBIT, adding non-linear and collective effects to the compression. An iterative process led to an optimised scheme and corresponding set of machine and simulation parameters. Of most interest in the investigation of the scheme was the impact of collective effects, particularly space charge. This was investigated for two compressor lattices operating above and below transition respectively. The strength of the space charge forces caused strong instabilities above transition, leading to microbunching, emittance growth and eventually the breaking up of the bunch. It was concluded that this was due to the negative mass instability. This finding prompted the investiga- tion of a lattice below transition, but in this case the space charge repulsion was found to hinder the compression entirely. In summary, the collective effects experienced during the compression were found to be too detrimental for realising the proposed scheme with the muon collider parameters in both cases. However, the compression scheme was also investigated at lower bunch intensities, reducing the strength of the collective effects. For the lattice above transition, this allowed for successful compression at 2 % of the original bunch intensity. Based on this, an alternative use case for the scheme is proposed for a neutrino factory, by modifying the repetition rate and bunch structure.