Emittance growth and preservation in a plasma-based linear collider

Abstract

Particle physics is addressing some of the grandest questions, armed with big science machines: high energy particle colliders. These machines have, however, ballooned in size, and new technologies for accelerating particles are therefore required. Plasma-based acceleration is a promising new concept in this regard, enabling higher-than-ever accelerating fields by surfing particles on plasma waves|or wakefields|promising smaller and potentially cheaper particle accelerators. Nevertheless, many challenges remain before plasma wakefield accelerators (PWFAs) can be used for the next linear electron-positron collider. One particularly important question is whether PWFAs can preserve the required beam quality - or emittance - to produce a sufficient collision rate. This thesis addresses questions about emittance growth in a plasma-based linear collider, specifically for three important aspects of such a machine. Firstly, staging of several plasma accelerator cells is a method suggested to reach high energies with moderate-energy drivers, but is made difficult by the large chromaticity and emittance growth induced during capture of highly diverging beams. Apochromatic corrective optics - where only linear optics elements are required - is proposed as a (partial) solution to this problem. Secondly, acceleration of positron beams is not trivial in a plasma accelerator, due to the charge asymmetry of ion-electron plasmas. Hollow channel plasmas have been proposed as a solution to this problem - symmetrizing the electron/positron plasma response. However, strong transverse wakefields in these hollow channels lead to rapid beam breakup, which was measured precisely in an experiment in the FACET facility at SLAC. Lastly, compact accelerating structures must be matched by similarly compact beam focusing devices. Active plasma lensing is a promising technique in this regard, but can suffer from aberrations and consequently emittance growth due to both nonuniform plasma temperatures and distortive plasma wakefields. This was studied experimentally at the CLEAR User Facility at CERN, where in particular it was found that the nonuniform plasma temperature aberration in an active plasma lens could be suppressed by changing from a light to a heavy gas species. As a consequence, emittance preservation in an active plasma lens was demonstrated for the first time.