High field phenomenology in linear accelerators for the compact linear collider

Abstract

The Compact Linear Collider (CLIC) is a 3 TeV linear electron-positron collider which is proposed to operate with loaded accelerating gradients up to 100 MV/m. These high gradients are accompanied by high field phenomena which limit the operation of the accelerating structures. Achieving reliable operation at these accelerating gradients requires an in-depth understanding of these phenomena and their effects on CLIC. This thesis investigates the phenomenology of high fields in CLIC accelerating structures through tests performed at the CERN's high gradient testing facilities.

The commissioning of a novel RF test stand will be presented. Using a unique RF pulse weaving method in combination with RF pulse compression, the new test stand offered the ability to test multiple accelerating structures in situ and at repetition rates up to 200 Hz. This offered a significant increase in the high gradient testing capacity at CERN. Using the new test stand, as well as existing infrastructure, four unique accelerating structures underwent conditioning to high gradients. These accelerating structures included a CLIC baseline design prototype, a structure with high order mode damping material, and two structures fabricated through novel machining and joining technologies. Three of the four structures were able to reliably operate at unloaded accelerating gradients of at least 100 MV/m with low breakdown rates.

Concurrent to the high gradient testing of accelerating structure, was an investigation into the radiation within the testing facilities, which was known to be the result of field emission capture. A series of measurements and simulations characterised the radiation produced during high power testing. A particular focus for the investigation was how the field emission capture varies with phase velocity. A model to describe the dependency of the capture of field emitted electrons on the phase velocity is presented. Measurements on the X-band test stands at CERN demonstrated that the capture increased ~20% for a 1 MHz increase in the RF driver frequency. These results were corroborated using a three dimensional RF and particle simulation.