UC Santa Cruz

As accelerator breakthroughs progress, Accelerator Physicists are greatly concerned with managing the so-called emittance of a particle beams. Emittance is defined as the momentum and position phase space area of a particle beam and is associated with the ability to focus and concentrate a beam. In general, lower emittance means higher beam quality. We care about emittance as it dictates many other key parameters that predict the success of an accelerator project. Collider experiments desire low emittance for higher luminosity, yielding more collisions to record. For free electron lasers (FEL), emittance directly determines how efficiently the electron beam microbunches, as microbunching is required to start the exponential growth of x-ray production in undulator magnets.

As FELs become more popular and scientifically in demand, the FEL user community is pushing for higher x-ray energies and higher repetition rates to enable new science. This presents a huge challenge for the accelerator community as the high repetition rate requirement coupled with the need for an ultra-low emittance beam is currently limiting the range of FELs. This dissertation explores this bottleneck for high repetition rate FEL light sources with the focus on a notable case study: SLAC's Linac Coherent Light Source II (LCLS-II). Simulations predict that decreasing the emittance at the undulators from the current estimate of 0.4 mm mrad to 0.1 mm mrad would expand SLACs LCLS-II x-ray energy upper bound from 15 keV to 22 keV. The benefit from lower emittance is even more pronounced for further linac energy upgrades.

With the LCLS-II emittance goals in mind, in this dissertation, I investigate emittance improvements for an electron injector system for FEL-like applications that would satisfy a high repetition rate requirement. I focused on two areas of potential improvement: using a Superconducting RF (SRF) gun cavity with a higher RF gradient on the cathode, or a higher quality cathode that produces a lower initial electron momentum distribution. I started with a detailed simulation study that decouples gun cavity improvements from cathode improvements. To do this, I utilized a genetic algorithm to optimize various injector lattices at different cathode qualities. I demonstrate that to realistically meet the demands of the FEL community, we will need both the higher gun gradients of SRF guns as well as improvements from the cathode.

I next present work on developing the solenoid component for a SRF gun developed at KEK in Japan to advance SRF gun cavity technology. I lay out the steps taken to design a superconducting solenoid that accommodates the unique requirement of a SRF gun cavity to have negligible magnetic field on the gun cavity wall material. The solenoid was tested and the operational results were compared to the expected performance of the solenoid design field with an acceptable agreement between the fields found. The solenoid will be used in a SRF gun test stand to add to experimental research for SRF gun cavities that can accommodate a higher RF gradient on the cathode.