UCLA

Over the course of the last century, particle accelerators have emerged as a ubiquitous technology in experimental physics. Electron linear accelerators (linacs) have been particularly useful historically for light sources such as free electron lasers (FELs) and high energy physics experiments such as linear colliders. The most useful of these machines are significant undertakings often requiring the investment of nation states and kilometer footprints which reduce accessibility. In addition, newer applications of linacs for example in compact machines for medical purposes and imaging such as ultrafast electron diffraction (UED) have become popular. As a result, scale reduction without loss of beam performance has become a significant interest. Performance figures of merit such as beam energy, current, and brightness are commonly considered. Operating electron linear accelerators at cryogenic temperatures offers a promising venue for obtaining significant increases in beam brightness and consequently accelerator miniaturization.

In the following dissertation, the general underpinnings for beam brightness as a figure of merit are explained and a derivation for the maximum achievable value for an ideal injector is described as introduction to the field. Relevant low temperature emission and surface physics are also introduced. We then draw specific attention to the brightness scaling of a cryogenic RF photogun and formulate this into expressions dependent on the cathode and accelerating cavity's low temperature behavior. We further use electromagnetic and beam dynamics simulations to augment this theory with analytic predictions. The general objective of this dissertation is not only to quantify the achievable beam brightness improvements from the cryogenic operation of a radio frequency photogun using a combination of theory, simulation, and experiment.

The primary experiments involve the commissioning of a new lab facility for Multi-Objective Testing for High gradient Radiofrequency Accelerators (MOTHRA) with several experiments. We performed multiple low power RF measurements of novel cavities at both room and cryogenic temperatures which illuminate a new regime of cryogenic cavity operation in C-band. We further developed a new specially designed CrYogenic Brightness-Optimized Radiofrequency Gun (CYBORG). Stable peak cathode fields $>90$ MV/m at temperatures between $80-82$ K have thus far been obtained with increasing gradient and lower temperatures possible based on iterative RF and cryogenic improvements. We have thus developed a previously unrealized environment for low temperature high gradient electron emission testing. The low temperature effects and shunt impedance optimized cavity geometry performed as expected validating existing theory. In addition, empirical observations of a poorly understood phenomenon, reduction of dark current electrons at cryogenic temperature, have been made, previously observed in DC breakdown experiments. Each of these is discussed in relevant sections with a particular eye towards the development of an ultra high brightness photoinjector. The next development phase will be discussed including the upgrades necessary for testing novel high quantum efficiency low mean transverse energy cathodes.

Additional surface physics effects are observed and discussed. Prominently featured is the work to explain an unexpected previously observed local minimum in surface resistance for resonant copper cavities, a necessary figure of merit for RF performance, at cryogenic temperatures. A new theoretical explanation of this phenomenon is introduced including a simplified thin film model. Considerations with regard to pushing to extreme high gradients and the implications for future linear accelerators are presented.