Theory and mitigation of electron back-bombardment in thermionic cathode radio frequency guns

Includes bibliographical references.2015 Summer.Photocathode RF guns are currently the standard for high- power, low-emittance beam generation in free-electron lasers. These devices require the use of high-power lasers (which are bulky and expensive to operate) and high-quantum-efficiency cathodes (which have limited lifetimes requiring frequent replacement). The use of RF-gated thermionic cathodes enables operation without a large drive laser and with long lifetimes. One major limitation of RF-gated thermionic cathodes is that electrons emitted late in the RF period will not gain enough energy to exit the gun before being accelerated back towards the cathode by the change in sign of the RF field. These electrons deposit their kinetic energy on the cathode surface in the form of heat, limiting the ability to control the output current from the cathode. This dissertation is aimed at understanding the fundamental design factors that drive the back-bombardment process and at exploring novel techniques to reduce its impact on a high-current system. This begins with the development of analytic models that predict the back-bombardment process in single-cell guns. These models are compared with simulation and with a measurement taken at a specific facility. This is followed by the development of analytic models that predict the effects of space-charge on back-bombardment. These models are compared with simulations. This is followed by an analysis of how the addition of multiple cells will impact the back-bombardment process. Finally, a two-frequency gun is studied for its ability to mitigate the back-bombardment process. This dissertation provides new insight on how the back-bombardment process scales as a function of the beam parameters and how space-charge affects this process. Additionally this dissertation shows how a second frequency can be used to mitigate the back-bombardment effect

Design and simulation of the Colorado State University linear accelerator system

2014 Fall.Includes bibliographical references.The University of Twente in the Netherlands recently donated a linear accelerator and free-electron laser system to Colorado State University. A detailed model and simulation of the system must be constructed in order to assist the re-commissioning process at CSU. An initial design of the beam-transport system must also be developed. This thesis begins with the basic theory needed to understand the context of the simulations and then works through the accelerator, starting from the point where the beam is generated and continuing through the whole system to the beam dump. Individual components are simulated, their parameters are characterized, and optimal initial settings are found. These individual simulations are then combined into a complete start-to-end simulation of the machine. The start-to-end simulation is then used to demonstrate the expected performance of the machine with the optimal settings. This provides a system design that will be used in the initial buildup of the accelerator, as well as a simulation tool that can be used for future studies (for example, testing of novel components) or for examining the impact of proposed design changes

FAST-GREENS: A High Efficiency Free Electron Laser Driven by Superconducting RF Accelerator

International audienceIn this paper we’ll describe the FAST-GREENS experimental program where a 4 m-long strongly tapered helical undulator with a seeded prebuncher is used in the high gain TESSA regime to convert a significant fraction (up to 10 %) of energy from the 240 MeV electron beam from the FAST linac to coherent 515 nm radiation. We’ll also discuss the longer term plans for the setup where by embedding the undulator in an optical cavity matched with the high repetition rate from the superconducting accelerator (3,9 MHz), a very high average power laser source can be obtained. Eventually, the laser pulses can be redirected onto the relativistic electrons to generate by inverse compton scattering a very high flux of circularly polarized gamma rays for polarized positron production

FAST-GREENS: A High Efficiency Free Electron Laser Driven by Superconducting RF Accelerator

International audienceIn this paper we’ll describe the FAST-GREENS experimental program where a 4 m-long strongly tapered helical undulator with a seeded prebuncher is used in the high gain TESSA regime to convert a significant fraction (up to 10 %) of energy from the 240 MeV electron beam from the FAST linac to coherent 515 nm radiation. We’ll also discuss the longer term plans for the setup where by embedding the undulator in an optical cavity matched with the high repetition rate from the superconducting accelerator (3,9 MHz), a very high average power laser source can be obtained. Eventually, the laser pulses can be redirected onto the relativistic electrons to generate by inverse compton scattering a very high flux of circularly polarized gamma rays for polarized positron production