PARMELA Based Simulations on Jefferson Lab Free Electron Laser Injector

Free Electron Lasers (FELs) have many applications and are under continuing development to meet various needs and in areas from materials processing to life sciences research. This is because of its unique capability of generating high-average power, coherent, and wavelength tunable light by employing relativistic beam of un-bounded electrons. The main challenge in FEL development is the generation of a high intensity, high-quality electron beam in an injector. Further acceleration to relativistic energies is accomplished in a linear accelerator (LINAC) for producing laser light in a device known as wiggler. The Jefferson Lab FEL team intents to increase the injector electron gun voltage where the electrons are generated for better electron beam quality. This effectively requires injector optimization. This optimization cannot be done empirically since, the many-body-physics nature of space-charge dominated electron beams is complex. Plus the optimization of all the components found in the FEL injector constitutes a multi-variant problem because of its interconnected elements. Modeling the electron beam dynamics with a computer code that handles space-charge physics is an essential step in both the design of a FEL injector layout and optimization of its multiple components. In this thesis, PARMELA (Phase and Radial Motion of Electrons in Linear Accelerator), a particle simulation dynamics code (originally developed by L. M Young, LANL and used extensively by researchers and scientists in the particle physics community), is used to optimize the injector input deck (originally created by B. Yunn and Y. Liu, CEBAF, TJNAF) element by element for the increased 500KV gun voltage. This exercise helps in analyzing the effect of each element on specific characteristics of the electron beam, especially normalized transverse emittance and intrinsic longitudinal emittance, which are the key parameters in the FEL gain. One of the primary contributions of this thesis is the extension and modeling of the existing injector input deck from the end of the injector to the end of the LINAC. The purpose is to study the longitudinal space-charge effects as the beam is accelerated to its final energy. The model and input deck for the LINAC was originally created by B. Yunn and H. Liu from TJNAF's CEBAF and more recently modified by K. B. Beard from JLab. Longitudinal space-charge effects cause the longitudinal emittance to increase, despite acceleration (unlike transverse emittance which remains fairly constant). Understanding the physics behind this effect, by modeling the injector input deck with its extension to the end of the LINAC in PARMELA and comparing with theoretical and experimental data, is of critical importance to properly configure the machine for maximum gain. Finally, optimization of injector input deck to the end of the LINAC is carried out successfully and demonstrates that the figures of merit for the electron beam quality, the longitudinal and transverse emittances, are indeed lower for the 500KV gun voltage as compared to the 350KV gun voltage, which helps increase the FEL gain.