The effects of a klyston on a high-current, high-gain free electron laser at SLAC

Abstract

A high current, high power X ray Free Electron Laser (FEL) using the Stanford linear accelerator (SLAC) as an electron beam source has been proposed. Such a system will provide the scientific community with its first realistic X ray laser. One dramatic use a hard X ray laser light at the proposed wavelength of 1.5 Angstrom would be to image DNA base pairs. With an accelerator already in place, the major cost of developing the SLAC system would be in the manufacturing of the undulator. The cost in building an undulator is proportional to its length. This thesis uses numerical simulation to evaluate the effectiveness of an FEL klystron in order to decrease the length of the undulator with a dispersive section. Simulations show that the quality of the electron beam injected into the undulator can greatly effect the ability of an FEL to produce coherent light, and is a factor in determining the length of an undulator needed for saturation. In the high gain FEL (such as proposed at SLAG), a dispersive section is sensitive to energy spread and may adversely affect the gain of the system. For moderate to low energy spread, it is found that a klystron is useful in shortening the undulator length. By examining how the strength of a klystron dispersive section affects the gain of a high-current system, we determine a critical strength above which the klystron can be detrimental. A dispersive section that is either too strong or not strong enough will result in less than optimum gain, or an increase in the required length of an undulator. Single mode, phase space simulations are used to investigate the effect on electron bunching and the onset of saturation. Longitudinal multimode simulations show the resulting coherence development.