Efficiency enhancement of a harmonic lasing free-electron laser

TL;DR

This paper investigates efficiency improvements in a harmonic lasing free-electron laser by optimizing wiggler tapering and using prebunched electron beams, supported by numerical simulations of nonlinear system evolution.

Contribution

It introduces a novel approach to enhance FEL efficiency through wiggler tapering and prebunching, validated by numerical solutions of coupled nonlinear equations.

Findings

Tapering the second wiggler increases third harmonic power.

Optimal tapering slopes significantly improve efficiency.

Prebunched beams reduce wiggler length requirements.

Abstract

The harmonic lasing free-electron laser amplifier, in which two wigglers is employed in order for the fundamental resonance of the second wiggler to coincide with the third harmonic of the first wiggler to generate ultraviolet radiation, is studied. A set of coupled nonlinear first-order differential equations describing the nonlinear evolution of the system, for a long electron bunch, is solved numerically by CYRUS code. Solutions for the non-averaged and averaged equations are compared. Remarkable agreement is found between the averaged and non-averaged simulation for the evolution of the third harmonic. Thermal effects in the form of longitudinal velocity spread are also investigated. For efficiency enhancement, the second wiggler field is set to decrease linearly and nonlinearly at the point where the radiation of the third harmonic saturates. The optimum starting point and the slope of the tapering of the amplitude of the wiggler are found by a successive run of the code. It is found that tapering can increase the saturated power of the third harmonic considerably. In order to reduce the length of the wiggler, the prebunched electron beam is considered.