Phase Jump Method for Efficiency Enhancement in Free-Electron Lasers

TL;DR

This paper introduces a physics-based model for the phase jump method in free-electron lasers, demonstrating how phase shifts can enhance efficiency by enabling continued energy extraction beyond initial saturation.

Contribution

The paper develops a comprehensive physics model for the phase jump method and compares it to undulator tapering, providing insights into optimizing FEL efficiency.

Findings

The phase jump method allows coherent radiation beyond initial saturation.

The model explains energy extraction via microbunch deceleration cycles.

Criteria for phase jump values and undulator segment length are established.

Abstract

The efficiency of a free-electron laser can be enhanced by sustaining the growth of the radiation power beyond the initial saturation. One notable method is undulator tapering, which involves the variation of the gap height and/or the period along the undulator. Another method is the introduction of phase jumps, using phase-shifting chicanes in the drift sections separating the undulator segments. In this article, we develop a physics model of this phase jump method, and verify it with numerical simulations. The model elucidates the energy extraction process in the longitudinal phase space. The main ingredient is the microbunch deceleration cycle, which enables the microbunched electron beam to decelerate and radiate coherently beyond the initial saturation. The ponderomotive bucket is stationary, and energy can even be extracted from electrons outside the bucket. The model addresses the selection criteria for the phase jump values, and the requirement on the undulator segment length. It also describes the mechanism of the final saturation. In addition, we discuss the similarities and differences between the phase jump method and undulator tapering, by comparing our phase jump model to the classic Kroll-Morton-Rosenbluth model.