Beam energy distribution influences on density modulation efficiency in seeded free-electron lasers

TL;DR

This paper investigates how different electron beam energy distributions affect density modulation efficiency in high-gain seeded free-electron lasers, revealing that uniform and saddle distributions significantly improve HGHG performance.

Contribution

It provides a comparative analysis of energy spread distributions on harmonic micro-bunching in various FEL schemes using theoretical and numerical methods.

Findings

Uniform and saddle energy distributions enhance HGHG-FEL performance.

These distributions have minimal impact on EEHG and PEHG schemes.

A 30th harmonic can be generated with 84keV RMS energy spread in a single-stage scheme.

Abstract

The beam energy spread at the entrance of undulator system is of paramount importance for efficient density modulation in high-gain seeded free-electron lasers (FELs). In this paper, the dependences of high harmonic micro-bunching in the high-gain harmonic generation (HGHG), echo-enabled harmonic generation (EEHG) and phase-merging enhanced harmonic generation (PEHG) schemes on the electron energy spread distribution are studied. Theoretical investigations and multi-dimensional numerical simulations are applied to the cases of uniform and saddle beam energy distributions and compared to a traditional Gaussian distribution. It shows that the uniform and saddle electron energy distributions significantly enhance the performance of HGHG-FELs, while they almost have no influence on EEHG and PEHG schemes. A numerical example demonstrates that, with about 84keV RMS uniform and/or saddle slice energy spread, the 30th harmonic radiation can be directly generated by a single-stage seeding scheme for a soft x-ray FEL facility.