Analysis of ultra-short bunches in free-electron lasers

L T Campbell,P Traczykowski,J R Henderson,H P Freund,B W J Mcneil,P J M Van Der Slot

doi:10.1088/1367-2630/ab9850

Abstract

Free-electron lasers (FELs) operate at wavelengths from millimeter waves through hard x-rays. At x-ray wavelengths, FELs typically rely on self-amplified spontaneous emission (SASE). Typical SASE emission contains multiple temporal ‘spikes’ which limit the longitudinal coherence of the optical output; hence, alternate schemes that improve on the longitudinal coherence of the SASE emission are of interest. In this paper, we consider electron bunches that are shorter than the SASE spike separation. In such cases, the spontaneously generated radiation consists of a single optical pulse with better longitudinal coherence than is found in typical SASE FELs. To investigate this regime, we use two FEL simulation codes. One (MINERVA) uses the slowly-varying envelope approximation (SVEA) which breaks down for extremely short pulses. The second (PUFFIN) is a particle-in-cell simulation code that is considered to be a more complete model of the underlying physics and which is able to simulate very short pulses. We first anchor these codes by showing that there is substantial agreement between the codes in simulation of the SPARC SASE FEL experiment at ENEA Frascati. We then compare the two codes for simulations using electron bunch lengths that are shorter than the SASE slice separation. The comparisons between the two codes for short bunch simulations elucidate the limitations of the SVEA in this regime but indicate that the SVEA can treat short bunches that are comparable to the cooperation length.

Highlights

While free-electron lasers (FELs) have been intensively studied since the 1970s, new developments and concepts keep the field fresh
We simulated the FEL interaction using ultra-short electron bunches and described a comparison between FEL models that rely on the slowly-varying envelope approximation (SVEA) formulation, which is represented by the MINERVA code, and on the complete Maxwell’s equations, which is represented by the PiC formulation code PUFFIN
The models have been applied to the Sorgente Pulsata ed Amplificata di Radiazione Coerente” (SPARC) spontaneous emission (SASE) FEL at ENEA Frascati

Summary

INTRODUCTION

While free-electron lasers (FELs) have been intensively studied since the 1970s, new developments and concepts keep the field fresh. Most of the physical/numerical models used to describe the FEL are based on either the slowly-varying envelope approximation (SVEA) or a particle-in-cell (PiC) formulation. While the SVEA codes require relatively modest computational resources the formulation breaks down for sufficiently short pulses because they employ an average of Maxwell’s equations over the time scale of the resonant wave. In this paper we present an analysis of ultra-short bunches in FELs using both the SVEA and PiC numerical formulations and use the MINERVA code and PUFFIN as representations of these approaches.

THE NUMERICAL FORMULATIONS

THE SPARC SASE FEL

SHORT PULSE SIMULATIONS

SUMMARY AND CONCLUSION