Realizing Quantum free-electron lasers: A critical analysis of experimental challenges and theoretical limits

TL;DR

This paper critically analyzes the experimental and theoretical challenges in realizing high-gain Quantum free-electron lasers, emphasizing the limitations imposed by physical effects and proposing a focus on seeded Quantum FELs.

Contribution

It provides a detailed assessment of experimental constraints and theoretical limits for Quantum FELs, and suggests a shift towards seeded configurations for practical realization.

Findings

Strong limits on quantum regime sustainability due to space-charge and spontaneous emission.

Optimized interaction geometries can improve Quantum FEL performance.

A need for rigorous quantum theory to accurately model spontaneous emission and space-charge effects.

Abstract

We examine the experimental requirements for realizing a high-gain Quantum free-electron laser (Quantum FEL). Beyond fundamental constraints on electron beam and undulator, we discuss optimized interaction geometries, include coherence properties along with the impact of diffraction, space-charge and spontaneous emission. Based on desired Quantum FEL properties, as well as current experimental capabilities, we provide a procedure for determining a corresponding set of experimental parameters. Even for an idealized situation, the combined constraints on space-charge and spontaneous emission put strong limits on sustaining the quantum regime over several gain lengths. Guided by these results we propose to shift the focus towards seeded Quantum FELs instead of continuing to aim for self-amplified spontaneous emission (SASE). Moreover, we point out the necessity of a rigorous quantum theory for spontaneous emission as well as for space-charge in order to identify possible loopholes in our line of argument.