High Efficiency, Multi-Terawatt X-ray free electron lasers

TL;DR

This paper investigates optimized undulator designs for high-efficiency, multi-terawatt X-ray free electron lasers, analyzing effects like synchrotron sidebands and proposing methods to maximize peak power and efficiency.

Contribution

It introduces an optimized superconducting helical undulator design and analyzes the impact of time-dependent effects on FEL performance, achieving higher peak power and efficiency.

Findings

Maximum peak power of 7.3 TW without time effects

Maximum efficiency of 9% with optimized tapering

Potential to reach 6 TW with enhanced seed power

Abstract

We study high efficiency, multi-terawatt peak power, few angstrom wavelength, X-ray Free Electron Lasers (X-ray FELs). To obtain these characteristics we consider an optimized undulator design: superconducting, helical, with short period and built-in strong focusing. This design reduces the length of the breaks between modules, decreasing diffraction effects, and allows using a stronger transverse electron focusing. Both effects reduce the gain length and the overall undulator length. The peak power and efficiency depend on the transverse electron beam distribution and on time dependent effects, like synchrotron sideband growth. The last effect is identified as the main cause for reduction of electron beam microbunching and FEL peak power. We show that the optimal functional form for the undulator magnetic field tapering profile, yielding the maximum output power, depends significantly on these effects. The output power achieved when neglecting time dependent effects for an LCLS-like X-ray FEL with a 100 m long tapered undulator is 7.3 TW, a 14 $\%$ electron beam energy extraction efficiency. When these effects are included the highest peak power is achieved reducing the tapering rate, thus minimizing the reduction in electron micro-bunching due to synchrotron sideband growth. The maximum efficiency obtained for this case is 9 $\%$, corresponding to 4.7 TW peak radiation power. Possible methods to suppress the synchrotron sidebands, and further enhance the FEL peak power, up to about 6 TW by increasing the seed power, are discussed.