CW Operation of the European XFEL: SC-Gun Injector Optimization, S2E Calculations and SASE Performance

TL;DR

This paper optimizes a superconducting gun injector for the European XFEL, performs start-to-end beam simulations including collective effects, and evaluates SASE performance for different operational parameters and improvements.

Contribution

It provides detailed optimization and simulation results for a superconducting gun injector, including effects of higher gradients and laser profiles, for cw XFEL operation.

Findings

Longer bunches with lower emittance survive compression.

Higher gun gradients improve beam quality and SASE output.

Micro-bunching instability is thoroughly modeled and understood.

Abstract

This note presents optimization results for an injector with a super-conducting gun for the cw operation of the European XFEL, the corresponding start-to-end simulation calculations for the beam transport to the undulators and SASE calculations of the X-ray intensities for the achievable photon energies. The optimizations for an injector with a super-conducting gun are done mainly for an already experimentally shown accelerating gradient of 40 MV/m. Different working points with respect to transverse emittance and bunch length are chosen to see whether the lower transverse emittance of the longer bunches survives the necessary stronger bunch compression to achieve comparable peak currents in the undulators. Also, results for possible further improvement of the gun gradient to 50 MV/m and a modified transverse laser profile (truncated Gaussian) are shown. The S2E calculations are taking into account all relevant collective effects, including a thorough treatment of the impact of the so called Micro-Bunching Instability. For that purpose, tracking with the real number of particles is done, with a resolution in space and time fine enough to calculate instability growth model-free.