Possible operation of the European XFEL with ultra-low emittance beams

TL;DR

This paper explores the potential of the European XFEL to operate with ultra-low emittance beams, enabling new modes such as high-energy SASE operation, multi-color generation, and broad-spectrum coverage from infrared to gamma-rays.

Contribution

It proposes novel configurations and techniques, including low-emittance beam utilization, a betatron switcher for multi-color operation, and a scheme for broad-spectrum radiation without major layout changes.

Findings

Ultra-low emittance beams enable SASE operation at 20-90 keV.

Multi-color X-ray generation from multiple electron bunches is feasible.

European XFEL can cover a broad electromagnetic spectrum with minimal modifications.

Abstract

Recent successful lasing of the Linac Coherent Light Source (LCLS) in the hard x-ray regime and the experimental demonstration of a possibility to produce low-charge bunches with ultra-small normalized emittance have lead to the discussions on optimistic scenarios of operation of the European XFEL. In this paper we consider new options that make use of low-emittance beams, a relatively high beam energy, tunable-gap undulators, and a multi-bunch capability of this facility. We study the possibility of operation of a spontaneous radiator (combining two of them, U1 and U2, in one beamline) in the SASE mode in the designed photon energy range 20-90 keV and show that it becomes possible with ultra-low emittance electron beams similar to those generated in LCLS. As an additional attractive option we consider the generation of powerful soft x-ray and VUV radiation by the same electron bunch for pump-probe experiments, making use of recently invented compact afterburner scheme. We also propose a betatron switcher as a simple, cheap, and robust solution for multi-color operation of SASE1 and SASE2 undulators, allowing to generate 2 to 5 x-ray beams of different independent colors from each of these undulators for simultaneous multi-user operation. We describe a scheme for pump-probe experiments, based on a production of two different colors by two closely spaced electron bunches (produced in photoinjector) with the help of a very fast betatron switcher. Finally, we discuss how without significant modifications of the layout the European XFEL can become a unique facility that continuously covers with powerful, coherent radiation a part of the electromagnetic spectrum from far infrared to gamma-rays.