A possible upgrade of FLASH for harmonic lasing down to 1.3 nm

TL;DR

This paper proposes using third harmonic lasing in a modified FLASH undulator to generate intense, narrow-band X-ray radiation at 1.3 nm, enhancing performance with phase shifters and standard undulator technology.

Contribution

It introduces a novel approach for harmonic lasing at 1.3 nm in FLASH, including specific undulator design and suppression techniques, with detailed simulation results.

Findings

Peak power reaches gigawatt levels.

Bandwidth is narrowed to 0.1% (FWHM).

Pulse duration is controllable from tens of femtoseconds.

Abstract

We propose the 3rd harmonic lasing in a new FLASH undulator as a way to produce intense, narrow-band, and stable SASE radiation down to 1.3 nm with the present accelerator energy of 1.25 GeV. To provide optimal conditions for harmonic lasing, we suggest to suppress the fundamental with the help of a special set of phase shifters. We rely on the standard technology of gap-tunable planar hybrid undulators, and choose the period of 2.3 cm and the minimum gap of 0.9 cm; total length of the undulator system is 34.5 m. With the help of numerical simulations we demonstrate that the 3rd harmonic lasing at 1.3 nm provides peak power at a gigawatt level and the narrow intrinsic bandwidth, 0.1% (FWHM). Pulse duration can be controlled in the range of a few tens of femtoseconds, and the peak brilliance reaches the value of 10^31 photons/(s mrad^2 mm^2 0.1%BW). With the given undulator design, a standard option of lasing at the fundamental wavelength to saturation is possible through the entire water window and at longer wavelengths. In this paper we briefly consider additional options such as polarization control, bandwidth reduction, self-seeding, X-ray pulse compression, and two-color operation. We also discuss possible technical issues and backup solutions.