Juliane Rönsch-Schulenburg

Experience with Multi-Beam and Multi-Beamline FEL-Operation

DESYs free-electron laser FLASH provides soft X-ray pulses for scientific users at wavelengths down to 4nm simultaneously in two undulator beamlines. They are driven by a common linear superconducting accelerator with a beam energy of up to 1.25 GeV. The superconducting technology allows the acceleration of electron bunch trains of several hundred bunches with a spacing of 1 microsecond or more and a repetition rate of 10 Hz. A fast kickerseptum system directs one part of the bunch train to FLASH1 and the other part to FLASH2 keeping the full 10 Hz repetition rate for both. The unique setup of FLASH allows independent FEL pulse parameters for both beamlines. In April 2016, simultaneous operation of FLASH1 and FLASH2 for external users started. This paper reports on our operating experience with this type of multi-beam, multi-beamline set-up.

First Operation of a Harmonic Lasing Self-Seeded FEL

Harmonic lasing is a perspective mode of operation of Xray FEL user facilities that allows to provide brilliant beams of higher energy photons for user experiments. Another useful application of harmonic lasing is so called Harmonic Lasing Self-Seeded Free Electron Laser (HLSS FEL) that allows to improve spectral brightness of these facilities. In the past, harmonic lasing has been demonstrated in the FEL oscillators in infrared and visible wavelength ranges, but not in high-gain FELs and not at short wavelengths. In this paper we report on the first evidence of the harmonic lasing and the first operation of the HLSS FEL at the soft X-ray FEL user facility FLASH in the wavelength range between 4.5 nm and 15 nm. Spectral brightness was improved in comparison with Self-Amplified Spontaneous emission (SASE) FEL by a factor of six in the exponential gain regime. A better performance of HLSS FEL with respect to SASE FEL in the post-saturation regime with a tapered undulator was observed as well. The first demonstration of harmonic lasing in a high-gain FEL and at short wavelengths paves the way for a variety of applications of this new operation mode in X-ray FELs.

Operation of Free Electron Laser FLASH Driven by Short Electron Pulses

The program of low charge mode of operation is under development at free electron laser FLASH aiming in single mode radiation pulses. A short pulse photoinjector laser has been installed at FLASH allowing production of ultrashort electron pluses with moderate compression factor of the beam formation system. Here we present pilot results of free electron laser FLASH operating at the wavelength λ = 13.1 nm and driven by 70 pC electron bunches. Relevant theoretical analysis has been performed showing good agreement with experimental results.

A new XUV-source for seeding a FEL at high repetition rates

Improved performance of Free Electron Laser (FEL) light sources in terms of timing stability, pulse shape and spectral properties of the amplified FEL pulses is of interest in many fields of science. A promising scheme is direct seeding with High-Harmonic Generation (HHG) in a noble gas target. A Free-Electron-Laser seeded by an external XUV-source is planned for FLASH II at DESY in Hamburg. The requirements for the XUV/soft X-ray source can be summarized as follows: A repetition rate of at least 100 kHz in a 10 Hz burst is needed at variable wavelengths from 10 to 40 nm and pulse energies of several nJ within single harmonics. This application requires a laser amplifier system with exceptional parameters, mJ-level pulse energy, sub-10 fs pulse duration at 100 kHz (1 MHz) burst repetition rate. A new OPCPA system is under development in order to meet these requirements, and very promising results has been achieved. In parallel to this development, a new High- Harmonic Generation concept is necessary to sustain the high average power of the driving laser system and for the need of high conversion efficiencies. Highest conversion efficiency in High Harmonic Generation has been shown using gas-filled capillary targets, up to now. For our application, only a free-jet target is applicable for high harmonic generation at high repetition rate, to overcome damage threshold limitations of HHG target optics. A new multi-jet target is under development and first tests show a good performance of this nozzle configuration.