Flash II: Perspectives and challenges

Abstract

FLASH has been a user facility since 2005, delivering radiation in the wavelength range between 7 and 47 nm using the SASE principle. After the present upgrade, the wavelength range is extended to 4.45 nm. With the 3rd harmonic accelerating module in place to linearize the longitudinal phase space, the stability and reproducibility of the machine is substantially improved. The user requests for beam time by far exceeds the time available. In order to increase user beam time and to improve the radiation properties delivered to users, a major extension of the user facility called FLASH II has been proposed by DESY in collaboration with the HZB. FLASH II is a seeded FEL in the parameter range of FLASH. As logical continuation, the seeding with HHG which started with sFLASH will result in direct seeding. Because in the foreseeable future there will probably not be HHG seed lasers available at high repetition rates down to wavelengths of 4 nm, a cascaded HGHG scheme is proposed to produce short wavelengths.After a first design report, the project now enters its technical design phase. During this time, the FLASH beam parameters after the present upgrade 2009/2010 will be characterized and the present design will be re-evaluated and adjusted. In addition, start-to-end simulations will complete the simulations which have been performed so far, including a design of the extraction area.

Ultrafast Dynamics of Magnetic Domain Structures Probed by Coherent Free-Electron Laser Light

“…One of those, the FLASH II project [34], currently under construction in Hamburg, will offer higher energies up to the L-edges of the transition metals of interest (Fe, Co, Ni) using the third harmonic of the undulator and an option for providing circularly polarized radiation is foreseen. The direct availability of circularly polarized radiation will not only further promote magnetic FTH imaging experiments but also will provide circularly polarized radiation independently of dichroic transitions.…”

Preparation and characterization of x-ray mirrors with three single layers of a-C, B4C, and Ni onto two 820-mm long Si substrate

“…The current challenge highlighted in this article is that two 820 mm long Si blanks should be coated by three single layers side-by-side. A mirror with such a stripe pattern is required in the FLASH II tunnel [6][7][8] . Before deposition onto the 820 mm long Si substrates, a lot of short calibration samples were coated over various areas to determine X-ray optical film properties such as thickness, density and roughness.…”

Pulsed operation of a high average power Yb:YAG thin-disk multipass amplifier

“…The thin-disk multipass amplifier is primarily designed to be operated in a 10 Hz burstmode as a pump amplifier for a seeding laser for FLASH2 [9] but can also be used with an increased burst repetition rate or for continuous operation with similar parameters. For this, modification of the multipass defocusing mirrors would be necessary, taking into account the increased thermal lensing of the thin disk introduced by cw-pumping.…”

“…Laser amplifiers with similar parameters can also be used as a high harmonic generation (HHG) driver for direct FEL XUV seeding [8]. The system described here is part of a planned seed laser system for FLASH2 [9]. The parameters desired are pulse energies of up to 2 mJ at burst repetition rates of 100 kHz to 1 MHz with pulse durations below 10 fs.…”