Soft x-ray free electron laser microfocus for exploring matter under extreme conditions

A J Nelson,L Juha,D Klinger,R Sobierajski,T Whitcher,M Fajardo,K Saksl,S Toleikis,M Jurek,O Renner,J Cihelka,R W Lee,T Dzelzainis,T Burian,P Heimann ,L Vyšín ,Th Tschentscher ,Bianca Iwan ,J Krzywiński ,David Riley ,Henry N Chapman ,Jakob Andreasson ,Nicuşor Tı̂mneanu ,A.r Khorsand ,V Hájková ,János Hajdu ,R R Fäustlin ,M Kozlová ,Saša Bajt ,Bob Nagler ,J S Wark ,J Chalupský ,S M Vinko ,J Pełka

doi:10.1364/oe.17.018271

Abstract

We have focused a beam (BL3) of FLASH (Free-electron LASer in Hamburg: lambda = 13.5 nm, pulse length 15 fs, pulse energy 10-40 microJ, 5 Hz) using a fine polished off-axis parabola having a focal length of 270 mm and coated with a Mo/Si multilayer with an initial reflectivity of 67% at 13.5 nm. The OAP was mounted and aligned with a picomotor controlled six-axis gimbal. Beam imprints on poly(methyl methacrylate) - PMMA were used to measure focus and the focused beam was used to create isochoric heating of various slab targets. Results show the focal spot has a diameter of < or =1 microm. Observations were correlated with simulations of best focus to provide further relevant information.

Highlights

The properties of 4th-generation Free Electron Laser (FEL) sources are revolutionizing the field of fast time-resolved x-ray science with the realization of sub-ps material response due to the ultrashort pulses and high intensity coherent monochromatic radiation
We have focused a beam (BL3) of free-electron laser in Hamburg (FLASH) (Free-electron LASer in Hamburg: λ = 13.5 nm, pulse length 15 fs, pulse energy 10-40 μJ, 5Hz) using a fine polished off-axis parabola having a focal length of 270 mm and coated with a Mo/Si multilayer with an initial reflectivity of 67% at 13.5 nm
Focused FEL beams are necessary for high intensity irradiation of samples to perform warm dense matter studies, single molecule X-ray diffraction imaging and various pump probe experiments since the samples are very small with weak scattering centers

Summary

Introduction

The properties of 4th-generation Free Electron Laser (FEL) sources are revolutionizing the field of fast time-resolved x-ray science with the realization of sub-ps material response due to the ultrashort pulses and high intensity coherent monochromatic radiation. The free-electron laser in Hamburg (FLASH) [1] in routine operation produces soft X-ray pulses at several wavelengths (32.5, 13.5, 7 nm) yielding 1014 W/cm at a 20 μm focus. This 4th-generation light source has allowed advanced studies of high-intensity laser-matter interactions on the femtosecond time scale [2,3]. The Maréchal criterion states that a system is regarded as well corrected if the Strehl ratio is greater than or equal to 0.8 [5,6] If these requirements are not met, the highest intensities that the 4th generation sources have to offer will be beyond reach. Better metrology is required for soft X-ray optics that meets the Maréchal criteria for attaining tight focus

Experimental method

Results and discussion

Conclusion