/en/research/projects/3-04/subprojects/3_Application-hard-X-rays/index.html
3-04 Transient Structures and Imaging with X-Rays
Project coordinator(s): H. Stiel, M. Wörner
Subprojects: Generation and application of hard x-rays with high repetition rate

M. Zhavarankov, Y. Gritsai, M. Bargheer, M. Wörner and T. Elsässer 
in collaboration with
E. Foerster, I. Uschmann, IQO FSU Jena

M. Faubel, Max-Planck-Institut für Strömungsforschung, Göttingen

M.C. Richardson, School of Optics and CREOL, University of Central Florida, Orlando (FL), USA 
 
Short triggered X-ray pulses provide a new and powerful tool for direct monitoring of atomic position. It opens a new possibility for time resolved investigation not only for solid state physic but also in chemistry, biology and medicine through X-ray probing of chemical reaction, magnification radiography or single pulse imaging of small objects.

In principle, there are three possibilities to generate short X-ray pulses:

  • Gated synchrotron radiation (- ps resolution)
  • Thompson scattering of intence laser pulses on a relativistic electron beam (0.5 ps resolution)
  • X-ray flashes from fs-laser driven plasmas (~100 fs resolution)
In laser produced plasmas a very large fraction of the laser energy can be transfered to hot electrons through resonance absorption and parametric instabilities processes. These supra-thermal electrons are generated near the critical density surface. They are emitted mainly into the corona region, down to plasma density gradient. However the electric field produced in this region due to space charge effects draw back a portion of these fast electrons into the solid target core, where the electrons lose their energy and produce incoherent x-rays. Depending on electron energy and the target material hot electrons typically penetrate several micrometer into the target material generating bremsstrahlung and Ka line radiation as they slow down via collisions with cold atoms. For the very short plasma gradient scale length, the duration of the Ka emission is governed by the thermalisation time of the electrons in the bulk of the target and by the duration of the laser pulse. The theory indicates that X-ray pulses below 100 fs might be obtained from femtosecond laser pulses with this technique. The efficient Ka generation is obtained now with the intensity of 1015-1019 W/cm2 depending on the atomic number of the target material. To utilise a Ka source operating at 1 kHz repetition rate 5-10 mJ laser pulses with typical pulse duration of 50 fs focused into spot of 10 µm are necessary.

A high average power laser system was developed. It consists of a chirp-mirror-controlled Ti-sapphire oscillator producing 20 fs pulses, which stretches before amplification to 250 ps, a regenerative amplifier with 1.5 W output power at 1 kHz, post-amplifier and compressor. After compression we get pulses with 46 fs duration and energy between 5 and 10 mJ. Spatial shape is Gaussian with M2 close to 1.1. For operation at high repetition rate a special target design is needed which provides for each laser pulse a virgin target surface and remain precisely aligned within the short Rayleigh range of the high numerical aperture focusing optics. As a target we used liquid Ga jet target or metal band target. For the Ga jet target a Ka lines photon flux as much as 5x1010 phot/sec was obtained. The size of X-ray source was estimated to be few tenths of µm. Successful application of the developed source for Bragg diffraction was demonstrated with GaAs and GaAs/AlGaAs superlatice samples.

 


 

Dr. Zhavoronkov

zhavoron@mbi-berlin.de

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