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X-RAY GENERATION

High intensity lasers can be used to generate intense bursts of X-rays when shooting on solid targets, preferably metals. After a pre-pulse or even the rising slope of the main laser pulse has created a plasma on the surface, electrons are accelerated in forward laser direction by the ponderomotive potential of an intense laser pulse. These electrons create X-rays by mainly two mechanisms:

  1. Bremsstrahlung - (German for slow-down-radiation) when electrons are stopped or just decelerated, they emit electromagnetic radiation. This radiation is in the X-ray regime for high energy electrons.
  2. K-shell radiation - fast electrons can "kick out" inner shell electrons when they collide with an atom. When the vacant position in the shell structure of the atom is filled by an electron of an outer shell (recombination), an X-ray photon is emitted. The most likely is a recombination from the L-shell, which leads to a Kα-photon (see figure below). This radiation is often preferred for further application, because the high energy and narrow bandwith allow to dominate even over the very bright emission of hot and dense plasmas.


Generation of Kα-radiation
REPLAY

The application of Kα-radiation for radiography (see below) is important for Z-Beamlet. In this context studies on the conversion efficiency from laser energy into X-ray energy are carried out. A desired wavelength of X-rays can be selected by chosing the right metal foil as production target. Several diagnostics have been fielded and they provided a consistent survey on conversion efficencies for Z-Beamlet from four to ten keV photon energies using foil targets ranging from Ca (20) to Ge (32):


Table with Kα conversion efficiencies at 4-10 keV achieved on Z-Beamlet; ref.: Ruggles (2003).
Efficiencies have been shown to drop dramatically beyond 10 keV (not shown).

Currently, the Z-Beamlet research activities use two different approaches to generate and apply X-rays:

High energy (>1kJ)
Long pulse (>0.5ns)
Z-Beamlet
Moderate energy (2-50J)
Short pulse (<1ps)
100 TW system

Intensities are already high at a maximum power of 2TW, and Kα-radiation can be generated. The vast amount of primary laser energy enables the production of extremely strong X-ray pulses at energies of ~ 4-8keV. This extraordinarily high flux of X-rays is used to take an "X-ray snapshot" - called radiograph - of fusion capsule and wire-array implosions (see figures below), which are done at the Z-Accelerator, which is located in the building adjacent to Z-Beamlet. Radiography is the main mission of the Z-Beamlet laser installation.



Ref.: Sinars 2004

Although working at much lower energies, these extremely short pulses with a power of up to 100TW lead to higher intensities. Thus higher electron energies and therefore higher X-ray energies are possible. Short pulse generated X-rays are planned to be used for the radiography of implosion pellets with the upcoming Z-Petawatt laser. This will lead to better time resolution and the analysis of higher compressed targets. The figure below shows a spectrum of Kα-radiation in copper generated with only 2J out of the front-end section of the 100TW system.

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