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:
Generation of Kα-radiation
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)
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 hν ~ 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.