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RADIOGRAPHY

The justification for Z-Beamlet (ZBL) originally arose from the need for backlighting/radiography on the Z-Accelerator. The Z-Accelerator is a large pulsed power device which efficiently converts electrical energy into x-rays. The x-ray burst, which qualifies as the world’s largest laboratory x-ray source, can be used for studies on basic high energy density science, laboratory astrophysics, inertial confinement fusion, and stockpile stewardship. Because the accelerator conditions are so volatile, it is difficult to get a good high resolution picture of what happens during these brief events. One approach is to use radiography, wherein a small x-ray source emits x-rays which go through the desired object before being recorded on film. By using x-rays, the image contains not only symmetry/structure information but also material thickness/transmission information.

While the neccessary study on laser generated X-rays as a suitable radiography source is a field of activity in itself, the actual radiography experiments concentrate on imaging techniques and their application for the characterization of dense plasmas. The currently used methods apply Kα radiation because of its good signal to noise ratio. There are two concepts: point projection (ref.: Bennett, 2001) and crystal imaging, which uses a single crystal to select one wavelength in the process of point projection (ref.: Sinars, 2004). Generally, the spatial resolution increases with smaller laser spot sizes.

  Point projection Crystal imaging
Schematic    
Concept A very small X-ray source illuminates an object. The shadow of the target is recorded by film. All generated X-ray wavelengths contribute. A point projection, but the crystal creates spatially seperate images of the object for each wavelength that has been created in the laser target. One separates only the intense image of the Kα line.
Drawbacks Every X-ray wavelength has a different penetration depth into matter, thus their different contributions to the shadow image lead to a slightly blurred composed result The imaging is dependent on fulfilling the bragg-condition for crystal planes. Thus the alignment is more difficult and sensitive. Besides, Z is a hostile environment and the crystal is destroyed in the exeriment.
Advantages
  • Robust and "simple" setup
  • Very high contrast
  • The focusing properties of the crystal improve the efficiency
  • Larger field of view

The images below show crystal imaged radiographs of a hydrodynamic jet exeriment and the early phase of an implosion of a hemispherical capsule. Other examples of images can be found under "laser generated X-rays". For additional information, please see the related news article, too.

Advanced Approaches

As new ultrashort pulse laser capabilities like Z-Petawatt come online, the radiography options will expand. Using a kiloJoule/picosecond class system, the x-ray efficiency begins to ramp up around 10 keV and can produce x-rays up to 100’s of keV. Realistically, Z-Petawatt will expand x-ray radiography options into the 10 to 100 keV range, allow the probing of denser matter on very short time-scales. Furthermore, a system like Z-Petawatt can produce proton beams which can also be used for radiography, with the added benefit that the protons can be used for electric field mapping.

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