Crystal Exposes Heart of Intense X-Ray Source
The Z machine at Sandia National Laboratory in Albuquerque, N.M., is the world's most powerful man-made x-ray source. In operation, laser-triggered switches release the charge stored in massive capacitor banks to channel 20 MA through an array of small wires, vaporizing them into a plasma. The magnetic field created by the current drives the plasma toward the target core, releasing nearly 250 TW of peak x-ray power along with several megajoules of kinetic energy in the form of flying debris. The energy released is dependent upon how uniformly the plasma is compressed.
The Z machine releases several megajoules of energy in the form of x-rays and flying debris. To investigate the process, Daniel B. Sinars (shown) and his team have developed a monochromatic x-ray imager based on a quartz spherical reflecting crystal, at which he is pointing. Most of the imaging hardware shown is destroyed each time the Z machine generates an implosion. Courtesy of Sandia National Laboratory.
But how to investigate the process in such a harsh environment? A new monochromatic diagnostic x-ray imager holds the answer.
Daniel B. Sinars, a researcher at the lab, has continued the development of a concept from Sergei A. Pikuz of the Lebedev Physical Institute in Moscow. Sinars places an x-ray source in the plane of the target, illuminating a spherical crystal. The radiation reflected from the quartz crystal produces a 10-µm-resolution image of a 20 × 4-mm region of the target at a shutter-protected focal plane. The x-ray imaging source is produced by illuminating a silicon or manganese target with a 1-ns pulse of 527-nm light from a 1-TW laser that is in a building 75 m away.
For most of the imaging tests, Kodak x-ray film is used. As Kodak phases out its film production, however, Sinars has begun to evaluate image plate detectors such as the Fuji BAS series.
Although the source and camera both presented nontrivial design challenges, the heart of the system is the spherical reflecting crystal. The quartz crystal serves a dual function. Because it is spherical, it produces an image, and because it is crystalline, the image is monochromatic. X-ray Bragg reflection from the crystal structure favorably reflects at a single wavelength. The narrow spectral bandpass is essential -- otherwise, the x-rays from the plasma would saturate the image.
The crystals begin as 1-mm-thick commercial parts that are polished to a final thickness of 50 to 100 µm and curved into a spherical shape. "The imaging properties allow us to get 10-µm resolution over a 4 × 20-mm field of view," said Sinars, "which is extraordinary for any high-energy-density plasma facility."
Because the Z machine releases so much energy, nearly every component of the imaging assembly is destroyed during a shot. The only reusable item is the camera housing, with its double shutter mechanism to protect the film.
A corollary to that destruction is the one-shot nature of the process. Each diagnostic image represents only one instant of the plasma implosion process. To characterize the development of the high-temperature plasma, Sinars and his team must build a new apparatus for each desired exposure. Future versions of the diagnostic system will incorporate double exposure and time-gated detection to track features in a single implosion.
The images produced show a progression far from the uniform shell implosion that was initially proposed as a system model. The wires, rather than disintegrating uniformly and simultaneously, generate hot spots, almost melting into a chain of globules that then serve as streaming sources of plasma. Sinars noted that the diagnostic already has revealed that the dense wire cores persist well into the current pulse. The Sandia group is using data from these images to refine mathematical models of the implosion process.
Given the extreme environment, the success of the imaging instrumentation is notable. Nevertheless, Sinars said, one of the most exciting aspects of these measurements is that the researchers can perform them at all.
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