An innovative approach harnessing crystals exposes the inner workings of the world's most powerful x-ray source.
Curved diffraction gratings designed in a Rowland circle configuration have the property that a source located on the Rowland circle will be imaged on the circle at a location dependent on wavelength. Daniel Sinars, a researcher at Sandia National Laboratories (SNL), Albuquerque, N.M., developed a monochromatic diagnostic x-ray imager based on that same principle. The diffraction grating is replaced with a quartz crystal, a customized version of a commercial part. The crystal is painstakingly aligned to a shuttered x-ray camera. The imager takes a single picture, then it's blown up.
To Sinars, who works with the "Z machine," the world's most powerful x-ray source, this is just part of the cost of doing business.
Inside "Z" The Z machine, more formally known as the particle beam fusion accelerator, Z-pinch version, is a multi-purpose machine intended to simulate conditions of a thermonuclear explosion. It is a component of the Dept. of Energy's nuclear stockpile stewardship program, and it also serves as a research tool for investigating high-temperature plasmas, a necessary precursor for producing controlled nuclear fusion.
![](/ProductImages/0503/pho1.jpg") Sandia National Laboratories researcher Daniel Sinars demonstrates the setup he and his team created to peer into the center of Sandia's Z machine at the moment of firing. The crystal under his finger is attached to portions of a Z target configuration. (Photo: Randy Montoya, SNL) |
At the heart of the 35-m-dia cylindrical building, jammed full of capacitor banks and magnetically-shielded transmission lines, is a 20-mm diameter cylinder typically composed of 300 7.4-µm tungsten filaments. The machine channels 20 MA of current through the target wires in a 100-nsec pulse. The wires vaporize to plasma, and the magnetic field created by the current compresses the plasma into the core of the cylinder. The dense, compressed plasma then releases more than 250 TW of peak x-ray energy. The compressed plasma is a nearly-perfect 2-million degree C blackbody source. The whole process, from initial current to completed reaction, takes less than 200 nsec. The problem comes in trying to understand exactly what's happening during those 200 nsec.
That's where Sinars' instrument comes in.
Extreme environments Researchers using the Z machine would like to follow the progress of plasma formation and compression. Ideally, an x-ray exposure would provide a snapshot of material density at a given moment, and stringing together a series of images would provide insight into the entire process. There are three significant challenges to investigating the Z-machine implosion. First is the short interaction time. Next is the several MJ of energy released in the form of flying debris. Last is the intense, broadband x-ray emission from the plasma source. The third of these problems is the most difficult to handle: any attempt to produce an x-ray image would first have to reject the incredibly high background levels of the implosion.
Sinars has continued development of a concept generated by Sergei Pikuz at the Lebedev Institute in Moscow, Russia. He placed an x-ray source in the plane of the target illuminating a spherical crystal. The light reflected off the 48 x 12-mm crystal produces an image of a 20 x 4-mm region of the target at a shutter-protected focal plane. The spectral bandwidth is determined by the size of the source on the Rowland circle.
"The unique feature of this technology is the narrow spectral bandpass (< 0.5 eV) for the radiation that the crystal allows us to use, coupled with the imaging properties of the spherically-bent crystal," says Sinars. "The imaging properties allow us to get 10-µm resolution over a 4 x 20-mm field-of-view, which is extraordinary for any high-energy-density plasma facility."
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Electrical discharges illuminate the surface of the Z machine, the world's most powerful X-ray source, during a recent accelerator shot. (Photo: Randy Montoya, SNL) |
To produce a sharp image of a rapidly-evolving target the researchers needed a very short exposure. Mechanical shuttering is orders of magnitude too slow; so they designed an x-ray source with an intrinsically short illumination time. The source is a "Beamlet" laser, a system developed as a proof-of-principle for the National Ignition Facility at Lawrence Livermore National Laboratory, Calif. The Z-Beamlet produces a kilojoule of energy in a 0.6-nsec pulse.
Green laser light from the Z-Beamlet is sent through a 75-m transmission path, then focused onto a 200-µm-dia spot on a silicon target. The target vaporizes and releases x-rays. The x-rays radiate through the cylindrical tungsten wire array and reflect off the spherically-curved crystal, then propagate through a baffle assembly to the camera. For most of the imaging tests Kodak x-ray film was used to capture the images, although as Kodak phases out its film production Sinars has begun work on evaluating image plate detectors. In the current design only one diagnostic image is produced for each Z-machine implosion, which means that the time evolution can only be studied by comparing images from different Z-machine firings.
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X-ray image of the edge of a cylindrical wire array. The image on the left is taken as the wires begin to vaporize, with plasma streaming from hot spots on the wires. In the image on the right, taken later in the process, the stretching tendrils of material emphasize the nonuniformity of the plasma compression. (Photo: SNL) |
According to Sinars, that may soon change. "Some areas we are working on for the near future include the development of two-frame imaging and time-gated (instead of time-integrated) detectors for this instrument. This would allow us to directly study the evolution of individual features in the plasmas, such as the growth of instabilities, that would be extremely valuable to constraining physical models for how fast these features should grow."
A necessary sacrifice So the high x-ray background and the short reaction time are dealt with, but how about the flying debris? "The energy release in the wire-array implosion is roughly equivalent to 0.5 to 1 kg of high-explosive," says Sinars, "and it is released in less than 1 cc of volume. That explosion produces a lot of debris and most of our backlighting hardware is destroyed each test." Meaning each new test requires a new customized crystal and another round of precision alignment.
Still the results are worth it. The path to inertial confinement fusion needs even larger facilities capable of handling even more current. Sinars summarizes, "we will need to understand how these sources scale to those larger currents to have confidence that the larger facilities will work."
—Richard Gaughan
Richard Gaughan is a freelance science writer and founder of Mountain Optical Systems Technology (
www.mountainoptical.com)
His column, Photonics Technology, will appear as a bi-monthly feature in R&D Magazine.