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To calibrate its fusion device, NIST is bending quartz crystals

The National Institute for Standards and Technology (NIST) has detailed its method of calibration of fusion research rigs, which uses an intriguing technique involving bent quartz crystals. These crystals are just what they sound like, and they can allow scientists to accurately measure the powerful x-ray signals coming from Sandia National Laboratories’ so-called Z-Machine, the largest x-ray generator in the world.
Basically, the Z Pulsed Power Facility pictured above (also called the Z-Machine, by people with stuff to do) works by holding a pellet a heavy hydrogen isotope called deuterium inside a large metal drum. Incredibly powerful capacitors store and instantaneously release more than 25 million amps of current all at once, causing the deuterium pellet to implode and fusing some of the pellet’s atoms. This causes the release of enormously powerful X-Ray signals, which can then be studied to learn bout the fusion process that created them — but how can we measure such signals accurately?


Sandia’s answer is bent quartz crystals, which are literally thin sheets of quartz that have been carefully bent over time. The crystals are rectangular wafers less than half a millimeter thick, which researchers loaded into metal cases that kept them from cracking as they ever-so-slowly acquired some curvature. Their crystal lattice, when bent in this way, acts as a prism for x-ray signals, splitting them into their specific sub-wavelengths so scientists can collect and analyze them individually.
For their calibration, these researchers basically shot the crystals with standard x-ray sources of known intensity and energy, and measured the resulting spectra. The intent wasn’t to learn about the x-rays themselves, but to create a response curve for the crystals and the energy/intensity of the radiation. These x-ray rainbows form a curve that then becomes the legend they use to interpret the quartz-split x-ray signals from actual experiments.
Each bent crystal needs to be individually calibrated, since it will have slightly different optical qualities and thus create a distinct scattering effect. If they didn’t make sure to quantify just how each crystal interacted with all the different varieties of x-rays, they’d end up with very slightly wrong results — and you know how much scientists hate those.

Those signals should grant insight into fusion power, among other things. Creating a working fusion reactor means keeping a fusion reaction stably contained, right now either with laser or magnetic confinement (Remember how they’re always talking about “confinement beams” on Star Trek?). That confinement can’t just be powerful enough, it has to be precisely shaped to fit the fusion reaction it’s containing — and to do that, you need to know a lot about how that reaction proceeds. These sorts of experiments can provide that kind of information.
More intriguingly, they could also let astronomers look into the elemental makeup of the Sun. Observations have implied certain elements fusing at the heart of the star, while theory predicts a very different distribution. Sandia’s Z-machine can create much the same sort of fusion reaction at “macro” scales. That could let astronomers derive some rules for fusion, and apply those rules to their understanding of our own star.
 


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