Sandia National Laboratories researchers have developed the ultrafast X-ray imager (UXI)—which captures images with an exposure time of only 1.5 nanoseconds, or 25 times faster than the best digital cameras.

Sandia physicist John Porter conceived and led the 10-year effort to capture plasma images more rapidly in the massive pulsed-power facility known as Z, an attempt to achieve controlled nuclear fusion. Denser groupings of observations at shorter time intervals are essential to more accurate numerical modeling, he says.

“There have been experiments where the best models predicted ignition, but it didn’t happen," Porter says. "There are too many ways a model unmoored from sufficient data can go from start point to end point. We need to feed simulations more data to ensure more accuracy.”

Sandia physicist John Porter sets in place a UXI camera in Sandia’s Z-beamlet laser facility. Image credit: Randy Montoya.A team of government, industry and academic experts from the National Diagnostic Plan have concurred, selecting further improvements to the camera as a top priority for accelerated development of next-generation diagnostics for high-energy density and inertial confinement fusion experiments.

“This project is important,” says Greg Rochau, program manager at Sandia, “because there are dynamics happening during the stagnation phase [when the fuel is at maximum compression] that we are unable to simulate with the best computational models because we don’t know what physics we’re missing. UXI enables diagnostics with better spatial and temporal resolution than we’ve ever had.”

Without UXI, several expensive, radiation-hardened CCD cameras coupled to microchannel plates—each with high-voltage power supplies and a bulky, expensive support system—would be needed to record data to the required precision. A single such camera might be used in successive experiments, with the camera programmed to fire a little later each time—but since no two experiments are exactly alike, it is hard to be sure how many nanoseconds into the second, third and fourth experiments the camera should capture. Then there is the expense of running the same experiment repeatedly.

The UXI camera's sensor consists of a radiation-hardened integrated circuit bonded to a silicon photodiode array. The bonding of these two integrated circuits joins two wafers, like two pancakes stitched together, into a monolithic hybrid structure.

“To date, we have created three generations of hybrid sensor cameras, each of which improves on its predecessor,” says Marcos Sanchez, team leader at Sandia’s Microsystems and Engineering Sciences Applications center.

The current sensor arrangement is a one-half-megapixel camera, with two frames of image storage per pixel. Each sensor’s shutter speed and inter-frame time can be set from 1.5 to 19 nanoseconds, making the sensors highly configurable to match the parameters of the experiment.

Work in progress with General Atomics in San Diego aims to shorten the image time to the 20-picosecond range—one seventy-fifth the exposure time of the current model—by coupling a UXI sensor to an innovative "pulse dilation" tube developed by researchers at General Atomics and the Lawrence Livermore National Laboratory.

The Sandia technology is available for licensing to government labs, industry and universities whose research could prosper from the ability to view a succession of chemical, nuclear or biological reactions that occur in nanoseconds.