Testing NIF’s Dual-Axis Imager

Two x-ray images of a NIF experiment are better than one.

That’s the theory behind the development of a new dual-axis x-ray camera designed to acquire two high-resolution images of NIF implosions. The camera, the Advanced Radiographic Capability (ARC) X-ray Imaging System, or AXIS, is capable of recording two Compton radiographs during a single NIF shot.

Compton radiography is an important diagnostic for inertial confinement fusion (ICF), as it provides a means to measure the density and asymmetries of the deuterium-tritium (DT) fuel in an ICF capsule near the time of peak compression. It will be a key diagnostic component of experiments using ARC, the NIF petawatt-class laser with peak power exceeding a quadrillion (10¹⁵) watts. ARC will produce brighter, more penetrating, higher-energy x rays than can be obtained with conventional radiographic techniques to image experiments driven by the main NIF laser.

NIF researchers conducted the first two tests of the new AXIS detector in combination with ARC-generated x-ray sources on March 1 and 2. In the first experiment, four 30-picosecond-long, 850-joules-per-beam ARC pulses irradiated two gold micro-wires to generate two short-pulse hard x-ray backlighting sources.

In the second experiment, ARC irradiated gold wires to backlight a 200-micron-diameter tungsten-carbide sphere. An excellent radiograph of the test object was produced.

Meeting the scientific requirements of the Compton radiography platform requires that two point-projection radiographs be obtained on a single experiment, allowing study of the implosion dynamics around the time when the fuel is densest and guaranteeing that a radiograph is always obtained when there is uncertainty in the timing of peak compression.

AXIS is a framing camera designed specifically for NIF Compton radiography with ARC. It will record two radiographs with an equivalent line of sight, a 300-micron-square field of view at the capsule, and 128× magnification. AXIS will achieve ≥20 signal to noise for neutron yields up to 4×10¹⁴, allowing fuel areal density (ρR) to be measured with an accuracy of 5 percent. This is achieved by minimizing noise onto the detector through a combination of shielding and time gating.

AXIS was developed primarily to meet the need for significantly improved detection quantum efficiency, or DQE, at high x-ray energies. The ARC laser will produce x-ray backlighter sources ranging from 50,000 to 200,000 electron volts, and AXIS will be capable of recording these high-energy x-rays with a DQE several times greater than other NIF x-ray cameras. AXIS also will provide a much larger field of view of the imploded capsule. AXIS’ larger signal-to-noise ratio will allow the density and distribution of the compressed DT fuel to be measured with significantly greater accuracy as ICF experiments are tuned for ignition.

The AXIS team members are Gareth Hall, Nobuhiko Izumi, Riccardo Tommasini, Joe Holder, Dana Hargrove, Perry Bell, Dave Bradley, Nino Landen, Alexander Lumbard, Jason Cruz, Ken Piston, Arthur Carpenter, Nathan Palmer, Brian Felker, Vern Rekow, Fred Allen, Rich Zacharias, Rick Montesanti, Bradford Petre, Maryum Ahmed, Tom Lee, Carl Hardy, and Cal Smith.

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