Tracking NIF Implosions @200 Billion Frames a Second

X-ray emissions from a high-neutron-yield NIF fusion reaction last less than 100 picoseconds (trillionths of a second). Before now, even the fastest x-ray framing cameras were unable to resolve the details of implosion velocity and symmetry in the core of a high-yield NIF implosion at the temporal resolution needed by inertial confinement fusion (ICF) researchers. To provide the necessary temporal resolution—the time scale over which changes in size or position can be measured—in order for NIF to continue moving toward ignition, an international team including researchers from General Atomics, the UK’s Kentech Instruments, Ltd., and LLNL has developed the world’s fastest x-ray framing camera: the Dilation X-ray Imager, or DIXI.

DIXI is a two-dimensional x-ray imager with a temporal resolution of about five picoseconds—it records a frame every five trillionths of a second, the equivalent of 200 billion images a second. “You would need a really big SD card on your camera to store that many images,” notes the DIXI system’s responsible engineer, Jay Ayers. The DIXI camera converts x-rays from an array of high-magnification 10-micron pinholes mounted 10 centimeters from Target Chamber Center (TCC) to electron images. These are dilated, or stretched, by a factor of 50 and then coupled to a conventional shuttered electron camera. This process results in a much shorter effective shutter speed and provides unprecedented x-ray time-resolution images.

DIXI is not only ten times faster than any other x-ray framing camera, it also is uniquely designed for operation in a hostile high-neutron environment, such as NIF as it approaches ignition. As currently installed, DIXI can diagnose fusion reactions producing up to approximately 10¹⁷ (100 quadrillion) neutrons.

Ayers and System Scientist Sabrina Nagel credited collaborator Jonathan Hares of Kentech with conceiving the instrument—“DIXI was his brainchild,” Nagel said—and Terry Hilsabeck of General Atomics with leading the design and fabrication of the detector platform under an LLNL contract. The NIF Engineering Group designed and fabricated the pinhole aperture, support, frame, and shielding, and performed the overall assembly and facility integration effort.

DIXI’s improved temporal resolution enables it to resolve implosion details close to the instant of extremely bright peak x-ray emission from the fuel core, which tends to wash out images just before and after it occurs. And to protect it from neutron and other radiation, the instrument is mounted 20 degrees off-axis on the outside of the NIF Target Chamber, “so the detector does not look directly at TCC,” Nagel explains. “It sits below the port, so it gets shielding from the 18 inches of Gunite (concrete) on the Target Chamber.” In addition, DIXI is shielded by 1,300 pounds of lead and high-density borated polyethylene.

DIXI’s complex instrumentation and off-axis positioning on the Target Chamber posed a number of timing, alignment, and logistics challenges. “It has been the most difficult instrument to time to date,” Ayers said. Aligning the pinhole array to ensure that images would be collected by the detector was another challenge, as were the space constraints involved in fitting DIXI into the crowded Target Bay. “There was a lot of optimization done with panels and shielding geometry,” Nagel said. In addition, new targets with special windows in the hohlraum are needed to provide DIXI with line-of-sight access to the x-rays from the fusion reaction.

Before it was installed on NIF in February, DIXI underwent intensive testing on the TITAN and COMET lasers at the Laboratory’s Jupiter Laser Facility (JLF) and demonstrated the temporal resolution needed by NIF. Since installation, Nagel said, it has “ridden along” on several high-yield NIF shots and recorded background neutron radiation levels as low as or lower than predicted by simulations. Nagel and LLNL’s Ken Piston led the testing at JLF. A second DIXI remains at JLF as a backup.

DIXI recorded its first NIF data during a high-yield deuterium-tritium (DT) experiment on Aug. 19-20. The instrument is well on its way to adding important capabilities to NIF by temporally resolving hot-spot formation, x-ray emission, fuel motion, and mix levels in the hot-spot at neutron yields of up to 10¹⁷. For a detailed description of DIXI, see the article in Review of Scientific Instruments.

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