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 uses a voltage ramp to impart a velocity gradient on
the signal-bearing electrons that are generated when x-rays hit the
transmission photocathode (PC). As the electron signal traverses a drift space,
it is stretched out in time by the velocity gradient, resulting in a signal
that is approximately 50 times longer. The high temporal resolution is achieved
by selecting only a small part of the stretched signal stream. (See the video for
a narrated animation). 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.
(Left) Mike Morris and Ken Piston install a film pack in
the DIXI diagnostic in preparation for an Aug. 19-20 NIF high-yield experiment.
(Right) Image of the first high-temporal-resolution implosion data recorded by
DIXI on a NIF shot. 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.
Outside the NIF Target Chamber: Jay Ayers (left), Sabrina
Nagel, Ken Piston, and DIXI. (Credit: Jim Pryatel) |
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