Lawrence Livermore National Laboratory
Diagnostics
The time it takes to complete a shot on the National Ignition Facility makes the blink of an eye seem almost like an eternity. From the moment the initial laser burst is created to the completion of ignition and burn in the NIF target, less than two millionths of a second will elapse. Implosion of the target capsule takes less than 20 billionths of a second, and the fusion process itself occurs in less than 100 picoseconds (trillionths of a second).
Many NIF target diagnostics will operate inside large vacuum-sealed diagnostic manipulator tubes. The instruments are loaded in the target bay and inserted in the target chamber through a gate valve after air is evacuated from the tube. Obtaining meaningful information about what actually happens in the target capsule in time scales measured in picoseconds requires a new generation of ultra-sensitive, ultra-fast, ultra-high resolution diagnostic equipment (see "Doing a Stretch of Time," Science & Technology Review, June 2007).
To measure the performance of the lasers, hohlraum and target capsule and to record the results of NIF experiments, the target chamber is surrounded by dozens of detectors, oscilloscopes, interferometers, streak cameras and other instruments designed to capture every last detail and nuance of the short-lived shot. 
Diagnostic equipment is aligned by opposed-port telescopes, with cameras viewing the target and reference points on the instrument.Reaction histories, dynamic temperatures of targets and dynamic opacities must be measured, and the instruments must be capable of capturing a wide dynamic range of signal strengths – far beyond the capabilities of existing systems.
Not only must the detectors be extra-fast and extra-sensitive, they have to be carefully positioned and aligned in order to capture data from the BB-sized fusion target; and they have to be operated remotely and be able to quickly transmit vast amounts of data to instruments kept at a safe distance from the target chamber's harsh radiation environment, which will include neutrons, X-rays, gamma rays and electromagnetic pulses.
Target diagnostics can image or otherwise analyze these emissions in order to understand the complex interaction between the laser and the superheated target. In NIF about 30 different diagnostics will be used to study all of these emissions with high accuracy and precision. These diagnostics have been designed to withstand the harsh environment created by the radiation that they are trying to measure.
Instruments like this pinhole camera are mounted on the front of gated X-ray detectors and inserted into DIMs to take X-ray movies of the imploded capsule.
Among the advanced ignition diagnostics being developed for the National Ignition Campaign (NIC) are:
- The Velocity Interferometer (Visar) diagnostic for shock timing. The intense X-rays produced by the laser beams
rapidly heat the outer surface of the spherical target, creating shock waves in the target. The Visar diagnostic uses sophisticated interferometric techniques to accurately measure the speed of these shock waves. Information obtained from this diagnostic will help to optimize the design of the target and improve performance. - A Cherenkov gamma-ray detector, which will measure the "history" of the hydrogen fuel's ignition and burn with 50-picosecond resolution.
- A CVD (chemical vapor deposited) diamond detector and magnetic recoil spectrometer for neutron spectroscopy.
The Dante soft X-ray power diagnostic. Characterizing the X-rays generated by the experiments helps scientists to understand how well the experiment performed. Precise X-ray power measurements will be made by Dante, a proven diagnostic that was used in preliminary experiments on NIF in 2004.
- X-ray emission and backlit imaging systems to obtain images of the microscopic core of the target as it ignites.
- A revolutionary "time microscope" that will artificially expand the time scale of the picosecond-length signals from the NIF detectors, so the details of their wave shape can be captured on oscilloscopes and streak cameras – extending existing capabilities by orders of magnitude.
Neutron detectors (left) are inserted in diagnostic wells on the NIF target chamber to make measurements close to the target. They are used to measure neutron yield, time of neutron production and neutron spectrum. The Magnetic Recoil Spectrometer (right), developed by MIT and the University of Rochester's Laboratory for Laser Energetics, measures the neutron spectrum from an implosion by measuring proton energy produced in a collision with the neutrons.
Other, even more advanced diagnostics, such as a high-energy petawatt (1015 watt) laser, the Advanced Radiographic Capability, are also under development to further enhance NIF's capabilities as it evolves during the 30 years of its expected lifetime.
NIF's Advanced Radiographic Capability (ARC) will conduct multiframe, hard-X-ray radiography of imploding NIF capsules. The short pulses for ARC will be produced by compressing the laser pulse with gratings in this large ARC compression chamber.
The NIC diagnostics effort is an international collaboration. All of the NIC participants – Lawrence Livermore, Los Alamos, and Sandia national laboratories, General Atomics, and the Laboratory for Laser Energetics – are contributing to the effort. Other participants are MIT and Lawrence Berkeley National Laboratory in the United States, and the Atomic Weapons Establishment in the UK and CEA, the French Atomic Energy Agency. Duke University, National Security Technologies, LLC, and Ohio University are supporting calibration and diagnostic development. Scientists from a number of universities also are contributing, including scientists from Colorado School of Mines, UCLA, and Geneseo State University.
Click here for more information on NIF's diagnostic equipment, including contact information for experimenters wishing to obtain additional details.
More Information
"Precision Diagnostics Tell All," Science & Technology Review, December 2010
"Doing a Stretch of Time," Science & Technology Review, June 2007
"Titan Leads the Way in Laser-Matter Science," Science & Technology Review, January/February 2007
