THE ability to capture data from high-energy-density experiments has always been of great importance to the Laboratory and its missions. During the years of underground nuclear testing, Livermore scientists developed advanced diagnostics such as specialized oscilloscopes, streak cameras, and detectors to measure key physical properties of nuclear blasts, including reaction time histories and the overall yield of an explosion. In the absence of underground testing, these sophisticated instruments are still crucial for national security research, especially for the novel experiments conducted at the National Ignition Facility (NIF).
Built to generate up to 1.8 megajoules of ultraviolet light at a peak power of 500 terawatts, NIF is the world’s largest, most energetic laser. With its one-of-a-kind capabilities, NIF enables researchers to explore new frontiers in high-energy-density science, from understanding the intricacies of astrophysics, hydrodynamics, and radiation transport to ensuring the continued safety and reliability of the nation’s nuclear weapons stockpile. In September 2010, NIF completed its first integrated ignition experiment, demonstrating that all of the facility’s complex systems function together as designed. This experiment was a key milestone in the National Ignition Campaign (NIC), a multi-institutional effort focused on achieving fusion ignition and energy gain.
In an ignition experiment, all of NIF’s 192 laser beams will be fired into a cylindrical case called a hohlraum, which contains a BB-sized capsule filled with deuterium–tritium (DT) fuel. Gathering data from ignition experiments requires an extensive suite of reliable, robust diagnostics that can image an experiment faster and in more detail than ever before. These devices detect and measure visible light, x rays, gamma rays, and nuclear products such as neutrons generated during an experiment. By studying the collected data, scientists and engineers can evaluate the performance characteristics of the laser, hohlraum, and target capsule. With this better understanding of the system’s performance, they can determine how to manipulate the laser and the target design to produce the precise conditions for initiating fusion burn that produces more energy than was used to create it.
NIF diagnostics are designed to withstand the harsh environment of the target chamber—conditions that would destroy traditional electronics—and to record micrometer-scale details within tens of picoseconds (trillionths of a second). Since 2005, an international team of scientists and engineers has been working to improve established diagnostic systems and build new devices that can meet these stringent requirements. (See the box below.) Says Livermore scientist Bob Kauffman, “As a result of this collaboration, we have designed and built over 30 diagnostics specifically for NIF, and the number keeps growing.”
Each diagnostic is being installed, calibrated, and tested in one of three stages. The first stage, completed in fall 2009, focused on instruments designed to measure the laser’s operational capabilities. (See S&TR, April/May 2010, A Stellar Performance.) Diagnostics tested in the second stage analyzed hohlraum energetics and how well the laser energy coupled to surrogate targets. (See S&TR, June 2010, Targets Designed for Ignition.) In 2011, the remaining diagnostics needed to evaluate the capsule implosion and neutron yield will be integrated into the target chamber.
Shock to the Heart
Optical diagnostics designed to detect visible light help determine the energy balance of an experiment as well as the implosion velocity of the fuel capsule, laser–plasma interactions, and instabilities that affect the target performance. In a perfect world, the amount of energy fired into the hohlraum would be transferred to the target with minimal loss. However, two types of optical effects—Raman and Brillouin scattering—can cause the light to scatter out of the hohlraum. Because scattered light can adversely affect the drive of an implosion, researchers have designed optical devices to measure the light’s power, spectrum, and angular distribution.
One such instrument is the full-aperture backscatter station. When light bounces off the hohlraum, some of it is reflected back through the laser’s final focusing lens and into the beamline. There, a turning mirror diverts it into the backscatter station, which combines time-resolved imaging, spectrometry, and calorimetry to accurately characterize the reflected light.
A near backscatter imager, on the other hand, uses specially coated plates inside the target chamber to measure light scattered outside the aperture of the focusing lens. Says Kauffman, “We need these data to calculate the total energy absorbed from the incident beam, which is important for determining energy balance.” A gated, intensified charge-coupled-device camera within the near backscatter imager provides time-resolved images recorded inside the hohlraum for analysis.
Another optical diagnostic is VISAR, the Velocity Interferometer for Any Reflector, which measures the speed of shock waves compressing the DT fuel. NIF can deliver pulses in a variety of shapes and lengths depending on the demands of a given experiment. For ignition, the laser produces four shocks that are timed to collapse the capsule in a specific sequence. If the shocks are too close together, they will coalesce in the ice layer that surrounds the DT gas. If they are too far apart, the ice will decompress between shocks.
VISAR is a time-resolved Doppler velocity camera that detects and images light reflected from the fuel capsule’s ablator surface. Two interferometers combine the reflected beam with a reference beam to create an interference pattern. When a shock hits the fuel capsule, the interference pattern changes, indicating a phase difference between the beams that is proportional to the shock velocity. A streak camera records the pattern, providing the information needed to optimize the target design and its overall performance.
Feeling Hot, Hot, Hot
Beamlines that miss the entrance holes produce an x-ray emission spot outside the hohlraum. A static x-ray imager is a filtered pinhole camera designed to record such emissions. These cameras typically measure x rays in the 2- to 3-kiloelectronvolt range. However, filters made from different materials can be placed in the imager to broaden the detection range. Time-integrated images show the x-ray emission from each laser beam as it irradiates the target, allowing shot controllers to verify beam positioning.
X-ray diagnostics also help measure the radiation temperature within the hohlraum, providing details on the time history and symmetry of the x-ray drive needed to implode the fuel capsule. A broadband, time-resolved x-ray spectrometer called Dante measures the x-ray flux emitted by the target throughout an experiment. “Dante is the workhorse diagnostic for measuring hohlraum temperature,” says Joe Kilkenny, diagnostic chief scientist and the NIC program leader at General Atomics. “Using data from Dante, we can determine the radiation temperature from the distribution of x-ray energies as a function of time.” Dante includes an array of filters and diodes for measuring radiation flux. Depending on the requirements for an experiment, filters made from various materials can be placed within the 18-channel instrument to detect a broad range of x-ray spectra emitted at the foot and peak of the laser pulse.
Without x-ray diagnostics, obtaining a clear picture of the physical processes occurring inside the hohlraum would be nearly impossible. Additional devices such as gated x-ray cameras capture details on the shape and velocity of the implosion. Streak x-ray cameras continually record a target as it evolves over trillionths of a second, producing time-resolved images that show the x-ray emissions of beams focused on the target. And an x-ray fluorescer characterizes broadband high-energy x rays that can preheat the capsule and degrade the quality of the implosion.
Are We There Yet?
“NIF experiments will produce about 1019 neutrons in a few tens of picoseconds,” says Trish Baisden, the deputy director for NIC Operations. “Nuclear diagnostics will allow us to measure physical properties such as neutron yield, ion temperature, bang time, core temperature, and reaction history to understand how well the experiment performed and how much energy was produced.”
One such diagnostic device is the neutron time-of-flight detector. Made with plastic scintillator and photomultiplier tubes or diamond photoconductors, this detector measures the total neutron yield and the energy broadening of the neutron signal from the time neutrons originate in the target to when they arrive at the detector. The signal’s travel time depends on the kinetic energy spread, which is a function of ion temperature. “Ion temperature is directly related to how fast the core implodes,” says David Meyerhofer, the director of the Experiment Division at the University of Rochester’s Laboratory for Laser Energetics. “Without the right temperature, ignition will not occur.”
The number of neutrons generated in an experiment depends on the combined thickness and density of the fuel shell. Known as the areal density, this characteristic is a function of how much energy is absorbed by the material, the accuracy of target conditions during implosion, and the attenuation of particles through the material. By detecting the dispersion and spread of neutrons, scientists can calculate a target’s areal density, which is essential for determining whether the capsule has enough mass to sustain the fusion reaction. “The temperature and shape of the implosion at peak compression and the areal density of the material are the most critical components for ensuring that we are on track to ignition,” says Meyerhofer.
The neutron time-of-flight detectors work in conjunction with neutron-based reaction history diagnostics to determine the burn time of the imploding capsule. Neutron imaging and spectroscopy devices further detail the compressed capsule’s shape and size.
To test these diagnostics, researchers are conducting experiments with dudded targets, in which hydrogen is added to the DT fuel mix. “The dudded targets allow us to study all the necessary experimental parameters for ignition in an environment with very low neutron yield,” says Baisden.
A Balanced View
The variety of diagnostic techniques provides a more complete picture of what is happening with the laser, hohlraum, and target. “During hohlraum energetics experiments, we simultaneously measured laser light entering the target, light backscattered from the target, and hohlraum temperature,” says Alex Hamza, a target fabrication manager at NIF. “We evaluated all three pieces of recorded data together. If the results had not matched, we would have needed to investigate the discrepancies.”
More than 20 devices can be installed for an experiment using diagnostic instrument manipulators. These vacuum-sealed tubes are attached to the target chamber and house specialized carts that can accommodate up to five diagnostics at a time. “The manipulators are like the Swiss Army knives of NIF,” says the facility’s operations manager Bruno Van Wonterghem. “With this equipment, we can tailor diagnostics to fit the requirements of a particular experiment.”
Diagnostic instrument manipulators allow the same diagnostic to view an experiment from different lines of sight, improving the operational availability and reliability of the detectors. In addition, they provide flexibility to quickly reconfigure and relocate instruments between shots.
Going beyond the Standard
Radiochemical techniques are also being adapted for NIF research. During an underground nuclear test, tracer elements incorporated into materials were activated by neutrons generated in the explosion, as one material transmuted into another. Researchers analyzed the resulting isotopes to determine whether the experiment achieved the desired conditions.
Using the same principles for NIF, scientists are adding tracer elements to target materials. “A big difference between applying these techniques for laser research as opposed to underground testing is that we can collect a signal within hours instead of weeks,” says Hamza. Future diagnostics may collect solid elements from an experiment by using a device that acts like a catcher’s mitt.
Some NIF diagnostics are new additions to inertial fusion research. For example, the magnetic recoil spectrometer, developed at the Massachusetts Institute of Technology, is a novel approach for measuring the neutron energy spectrum. Because neutrons cannot be dispersed according to energy by a magnet, they are first converted to protons. A diagnostic instrument manipulator places a plastic foil about 1 meter from the target. Neutrons from the experiment collide with atoms in the foil, causing protons to recoil. This process converts the neutron energy to protons, which are dispersed by a magnet onto a detector that measures the proton energy spectrum. The probability of detecting a proton is much higher than it is for detecting a neutron, allowing the magnetic recoil spectrometer to capture more information. When fielded on the OMEGA laser during DT experiments, the device proved to be extremely successful.
The most highly anticipated NIF diagnostic is the advanced radiographic capability. This high-energy, short-pulse x-ray backlighter uses one of the NIF beamlines to x ray a target during an experiment. Each pulse of x-ray energy lasts just a few picoseconds. Coupled with a backlighter, the device provides high-resolution images of the capsule as it implodes.
A Future Filled with Possibilities
The full suite of NIF diagnostics will be essential to making ignition a success. “Because of the hard work and ingenuity of scientists and engineers all over the country and abroad designing and building instruments that can help us characterize the laser, hohlraum, and capsule, we are closer to achieving ignition than ever before,” says Ed Moses, principal associate director for NIF and Photon Science. As anticipation builds for the first ignition shot, scientists look forward to analyzing the impressive data these precision diagnostics will reveal.
Key Words: advanced radiographic capability, Dante spectrometer, diagnostic instrument manipulator, full aperture backscatter station, high-energy-density science, hohlraum, ignition, inertial confinement fusion, laser, magnetic recoil spectrometer, National Ignition Campaign (NIC), National Ignition Facility (NIF), near backscatter imager, neutron, OMEGA laser, target, Velocity Interferometer for Any Reflector (VISAR).
For further information contact Robert L. Kauffman (925) 422-0419 (firstname.lastname@example.org).
Lawrence Livermore National Laboratory
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