In a few trillionths of a second, a dramatic event within the Target Chamber has passed. Yet, within that instant lies an astounding mix of physics interactions that create ignition: a self-sustained fusion reaction that generates more energy than the amount used to initiate it. On December 5, 2022, Lawrence Livermore made history with the laser shot at the National Ignition Facility (NIF) that successfully demonstrated ignition in a laboratory setting for the first time. (See the article The Future of Ignition.) Key to determining the success of this shot, and all experiments at NIF, is a suite of diagnostics that tune and measure critical aspects of high-energy-density (HED) experiments with unprecedented detail. These diagnostics are required to provide data that support stockpile stewardship and weapons science for the Department of Energy’s National Nuclear Security Administration.
NIF is home to more than 100 exquisitely sensitive and robust diagnostics, all of which were designed, built, and honed with the help of collaborators, including General Atomics; Los Alamos and Sandia national laboratories; academic partners such as the University of Rochester’s Laboratory for Laser Energetics (LLE) and the Massachusetts Institute of Technology (MIT); as well as the United Kingdom’s Atomic Weapons Establishment (AWE) and France’s French Alternative Energies and Atomic Energy Commission (CEA). Since 2013, as part of the National Diagnostics Working Group, this international team of technical experts has been improving established diagnostic systems and building new instruments to meet the stringent requirements of NIF experiments, including those for ignition.
Approximately 10 to 20 optical, x-ray, and nuclear diagnostics are used routinely to measure key variables of an ignition shot—from how well the laser energy couples to the target to implosion symmetry and the final fusion energy yield. These instruments collect diverse data from multiple distances and angles to provide a holistic picture of the event. “Diagnostics are our eyes and ears, helping us develop insight into our experiments,” says NIF’s Operations Manager Bruno Van Wonterghem. As such, they provide crucial details of the underlying physics, enabling scientists to simulate, adapt, and modify experimental platforms in pursuit of repeatable, predictable fusion burn for applications in stockpile stewardship.
Ready, Aim, Fire
During an ignition experiment, NIF’s 192 laser beams converge into either end of a pencil eraser–sized gold hohlraum—a tiny cylinder that holds a fuel capsule containing hydrogen isotopes deuterium and tritium (DT). When the carefully pulsed beams contact the hohlraum’s interior walls, x-rays are generated that heat and vaporize the fuel capsule’s outer high-density carbon surface—called the ablator—producing an inward force that compresses the cryogenic DT fuel. Compression generates a series of shock waves that heat the capsule’s inner core, creating an ignition “hot spot” that initiates and promotes the self-sustaining fusion reaction—a process typically found only in the center of stars, such as the Sun.
Prior to any NIF shot, sophisticated alignment systems ensure the target is positioned with pinpoint accuracy relative to the laser. In addition, an entire class of diagnostics provides data for fine-tuning experimental parameters. For example, instruments such as calorimeters measure the total energy produced, while beam profilers and wavefront sensors optimize the laser’s spatial and temporal profile. Streak cameras and oscilloscopes help refine the laser pulse shape, and spectrometers measure the efficiency of frequency conversion as the laser beams travel through the NIF system to the target.
After an ignition shot has fired, diagnostics collect data on the interactions between the laser, the hohlraum, and the target, and their effects on four key parameters of the implosion: shape, velocity, adiabat (thermodynamic state), and mix (ablator material entrained into the fuel). The neutron yield, distribution, and spectra measurements along with time-integrated and time-resolved x-ray imaging all contribute to determining the experiment’s success. Andrew MacKinnon, who leads NIF’s HED Science and Technology organization, says, “The combination of optical, x-ray, and neutron diagnostics provides powerful insights into the formation and evolution of the implosion over time.”
Robust and Reliable
NIF scientists use diagnostic data to inform computational models and theory to predict and improve implosion performance and experimental platforms. For example, the optical VISAR (velocity interferometer system for any reflector) diagnostic is critical for understanding the shock timing that leads to compression of the fuel capsule. In ignition experiments, multiple shocks from the laser are used to drive the implosion and compress the fuel uniformly. VISAR is based on a design originally developed at Sandia, which uses Doppler velocity imaging to detect light reflected from the ablator’s surface as shockwaves propagate through the material.
As the target approaches peak compression, NIF diagnostics reliably and accurately record x-rays and neutrons, which are generated in just 100 picoseconds or less as the target ignites. X-ray and neutron imaging diagnostics work in tandem to provide in-depth analysis of implosion symmetry. Neutron diagnostics, such as the MIT-designed magnetic recoil spectrometer and multiple neutron time-of-flight (nTOF) detectors, measure the neutron yield and spectrum produced during the fusion reaction. Developed in collaboration with LLE, nTOFs are placed at various distances from the target and use scintillators to detect neutrons. (Scintillator interactions create bursts of light that are converted into electrical signals that are recorded by the detector.) By analyzing the time it takes for neutrons to travel from the target to the detector, scientists can calculate their energy and determine the ion temperature of the implosion. NIF experimentalist Dan Casey says, “The 14-megaelectronvolt (MeV) neutrons emitted from the implosion have a modified spectrum because of Doppler broadening. The width of the 14-MeV peak provides a measurement of the ion temperature of the hot spot.”
X-ray diagnostics complement neutron measurements, providing detailed images of implosion symmetry. The 2D convergent ablator back-lit radiography platform utilizes a backlighter foil and a time-gate pinhole camera to capture images of the implosion in real time. Casey says, “This platform allows us to see what the implosion looks like in flight,” enabling the team to tune the implosion and optimize symmetry before the full DT layered experiments. X-ray spectrometers, specifically NIF’s two workhorse Dante diagnostics, measure the brightness and energy of the x-rays that heat and vaporize the ablator’s surface. Developed in collaboration with AWE, Dante uses an array of filters and diodes for measuring radiation flux to determine the hohlraum temperature from the distribution of x-ray energies as a function of time. One of Dante’s detection channels uses a multilayer mirror developed by CEA to directly measure the x-ray drive spectrum that can preheat the capsule and produce hydrodynamic instabilities that negatively affect the implosion. These measurements, combined with neutron spectra data, allow scientists to evaluate the overall plasma conditions of the fusion fuel. Extremely high temperatures (tens to hundreds of millions of Kelvin) are required to drive the implosions to high enough velocities to trigger ignition and self-sustained burn.
Outside neutron and x-ray imaging, NIF instruments can also measure the radiochemistry of the fusion reaction and gamma rays emitted from the DT fuel or from neutrons interacting with the carbon ablator. The gamma reaction history diagnostic, which was designed by Los Alamos, records the timing and energy of gamma rays emitted during the fusion reaction to help scientists determine the reaction rate and assess whether the target conditions are optimal for efficient fusion burn. “For experiments with significantly high yield, we see the reaction history become shorter as the burn happens faster,” says Casey. “We are using this data to increase the compression of the capsule. The idea being if the assembly can be held together for a little longer, then it will burn significantly more.” These efforts are critical for optimizing the efficiency of the fusion burn inside different types of fusion platforms.
The Stable Grows
As NIF ignition experiments achieve higher yields, diagnostics are constantly being evaluated, upgraded, and developed to withstand the increasingly intense x-ray and debris fluences inside the Target Chamber. Van Wonterghem says, “We are achieving yields around 5.2 megajoules, and the diagnostics perform reasonably well at these levels, but we are working to harden or redesign diagnostics to handle yields of 10 megajoules or higher.”
One diagnostic under revision is DIXI (dilation x-ray imager), which records x-ray emissions with less than 10-picosecond temporal resolution. DIXI, which was developed in collaboration with General Atomics, was initially equipped with a CCD (charged-couple device) camera and was effective for yields up to 1016 neutrons. To push yields higher, diagnostics experts transitioned the system to CMOS (complementary metal oxide semiconductor)-based detectors, which are radiation-hardened and can handle yields up to about 1017 neutrons (approximately 200 kilojoules). “We have gone back to using film,” says MacKinnon, “which is extremely radiation-hard due to its thinness and reduced sensitivity to neutrons. However, even film will eventually saturate, so we are conducting research and development to achieve the required engineering and technology improvements needed to push to higher yields, greater than 20 megajoules, for example.”
NIF diagnostic experts have also made inroads in improving technology to more efficiently and accurately capture implosion dynamics related to mix. NIF’s neutron imaging system (NIS), designed by Los Alamos, provides twin capabilities for capturing both neutrons and gamma rays emitted from the fusion fuel. “As the hohlraum bakes, some of the carbon burns off and leads to the rocket effect that implodes the capsule, but some of it is left behind at the time of peak burn,” says former Livermore physicist Michael Rubery. “Gamma-ray imaging can tell us about how the fuel is being compressed and what might be helping or hindering implosion.”
NIS uses a gold pinhole (penumbral) array, stationed about 30 centimeters from the target center to image both neutrons and gamma rays. These fusion products are independently directed to an optical table where scintillators convert their energy to light. Highly sensitive gated cameras then capture the signals for analysis. Rubery worked with Livermore researchers in the Materials Science Division to replace a traditional plastic scintillator with a novel Laboratory-developed scintillator for gamma detection. The material is high-Z and high density (having numerous electrons with which the gammas can interact), and has a high light output (more light given out per interaction). The scintillator material is 6 times denser than plastic and greater than 30 times more efficient at generating a signal. Rubery says, “We are now fielding this diagnostic regularly for ignition experiments, allowing us to more finely tune the quality of upcoming shots.”
Also under development is the high yield x-ray imager (HYXI) diagnostic—an advanced system for high-fidelity, time-resolved x-ray imaging of inertial confinement fusion experiments producing yields of 10 megajoules and above. “Combining electron pulse-dilation technology with radiation-hardened hybrid-CMOS imaging sensors, HYXI is designed to overcome the limitations of existing diagnostics, such as the polar and equatorial DIXI diagnostics, by significantly improving the signal-to-noise ratio and dynamic range in high-radiation environments,” says Robert Plummer, the section manager for the Target Diagnostics, Engineering, and Science organization. HYXI will allow precise measurements of hot spot formation dynamics and burn propagation, providing critical insights into ignition conditions and variability in fusion performance. Plummer continues, “Beyond its immediate application, HYXI’s deployment will demonstrate the performance of next-generation Hyperion CMOS radiation-tolerant imaging sensors, paving the way for future diagnostic systems in high fusion yield applications.”
Teamwork Vital to Success
Through innovation and collaboration, NIF is making great strides in fusion research. As scientists continue to push toward higher yields, NIF diagnostics provide the understanding needed to modify experimental platforms and achieve progressive success. “We are continually tweaking ways to adjust the symmetry as we see the strong role it plays in a well-performing implosion,” says Van Wonterghem. In addition to efforts to improve implosion stability, scientists are modifying the hohlraum geometry to enhance energy efficiency. Such modifications are all aided by the data captured by NIF’s precision diagnostics.
Looking ahead, NIF’s Extended Yield Capability project aims to also increase the laser’s energy beyond its initial design specification to 2.6 megajoules or higher. As these advances materialize, NIF’s diagnostics will remain indispensable for revealing the physics of an artificial Sun, helping to realize the transformative potential of fusion for HED weapons science and potential fusion energy generation.
— Caryn Meissner
For further information contact Bruno Van Wonterghem (925) 423-9494 (vanwonterghem1 [at] llnl.gov (vanwonterghem1[at]llnl[dot]gov)).
