ON THE PATH TO IGNITION

10 S&TR March 2013 National Ignition Campaign

National Ignition Facility experiments produce new states

of matter as scientists close in on creating the conditions

required to ignite fusion fuel.

MONG the most challenging scientific closer to achieving ignition and fulfilling A quests over the past 50 years has the vision of early fusion pioneers such as been the international effort to create, in former Laboratory Director John Nuckolls. a laboratory setting, a miniature “star on Shortly after the ’s invention in 1960, Earth.” The goal of this endeavor is to Nuckolls conceived of using the x rays surpass the extreme mix of temperature, generated by a powerful laser pulse to fuse density, and pressure at the center of the hydrogen isotopes, convert matter into Sun and generate conditions in which energy (as in Einstein’s famous equation, hydrogen fusion reactions can start and E = mc2), and thereby liberate more energy sustain themselves, thus creating a fusion than is delivered by the laser pulse. fire in the laboratory. The ongoing fusion “NIF was designed to be the world’s reaction in the Sun’s center provides largest laser,” says NIF chief scientist John all the energy needed for life on Earth. Lindl. “In fact, it now operates as such. Replicating this sustained reaction in a We have known from the outset that the laboratory requires conditions that are even energy it delivers does not give us a large more extreme: temperatures in the tens of margin of performance to achieve ignition. millions of degrees, pressures hundreds Everything that occurs during the implosion of billions of times Earth’s atmosphere, of hydrogen fuel must be nearly perfect.” and a density of burning matter that is Lindl notes that significant progress has more than 100 times the density of lead. been made since precision experiments In their quest to bring “star power” to began in May 2011, as part of the National Earth, researchers at the National Ignition Ignition Campaign (NIC). (See the box Facility (NIF) have worked over the past on p. 12.) He attributes that success to 18 months to produce states of matter the heroic efforts of the entire NIF staff. never before achieved in a laboratory “In all areas of the organization—, setting. Using all 192 beams of the giant diagnostics, cryogenics, and operations— laser, experimenters are generating people have worked incredibly hard and temperatures and densities inside an with incredible skill. As a result, we imploding, peppercorn-size capsule of have made huge advances in technology, frozen hydrogen isotopes ( and materials, and scientific understanding. , or D–T) that when compressed to We have come a long way and routinely the diameter of a human hair will sustain achieve environments that are extreme by fusion burn. any measure.” Edward Moses, principal associate director for NIF and Photon Science and Indirect Drive Heats the Fuel leader of the fusion program effort, says, To achieve the extreme conditions “The ultimate goal of these experiments required for fusion, NIF’s 192 laser beams is to ignite a self-sustaining burn wave of are focused into laser entrance holes at fusion fuel, producing more energy than each end of a 1-centimeter-long cylindrical is delivered to the target—an event called target, called a hohlraum. The laser ignition.” NIF researchers are moving ever beams irradiate the interior surface of the

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hohlraum, raising the interior temperature volume of the capsule collapses by almost a conditions are hotter and nearly as dense as to more than 3 million degrees Celsius. factor of 100,000, heating and compressing those in the center of the Sun. At these temperatures, the hohlraum the frozen D–T ice layer, the fuel for The hot spot is surrounded by a works much like an oven, radiating x rays ignition, lying just inside the outer plastic region containing about 90 percent of into its own interior. The x rays in turn layer. In experiments to date, the resulting the fusion fuel, which has a density of irradiate the plastic shell of the fuel capsule hot center, called the hot spot, reaches 600 to 800 grams per cubic centimeter at mounted in the hohlraum’s center, ablating temperatures of nearly 40 million degrees a temperature of 1 to 2 million degrees the surface and causing the capsule to Celsius with densities of 50 to 100 grams Celsius. The fuel’s density is about implode through a rocketlike reaction. per cubic centimeter (5 to 10 times the 3,000 times the initial density of the X-ray and emission images from density of lead) and pressures 150 to frozen D–T layer—well in excess of that ignition experiments show that the interior 200 billion times Earth’s atmosphere. These in the Sun’s center and by far the highest

The National Ignition Campaign

Ignition experiments on the National Ignition Facility (NIF) began achieved in the laboratory, they remain a factor of two to three below as part of the National Nuclear Security Administration’s (NNSA’s) those needed for ignition. National Ignition Campaign (NIC). This campaign, which began in Over the course of NIC, a large body of scientific knowledge 2006 and ended September 30, 2012, had two principal goals: transition and major new experimental, diagnostic, and target manufacturing NIF to routine operations as the world’s preeminent high-energy-density capabilities were developed and validated. NIF scientists and engineers user facility for the nuclear weapons programs, fundamental science, steadily increased the laser’s energy and power, culminating on July 5, nonproliferation, and other national security purposes and develop the 2012, when the system’s 192 beams delivered more than 1.8 megajoules capability to study and achieve ignition on NIF. The campaign involved of ultraviolet light (in excess of 50 times more energy than any laser the participation of Sandia and Los Alamos national laboratories, has demonstrated) and more than 500 trillion watts of power to the General Atomics, and the Laboratory center of the target chamber. (See S&TR, for Laser Energetics at the University of September 2012, p. 2.) The combination 2012 Rochester. Other key contributors included 500 of energy and power, along with the the Massachusetts Institute of Technology, 2011 precision of laser beam pointing and Lawrence Berkeley National Laboratory, power balance of all the beams, met Atomic Weapons Establishment in 400 the NIC specifications. International England, and the French Commissariat à 2010 review committees have agreed that l’Énergie Atomique. the laser has proven exceptionally 300 Following completion of NIF reliable, durable, precise, and flexible in construction in March 2009, researchers 2009 accommodating the unusually diverse focused on installing, qualifying, and 200 needs of various research groups. integrating the facility’s many systems An NNSA report to Congress in as well as developing the experimental December 2012 stated that “The NIF platforms necessary to study and 100 laser performed reliably and with great Power at target, trillion watts control, with adequate precision, the precision and executed thirty-seven key processes necessary for achieving Nova, OMEGA lasers cryogenic implosion experiments. Power ignition. (See S&TR, September 2012, 0 and energy have exceeded initial design 0 0.5 1.0 1.5 2.0 pp. 14–21.) Precision implosion- specifications. Target quality is superb, optimization experiments using these Energy at target, megajoules and diagnostics have been developed platforms steadily increased the pressure that are returning experimental data of in the frozen hydrogen fuel from about Scientists have steadily raised the power and unprecedented quality.” 1,000 terapascals (or nearly 10 billion energy levels at the National Ignition Facility The report added, “The pursuit of times atmospheric pressure) in the (NIF) since it was completed in September ignition and high fusion yields in the early implosions to currently about laboratory is a major objective of the 2009. On July 5, 2012, the laser system’s 15,000 terapascals, achieving densities in SSP [Stockpile Stewardship Program] 192 beams delivered more than 1.8 megajoules excess of 600 grams per cubic centimeter and ICF [Inertial Confinement Fusion] or about 60 times the density of lead. of ultraviolet (3-omega) light and more than Program and is a grand challenge Although the pressures achieved are 500 trillion watts of power to a target. This scientific problem that tests our codes, approaching those in the center of the unprecedented combination of energy and our people, our facilities, and our Sun and are by far the highest ever power met NIF’s original design specifications. integrated capabilities.”

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3 density ever achieved in laboratory NIF researchers are continuing to work experiments. on elevating the hot-spot temperature and The goal of current experiments density through increased alpha heating to is to increase the hot-spot density by improve deuterium–tritium (D–T) yield. The another factor of three at about the same 2 yield attained in current experiments is temperature already being achieved. Under about 50 times greater than that achieved these conditions, the fusion reaction rate on the first cryo-layered experiment in would be sufficient to generate ignition, 2010 and is within a factor of 5 to 10 of the yield needed to initiate ignition. or burn conditions. The precursor step 1 to ignition is attaining measurable alpha D–T yield, kilojoules heating, in which the helium nuclei (alpha particles) produced by the fusing of deuterium and tritium nuclei deposit enough kinetic energy to increase the fuel’s 0 0 1 2 3 temperature well above that produced Hot-spot energy, kilojoules by the laser-generated implosion alone. “Alpha-particle heating is required for must have a uniformity of better than Angle of incident sustained fusion burn and the release of 1 percent. Laser light thus needs to be laser beams Laser beams 23.5° more energy than was necessary to initiate absorbed by the gold hohlraum in a precise 30° 44.5° the reaction,” explains Lindl. and prescribed manner. Achieving the 50° Alpha heating can only happen when required precision is challenging because the hot-spot fuel is dense enough and of instabilities that inevitably occur when has a sufficient radius to capture the intense laser light interacts with the hot alpha particles and absorb their energy. plasma, filling the hohlraum. For the first time in any laboratory, NIF This complex, interactive laser experiments are routinely producing propagation phenomenon is particularly fuel conditions sufficient to stop fusion- difficult to model because the laser Capsule fill tube produced alpha particles, a critical itself produces the hot plasma in requirement for self-heating of fusion fuel. the hohlraum. Codes developed to In the better-performing implosions to date, simulate laser–plasma interactions are more thermonuclear energy is produced at the edge of the Laboratory’s current 10 millimeters from the hot spot than is delivered by calculational capabilities. They have a compression heating from the laser energy voracious appetite for the computing alone. About one-fifth of this hot-spot capacity available on Livermore’s high- energy is in the form of alpha particles. performance machines. Calculations indicate that the alpha-particle Models describing hohlraum physics Hohlraum yield must be about a factor of 10 higher must accurately simulate a host of intricate to initiate ignition and a self-sustaining processes. For example, laser light passes burn wave that starts in the hot spot and through the laser entrance holes and propagates into the surrounding main fuel, interacts with the ions and electrons that a process Lindl compares to lighting a fire make up the plasma. Plasma energy heated NIF laser light enters the two laser with a match. by the laser is conducted away from the entrance holes to form an inner absorption regions. Absorbed light is cone that illuminates the hohlraum Perfecting Symmetry subsequently converted into x rays that wall near the equator of the capsule. Implosion velocities greater than flow within the hohlraum and are absorbed An outer cone is also formed that 300 kilometers per second (or nearly into the fuel capsule’s ablator layer. Codes illuminates an area of the wall 700,000 miles per hour) and a uniformly must also model the physics during capsule closer to each laser entrance hole. spherical implosion are essential for implosion and the thermonuclear burn of the The outer cone is composed of producing the fuel conditions needed for D–T fuel. two subcones, one at 50 degrees ignition. The hohlraum must be heated When NIC began, models of hohlraum and one at 44.5 degrees. The two to 3 million degrees Celsius, and the flux physics had been developed and tested inner subcones enter at 30 and of x rays streaming onto the fuel capsule using the OMEGA laser at the University 23.5 degrees.

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of Rochester and, before that, Livermore’s inner and outer cone is composed of two 10-beam Nova laser. Those experiments subcones. Beams forming the outer cone were performed on much smaller targets, enter the top and bottom laser entrance using 50 to 100 times less laser energy than holes at angles of 50 and 44.5 degrees with NIF routinely produces. Some researchers respect to the hohlraum’s vertical axis. The were concerned that these experiments and, inner beam energy is split equally between consequently, the models would not be 30 and 23.5 degrees. This illumination applicable when scaled to NIF. To address pattern ensures that the x-ray drive on this concern, the NIF team tested and the target remains uniform in space and improved the models as NIC experiments controllable in time. gathered data at higher energy and on Experiments show hohlraums absorbing larger targets. about 85 percent of the laser energy. Losses During an ignition experiment, laser are caused by instabilities that occur when light enters each laser entrance hole in laser light interacts with plasma ions plasma electrons to energies high enough to the form of two cones. An inner cone and electrons emanating from the gold preheat the capsule and make compression containing one-third of the energy travels hohlraum, with the outer plastic layer of more difficult. The main type of laser– at a low angle into the hohlraum and the fuel capsule, and with helium ions plasma interaction is stimulated Raman illuminates the wall near the equator. inside the hohlraum. Most interactions scattering on the inner beams, which An outer cone containing two-thirds of are unwelcome because they scatter laser produces energetic, superhot electrons. the energy enters at a high angle and light, thereby interfering with the uniform illuminates an area of the hohlraum wall x-ray field needed to evenly compress the Taming Laser–Plasma Instabilities closer to each laser entrance hole. Each capsule. Interactions can also accelerate The first NIF hohlraum energetics shots fielded from 2009 to 2010 yielded critical data for efforts to control laser–plasma (top) Hohlraums, whether 1.0 instabilities. Some of the earliest results manufactured from gold or were not consistent with predictions from depleted uranium, absorb 0.9 the standard model experimenters had been about 85 percent of the using with the less-energetic lasers. The laser energy, a percentage NIF shots experienced more stimulated 0.8 that is nearly independent Raman scattering and much cooler plasma of laser energy. The temperatures than predicted as well shaded band shows the 0.7 as a tendency toward pancake-shaped best fit to the data with Absorbed fraction implosions (instead of the desired round ±1 standard deviation. 0.6 shape). In addition, the x-ray drive on the Gold (bottom) In contrast, fuel capsule differed from predictions. Depleted uranium hohlraum radiation The standard model was originally 0.5 temperatures rise with 1.0 1.2 1.4 1.6 1.8 2.0 developed in the 1970s—an era when increased laser energy, Incident laser energy, megajoules computer resources were much less as expected, and higher capable than the current generation of 340 temperatures are achieved Gold supercomputers at Livermore. “Laser– with depleted uranium Depleted uranium plasma interactions such as stimulated compared with gold. Raman scattering are difficult to 320 model, and to predict them accurately requires a correct notion of the plasma conditions,” explains physicist Mordy Rosen. “Although we knew the standard 300 model had shortcomings, its results were considered conservative.” In 2010, Rosen and colleagues

Radiation temperature, electronvolts developed the high flux model (HFM), 280 which contains two important advances. 1.0 1.2 1.4 1.6 1.8 Absorbed laser energy, megajoules It more realistically accounts for the

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60 micrometers

complex atomic physics occurring in the lasers traveling in different directions A series of micrographs shows the dramatic hot, partially ionized gold atoms from overlap. This phenomenon permits improvements (from left to right) made in the hohlraum wall. It also incorporates redirecting energy between the outer and implosion symmetry on the first four ignition a modified model for the flow of heat inner cone beams where they overlap at experiments conducted September 2–5, 2009. carried by the plasma electrons. The new each laser entrance hole. Controlling the The pancake-shaped implosions were inconsistent model accurately predicted a much cooler shift in the energy balance of the two with predictions from the guiding standard model. hohlraum plasma (25 million degrees cones requires adjusting the color, or Scientists used crossbeam energy transfer to Celsius compared with the 40 million wavelength, of beams forming the outer make small adjustments in wavelengths and predicted by the standard model). This cone by 0.6 to 0.9 nanometers to provide redirect energy from outer to inner cone beams. cooler plasma helped plasma physicists 25 percent more power to the inner cone. The technique, together with other adjustments, Denise Hinkel and Ed Williams explain “We had always assumed we would have results in much more spherical implosions. the spectrum and level of the Raman light to minimize crossbeam transfer,” says found in the data. HFM also predicted physicist Debbie Callahan. “Then we a higher flux of x-ray emission, as realized it could be a tool to improve from the poles of the capsule in the was indeed observed flowing from the implosion symmetry.” hohlraum (when viewed up or down hohlraums through the laser entrance holes. Says Lindl, “Until recently, there was the hohlraum’s axis). In addition, HFM accurately predicted never a time when plasma phenomena The final arrangement involves the unexpected pancaking of the fuel could help us; it usually made life more first transferring energy from the 44.5- capsule that occurs because the outer beams difficult. We could try turning up the and 50-degree beams into the 23.5- and convert laser light to x rays more efficiently energy on the inner beams, but that 30-degree beams. Energy is then than the inner beams. These x rays shone approach would drive the laser harder. transferred from the 30-degree beam to more brightly on the poles of the capsule, NIF works more efficiently when all the 23.5-degree beam deeper into the driving it toward a pancake shape instead of beams have the same power. Instead, we hohlraum plasma, where only these beams a uniformly round sphere. The cooler and shifted energy from the outer to the inner continue to overlap. Together, the energy- denser plasma absorbed more light from the cones. NIF was built with the capability transfer measures produce a much brighter inner beams, preventing those beams from to adjust the wavelength of internal and beam that propagates more deeply into penetrating into the hohlraum’s midplane. external beams to minimize crossbeam the hohlraum’s center. For this pioneering As a result, the inner beams could not transfer. Adjusting the wavelength work, a team of NIF researchers earned the provide enough drive on the central region separation to control symmetry wasn’t American Physical Society’s 2012 John of the hohlraum to counter the outer beams’ part of the original NIF plan, but it Dawson Award for Excellence in Plasma drive on the pole regions. A pancake-shaped works beautifully.” With these insights, Physics Research. implosion thus ensued. researchers are working to further reduce Adjusting the power levels among The NIC team took the unusual step laser–plasma interactions. the subcones controlled only one aspect of using laser–plasma interactions to Successful experiments in 2009 of implosion symmetry. Another aspect overcome the uneven distribution of prompted scientists to add another was achieved by more precisely pointing laser light on the hohlraum walls. The refinement. They adjusted the relative beams at spots on the hohlraum walls. researchers used a technique called wavelengths on the 23.5- and 30-degree Additionally, the team made small design crossbeam energy transfer that can occur inner beams by 0.1 nanometers to more changes to the hohlraum geometry. A in plasma when two or more high-power precisely control the asymmetry as seen somewhat shorter and wider hohlraum

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allows the inner beams better access to the hohlraum wall is not yet optimal for heating, we must gradually increase the the waist region. Together, the more- ensuring a symmetric implosion. Upcoming pressure. The NIF ignition pulse achieves precise beam pointing, crossbeam experiments will focus on improving the the desired pressure of 10 terapascals in a energy transfer, and modified hohlraum D–T layer’s shape. sequence of four weaker shocks.” dimensions are key steps for meeting The goal of ignition shock-timing the stringent uniformity requirements Pulse Shape of Four Precise Shocks experiments has been to precisely set the of 99-percent capsule radiation flux A nearly perfect x-ray drive on the outer power levels for each of the four shocks, to achieve ignition. Because HFM plastic shell is only one essential element which together create an overall pulse predictions have been consistent with of ensuring ignition. In addition, the laser lasting about 20 nanoseconds. The pulse various experimental observations, pulse must be carefully shaped to send a consists of an initial small shock (called the model allows researchers to better precisely timed series of shocks through the a picket), followed by a relatively low- understand hohlraum performance and the frozen D–T layer. The timing is such that energy but important 9-nanosecond-long role of laser–plasma interactions. the shocks overtake each other soon after trough to maintain a constant first-shock Lindl cautions that optimizing implosion they travel into the D–T gas comprising the velocity in the capsule. Second, third, symmetry is not yet complete. For example, very center of the fuel capsule. and fourth shocks at increasing power are the D–T fuel layer surrounding the hot “If we merely hit the capsule with followed by a 3-nanosecond peak power spot can have a different shape than the the power required to generate the pulse, which provides most of the drive hot spot. The NIF team is currently 10-terapascal pressure needed to achieve for final acceleration of the shell. The developing techniques such as Compton the desired implosion velocity, the shock symmetry of the imploded core (hot spot) radiography (an x-ray shadowing technique) from that pressure would generate too is affected by the symmetry of each shock, to provide data on the D–T layer’s shape much internal heat and decompress the but it is most sensitive to the initial picket at peak compression. Early results show frozen D–T layer,” says Lindl. “To obtain and the x-ray drive produced by the final that the position of the outer beams on high velocity without significant shock peak power pulse. Physicist Harry Robey notes that each shock wave propagates through the capsule 400 1,000 ablator and compresses the fuel layer. The first three shocks are adjusted to merge Laser power just after passing through the D–T layer. “We want these three shocks to merge at one time in a single radial location inside 300 the capsule,” says Robey. If they are not 100 spaced correctly (and at ±50 picoseconds, or 50-trillionths of a second), the D–T fuel layer will not reach the required 200 density at the end of the implosion. A Pressure correctly timed fourth shock, designed to overtake the first three shocks after they Power, trillion watts Power, coalesce, is critical for keeping the fuel at Temperature 10 Temperature, electronvolts Temperature,

Pressure, million atmospheres maximum compression. 100 Experiments to measure the strength (velocity) and timing of these shocks are conducted in a keyhole target platform. Keyholes have a cone inserted through the side of the hohlraum wall and into a capsule 0 1 0 5 10 15 20 filled with cooled liquid deuterium—an Time, nanoseconds excellent substitute to the D–T ice layer. Achieving ignition requires a 20-nanosecond-long laser pulse that sends a carefully timed These experiments use the velocity series of shocks to efficiently compress the fuel capsule. As each shock passes through the interferometer system for any reflector capsule, the fuel is further compressed. The 3-nanosecond peak power pulse that follows this (VISAR), where the shock in the plastic sequence provides most of the drive for final acceleration of the capsule. shell or the surrogate fuel reflects the

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A NIF technician holds a keyhole target attached to equipment that is part of the cryogenic cooling system. Keyhole experiments measure the strength (velocity) and timing of the four shock waves from the laser pulse as they transit the capsule. For these experiments, a cone is inserted through the side of the hohlraum wall and into a capsule filled with cooled liquid deuterium. The inset shows the keyhole cone and capsule without a hohlraum around them.

2 millimeters

VISAR laser. The instrument then precisely fuel layer surrounding the hot spot is not round-the-clock operations are producing records the timing of all four shocks. as spherical as required. “We believe data of unprecedented quality.” that understanding and controlling three- “Discovery science is the exploration of Ignition Ahead dimensional departures from spherical the unknown,” continues Lindl. “Ignition Lindl notes that scientists have implosions is a critical milestone on the remains a grand scientific challenge that already achieved the required x-ray drive path to ignition,” says physicist John requires unprecedented precision from the temperature, hot-spot shape, and shock Edwards, associate program leader for laser, diagnostics, targets, and experiments. timing, and they have nearly attained the Inertial Confinement Fusion in the NIF We can’t say exactly when we will achieve needed implosion velocity. However, and Photon Science Principal Directorate. ignition, but we have made tremendous challenges remain. Densities in the D–T Radiography of the D–T fuel at peak progress and have developed an exciting layer must reach 1,000 grams per cubic compression will better record the hot-spot experimental plan to take us the rest of centimeter, somewhat higher than the shape and distinguish among different the way.” 600 to 800 grams per cubic centimeter mechanisms that can affect the degree and —Arnie Heller achieved in current experiments. uniformity of compression. When NIF’s Likewise, hot-spot pressures are a Advanced Radiographic Capability goes factor of two to three lower than those online, it will generate a much brighter Key Words: Advanced Radiographic required for ignition. Looked at another source of x rays for Compton radiography Capability, alpha heating, crossbeam transfer, high flux model (HFM), hohlraum, ignition, way, because the measured hot-spot than can be obtained with the standard National Ignition Campaign (NIC), National temperatures are close to those needed to NIF beams. (See S&TR, December 2011, Ignition Facility (NIF), stimulated Raman reach ignition, the hot-spot density is a pp. 12–15.) scattering, velocity interferometer system for factor of two to three lower than required. “NIF continues to make outstanding any reflector (VISAR), wavelength tuning. Recent experimental evidence also progress toward the goal of ignition,” says indicates that the achieved pressure is Lindl. “The laser, diagnostic systems, For further information contact insufficient because the imploding D–T target design and fabrication, and (925) 422-5430 ([email protected]).

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