National Ignition Facility
The National Ignition Facility, or NIF, is a large, laser-based inertial confinement fusion (ICF) research device located at the Lawrence Livermore National Laboratory in Livermore, California, USA. NIF uses powerful lasers to heat and compress a small amount of hydrogen fuel to the point where nuclear fusion reactions take place. NIF's mission is to achieve fusion ignition with high energy gain, and to support nuclear weapon maintenance and design by studying the behavior of matter under the conditions found within nuclear weapons.1 NIF is the largest and most energetic ICF device built to date.
Construction began in 1997 but management problems and technical delays slowed progress into the early 2000s, when a new management team took over. Progress after 2000 was much smoother, but compared to initial estimates, NIF was completed five years behind schedule and was almost four times more expensive than originally budgeted. Construction was certified complete on 31 March 2009 by the U.S. Department of Energy,2 and a dedication ceremony took place on 29 May 2009.3 The first large-scale laser target experiments were performed in June 20094 and the first "integrated ignition experiments" (which tested the laser's power) were declared completed in October 2010.5
Inertial confinement fusion (ICF) devices use "drivers" to rapidly heat the outer layers of a "target" in order to compress it. The target is a small spherical pellet containing a few milligrams of fusion fuel, typically a mix of deuterium and tritium. The energy of the laser heats the surface of the pellet into a plasma, which explodes off the surface. The remaining portion of the target is driven inward, eventually compressing it into a small point of extremely high density. The rapid blowoff also creates a shock wave that travels toward the center of the compressed fuel from all sides. When it reaches the center of the fuel, a small volume is further heated and compressed to a great degree. When the temperature and density of that small spot are raised high enough, fusion reactions will occur and release energy.7
The fusion reactions release high-energy particles, some of which, primarily alpha particles, collide with the surrounding high density fuel and heat it further. If this process deposits enough energy in a given area it can cause that fuel to undergo fusion as well. Given the right overall conditions of the compressed fuel—high enough density and temperature—this heating process will result in a chain reaction, burning outward from the center where the shock wave started the reaction. This is a condition known as "ignition", which will lead to a significant portion of the fuel in the target undergoing fusion and releasing large amounts of energy.8
To date most ICF experiments have used lasers to heat the target. Calculations show that the energy must be delivered quickly in order to compress the core before it disassembles. The laser energy also must be focused extremely evenly across the target's outer surface in order to collapse the fuel into a symmetric core. Although other "drivers" have been suggested, notably heavy ions driven in particle accelerators, lasers are currently the only devices with the right combination of features.910
NIF aims to create a single 500 terawatt (TW) peak flash of light that reaches the target from numerous directions at the same time, within a few picoseconds. The design uses 192 beamlines in a parallel system of flashlamp-pumped, neodymium-doped phosphate glass lasers.11
To ensure that the output of the beamlines is uniform, the initial laser light is amplified from a single source in the Injection Laser System (ILS). This starts with a low-power flash of 1053 nanometers (nm) infra-red light generated in an ytterbium-doped optical fiber laser known as the Master Oscillator.12 The light from the Master Oscillator is split and directed into 48 Preamplifier Modules (PAMs). Each PAM contains a two-stage amplification process. The first stage is a regenerative amplifier in which the pulse circulates 30 to 60 times, increasing in energy from nanojoules to tens of millijoules. The light then passes four times through a circuit containing a neodymium glass amplifier similar to (but much smaller than) the ones used in the main beamlines, boosting the nanojoules of light created in the Master Oscillator to about 6 joules. According to LLNL, the design of the PAMs was one of the major challenges during construction. Improvements to the design since then have allowed them to surpass their initial design goals.13
The main amplification takes place in a series of glass amplifiers located at one end of the beamlines. Before "firing", the amplifiers are first optically pumped by a total of 7,680 xenon flash lamps (the PAMs have their own smaller flash lamps as well). The lamps are powered by a capacitor bank which stores a total of 422 megajoules (MJ) of electrical energy. When the wavefront passes through them, the amplifiers release some of the light energy stored in them into the beam. To improve the energy transfer the beams are sent though the main amplifier section four times, using an optical switch located in a mirrored cavity. In total these amplifiers boost the original 6 J provided by the PAMs to a nominal 4 MJ.7 Given the time scale of a few billionths of a second, the peak UV power delivered to the target is correspondingly very high, 500 TW.
After the amplification is complete the light is "switched" back into the beamline, where it runs to the far end of the building to the target chamber. The target chamber weighs 287,000 pounds (130,000 kg), with a diameter of 10 meters.14 The total length of the path the laser beam propagates from one end to the other is about 5,000 feet (1500 meters). A considerable amount of this length is taken up by "spatial filters", small telescopes that focus the laser beam down to a tiny point, with a mask cutting off any stray light outside the focal point. The filters ensure that the image of the beam when it reaches the target is extremely uniform, removing any light that was mis-focused by imperfections in the optics upstream. Spatial filters were a major step forward in ICF work when they were introduced in the Cyclops laser, an earlier LLNL experiment. The various optical elements in the beamlines are generally packaged into Line Replaceable Units (LRUs), standardized boxes about the size of a vending machine that can be dropped out of the beamline for replacement from below.15
Just before reaching the Target Chamber the light is reflected off various mirrors in the switchyard and target area in order to impinge on the target from different directions. Since the length of the overall path from the Master Oscillator to the target is different for each of the beamlines, optics are used to delay the light in order to ensure all of them reach the center within a few picoseconds of each other.16 As can be seen in the layout diagram above, NIF normally directs the laser into the chamber from the top and bottom. The target area and switchyard system can be reconfigured by moving half of the 48 beamlines to alternate positions closer to the equator of the target chamber.
One of the last steps in the process before reaching the target chamber is to convert the infrared (IR) light at 1053 nm into the ultraviolet (UV) at 351 nm in a device known as a frequency converter.17 These are made of thin sheets (about 1 cm thick) cut from a single crystal of potassium dihydrogen phosphate. When the 1053 nm (IR) light passes through the first of two of these sheets, frequency addition converts a large fraction of the light into 527 nm light (green). On passing through the second sheet, frequency combination converts much of the 527 nm light and the remaining 1053 nm light into 351 nm (UV) light. IR light is much less effective than UV at heating the targets, because IR couples more strongly with hot electrons which will absorb a considerable amount of energy and interfere with compressing the target. The conversion process can reach peak efficiencies of about 80 percent for a laser pulse that has a flat temporal shape, but the temporal shape needed for ignition varies significantly over the duration of the pulse. The actual conversion process is about 50 percent efficient, reducing delivered energy to a nominal 1.8 MJ.18
One important aspect of any ICF research project is ensuring that experiments can actually be carried out on a timely basis. Previous devices generally had to cool down for many hours to allow the flashlamps and laser glass to regain their shapes after firing (due to thermal expansion), limiting use to one or fewer firings a day. One of the goals for NIF is to reduce this time to less than four hours, in order to allow 700 firings a year.19
The name "National Ignition Facility" refers to the goal of "igniting" the fusion fuel, and releasing more fusion energy than the UV laser delivers to the target, a long-sought threshold in fusion research. In existing (non-weapon) fusion experiments the heat produced by the fusion reactions rapidly escapes from the plasma, meaning that external heating must be applied continually in order to keep the reactions going. "Ignition" refers to the point at which the energy given off in the fusion reactions currently underway is high enough to sustain the temperature of the fuel against all losses of energy, so that fusion reactions can continue. This causes a chain-reaction that allows the majority of the fuel to undergo a nuclear "burn". Ignition is considered a key requirement if fusion power is to ever become practical.8
NIF is designed primarily to use the indirect drive method of operation, in which the laser heats a small metal cylinder instead of the capsule inside it. The heat causes the cylinder, known as a hohlraum (German for "hollow room", or cavity), to re-emit the energy as intense X-rays, which are more evenly distributed and symmetrical than the original laser beams. Experimental systems, including the OMEGA and Nova lasers, validated this approach through the late 1980s.20 In the case of the NIF, the large delivered power allows for the use of a much larger target; the baseline pellet design is about 2 mm in diameter, chilled to about 18 Kelvin (-255 degrees Celsius) and lined with a layer of solid deuterium-tritium (DT) fuel. The hollow interior also contains a small amount of DT gas.
This conversion process is fairly efficient. In a typical experiment, the laser will generate 3 megajoules of infrared laser energy. About 1.5 megajoules of this is left after conversion to UV, and about 15 percent of this is lost in the x-ray conversion in the hohlraum. About 15 percent of the resulting x-rays (or about 150 kilojoules) will be absorbed by the outer layers of the target.21 The resulting inward directed compression is expected to compress the fuel in the center of the target to a density of about 1,000 g/cm³ (or 1,000,000 kg/m³);22 for comparison, lead has a normal density of about 11 g/cm³ (11,340 kg/m³). It is expected this will cause about 20 MJ of fusion energy to be released, resulting in a net fusion energy gain of about 15 (G=Fusion energy/UV laser energy).21 Improvements in both the laser system and hohlraum design are expected to improve the energy absorbed by the capsule to about 420 kJ, which, in turn, could generate up to 100-150 MJ of fusion energy.22 However, the baseline design allows for a maximum of about 45 MJ of fusion energy release, due to the design of the target chamber.23 This is the equivalent of about 11 kg of TNT exploding.
These output energies are still less than the 422 MJ of input energy required to charge the system's capacitors that power the laser amplifiers. The net "wall-plug" efficiency of NIF (UV laser energy out divided by the energy required to pump the lasers from an external source) is less than one percent, and the total wall-to-fusion efficiency is under 10% at its maximum performance. An economical fusion reactor would require that the fusion output be at least an order of magnitude more than this input. Commercial laser fusion systems would use the much more efficient diode-pumped solid state lasers (DPSSL), where "wall-plug" efficiencies of 10 percent have been demonstrated, and efficiencies 16-18 percent are expected with advanced concepts under development.24
NIF is also exploring new types of targets. Previous experiments generally used plastic ablators, typically polystyrene (CH). NIF's targets also are constructed by coating a plastic form with a layer of sputtered beryllium or beryllium-copper alloys, and then oxidizing the plastic out of the center.2526 In comparison to traditional plastic targets, beryllium targets offer higher overall implosion efficiencies for the indirect-drive mode where the incoming energy is in the form of x-rays.
Although NIF was primarily designed as an indirect drive device, the energy in the laser is high enough to be used as a direct drive system as well, where the laser shines directly on the target. Even at UV wavelengths the power delivered by NIF is estimated to be more than enough to cause ignition, resulting in fusion energy gains of about 40 times,27 somewhat higher than the indirect drive system. A more uniform beam layout suitable for direct drive experiments can be arranged through changes in the switchyard that move half of the beamlines to locations closer to the middle of the target chamber.
It has been shown, using scaled implosions on the OMEGA laser and computer simulations, that NIF should also be capable of igniting a capsule using the so-called polar direct drive (PDD) configuration where the target is irradiated directly by the laser, but only from the top and bottom, with no changes to the NIF beamline layout.28 In this configuration the target suffers either a "pancake" or "cigar" anisotropy on implosion, reducing the maximum temperature at the core.
Other targets, called saturn targets, are specifically designed to reduce the anisotropy and improve the implosion.29 They feature a small plastic ring around the "equator" of the target, which quickly vaporizes into a plasma when hit by the laser. Some of the laser light is refracted through this plasma back towards the equator of the target, evening out the heating. Ignition with gains of just over thirty-five times are thought to be possible using these targets at NIF,28 producing results almost as good as the fully symmetric direct drive approach.
LLNL's history with the ICF program starts with physicist John Nuckolls, who started considering the problem after a 1957 meeting on the peaceful use of nuclear weapons, arranged by Edward Teller at LLNL. During these meetings, the idea that would later be known as PACER would be developed - the explosion of 1 MT hydrogen bombs in large underground caverns to generate steam that would be converted into electrical power. After identifying several problems with this approach, Nuckolls became interested in understanding how small a bomb could be made that would still generate net positive power.30
There are two parts to a typical hydrogen bomb, a plutonium-based atomic bomb known as the primary, and a cylindrical arrangement of fusion fuels known as the secondary. Triggering the bomb causes the primary to explode. During the initial instants of this process it gives off a huge burst of x-rays. The x-rays are deliberately trapped between two layers of heavy metal, often depleted uranium or lead, lining the inner side of the bomb casing and the outer side of the secondary. The metals are heated to millions of degrees by the x-rays, causing them to vaporize with explosive force. When this occurs in the layer on the outside of secondary, a shock wave is created that travels into the secondary, squeezing a small amount of fission fuel in its center. This gives off neutrons that spread into the surrounding lithium deuteride (LiD) fusion fuel, causing it to turn into D and T, which then fuse and release more neutrons. This entire reaction takes place extremely rapidly, before the mechanical effects of the primary's explosion reach the bomb's casing and destroy it.
Nuckolls's idea was to explore how small the secondary could be made, and what effects this would have on the energy needed from the primary. The simplest change is to replace the LiD fuel with D-T, thereby eliminating the need for the external neutron source. At that point there is no theoretical smallest size to which the secondary can be scaled down. As the secondary got smaller, so did the amount of energy needed to cause the implosion to reach the required conditions. At the milligram level, the energy levels started to approach those available though several known devices.30
By the early 1960s, Nuckolls and several other weapons designers had developed the outlines of the ICF approach. The D-T fuel would be placed in a small capsule, designed to rapidly ablate when heated and thereby maximize compression and shock wave formation. This capsule would be placed within an engineered shell, the hohlraum, which acted similar to the metal layer on the outside of the secondary. However, the hohlraum did not have to be heated by x-rays; any source of energy could be used as long as it delivered enough energy to cause the hohlraum itself to heat up and start giving off x-rays. Ideally the energy source would be located some distance away from the hohlraums, to mechanically isolate both ends of the reaction. In theory a small atomic bomb could be used as the energy source, as it is in a hydrogen bomb, but ideally smaller energy sources would be used. Using computer simulations, the teams estimated that about 5 MJ of energy would be needed from the primary, generating a 100,000 kJ beam onto the target.30 To put this in perspective, a small fission primary of 0.5 kT releases 2 million MJ in total.31
By the early 1970s further study had made great advances in the theoretical understanding of the implosion process. In particular, by "shaping" the pulse to have more or less energy over time, the implosion process could be greatly improved.30 Using these concepts, drivers in the kJ range were expected to cause ignition, while "high gain" would require energies around 1 MJ.3233 Meanwhile, Ray Kidder developed the direct implosion concept and made several calculations related to this concept.30
None of this work was taken very seriously at the time, and only small experimental systems were developed through the 1960s. However, in the early 1970s Kidder formed KMS Fusion to directly commercialize his direct implosion concept using lasers. This sparked off intense rivalry between the existing weapons establishment, of which Kidder was formerly a member, and the various large weapons labs, who saw this as a threat to their field of research. This led to a rapid development program at all of these labs and others.30 LLNL decided early on to concentrate on glass lasers, while other facilities studied gas lasers using carbon dioxide (e.g. Antares laser, Los Alamos National Laboratory) or KrF (e.g. Nike laser, Naval Research Laboratory).
Throughout these early stages of development, much of the understanding of the fusion process was the result of computer simulations in a program known as LASNEX. LASNEX greatly simplified the reaction to a 2-dimensional simulation, which was all that was possible given the amount of computing power at the time. According to LASNEX, laser drivers in the kJ range would have the required properties, which was just within the state of the art. This led to the Shiva laser project which was completed in 1977. Contrary to predictions, Shiva was unable to achieve ignition and fell far off of the fusion outputs that were expected. This was traced to issues with the way the laser delivered heat to the target, which delivered most of its energy to electrons rather than the entire fuel mass. Further experiments and simulations demonstrated that this process could be dramatically improved by using shorter wavelengths of laser light.
Further upgrades to the simulation programs, accounting for these effects, predicted a new design that would reach ignition. This new system emerged as the 20-beam 200 kJ Nova laser concept. During the initial construction phase, Nuckolls found an error in his calculations, and an October 1979 review chaired by John Foster Jr. of TRW confirmed that there was no way Nova would reach ignition. The Nova design was then modified into a smaller 10-beam design that added frequency conversion to 351 nm light, which would increase coupling efficiency.34 In operation, Nova was able to deliver about 30 kJ of UV laser energy, about half of what was initially expected, primarily due to limits set by optical damage to the final focusing optics. Even at those levels, it was clear that the predictions for fusion production were still wrong, even at the limited powers available, fusion yields were far below predictions.
Throughout these efforts, the amount of energy needed to reach ignition had continually risen and it was unclear whether the current 200 kJ estimate was more reliable than earlier ones. The Department of Energy (DOE) decided that direct experimentation was the best way to settle the issue, and started in 1978 a series of underground experiments at the Nevada Test Site (now known as Nevada National Security Site), that used small nuclear bombs to illuminate spherical targets whose size was that intended for the MFL (Micro Fusion Laboratory) project. These experiments were very similar to some of Nuckoll's original concepts. Initial data were available by mid-1984, and the testing ceased in 1988.
These experiments are referred to in a fundamental document for the design of the NIF facility: the 91 page paper of John Lindl entitled “Development of the indirect-drive approach to inertial confinement and the target physics basis for ignition and gain”, published in 1995 in the AIP/Physics of plasmas.35
- A joint Los Alamos/ LLNL program using nuclear experiments, called Halite at LLNL and Centurion at Los Alamos (collectively called H/C), demonstrated excellent performance, putting to rest fundamental questions about the feasibility of achieving high gain. It performed inertial fusion experiments using nuclear explosives at the Nevada Test Site at higher energies than those available in the laboratory. (Lindl, 1995, p. 3939)
In his paper, Lindl mentions and introduces several unpublished LLNL reports:
- In 1979, when it became clear that ignition would not be achieved on Nova, we derived a strategy for obtaining the database that would be required for ignition in a future facility 36 This strategy tests the physics of high gain targets by using a series of Nova experiments on targets that are as close as possible to being “hydrodynamically equivalent targets” (HETs) and by using a series of underground experiments (Halite/Centurion) at much higher energies. (Lindl, 1995, p. 3948)
Although there is little publicly available data from the Halite/Centurion series, the results, augmented by experiments on the Nova laser, supported detailed simulations that ignition and net energy gains could be achieved with a few MJ.37 Based on numerous articles referring to the Halite/Centurion project, and citations from numerous weapons specialists, the X-ray power required for ignition of a microcapsule is around 10MJ.3839404142434445
Applied to indirect drive laser fusion, it would correspond to a 100MJ driver, which is currently beyond technological capabilities. While both the thermonuclear stage of a weapon device and NIF work through inertial confinement via an indirect drive, they are fairly different: In weapon devices, the radiative wave filling a hohlraum, is supplied by the primary X-ray emission burst. In laser-based devices X-ray is produced from the conversion of UV radiation by the hohlraum’s inner wall in a laser-matter interaction. The energy injection timing can be precisely controlled for better compression efficiency and to cope with the geometry difference between the cylindrical hohlraum and the spherical target. In NIF same-size targets have been designed with thinner capsules. On those two grounds, numerical simulations showed that ignition and net energy gains could be achieved with a few MJ.4647
Nova's partial success, combined with the Halite-Centurion numbers, prompted DOE to request a custom military ICF facility they called the "Laboratory Microfusion Facility" (LMF) that could achieve fusion yields of between 100 and 1,000 MJ. Based on modeling runs using the LASNEX computer program developed at LLNL,48 it was estimated that LMF would require a driver of about 10 MJ.34 Building such a device was within the state of the art, but would be expensive, approximately $1 billion.49 LLNL submitted a design with a 5 MJ 350 nm (UV) driver laser that would be able to reach about 200 MJ yield, which was enough to attain the majority of the LMF goals. The program was estimated to cost about $600 million FY 1989 dollars, and an additional $250 million to upgrade it to a full 1,000 MJ if needed, and would grow to well over $1 billion if LMF was to meet all of the goals requested by the DOE.49 Other labs also proposed their own LMF designs using other technologies.
In 1989/90 the National Academy of Sciences conducted a second review of the US ICF efforts on behalf of the US Congress. The report concluded that "considering the extrapolations required in target physics and driver performance, as well as the likely $1 billion cost, the committee believes that an LMF [i.e., a Laser Microfusion Facility with yields to one gigajoule] is too large a step to take directly from the present program". Their report suggested that the primary goal of the program in the short term should be resolving the various issues related to ignition, and that a full-scale LMF should not be attempted until these problems were resolved.50 The report was also critical of the gas laser experiments being carried out at LANL, and suggested they, and similar projects at other labs, be dropped. The report accepted the LASNEX numbers and continued to approve an approach with laser energy around 10 MJ. Nevertheless, the authors were aware of the potential for higher energy requirements, and noted "Indeed, if it did turn out that a 100 MJ driver were required for ignition and gain, one would have to rethink the entire approach to, and rationale for, ICF".50
In July 1990, LLNL responded to these suggestions with the Nova Upgrade, which would reuse the majority of the existing Nova facility, along with the adjacent Shiva facility. The resulting system would be much lower power than the LMF concept, with a driver of about 1 MJ.51 The new design included a number of features that advanced the state of the art in the driver section, including the multi-pass design in the main amplifiers, and 18 beamlines (up from 10) that were split into 288 "beamlets" as they entered the target area in order to improve the uniformity of illumination. The plans called for the installation of two main banks of laser beamlines, one in the existing Nova beamline room, and the other in the older Shiva building next door, extending through its laser bay and target area into an upgraded Nova target area. The lasers would deliver about 500 TW in a 4 ns pulse. The upgrades were expected to allow the new Nova to produce fusion yields of between 2 and 10 MJ.49 The initial estimates from 1992 estimated construction costs around $400 million, with construction taking place from 1995 to 1999.
Throughout this period, the ending of the Cold War led to dramatic changes in defense funding and priorities. As the need for nuclear weapons was greatly reduced and various arms limitation agreements led to a reduction in warhead count, the US was faced with the prospect of losing a generation of nuclear weapon designers able to maintain the existing stockpiles, or design new weapons.52 At the same time, progress was being made on what would become the Comprehensive Nuclear-Test-Ban Treaty, which would ban all criticality testing. This would make the reliable development of newer generations of nuclear weapons much more difficult.
Out of these changes came the Stockpile Stewardship and Management Program (SSMP), which, among other things, included funds for the development of methods to design and build nuclear weapons that would work without having to be explosively tested. In a series of meetings that started in 1995, an agreement formed between the labs to divide up the SSMP efforts. An important part of this would be confirmation of computer models using low-yield ICF experiments. The Nova Upgrade was too small to use for these experiments,53 and a redesign emerged as NIF in 1994. The estimated cost of the project remained just over $1 billion,54 with completion in 2002. Physicist Richard Garwin described the outcome this way, "Sandia got the microelectronics research center [MESA], which had minimal relevance to the CTBT. Los Alamos got the Dual-Axis Radiographic Hydrodynamic Test facility. Livermore got the National Ignition Facility. They all maintained that these projects were essential to stockpile stewardship, which they are not".55
In spite of the "agreement", the large project cost combined with the ending of similar projects at other labs resulted in several highly critical comments by scientists at other weapons labs, Sandia National Laboratories in particular. In May 1997, Sandia fusion scientist Rick Spielman publicly stated that NIF had "virtually no internal peer review on the technical issues" and that "Livermore essentially picked the panel to review themselves".56 Similar complaints about the makeup of the "oversight" committees, consisting largely of LLNL contractors, led to a lawsuit being filed by the Natural Resources Defense Council.57 As a result, in October 2002, a U.S. District Court ordered DOE to disclose committee documents and to include a statement in the committees' reports that the department had not complied with the Federal Advisory Committee Act (FACA). (The District Court order was partially set aside on appeal in January 2004 58 and had no effect on the progress of the NIF project. A retired Sandia manager, Bob Puerifoy, was even more blunt than Spielman: "NIF is worthless ... it can't be used to maintain the stockpile, period".59
A contrasting view was expressed by Victor Reis, assistant secretary for Defense Programs within DOE and the chief architect of the Stockpile Stewardship Program. Reis told the U.S. House Armed Services Committee in 1997 that NIF was “designed to produce, for the first time in a laboratory setting, conditions of temperature and density of matter close to those that occur in the detonation of nuclear weapons. The ability to study the behavior of matter and the transfer of energy and radiation under these conditions is key to understanding the basic physics of nuclear weapons and predicting their performance without underground nuclear testing.60 And DOE Secretary Federico Peña testified the following year, "Experiments at the NIF will test the simulation codes developed under the Accelerated Strategic Computing Initiative (now known as Advanced Simulation and Computing program) and demonstrate how aged or changed materials in weapons could behave under the unique conditions of nuclear weapons detonation. Two JASON panels, which are composed of scientific and technical national security experts, have stated that the NIF is the most scientifically valuable of all programs proposed for science-based stockpile stewardship."61 Despite the initial criticism, Sandia, as well as Los Alamos, provided support in the development of many NIF technologies,62 and both laboratories later became partners with NIF in the National Ignition Campaign.63
Work on the NIF started with a single beamline demonstrator, Beamlet. Beamlet operated between 1994 and 1997 and was entirely successful. It was then sent to Sandia National Laboratories as a light source in their Z machine. A full-sized demonstrator then followed, in AMPLAB, which started operations in 1997.64 The official groundbreaking on the main NIF site was in May 29, 1997.65
At the time, the DOE was estimating that the NIF would cost approximately $1.1 billion and another $1 billion for related research, and would be complete as early as 2002.66 Later in 1997 the DOE approved an additional $100 million in funding and pushed the operational date back to 2004. As late as 1998 LNLL's public documents stated the overall price was $1.2 billion, with the first eight lasers coming online in 2001 and full completion in 2003.67
The physical scale of the facility alone made the construction project challenging. By the time the “conventional facility” (the shell for the laser) was complete in 2001, more than 210,000 cubic yards of soil had been excavated, more than 73,000 cubic yards of concrete had been poured, 7,600 tons of reinforcing steel rebar had been placed, and more than 5,000 tons of structural steel had been erected. In addition to its sheer size, building NIF presented a number of unique challenges. To isolate the laser system from vibration, the foundation of each laser bay was made independent of the rest of the structure. Three-foot-thick, 420-foot-long and 80-foot-wide slabs, each containing 3,800 cubic yards of concrete, required continuous concrete pours to achieve their specifications. There were also unexpected challenges to cope with: In November, 1997, an El Niño weather front dumped two inches of rain in two hours, flooding the NIF site just three days before the scheduled concrete foundation pour. More than 200,000 gallons of water filled the 800x400-foot excavation. The earth was so soaked that the framing for the retaining wall sank six inches, forcing the crew to disassemble and reassemble it in order to pour the concrete.68 A month later, another surprise was in store. Construction was halted in December, 1997, when 16,000-year-old mammoth bones were discovered on the construction site. Paleontologists were called in to carefully remove and preserve the bones, and construction restarted within four days.69
A variety of research and development, technology and engineering challenges also had to be overcome, such as working with the optics industry to create a precision large optics fabrication capability to supply the laser glass for NIF’s 7,500 meter-sized optics. State-of-the-art optics measurement, coating and finishing techniques were needed to withstand NIF’s high-energy lasers, as were methods for amplifying the laser beams to the needed energy levels.70 Continuous-pour glass, rapid-growth crystals, innovative optical switches, and deformable mirrors were among the technology innovations developed for NIF.71
Sandia, with extensive experience in pulsed power delivery, designed the capacitor banks used to feed the flashlamps, completing the first unit in October 1998. To everyone's surprise, the Pulsed Power Conditioning Modules (PCMs) suffered capacitor failures that led to explosions. This required a redesign of the module to contain the debris, but since the concrete structure of the buildings holding them had already been poured, this left the new modules so tightly packed that there was no way to do maintenance in-place. Yet another redesign followed, this time allowing the modules to be removed from the bays for servicing.34 Continuing problems of this sort further delayed the operational start of the project, and in September 1999, an updated DOE report stated that NIF would require up to $350 million more and completion would be pushed back to 2006.66
The 287,000-pound, 10-meter-diameter target chamber was hoisted out of its silo by one of the largest cranes in the world and gently installed into its berth in the NIF target bay on June 17, 1999, a breathtaking event that took only about 30 minutes. The seven-story walls and roof of the target bay were then completed.72
Throughout this period the problems with NIF were not being reported up the management chain. In 1999 then Secretary of Energy Bill Richardson reported to Congress that the NIF project was on time and budget, following the information that had been passed onto him by NIF's management. In August that year it was revealed that NIF management had misled Richardson, and in fact neither claim was close to the truth.73 As the GAO would later note, "Furthermore, the Laboratory's former laser director, who oversaw NIF and all other laser activities, assured Laboratory managers, DOE, the university, and the Congress that the NIF project was adequately funded and staffed and was continuing on cost and schedule, even while he was briefed on clear and growing evidence that NIF had serious problems".66 Richardson later commented "I have been very concerned about the management of this facility... bad management has overtaken good science. I don't want this to ever happen again". A DOE Task Force reporting to Richardson late in January 2000 summarized that "organizations of the NIF project failed to implement program and project management procedures and processes commensurate with a major research and development project... [and that] ...no one gets a passing grade on NIF Management: not the DOE's office of Defense Programs, not the Lawrence Livermore National Laboratory and not the University of California".74
Early technical delays and project management issues caused the DOE to begin a comprehensive "Rebaseline Validation Review of the National Ignition Facility Project" in 2000, which took a critical look at the project, identifying areas of concern and adjusting the schedule and budget to ensure completion. John Gordon, National Nuclear Security Administrator, stated "We have prepared a detailed bottom-up cost and schedule to complete the NIF project... The independent review supports our position that the NIF management team has made significant progress and resolved earlier problems".75 The report revised their budget estimate to $2.25 billion, not including related R&D which pushed it to $3.3 billion total, and pushed back the completion date to 2006 with the first lines coming online in 2004.7677
Given the budget problems, the US Congress requested an independent review by the General Accounting Office (GAO). They returned a highly critical report in August 2000 stating that the budget was likely $3.9 billion, including R&D, and that the facility was unlikely to be completed anywhere near on time.6678 The report, "Management and Oversight Failures Caused Major Cost Overruns and Schedule Delays," identified management problems for the overruns, and also criticized the program for failing to include a considerable amount of money dedicated to target fabrication in the budget, including it in operational costs instead of development.73 A follow-up report the next year included all of these items, pushing the budget to $4.2 billion, and the completion date to around 2008.
A new management team took over the NIF project7980 in September 1999, headed by George Miller (who later became LLNL director 2006-2011), who was named acting associate director for lasers. Current NIF Director Ed Moses, former head of the Atomic Vapor Laser Isotope Separation (AVLIS) program at LLNL, became NIF project manager. Since the rebaselining, NIF's management has received many positive reviews and the project has met the budgets and schedules approved by Congress. In October 2010, the project was named "Project of the Year" by the Project Management Institute, which cited NIF as a "stellar example of how properly applied project management excellence can bring together global teams to deliver a project of this scale and importance efficiently."81
Recent reviews of the project have been positive, generally in keeping with the post-GAO Rebaseline schedules and budgets. However, there were lingering concerns about the NIF's ability to reach ignition, at least in the short term. An independent review by the JASON Defense Advisory Group was generally positive about NIF's prospects over the long term, but concluded that "The scientific and technical challenges in such a complex activity suggest that success in the early attempts at ignition in 2010, while possible, is unlikely".82 The group suggested a number of changes to the completion timeline to bring NIF to its full design power as soon as possible, skipping over a testing period at lower powers that they felt had little value.
In May 2003, the NIF achieved "first light" on a bundle of four beams, producing a 10.4 kJ pulse of IR light in a single beamline.19 In 2005 the first eight beams (a full bundle) were fired producing 153 kJ of infrared light, thus eclipsing OMEGA as the highest energy laser (per pulse) on the planet. By January 2007 all of the LRUs in the Master Oscillator Room (MOOR) were complete and the computer room had been installed. By August 2007 96 laser lines were completed and commissioned, and "A total infrared energy of more than 2.5 megajoules has now been fired. This is more than 40 times what the Nova laser typically operated at the time it was the world's largest laser".83
On January 26, 2009, the final line replaceable unit (LRU) was installed, completing one of the final major milestones of the NIF construction project84 and meaning that construction was unofficially completed.85 On February 26, 2009, for the first time NIF fired all 192 laser beams into the target chamber.86 On March 10, 2009, NIF became the first laser to break the megajoule barrier, firing all 192 beams and delivering 1.1 MJ of ultraviolet light, known as 3ω, to the target chamber center in a shaped ignition pulse.87 The main laser delivered 1.952 MJ of infrared energy.
On 29 May 2009 the NIF was dedicated in a ceremony attended by thousands, including California Governor Arnold Schwarzenegger and Senator Dianne Feinstein.3 The first laser shots into a hohlraum target were fired in late June 2009.4
On January 28, 2010, the facility published a paper reporting the delivery of a 669 kJ pulse to a gold hohlraum, setting new records for power delivery by a laser, and leading to analysis suggesting that suspected interference by generated plasma would not be a problem in igniting a fusion reaction.8889 Due to the size of the test hohlraums, laser/plasma interactions produced plasma-optics gratings, acting like tiny prisms, which produced symmetric X-ray drive on the capsule inside the hohlraum.89
After gradually altering the wavelength of the laser, they were able to compress a spherical capsule evenly, and were able to heat it up to 3.3 million Kelvin.90 The capsule contained cryogenically cooled gas, acting as a substitute for the deuterium and tritium fuel capsules that will be used later on.89 Plasma Physics Group Leader Dr. Siegfried Glenzer said they've shown they can maintain the precise fuel layers needed in the lab, but not yet within the laser system.90
As of January 2010, the NIF could run as high as 1.8 megajoules. Glenzer said that experiments with slightly larger hohlraums containing fusion-ready fuel pellets would begin before May 2010, slowly ramping up to 1.2 megajoules — enough for ignition according to calculations. But first the target chamber needed to be equipped with shields to block neutrons that a fusion reaction would produce.88 On June 5, 2010 the NIF team fired lasers at the target chamber for the first time in six months; realignment of the beams took place later in June in preparation for further high-energy operation.91
With the main construction complete, NIF started working on the "National Ignition Campaign" (NIC), the quest to successfully produce more fusion energy than the beamlines deposit on the target. On October 8, 2010 the first integrated ignition test was announced to have been completed successfully. The 192-beam laser system fired over a million joules of ultraviolet laser energy into a capsule filled with the hydrogen fuel. However, a number of problems slowed the drive toward ignition-level laser energies in the 1.4 to 1.5 million Joule range.
Progress was initially slowed by the potential for damage from overheating due to a concentration of energy on optical components that is greater than anything previously attempted.92 Other issues included problems layering the fuel inside the targets, and minute quantities of dust being found on the capsule surface.93
As the power was increased and targets of increasing sophistication were used, another problem appeared that was causing asymmetric implosion. This was eventually traced to minute amounts of water vapor in the target chamber which froze to the windows on the ends of the hohlraums. This was solved by re-designing the hohlraum with two layers of glass on either end, in effect creating a storm window.93 Steven Koonin, DOE undersecretary for science, visited the lab for an update on the NIC on 23 April, the day after the window problem was announced as solved. On 10 March he had described the NIC as "a goal of overriding importance for the DOE" and expressed that progress to date "was not as rapid as I had hoped".93
NIC shots halted in February 2011, as the machine was turned over to SSMP materials experiments. As these experiments wound down, a series of planned upgrades were carried out, notably a series of improved diagnostic and measurement instruments. Among these changes were the addition of the ARC system, which uses 4 of the NIF's 192 beams as a backlighting source for high-speed imaging of the implosion sequence. NIC runs restarted in May 2011 with the goal of timing the four laser shock waves that compress the fusion target to very high precision. The shots tested the symmetry of the X-ray drive during the first three nanoseconds. Full-system shots fired in the second half of May achieved unprecedented peak pressures of 50 megabars.94
According to an article in Science magazine published in October 2011, there are 17 parameters (14 on the laser, and 3 on the hohlraum) that can be tweaked to help the NIF achieve the four identified conditions necessary for ignition. The 4 conditions that need to be achieved for ignition to take place are: "The imploding fuel must maintain its spherical shape; it must achieve a certain speed; the amount of mixing between the fuel and the capsule material must be kept low; and the entropy of the system must be kept down—in other words, the energy applied needs to be focused on compressing the fuel and not raising its temperature, which would impede compression".
The plan to achieve these four conditions involves:
- Implosion velocity needs to be increased from 300 km/s to 370 km/s -- this can be tweaked by the pulse shape.
- The power of the fourth and final burst of the laser pulse has to be 300 times the power of the initial bursts. It was presently at 50x.
- The hohlraum shape has to be made stubbier so that incoming laser beams are not subject to interference by material blow off.
- The hohlraum material has to be tweaked to avoid the fuel being heated so that compression can be increased by 30 km/s.95
In January 2012, Mike Dunne, director of NIF's laser fusion energy program, predicted in a Photonics West 2012 plenary talk that ignition would be achieved at NIF by October 2012.96 In the same month, the NIF fired a record high of 57 shots, more than in any month up to that point.97
On March 15, 2012, the NIF's array of 192 lasers fired a shaped pulse of energy that generated 411 trillion watts of peak power - 1,000 times more than the United States uses at any one moment.98 The total energy created as the pulse was generated, was calculated to be 2.03 million joules, making the NIF the world's first 2MJ ultraviolet laser – about 100 times more powerful than any other laser in existence.99 On July 5, 2012, the NIF system delivered more than 500 trillion watts (terawatts or TW) of peak power and 1.85 megajoules (MJ) of ultraviolet laser light to its target.100
"The 500 TW shot is an extraordinary accomplishment by the NIF Team, creating unprecedented conditions in the laboratory that hitherto only existed deep in stellar interiors," said Dr. Richard Petrasso, senior research scientist and division head of high energy density physics at the Massachusetts Institute of Technology. "For scientists across the nation and the world who, like ourselves, are actively pursuing fundamental science under extreme conditions and the goal of laboratory fusion ignition, this is a remarkable and exciting achievement."101
"Already the most incredibly tightly controlled and most energetic laser in the world, it is remarkable that NIF has achieved the 500 TW milestone - quite a significant achievement," said Dr. Raymond Jeanloz, professor of astronomy and earth and planetary science at the University of California, Berkeley. "This breakthrough will give us incredible new opportunities in studying materials at extreme conditions." 102
The NIC campaign has been periodically reviewed by a team led by Dr. Steven E. Koonin, Under Secretary of Science. The 6th review, May 31, 2012 was chaired by David H. Crandall, Advisor on National Security and Inertial Fusion, Dr. Koonin being precluded to chair the review because of a conflict of interest. The review was conducted with the same external reviewers, who had previously served Dr. Koonin. Each provided their report independently, with their own estimate of the probability of achieving ignition within the plan, i.e. before December 31, 2012. The conclusion of the review was published on July 19, 2012.103
The previous review dated January 31, 2012, identified a number of experimental improvements that have been completed or are under way.103 The new report unanimously praised the quality of the installation: lasers, optics, targets, diagnostics, operations have all been outstanding, however:
- The integrated conclusion based on this extensive period of experimentation, however, is that considerable hurdles must be overcome to reach ignition or the goal of observing unequivocal alpha heating. Indeed the reviewers note that given the unknowns with the present 'semi-empirical' approach, the probability of ignition before the end of December is extremely low and even the goal of demonstrating unambiguous alpha heating is challenging. (Crandall Memo 2012, p. 2)
Further, the report members express deep concerns on the gaps between observed performance and ICF simulation codes such that the current codes are of a limited utility going forward. The report members find faulty prediction value in the computer codes. Specifically, they found a lack of predictive ability of the radiation drive to the capsule and inadequately modeled laser-plasma interactions. These effects lead to pressure being one half to one third of that required for ignition, far below the predicted values. The memo page 5 discusses the mix of ablator material and capsule fuel due likely to hydrodynamics instabilities in the outer surface of the ablator.103
The report goes on to suggest that using a thicker ablator may improve performance, but this increases its inertia. To keep the required implosion speed, they request that the NIF energy be increased to 2MJ. One must also keep in mind that neodymium lasers can withstand only a limited amount of energy or risk permanent damage to the optical quality of the lasing medium. The reviewers question whether or not the energy of NIF is sufficient to indirectly compress a large enough capsule to avoid the mix limit and reach ignition.104 The report concluded that ignition within the calendar year 2012 is 'highly unlikely'.103
The NIC officially ended on September 30, 2012 without achieving ignition. According to numerous articles in the press, Congress is concerned about the project's progress and funding arguments may begin anew.105106107 These reports also suggest that NIF will shift its focus away from ignition back toward materials research.108
Meanwhile, Christopher Deeney, NNSA assistant deputy administrator for stockpile stewardship, said that NNSA is expected to deliver a report to Congress by 30 November that details its new path forward to ignition.113 A newer "Fast ignition" approach may have advantages over NIF's current "central hot spot" ignition approach.114
In February 2013, the National Research Council issued a report called “An Assessment of the Prospects for Internal Fusion Energy,” which highlights the significant impact of successful development of inertial fusion energy. According to the report, the target physics programs at the National Ignition Facility, Naval Research Laboratories (NIKE laser), Laboratory for Laser Energetics (OMEGA Laser) and Sandia National Laboratory (Z machine) should receive continued high priority.115
It was also noted that there is unanimity among the expert review committees on NIF’s potential to achieve ignition. The committee asserts that while NIF has not yet achieved ignition (since its construction completion in 2009) that does not lessen the long-term technical prospects for IFE.116
Other fusion reactor designs could also be potential sources of energy in the future. Some similar experimental projects are:
- DEMOnstration Power Plant (DEMO).117
- High Power laser Energy Research facility (HiPER).118
- International Fusion Materials Irradiation Facility (IFMIF).119
- Joint European Torus (JET).120
- Tore Supra.121
- Laboratory for Laser Energetics (LLE).
- Laser Mégajoule (LMJ).122
- Wendelstein 7-X.123
- Magnetized Liner Inertial Fusion (MagLIF).124
Panorama taken outside the fusion chamber.
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