Nova was a high-power laser built at the Lawrence Livermore National Laboratory (LLNL) in California, United States, in 1984 which conducted advanced inertial confinement fusion (ICF) experiments until its dismantling in 1999. Nova was the first ICF experiment built with the intention of reaching "ignition", the condition where self heating of the fusion plasma exceeds all losses. Although Nova failed in this goal, the data it generated clearly defined the problem as being mostly a result of Rayleigh–Taylor instability, leading to the design of the National Ignition Facility, Nova's successor. Nova also generated considerable amounts of data on high-density matter physics, regardless of the lack of ignition, which is useful both in fusion power and nuclear weapons research.
Background
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 heat of the driving laser burns the surface of the pellet into a plasma, which explodes off the surface. The remaining portion of the target is driven inwards due to Newton's third law, eventually collapsing into a small point of very high density."How NIF works" , Lawrence Livermore National Laboratory. Retrieved on October 2, 2007.
The rapid blowoff also creates a shock wave that travels towards the center of the compressed fuel. When it reaches the center of the fuel and meets the shock from the other side of the target, the energy in the shock wave further heats and compresses the tiny volume around it. If the temperature and density of that small spot can be raised high enough, fusion reactions in a small portion of the fuel will occur.
The fusion reactions release high-energy particles, some of which (primarily alpha particles) collide with the remaining high-density fuel around it and slow down. This heats the fuel, and can potentially cause that fuel to undergo fusion as well. Given the right overall conditions of the compressed fuel—high enough temperature and high enough product of density and time during which the plasma is confined by its own inertia—this heating process can result in ignition, initiating a burn wave outward from the central hot spot.Per F. Peterson, "Inertial Fusion Energy: A Tutorial on the Technology and Economics" , University of California, Berkeley, 1998. Retrieved on May 7, 2008.
To date, most ICF experiments have used lasers to heat the targets. Calculations show that the energy must be delivered quickly in order to compress the core before it disassembles, as well as creating a suitable shock wave. The energy must also 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.Per F. Peterson, "How IFE Targets Work" , University of California, Berkeley, 1998. Retrieved on May 8, 2008.Per F. Peterson, "Drivers for Inertial Fusion Energy" , University of California, Berkeley, 1998. Retrieved on May 8, 2008.
History LLNL's history with the ICF program starts with physicist John Nuckolls, who predicted in 1972 that ignition could be achieved with laser energies about 1 kJ, while "high gain" would require energies around 1 MJ.Nuckolls et al., "Laser Compression of Matter to Super-High Densities: Thermonuclear (CTR) Applications", Nature Vol. 239, 1972, pp. 129John Lindl, "The Edward Teller Medal Lecture: The Evolution Toward Indirect Drive and Two Decades of Progress Toward ICF Ignition and Burn", 11th International Workshop on Laser Interaction and Related Plasma Phenomena, December 1994. Retrieved on May 7, 2008. Although this sounds very low powered compared to modern machines, at the time it was just beyond the state of the art, and led to a number of programs to produce lasers in this power range.
Prior to the construction of Nova, LLNL had designed and built a series of ever-larger lasers that explored the problems of basic ICF design. LLNL was primarily interested in the Nd:glass laser, which, at the time, was one of a very few high-energy laser designs known. LLNL had 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). Building large Nd:glass lasers had not been attempted before, and LLNL's early research focused primarily on how to make these devices."Building increasingly powerful lasers" , Year of Physics, 2005, Lawrence Livermore National Laboratory
One problem was the homogeneity of the beams. Even minor variations in intensity of the beams would result in "self-focusing" in the air and glass optics in a process known as Kerr lensing. The resulting beam included small "filaments" of extremely high light intensity, so high it would damage the glass optics of the device. This problem was solved in the Cyclops laser with the introduction of the spatial filtering technique. Cyclops was followed by the Argus laser of greater power, which explored the problems of controlling more than one beam and illuminating a target more evenly. All of this work culminated in the Shiva laser, a proof-of-concept design for a high power system that included 20 separate "laser amplifiers" that were directed around the target to illuminate it.J. A. Glaze, "Shiva: A 30 terawatt glass laser for fusion research", presented at the ANS Annual Meeting, San Diego, 18–23 June 1978
It was during experiments with Shiva that another serious unexpected problem appeared. The infrared light generated by the Nd:glass lasers was found to interact very strongly with the electrons in the plasma created during the initial heating through the process of stimulated Raman scattering. This process, referred to as "hot electron pre-heating", carried away a great amount of the laser's energy, and also caused the core of the target to heat before it reached maximum compression. This meant that much less energy was being deposited in the center of the collapse, both due to the reduction in implosion energy, as well as the outward force of the heated core. Although it was known that shorter wavelengths would reduce this problem, it had earlier been expected that the IR frequencies used in Shiva would be "short enough". This proved not to be the case."Empowering Light: Historical Accomplishments in Laser Research", Science & Technology Review, September 2002, pp. 20-29
A solution to this problem was explored in the form of efficient frequency multipliers, optical devices that combine several photons into one of higher energy, and thus frequency. These devices were quickly introduced and tested experimentally on the OMEGA laser and others, proving effective. Although the process is only about 50% efficient, and half the original laser power is lost, the resulting ultraviolet light couples much more efficiently to the target plasma and is much more effective in collapsing the target to high density.
With these solutions in hand, LLNL decided to build a device with the power needed to produce ignition conditions. Design started in the late 1970s, with construction following shortly starting with the testbed Novette laser to validate the basic beamline and frequency multiplier design. This was a time of repeated energy crises in the U.S. and funding was not difficult to find given the large amounts of money available for alternative energy and nuclear weapons research.
Design
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 design that added frequency conversion to 351 nm light, which would increase coupling efficiency.Matthew McKinzie and Christopher Paine, "When Peer Review Fails", NDRC. Retrieved on May 7, 2008. The "new Nova" emerged as a system with ten laser amplifiers, or beamlines. Each beamline consisted of a series of Nd:glass amplifiers separated by spatial filters and other optics for cleaning up the resulting beams. Although techniques for folding the beamlines were known as early as Shiva, they were not well developed at this point in time. Nova ended up with a single fold in its layout, and the laser bay containing the beamlines was long. To the casual observer it appears to contain twenty long beamlines, but due to the fold each of the ten is actually almost long in terms of optical path length.Ted Perry, Bruce Remington, "Nova Laser Experiments and Stockpile Stewardship", Science & Technology Review, September 1997, pp. 5-13
Prior to firing, the Nd:glass amplifiers are first pumped with a series of Xenon flash lamps surrounding them. Some of the light produced by the lamps is captured in the glass, leading to a population inversion that allows for amplification via stimulated emission. This process is quite inefficient, and only about 1 to 1.5% of the power fed into the lamps is actually turned into laser energy. In order to produce the sort of laser power required for Nova, the lamps had to be very large, fed power from a large bank of capacitors located under the laser bay. The flash also generates a large amount of heat which distorts the glass, requiring time for the lamps and glass to cool before they can be fired again. This limits Nova to about six firings a day at the maximum.
Once pumped and ready for firing, a small pulse of laser light is fed into the beamlines. The Nd:glass disks each dump additional power into the beam as it passes through them. After passing through a number of amplifiers the light pulse is "cleaned up" in a spatial filter before being fed into another series of amplifiers. At each stage additional optics were used to increase the diameter of the beam and allow the use of larger and larger amplifier disks. In total, Nova contained fifteen amplifiers and five filters of increasing size in the beamlines, with an option to add an additional amplifier on the last stage, although it is not clear if these were used in practice.
From there all ten beams pass into the experiment area at one end of the laser bay. Here a series of mirrors reflects the beams to impinge in the center of the bay from all angles. Optical devices in some of the paths slow the beams so that they all reach the center at the same time (within about a picosecond), as some of the beams have longer paths to the center than others. Frequency multipliers upconvert the light to green and blue (UV) just prior to entering the "target chamber". Nova is arranged so any remaining IR or green light is focused short of the center of the chamber.
The Nova laser as a whole was capable of delivering approximately 100 kilojoules of infrared light at 1054 nm, or 40-45 kilojoules of frequency tripled light at 351 nm (the third harmonic of the Nd:Glass fundamental line at 1054 nm) in a pulse duration of about 2 to 4 nanoseconds and thus was capable of producing a UV pulse in the range of 16 trillion watts.
Fusion in Nova