Nova (laser)

Nova was a high-power laser built at the Lawrence Livermore National Laboratory (LLNL) 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', a chain reaction of nuclear fusion that releases a large amount of energy. 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. Nova was a high-power laser built at the Lawrence Livermore National Laboratory (LLNL) 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', a chain reaction of nuclear fusion that releases a large amount of energy. 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. 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 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. 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 will occur. The fusion reactions release high-energy particles, some of which (primarily alpha particles) collide with the 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 density and temperature—this heating process can 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 can lead to a significant portion of the fuel in the target undergoing fusion, and the release of significant amounts of energy. 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. 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. 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 focussed primarily on how to make these devices. 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. 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.