Nova laser
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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 long-sought goal in fusion research. Although Nova failed in this goal, the data it generated clearly defined the problem as being mostly a result of hydrodynamic instability, leading to the design of the National Ignition Facility, Nova's successor.
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[edit] Background
The basic idea of any ICF device is to rapidly heat the outer layers of a "target", typically made of plastic with a small amount (a few milligrams) of fusion fuel on the inside. The heat burns the plastic into a plasma, which explodes off the surface. Due to Newton's Third Law, the remaining portion of the target is driven inwards, eventually collapsing into a small point of very high density, many times the density of lead in modern experiments. The rapid blowoff also creates a shock wave that travels into the very center of the compressed fuel. The energy in the shock wave further heats and compresses the very center of the fuel. If the temperature and density of that small spot is raised high enough, fusion reactions will occur.
The fusion reactions release high-energy particles, which collide with the high density fuel and slow down. This heats the fuel further, 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 and to create a shock wave. The laser beams must also be focussed evenly across the target's outer surface in order to collapse the fuel into a symmetric core. Although other "drivers" have been suggested, lasers are currently the only devices with the right combination of features.
[edit] History
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. Building large Nd:glass devices had not been attempted before, and LLNL's early research focussed primarily on how to build these devices.
One problem was the homogeneity of the beams. Even minor variations in intensity of the beams would result in Kerr lensing (self focussing beam collapse) as the beam passed through air and glass. The areas of the beam that collapsed to super high intensity fillaments then severely damaged 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 an unexpected serious problem appeared. The infrared light generated by the Nd:glass lasers was found to interact very strongly via stimulated Raman scattering, with the electrons in the plasma blowing off the target during the initial heating. This carried away a great amount of the laser's energy and also actually caused the core of the target to heat before it reached maximum compression thus ruining the implosion, referred to as hot electron pre-heating. 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.
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, 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. 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.
[edit] Design
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 300 feet long. To the casual observer it appears to contain twenty 300 foot long beamlines, but due to the fold each of the ten is actually almost 600 feet long in terms of optical path length.
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 entering the beamline and passing through an initial spatial filter, it encounters its first amplifier, 4 cm in diameter. Another filter passes it into two 9.2 cm amplifiers, then a single 15 cm one, and then three 20.8 cm ones. A much larger filter then cleans up this initial beam. From there it is reflected back towards the "front" where the main amplifiers lie, a set of four 31.5 cm amplifiers, another filter, and then four 46 cm ones. These last two sets of amplifiers were designed to allow an additional stage each (to five) to further boost power, although it is not clear if these were used in practice. A final filter cleans up the beam again.
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 a few tens of terawatts.
[edit] Fusion in Nova
As had happened with the earlier Shiva, Nova failed to meet expectations in terms of fusion output. In this case the problem was tracked to instabilities in the collapse that mixed the fuel and therefore upset the even collapse of the fuel and the formation of the shock wave. The maximum fusion yield on NOVA was about 1013 neutrons per shot.
The problem was caused by Nova's inability to closely match the output energy of each of the beamlines, which meant that different areas of the pellet received different amounts of heating across its surface. This led to "hot spots" on the pellet which were imprinted into the imploding plasma, seeding Rayleigh-Taylor instabilities and thereby mixing the plasma so the center did not collapse uniformly.
Research on Nova then turned to the "indirect drive" approach, where the lasers do not shine directly on the target. Instead they shine on the inside surface of a thin metal foil, typically made of gold, lead, or another "high-z" metal. When heated by the laser, the metal re-radiates this energy as diffuse x-rays, which are even more efficient than UV at compressing the fuel pellet. In order to emit x-rays, the metal must be heated to very high temperatures, which uses up a considerable amount of the laser energy. So while the compression is more efficient, the overall energy delivered to the target is nevertheless much smaller. The reason for the x-ray conversion is not to improve energy delivery, but to "smooth" the energy profile; since the metal foil spreads out the heat somewhat, the anisotropies in the original laser are greatly reduced.
The foil shells, or "hohlraums", are generally formed as small open-ended cylinders, with the laser arranged to shine in the open ends at an oblique angle in order to strike the inner surface. In order to support the indirect drive research at Nova, a second experimental area was built "past" the main one, opposite the laser bay. The system was arranged to focus all ten beams into two sets of five each, which passed into this second area and then into either end of the target chamber, and from there into the hohlraums.
The indirect drive approach appeared to solve the problems with the original design, now referred to as "direct drive". However, Nova's laser had been designed in order to deliver the required amount of UV light onto the target in order to cause ignition, so when the losses due to the hohlraum were included it was no longer nearly powerful enough. Yet the hohlraum solution seemed to largely solve the instabilities, so all that was needed for a successful fusion campaign was a larger laser, a much larger laser. Design of such a system started in the late 1980s and eventually emerged as the National Ignition Facility, which is currently under construction.
