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Order a copy Copyright or permission restrictions may apply. We will contact you if necessary. To learn more about Copies Direct watch this short online video. Need help? Before firing, the amplifiers are first optically pumped by a total of 7, xenon flash lamps the PAMs have their own smaller flash lamps as well.
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.
Near the center of each beamline, and taking up the majority of the total length, are spatial filters. These consist of long tubes with small telescopes at the end that focus the laser beam down to a tiny point in the center of the tube, where a mask cuts 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 misfocused by imperfections in the optics upstream. 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.
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. 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.
Lawrence Livermore National Laboratory`s (LLNL`s) Inertial Confinement Fusion (ICF) Program is a Department of Energy (DOE) Defense. Available in the National Library of Australia collection. Format: Journal, Microform, Online; v.: ill. ; 28 cm.
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. Infrared 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 compression. 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. 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 firings a year. The name National Ignition Facility refers to the goal of igniting the fusion fuel, 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 those losses. 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. 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. The hollow interior also contains a small amount of DT gas.
About 1. The pressure is the equivalent of billion atmospheres. 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 , where wall-plug efficiencies of 10 percent have been demonstrated, and efficiencies percent are expected with advanced concepts under development. 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.
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,  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.
Other targets, called saturn targets , are specifically designed to reduce the anisotropy and improve the implosion. 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,  producing results almost as good as the fully symmetric direct drive approach.
PACER envisioned the explosion of small 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.
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. The primary releases significant amounts of x-rays, which are trapped within the bomb casing and heat and compress the secondary until it undergoes ignition.
The secondary consists of lithium deuteride fuel, which requires an external neutron source to begin the reaction.
This is normally in the form of a D-T "spark plug" in the center of the fuel. 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 to cause ignition. The simplest change is to replace the LiD fuel with D-T gas, essentially making the spark plug the entire secondary.
At that point there is no theoretical smallest size - as the secondary got smaller, so did the amount of energy needed to reach ignition. At the milligram level, the energy levels started to approach those available through several known devices. By the early s, 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 bomb casing. 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, to mechanically isolate both ends of the reaction. 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.
While Nuckolls and LLNL were working on hohlraum-based concepts, former weapon designer Ray Kidder was working on the direct drive concept, using a large number of laser beams to evenly heat the target capsule. This sparked off intense rivalry between Kidder and the weapons labs.
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, primarily 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 to reach low gain, which was just within the state of the art. This led to the Shiva laser project which was completed in Contrary to predictions, Shiva fell far short of its goals, and the densities reached were thousands of times smaller than predicted. 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. During the initial construction phase, Nuckolls found an error in his calculations, and an October review chaired by John Foster Jr. 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.
With each experiment, the predicted energy needed to reach ignition rose, and it was not clear that post-Nova predictions were any more accurate than earlier ones. The Department of Energy DOE decided that direct experimentation was the best way to settle the issue, and in they started a series of underground experiments at the Nevada Test Site that used small nuclear bombs to illuminate ICF targets. Each test was able to simultaneously illuminate many targets, allowing them to test the amount of x-ray energy needed by placing the targets at different distances from the bomb.
Another question was how large the fuel assembly had to be in order for the fuel to self-heat from the fusion reactions and thus reach ignition. Initial data were available by mid, and the testing ceased in Ignition was achieved for the first time during these tests, but the amount of energy and the size of the fuel targets needed to reach ignition was far higher than predicted.
A great debate broke out in the ICF establishment as a result. Nevertheless, the authors were aware of the potential for higher energy requirements, and noted "Indeed, if it did turn out that a MJ driver were required for ignition and gain, one would have to rethink the entire approach to, and rationale for, ICF". The National Academy of Sciences review led to a reevaluation of these plans, and in July , LLNL responded with the Nova Upgrade, which would reuse the majority of the existing Nova facility, along with the adjacent Shiva facility.
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. 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.
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 , an agreement formed between the labs to divide up the SSMP efforts.