National Ignition Facility
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The National Ignition Facility, or NIF, is an ultra-high energy, very high-power laser research device currently under construction at the Lawrence Livermore National Laboratory, in Livermore, California. The device's main roles will be exploration of inertial confinement fusion and, through these experiments, exploring the science/physics underlying high energy density physics and nuclear weapons for the United States.
Construction of the NIF has been fraught with problems, and is about seven years behind the first schedule estimate and almost ten times over the first budget estimates, as of September 2006. Its potential role in nuclear weapon research has also made it a controversial political topic. Nevertheless NIF achieved first light in August 2005. As of May 2006, sixteen of the lasers have been completed. Construction of the NIF is currently estimated to be completed in 2009 with the first fusion ignition tests planned for 2010.
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[edit] Description
The basic goal of any inertial confinement fusion (ICF) system is to quickly heat the outer layers of a "target" with the laser. This heat explosively vaporizes the outer surface of the target and heat it into a plasma. The rapid expansion of the resulting plasma "explodes" off the target, pushing the rest of the target in the opposite direction due to Newton's Third Law. This compresses the interior of the target to very high density (many times the density of lead for instance), while at the same time creating a shock wave traveling into the center. When the shock wave reaches the center of the target its energy further heats and compresses the very center of the compressed fuel, raising the temperature at that spot to hundreds of millions of kelvins. The combination of heating and compression create the required conditions for fusion in the center of the target.
In order to make this process efficient, the compression must be extremely symmetrical, a process that has been a major design problem with previous ICF attempts. To address this, NIF aims to create a single ultrabright flash of light that reaches the target from several directions at precisely the same time. The original design called for a single laser source to be redirected into 256 "beamlines", each of which would amplify the power of this single source through a series of 19 neodymium-doped phosphate glass amplifiers. During one of several redesigns the number of beamlines and amplifiers was reduced to the current design's 192 beamlines and 16 amplifiers per line. This number is nonetheless far and away beyond the number and size of beams of any preceding ICF laser.
The initial laser light is provided by the Injection Laser System (ILS), the original source being created in a ytterbium-doped optical fiber known as the Master Oscillator. The light from this source is directed into 48 small Preamplifier Modules (PAMs). The modules pass the light four times through a circuit containing a neodymium glass amplifier similar to (but much smaller than) the ones used in the main beamlines. The amplifiers operate in the infra red region, at 1054 nanometers. The microjoules of light created in the Master Oscillator is boosted to about 10 joules by the time it leaves the PAMs. According to LLNL, the design of the PAMs has been one of the major stumbling blocks during construction.
The main amplification takes place in a series of glass amplifiers located at one end of the beamlines. Prior to "firing", the amplifiers are first optically pumped by a total of 7,680 Xenon flash lamps (the PAMs have their own smaller pumps as well). The lamps are powered by a capacitor bank which stores a total of 330 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. This is not a particularly efficient process, and in order 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 10 J provided by the PAMs to a nominal 4 MJ (1000 terawatts).
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 total length of the laser from one end to the other is about 1,000 feet (305 meters). Diagnostic and wave-shaping elements are spread through the entire length of the beamline, which allows the wavefront to be accurately focused in order to ensure that the image of the beam as it reaches the target is extremely uniform. Most of the equipment is packaged into Line Replaceable Units, standardized boxes about the size of a small car that can be dropped out of the beamline for replacement from below.
On reaching the Target Chamber the light is reflected off various turning mirrors in order to impinge on the target from different directions. As can be seen in the layout diagram above, NIF directs the laser into the chamber primarily from the top and bottom. 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 "slow" the light in order to ensure all of them reach the center within a picosecond of each other.
One of the last steps in the process before reaching the target chamber is to convert the infrared light at 1054 nm into the ultraviolet at 351 nm in a device known as a optical frequency multiplier. These are made of thin sheets cut from a monocrystal of potassium dihydrogen phosphate placed in the beamlines. When the laser light passes through them they essentially combine three of the IR photons into a single UV one. 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.[1] The conversion process is about 50% effective, reducing delivered energy to a nominal 1.8 MJ (500 terawatts).
[edit] NIF and ICF
The name "NIF" 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 where the rate of fusion is high enough that the heat generated by the fusion reaction itself is enough to continually sustain the continued fusing of the surrounding fuel. In this case the majority of the fuel undergoes a "burn", in much the same way wood will burn to ash after being ignited by a match. Ignition is considered a key requirement if fusion power is to ever become practical.
The NOVA laser, NIF's predecessor built in the early 1980's, was the first experiment to be built with the deliberate intention of creating the conditions needed for ignition. NOVA failed to achieve this goal due to serious unforeseen problems caused by non-uniformities in the compression of the target (hydrodynamic instabilities). NOVA was unable to closely match the power from each beamline, which meant that different areas of the pellet received different amounts of implosion force and different x-ray energy irradiation when in the direct drive mode. The anisotropies in beam intensity caused "hot spots" to develop 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.
Today, the physics governing these plasma instability problems are much more thoroughly understood, due both to the lessons learned with NOVA as well as the dramatically improved computer simulations available recently. At the same time, the art of focusing and smoothing laser irradiation on the target has progressed dramatically through a variety of methods pioneered using the OMEGA laser and several other experimental setups. These challenges are believed to be well understood now, and it is expected NIF will be able to ignite its fuel capsules. 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 3 mm in diameter, chilled to a few degrees above absolute zero and lined with a layer of solid deuterium-tritium (DT) fuel. The hollow interior also contains a small amount of DT gas.
NIF mainly plans to use the "indirect drive" method of compressing the target. In the indirect drive system, the laser light does not shine on the target directly, but onto the inside a thin metal shell made of a heavy metal such as lead or gold, known as a "hohlraum". The hohlraum is typically in the form of a small open-ended cylinder, and NIF is arranged to shine the laser into the open ends. The laser energy creates an intense flux of x-rays inside the hohlraum, which heat the outer layer of the fuel pellet inside. X-rays are much more efficient at heating the pellet than the original ultraviolet light, which is why it is considered a practical solution even though considerable energy is lost to heating the hohlraum. This conversion process is fairly efficient, of the original ~4 MJ of laser energy created in the beamlines, 1.8 MJ is left after conversion to UV, and about half of the rest lost in the x-ray conversion in the hohlraum. Of the rest, perhaps 10 to 20% of the resulting x-rays will be absorbed by the outer layers of the target (see image below).[2] The shockwave created by this heating absorbs about 140 kJ, which is expected to compress the fuel in the center of the target to a density of about 1000 g/mL;[3] for comparison, lead has a normal density of about 11 g/mL. It is expected this will cause about 20 MJ of fusion energy to be released.[2] Improvements in both the laser system and hohlraum design are expected to improve the shockwave to about 420 kJ, in turn improving the fusion energy to about 100 MJ.[3] However the baseline design allows for a maximum of about 45 MJ of fusion energy release, due to the design of the target chamber.[4]
NIF was primarily designed as an indirect drive device, largely because x-rays are a major driver in the nuclear bombs that the NIF will be simulating. Nevertheless, 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 forty times, somewhat higher than the indirect drive system. However, as the NIF was designed primarily with hohlraum's in mind, the beam layout is arranged to shine into the chamber from the top and bottom, as opposed to from all sides. An additional set of ports is available for these experiments, but changing the system to use them is a time consuming process that makes such experiments unlikely to be scheduled in the short term.
It has recently been shown, using scaled implosions on the OMEGA laser and multidimensional computer simulations, that NIF should also be capable of igniting a capsule, albeit with a lower gain factor, 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.[5] In this configuration the target suffers either a "pancake" or "cigar" anisotropy on implosion, reducing the maximum temperature at the core. However, the amount of energy being dumped into the target by the laser is so high that it ignites anyway. Fusion gains in this configuration are estimated to be anywhere between ten and thirty times, less than the symmetrical direct drive approach, but operable with no changes to the NIF beamline layout.
Certain other targets called "saturn targets" are specifically designed to reduce the anisotropy.[6] 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 just over thirty-five are thought to be possible using these targets on NIF,[5] producing results almost as good as the fully symmetric direct drive approach.
[edit] Construction problems
When it was first proposed in 1993, the Department of Energy (DOE) estimated NIF was to cost about $667 million dollars, and be completed by 2002. By 1995, still during the planning stages, the estimates had already risen to about $1 billion, and when construction on the main buildings started in May 1997 the number had again crept upward to $1.1 billion. In January 2000 the Secretary of Energy Bill Richardson, then the director of DOE, claimed that the NIF was "on time and within budget", but was soon advised by project managers that neither claim was actually close to the truth. A June 2000 report by DOE further revised their estimate to $2.25 billion, and pushed back the completion date.[7]
In-fighting between the various Department of Energy laboratories soon started, with Sandia and Los Alamos publicly attacking the facility as ill-conceived. On 25 May Sandia vice president Tom Hunter told the Albuquerque Tribune that the NIF should be downsized so that it would not "disrupt the investment needed" in other labs.[8] Criticism of the project also came from politicians, government officials and review panels, some going so far as to refer to the project as being "out of control".[9]
Given the budget problems, the US Congress requested an independent review by the General Accounting Office (GAO). They returned a report in August 2000 stating that the budget was likely $4 billion and was unlikely to be completed anywhere near on time.[10] A follow-up report the next year pushed the budget up again to $4.2 billion, and the completion date to around 2007.
In August of 2005, the NIF achieved "first light" on a bundle of 8 beams, producing a 10 nanosecond, 152.8 kJ pulse of IR light, thus eclipsing OMEGA as the highest energy laser (per pulse) on the planet. The energy after conversion to the third harmonic at 351 nm would be around 70 kJ, approximately double that of the highest energy pulses either the OMEGA or NOVA lasers ever produced.
As of May 2006, sixteen of the eventual 192 lasers had been completed, and by July 2, 300 of 6,216 LRUs have been installed.[11] The lab currently calls for construction to be complete in "1,000 days", which puts the date some time in 2009, with the "Ignition Campaign" starting the next year. Significant effort continues to ensure that this campaign is successful, though by the nature of scientific research its success cannot be guaranteed.
These delays have led to something of a race with the French Laser Mégajoule, which has very similar energies as NIF. Mégajoule started construction later than NIF but has a shorter planned building time, estimated to be complete in 2010.
[edit] Criticisms
Critics argue that the most promising electricity-generating fusion technology is that of magnetic confinement, and as such money could arguably be better spent on facilities relevant to ITER. Proponents point to the long and unsuccessful history of magnetic fusion experiments in terms of generating net electrical output (although inertial fusion has been equally disappointing in this respect), and the many decades which will have to pass before viable commercial energy is predicted to flow from ITER and its successors (the first commercial fusion power plant from ITER is not expected before 2050).
Critics also point out that it appears that the primary basis for the construction of NIF is to help with the Stockpile Stewardship and Management Program (in particular the secondary – or fusion – stage of hydrogen bombs), and since this second stage is extremely resilient, it appears there is no need for testing the second stage in the manner that NIF would. Additionally, if problems with the fusion component of bombs did develop in the future, there is doubt as to how much the information learned from NIF would be of aid in maintaining the stockpile. However, in 2001 it was learned that LLNL was pursuing a method to allow the use of plutonium and uranium in experiments on NIF[12] (so called "subcriticals"); this would allow a direct examination of equation of state parameters for these materials at extremely high pressures and densities not currently allowed by subcritical experiments which compress the fissile material using conventional explosives. The decision does not appear to be finalized at this time though.
Some people think that any further expenditures in ensuring the capabilities of nuclear weapons (as theoretically done by NIF)is a waste of money and step away from the ultimate goal of nuclear disarmament.
[edit] References
- ^ UV SOURCES: World’s largest laser to generate powerful ultraviolet beams
- ^ a b Feasibility of High Yield/High Gain NIF Capsules
- ^ a b NIF Ignition Physics Program
- ^ Target Area Design Basis and System Performance for NIF
- ^ a b Polar Direct Drive—Ignition at 1 MJ
- ^ The Saturn Target for Polar Direct Drive on the National Ignition Facility
- ^ New Cost and Schedule Estimates for National Ignition Facility
- ^ Family Quarel; internal fight over the future of the world's largest laser
- ^ NIF Moves Forward Amid Controversy
- ^ GAO Report Cites New NIF Cost Estimate
- ^ NIF Project Completion in 1,000 Days
- ^ Plutonium, HEU, and Lithium Hydride
