National Ignition Facility

The National Ignition Facility, or NIF, is a high-energy, high-power laser research device under construction at the Lawrence Livermore National Laboratory, in Livermore, California. Its main roles will be the exploration of inertial confinement fusion and, through these experiments, exploring high-energy high-density physics and nuclear weapons for the United States.

Construction of the NIF has been fraught with problems; compared to initial estimates, NIF is over seven years behind schedule and almost ten times over budget. Its potential role in nuclear weapon research has also made it a controversial political topic. As of November 2007, ninety-six of the lasers (out of a planned 192) had been completed and commissioned. A further forty-eight (for a total of 144) are nearing completion. 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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Background

Main article: ICF mechanism

The basic idea behind any inertial confinement fusion (ICF) device is to rapidly heat the outer layers of a "target", a small plastic sphere containing a few milligrams of fusion fuel, typically a mix of deuterium and tritium. 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. The rapid blowoff also creates a shock wave that travels towards the center of the compressed fuel. When it meets itself in the center of the fuel, the energy in the shock wave further heats and compresses the tiny volume around it. 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 around it 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, as well as creating a suitable shock wave. The laser beams must also be focused 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.

Description

  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.^[1] 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.^[2]

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.^[1] Given the time scale of a few billionths of a second, the power is correspondingly very high (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 (LRUs), standardized boxes about the size of a small car that can be dropped out of the beamline for replacement from below.^[3]

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.^[4] The conversion process is about 50% effective, reducing delivered energy to a nominal 1.8 MJ (500 terawatts).

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.

NIF uses 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, to re-emit the energy as intense X-rays, which are more evenly distributed and symmetrical than the original laser beams. Experimental systems, including the OMEGA and Nova lasers, validated this approach through the late 1980s. 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 2 mm in diameter, chilled to about 18 degrees C 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 is arranged to shine the laser into the open ends of the hohlraum for conversion into X-rays.  

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).^[5] 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;^[6] 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.^[5] 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.^[6] However the baseline design allows for a maximum of about 45 MJ of fusion energy release, due to the design of the target chamber.^[7]

NIF was primarily designed as an indirect drive device. 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 hohlraums 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.^[8] 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.^[9] 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,^[8] producing results almost as good as the fully symmetric direct drive approach.

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.^[10]

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.^[11] 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".^[12]

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.^[13] 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. 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.^[14] As of November 2007 96 laser lines have been completed and commissioned. 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.

Criticisms

Critics 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, see Teller-Ulam design for details), 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^[15]; 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.

References