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National Ignition Facility

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NIF's basic layout. The laser pulse is generated in the room just right of center, and is sent into the beamlines (blue) on either side. After several passes through the beamlines the light is sent into the "switchyard" (red) where it is aimed into the target chamber (silver). Three football fields could fit inside NIF.

The National Ignition Facility, or NIF, is a laser-based inertial confinement fusion (ICF) research device located at the Lawrence Livermore National Laboratory in Livermore, California. NIF uses powerful lasers to heat and compress a small amount of hydrogen fuel to the point where nuclear fusion reactions take place. NIF is the largest and most energetic ICF device built to date, and the first that is expected to reach the long-sought goal of "ignition", producing more energy than was put in to start the reaction.

Construction began in 1997 but was fraught with problems and ran into a series of delays that greatly slowed progress into the early 2000s. Progress since then has been much smoother, but compared to initial estimates, NIF is five years behind schedule and almost four times more expensive than budgeted. The construction of the National Ignition Facility was certified complete on 31 March 2009 by the U.S. Department of Energy^[1], and a dedication ceremony took place on 29 May 2009.^[2] The first large-scale laser target experiments were performed in June 2009^[3] and ignition experiments are expected to begin in 2010.^[4]

Contents [hide] 1 Description 1.1 Background 1.2 Driver laser 1.3 NIF and ICF 2 History 2.1 Impetus 2.2 LMF and Nova Upgrade 2.3 NIF emerges 2.4 Early construction problems 2.5 Re-baseline and GAO report 2.6 Progress 2.7 Completion 3 Criticisms 4 Pictures 5 References 6 External links

[edit] Description

[edit] Background

Main article: ICF mechanism

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 from all sides. When it reaches the center of the fuel, a small volume is further heated and compressed to a great degree. If the temperature and density of that small spot can be raised high enough, fusion reactions will occur.^[5]

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 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.^[6]

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.^[7][8]

[edit] Driver laser

NIF aims to create a single 500 terawatt (TW) flash of light that reaches the target from numerous directions at the same time, within a few picoseconds. The design uses 192 individual "beamlets", which are amplified in 48 beamlines containing 16 laser amplifiers per line, each one amplifying four of the beamlets.^[5]

To ensure that the output of the beamlines is uniform, the initial laser light is amplified from a single source in the Injection Laser System (ILS). This starts with a low-power flash of 1053 nanometers (nm) infra-red light generated in an ytterbium-doped optical fiber laser known as the Master Oscillator.^[9] The light from the Master Oscillator is split and directed into 48 Preamplifier Modules (PAMs). The PAMs 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, boosting the nanojoules of light created in the Master Oscillator to about 6 joules. According to LLNL, the design of the PAMs was one of the major stumbling blocks during construction. Improvements to the design since then have allowed them to surpass their initial design goals.^[10]

Simplified diagram of the beampath of a NIF laser beam, one of 48 similar beamlines. On the left are the amplifiers and optical switch, and on the right is the final spatial filter, switchyard and optical frequency converter.

The main amplification takes place in a series of glass amplifiers located at one end of the beamlines. Before "firing", the amplifiers are first optically pumped by a total of 7,680 xenon flash lamps (the PAMs have their own smaller flash lamps as well). The lamps are powered by a capacitor bank which stores a total of 400 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 less than a quarter of the stored energy is transferred into the beam as it passes through; 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 6 J provided by the PAMs to a nominal 4 MJ.^[5] Given the time scale of a few billionths of a second, the power is correspondingly very high, 500 TW.

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). A considerable amount of this length is taken up by "spatial filters", small telescopes that focus the laser beam down to a tiny point, with a mask cutting 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 mis-focussed by imperfections in the optics upstream. Spatial filters were a major step forward in ICF work when they were introduced in the Cyclops laser, an earlier LLNL experiment. The various optical elements in the beamlines are generally 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.^[11]

Just before reaching the Target Chamber the light is reflected off various mirrors in the switchyard in order to impinge on the target from different directions. 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 few picoseconds of each other.^[12] As can be seen in the layout diagram above, NIF normally directs the laser into the chamber from the top and bottom. 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.

A large KDP crystal grown at LLNL to be cut into slices and used on NIF for frequency conversion from the IR fundamental line at 1053 nm to UV at 351 nm.

One of the last steps in the process before reaching the target chamber is to convert the infrared light at 1053 nm into the ultraviolet (UV) at 351 nm in a device known as a frequency converter.^[13] These are made of thin sheets cut from a single crystal of potassium dihydrogen phosphate. When the 1053 nm (IR) light passes through the first of two of these sheets, frequency addition converts a large fraction of the light into 527 nm light (green). On passing through the second sheet, frequency combination converts much of the 527 nm light and the remaining 1053 nm light into 351 nm (UV) light. 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. The conversion process is about 50% efficient, reducing delivered energy to a nominal 1.8 MJ.^[14]

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 hours to allow the flashlamps and laser glass to regain their shapes after firing-caused thermal expansion, limiting use to one or fewer firings a day. One of the goals for NIF is to reduce this time to 5 hours, in order to allow 700 firings a year.^[15]

[edit] NIF and ICF

Laser energy to hohlraum x-ray to target capsule energy coupling efficiency. Note the "laser energy" is after conversion to UV, which loses about 50% of the original IR power.

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 cause fusion reactions in the surrounding fuel. 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.^[6]

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. Experimental systems, including the OMEGA and Nova lasers, validated this approach through the late 1980s.^[16] 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 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.

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 remainder is 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).^[17] 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 1,000 g/mL (or 1,000,000 kg/m³);^[18] for comparison, lead has a normal density of about 11 g/mL (11,340 kg/m³). It is expected this will cause about 20 MJ of fusion energy to be released.^[17] 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.^[18] However, the baseline design allows for a maximum of about 45 MJ of fusion energy release, due to the design of the target chamber.^[19] This is the equivalent of about 11 kg of TNT exploding.

Mockup of the gold-plated hohlraum designed for the NIF.

NIF's fuel "target", filled with either D-T gas or D-T ice. The capsule is held in the hohlraum using thin plastic webbing.

NIF is also exploring new types of targets. Previous experiments generally used plastic ablators, typically polystyrene (CH). NIF's targets are constructed by coating a plastic form in a layer of sputtered beryllium or beryllium-copper alloys, and then oxidizing the plastic out of the center.^[20][21] In comparison to traditional plastic targets, beryllium targets offer higher density, high transparency to x-rays, and high thermal conductivity. All of these are advantageous in the indirect-drive mode where the incoming energy is in the form of x-rays. They also have a higher leftover ablative mass compared to the fuel inside, which has the benefit of being less sensitive to instability growth from the roughness of the DT ice (although plastic targets of the same mass also show this effect). A more practical benefit is that the mechanical strength of a Be target is high enough to contain the fuel in gaseous form at room temperature. This could allow the targets to be filled with fuel and stored for periods before being chilled to freeze the DT just before firing. In practice, however, the ice has to be carefully grown from an initial seed.^[22]

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 forty times,^[23] somewhat higher than the indirect drive system. In this case the value of the Be target is reduced and more traditional plastic targets are more appropriate. 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.^[24] 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 obtainable with no changes to the NIF beamline layout.

Other targets, called saturn targets, are specifically designed to reduce the anisotropy and improve the implosion.^[25] 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 of just over thirty-five times are thought to be possible using these targets at NIF,^[24] producing results almost as good as the fully symmetric direct drive approach.

[edit] History

[edit] Impetus

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.^[26][27] 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. LLNL decided early on to concentrate on glass lasers, while other facilities studied gas lasers using carbon dioxide (e.g. science & technology - the latest revolutionary experiments