National Ignition Facility

[goat balls]

There's a lot of talk out there that this has worked.


The National Ignition Facility, or NIF, is a large, laser-based inertial confinement fusion (ICF) research device located at the Lawrence Livermore National Laboratory in Livermore, California, USA. 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. Its mission is to achieve fusion with high energy gain, and to support nuclear weapon maintenance and design by studying the behavior of matter under the conditions found within nuclear weapons.[1]

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 after 2000 was much smoother, but compared to initial estimates, NIF was completed five years behind schedule and was almost four times more expensive than budgeted. Construction was certified complete on 31 March 2009 by the U.S. Department of Energy,[2] and a dedication ceremony took place on 29 May 2009.[3] The first large-scale laser target experiments were performed in June 2009[4] and the first integrated ignition experiments were declared completed in October 2010.[5]

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 energy of the laser heats the surface of the pellet into a plasma, which explodes off the surface. The remaining portion of the target is driven inwards, eventually compressing it into a small point of extremely 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. When the temperature and density of that small spot are raised high enough, fusion reactions will occur and release energy.[6]

The fusion reactions release high-energy particles, some of which, primarily alpha particles, collide with the surrounding high density fuel and heat it further. If this process deposits enough energy in a given area it can 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 will 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 will lead to a significant portion of the fuel in the target undergoing fusion and releasing large amounts of energy.[7]

To date most ICF experiments have used lasers to heat the target. Calculations show that the energy must be delivered quickly in order to compress the core before it disassembles. The laser 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.[8][9]

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 all losses of energy, so that fusion reactions can continue. 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.[7]

Completion

On January 26, 2009, the final line replaceable unit (LRU) was installed, completing one of the final major milestones of the NIF construction project[58] and meaning that construction was unofficially completed.[59] On February 26, 2009, for the first time NIF fired all 192 laser beams into the target chamber.[60] On March 10, 2009, NIF became the first laser to break the megajoule barrier, firing all 192 beams and delivering 1.1 MJ of ultraviolet light, known as 3ω, to the target chamber center in a shaped ignition pulse.[61] The main laser delivered 1.952 MJ of infrared energy.

On 29 May 2009 the NIF was dedicated in a ceremony attended by thousands, including California Governor Arnold Schwarzenegger and Senator Dianne Feinstein.[3] The first laser shots into a hohlraum target were fired in late June 2009.[4]
[edit] Buildup to main experiments

On January 28, 2010, the facility published a paper reporting the delivery of a 669 kJ pulse to a gold hohlraum, setting new records for power delivery by a laser, and leading to analysis suggesting that suspected interference by generated plasma would not be a problem in igniting a fusion reaction.[62][63] Due to the size of the test hohlraums, laser/plasma interactions produced plasma-optics gratings, acting like tiny prisms, which produced symmetric X-ray drive on the capsule inside the hohlraum.[63]

After gradually altering the wavelength of the laser, they were able to compress a spherical capsule evenly, and were able to heat it up to 3.3 million Kelvin.[64] The capsule contained cryogenically cooled gas, acting as a substitute for the deuterium and tritium fuel capsules that will be used later on.[63] Plasma Physics Group Leader Dr. Siegfried Glenzer said they've shown they can maintain the precise fuel layers needed in the lab, but not yet within the laser system.[64]

As of January 2010, the NIF could run as high as 1.8 megajoules. Glenzer said that experiments with slightly larger hohlraums containing fusion-ready fuel pellets would begin before May 2010, slowly ramping up to 1.2 megajoules — enough for ignition according to calculations. But first the target chamber needed to be equipped with shields to block neutrons that a fusion reaction would produce.[62] On June 5, 2010 the NIF team fired lasers at the target chamber for the first time in six months; realignment of the beams took place later in June in preparation for further high-energy operation.[65]
[edit] National Ignition Campaign

With the main construction complete, NIF started working on the "National Ignition Campaign" (NIC), the quest to successfully produce more fusion energy than the beamlines deposit on the target. On October 8, 2010 the first integrated ignition test was announced to have been completed successfully. The 192-beam laser system fired over a million joules of ultraviolet laser energy into a capsule filled with the hydrogen fuel. However, a number of problems slowed the drive toward ignition-level laser energies in the 1.4 to 1.5 million Joule range.

Progress was initially slowed by the potential for damage from overheating due to a concentration of energy on optical components that is greater than anything previously attempted.[66] Other issues included problems layering the fuel inside the targets, and minute quantities of dust being found on the capsule surface.[67]

As the power was increased and targets of increasing sophistication were used, another problem appeared that was causing asymmetric implosion. This was eventually traced to minute amounts of water vapor in the target chamber which froze to the windows on the ends of the hohlraums. This was solved by re-designing the hohlraum with two layers of glass on either end, in effect creating a storm window.[67] Steven Koonin, DOE undersecretary for science, visited the lab for an update on the NIC on 23 April, the day after the window problem was announced as solved. On 10 March he had described the NIC as "a goal of overriding importance for the DOE" and expressed that progress to date "was not as rapid as I had hoped."[67]

NIC shots halted in February 2011, as the machine was turned over to SSMP materials experiments. As these experiments wound down, a series of planned upgrades were carried out, notably a series of improved diagnostic and measurement instruments. Among these changes were the addition of the ARC system, which uses 4 of the NIF's 192 beams as a backlighting source for high-speed imaging of the implosion sequence. NIC runs re-started in May 2011 with the goal of timing the four laser shock waves that compress the fusion target to very high precision. The shots tested the symmetry of the X-ray drive during the first three nanoseconds. Full-system shots fired in the second half of May achieved unprecedented peak pressures of 50 megabars.[68]

According to an article in Science magazine published in October 2011, there are 17 parameters (14 on the laser, and 3 on the hohlraum) that can be tweaked to help the NIF achieve the four identified conditions necessary for ignition. The 4 conditions that need to be achieved for ignition to take place are: "The imploding fuel must maintain its spherical shape; it must achieve a certain speed; the amount of mixing between the fuel and the capsule material must be kept low; and the entropy of the system must be kept down—in other words, the energy applied needs to be focused on compressing the fuel and not raising its temperature, which would impede compression."

The plan to achieve these four conditions involves:

Implosion velocity needs to be increased from 300 km/s to 370 km/s -- this can be tweaked by the pulse shape.
The power of the fourth and final burst of the laser pulse has to be 300 times the power of the initial bursts. It was presently at 50x.
The hohlraum shape has to be made stubbier so that incoming laser beams are not subject to interference by material blow off.
The hohlraum material has to be tweaked to avoid the fuel being heated so that compression can be increased by 30 km/s.[69]

In January 2012, NIF Director Mike Dunne predicted in a Photonics West 2012 plenary talk that ignition would be achieved at NIF by October 2012.[70] In the same month, the NIF fired a record high of 57 shots, more than in any month up to that point.[71]