The National Ignition Facility (NIF) is a laser-based inertial confinement fusion (ICF) research facility, located at Lawrence Livermore National Laboratory in Livermore, California, United States. NIF's mission is to achieve fusion ignition with high energy gain. It achieved the first instance of scientific breakeven controlled fusion in an experiment on December 5, 2022, with an energy gain factor of 1.5. It supports nuclear weapon maintenance and design by studying the behavior of matter under the conditions found within nuclear explosions.
NIF is the largest and most powerful ICF device built to date. The basic ICF concept is to squeeze a small amount of fuel to reach the pressure and temperature necessary for fusion. NIF hosts the world's most energetic laser, which indirectly heats the outer layer of a small sphere. The energy is so intense that it causes the sphere to implode, squeezing the fuel inside. The implosion reaches a peak speed of , raising the fuel density from about that of water to about 100 times that of lead. The delivery of energy and the adiabatic process during implosion raises the temperature of the fuel to hundreds of millions of degrees. At these temperatures, fusion processes occur in the tiny interval before the fuel explodes outward.
Construction on the NIF began in 1997. NIF was completed five years behind schedule and cost almost four times its original budget. Construction was certified complete on March 31, 2009, by the U.S. Department of Energy. The first large-scale experiments were performed in June 2009 and the first "integrated ignition experiments" (which tested the laser's power) were declared completed in October 2010.
From 2009 to 2012 experiments were conducted under the National Ignition Campaign, with the goal of reaching ignition just after the laser reached full power, some time in the second half of 2012. The campaign officially ended in September 2012, at about the conditions needed for ignition. Thereafter NIF has been used primarily for materials science and weapons research.
In 2021, after improvements in fuel target design, a NIF experiment produced 70% of the energy of the laser, beating the energy gain record set in 1997 by the JET reactor at 67% and achieving the first ignited plasma in a laboratory. On December 5, 2022, after further technical improvements, NIF reached scientific breakeven for the first time, producing 154% of the energy of the laser. While this was scientifically a success, the experiment produced less than 1% of the energy the facility used to create it: while 3.15 MJ of energy was yielded from 2.05 MJ input, the 2.05 MJ laser pulse took about 300 MJ to produce. Subsequent shots have seen continued increases in yield; a February 2024 shot exceeded 5 MJ.
Inertial confinement fusion basics
Inertial confinement fusion (ICF) devices use intense energy to rapidly heat the outer layers of a target in order to compress it. Nuclear fission provides the energy source for thermonuclear warheads, while sources such as laser beams and particle beams are used in non-weapon devices.
The target is a small spherical pellet containing a few milligrams of fusion fuel, typically a mix of deuterium (D) and tritium (T), as this composition has the lowest ignition temperature.
The lasers can either heat the surface of the fuel pellet directly – known as direct drive – or heat the inner surface of a hollow metal cylinder around the pellet – known as indirect drive. In the indirect drive case, the cylinder, called a hohlraum (German for 'hollow room' or 'cavity'), becomes hot enough to re-emit the energy as even higher frequency X-rays. These X-rays, which are more symmetrically distributed than the original laser light, heat the surface of pellet.
In either case, the material on the outside of the pellet is turned into a plasma, which explodes away from the surface. The rest of the pellet is driven inward on all sides, into a small volume of extremely high density. The surface explosion creates shock waves that travel inward. At the center of the fuel, a small volume is further heated and compressed. When the temperature and density are high enough, fusion reactions occur. The energy must be delivered quickly and spread extremely evenly across the target's outer surface in order to compress the fuel symmetrically.
The reactions release high-energy particles, some of which, primarily alpha particles, collide with unfused fuel and heat it further, potentially triggering additional fusion. At the same time, the fuel is also losing heat through x-ray losses and hot electrons leaving the fuel area. Thus the rate of alpha heating must be greater than the loss rate, termed bootstrapping. Given the right conditions—high enough density, temperature, and duration—bootstrapping results in a chain reaction, burning outward from the center. This is known as ignition, which fuses a significant portion of the fuel and releases large amounts of energy.
As of 1998, most ICF experiments had used laser drivers. Other drivers have been examined, such as heavy ions driven by particle accelerators.Per F. Peterson, "How IFE Targets Work", University of California, Berkeley, 1998. Retrieved on May 8, 2008. Per F. Peterson, "Drivers for Inertial Fusion Energy", University of California, Berkeley, 1998. Retrieved on May 8, 2008.
Design
System
NIF primarily uses the indirect drive method of operation, in which the laser heats a small metal cylinder surrounding the capsule inside it, which then emits X-rays that heat the fuel pellet. Experimental systems, including the OMEGA and Nova lasers, validated this approach.J.D. Lindl et al., The physics basis for ignition using indirect-drive targets on the National Ignition Facility, Physics of Plasmas, Vol. 11, February 2004, page 339. Retrieved on May 7, 2008. The NIF's high power supports a much larger target than OMEGA or Nova; the baseline pellet design is about 2 mm in diameter. It is chilled to about 18 kelvin (−255 °C) and lined with a layer of frozen deuterium–tritium (DT) fuel. The hollow interior contains a small amount of DT gas.
In a typical experiment, the laser generates 3 MJ of infrared laser energy of a possible 4. About 1.5 MJ remains after conversion to UV, and another 15 percent is lost in the hohlraum. About 15 percent of the resulting x-rays, about 150 kJ, are absorbed by the target's outer layers. The coupling between the capsule and the x-rays is lossy, and ultimately only about 10 to 14 kJ of energy is deposited in the fuel.
The fuels in the center of the target are compressed to a density of about 1000 g/cm3. For comparison, lead has a density of about 11 g/cm3. The pressure is the equivalent of 300 billion atmospheres.
Before NIF was constructed, it was expected based on simulations that 10–15 MJ of fusion energy would be released, resulting in a net fusion energy gain, denoted Q, of about 5–8 (fusion energy out/UV laser energy in). Due to the design of the target chamber, the baseline design limited the maximum possible fusion energy release to 45 MJ, equivalent to about 11 kg of TNT exploding. When NIF was built and used in 2011, the fusion energy was far lower than expected – less than 1 kJ. Performance was gradually improved until, as of 2024, the fusion energy routinely exceeded 2 MJ.
To be useful for energy production, a fusion facility must produce fusion output at least an order of magnitude more than the energy used to power the laser amplifiers – 400 MJ in the case of NIF. Commercial laser fusion systems would use much more efficient diode-pumped solid state lasers, where wall-plug efficiencies of 10 percent have been demonstrated, and efficiencies 16–18 percent were expected with advanced concepts under development in 1996.
Laser
As of 2010 NIF aimed to create a single 500 terawatt (TW) peak flash of light that reaches the target from numerous directions within a few picoseconds. The design uses 192 beamlines in a parallel system of flashlamp-pumped, neodymium-doped phosphate glass lasers.
To ensure that the output of the beamlines is uniform, the laser is amplified from a single source in the Injection Laser System (ILS). This starts with a low-power flash of 1053-nanometer (nm) infrared light generated in an ytterbium-doped optical fiber laser termed Master Oscillator. Its light is split and directed into 48 Preamplifier Modules (PAMs). Each PAM conducts a two-stage amplification process via xenon flash lamps. The first stage is a regenerative amplifier in which the pulse circulates 30 to 60 times, increasing its energy from nanojoules to tens of millijoules. The second stage sends 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 millijoules to about 6 joules. According to LLNL, designing the PAMs was one of the major challenges. Subsequent improvements allowed them to surpass their initial design goals., Lawrence Livermore National Laboratory. Retrieved on October 2, 2007
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 flash lamps. The lamps are powered by a capacitor bank that stores 400 MJ (110 kWh). When the wavefront passes through them, the amplifiers release some of the energy stored in them into the beam. The beams are sent through the main amplifier four times, using an optical switch located in a mirrored cavity. These amplifiers boost the original 6 J to a nominal 4 MJ. Given the time scale of a few nanoseconds, the peak UV power delivered to the target reaches 500 TW.
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 beam 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 beam image is extremely uniform. Spatial filters were a major step forward. They were introduced in the Cyclops laser, an earlier LLNL experiment.
The end-to-end length of the path the laser beam travels, including switches, is about . 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 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 target chamber is a multi-piece steel sphere weighing . Just before reaching the target chamber, the light is reflected off mirrors in the switchyard and target area in order to hit the target from different directions. Since the path length from the Master Oscillator to the target is different for each beamline, optics are used to delay the light in order to ensure that they all reach the center within a few picoseconds of each other.Arnie Heller, Orchestrating the World's Most Powerful Laser , Science & Technology Review, July/August 2005. Retrieved on May 7, 2008