Inertial confinement fusion (ICF) is a fusion energy process that initiates nuclear fusion reactions by compressing and heating targets filled with fuel. The targets are small pellets, typically containing deuterium (2H) and tritium (3H).
Typically, short pulse lasers deposit energy on a hohlraum. Its inner surface vaporizes, releasing X-rays. These converge on the pellet's exterior, turning it into a plasma. This produces a reaction force in the form of shock waves that travel through the target. The waves compress and heat it. Sufficiently powerful shock waves achieve the Lawson criterion for fusion of the fuel.
ICF is one of two major branches of fusion research; the other is magnetic confinement fusion (MCF). When first proposed in the early 1970s, ICF appeared to be a practical approach to power production and the field flourished. Experiments demonstrated that the efficiency of these devices was much lower than expected. Throughout the 1980s and '90s, experiments were conducted in order to understand the interaction of high-intensity laser light and plasma. These led to the design of much larger machines that achieved ignition-generating energies. Nonetheless, MCF currently dominates power-generation approaches.
Unlike MCF, ICF has direct dual-use applications to the study of thermonuclear weapon detonation. For nuclear states, ICF forms a component of stockpile stewardship. This allows the allocation of not only scientific but military funding.
California's Lawrence Livermore National Laboratory has dominated ICF history, and operates the largest ICF experiment, the National Ignition Facility (NIF). In 2022, an NIF deuterium-tritium shot yielded 3.15 megajoules (MJ) from a delivered energy of 2.05 MJ, the first time that any fusion device produced an energy gain factor above one.
Description Fusion basics Fusion reactions combine smaller atoms to form larger ones. This occurs when two atoms (or ions, atoms stripped of their electrons) come close enough to each other that the nuclear force dominates the electrostatic force that otherwise keeps them apart. Overcoming electrostatic repulsion requires kinetic energy sufficient to overcome the Coulomb barrier or fusion barrier.
Less energy is needed to cause lighter nuclei to fuse, as they have less electrical charge and thus a lower barrier energy. Thus the barrier is lowest for hydrogen. Conversely, the nuclear force increases with the number of nucleons, so isotopes of hydrogen that contain additional neutrons reduce the required energy. The easiest fuel is a mixture of 2H, and 3H, known as D-T.
The odds of fusion occurring are a function of the fuel density and temperature and the length of time that the density and temperature are maintained. Even under ideal conditions, the chance that a D and T pair fuse is very small. Higher density and longer times allow more encounters among the atoms. This cross section is further dependent on individual ion energies. This combination, the fusion triple product, must reach the Lawson criterion, to reach ignition.
Thermonuclear devices
The first ICF devices were the hydrogen bombs invented in the early 1950s. A hydrogen bomb consists of two bombs in a single case. The first, the primary stage, is a fission-powered device normally using plutonium. When it explodes it gives off a burst of thermal X-rays that fill the interior of the bomb casing. These X-rays are absorbed by a special material (like Fogbank) surrounding the secondary stage, which consists of fusion fuel, sandwiched between a fission fuel sparkplug and tamper. The X-rays heat this secondary and initiate further fission. Due to Newton's third law, this causes the fuel inside to be driven inward, compressing and heating it. This causes the fusion fuel to reach the temperature and density where fusion reactions begin.
In the case of D-T fuel, most of the energy is released in the form of alpha particles and neutrons. Under normal conditions, an alpha can travel about 10 mm through the fuel, but in the ultra-dense conditions in the compressed fuel, they can travel about 0.01 mm before their electrical charge, interacting with the surrounding plasma, causes them to lose their speed. This means the majority of the energy released by the alphas is redeposited in the fuel. This transfer of kinetic energy heats the surrounding particles to the energies they need to undergo fusion. This process causes the fusion fuel to burn outward from the center. The electrically neutral neutrons travel longer distances in the fuel mass and do not contribute to this self-heating process. In a bomb, they are instead used to either breed tritium through reactions in a lithium-deuteride fuel, or are used to split additional fissionable fuel surrounding the secondary stage, often part of the bomb casing.
The requirement that the reaction has to be sparked by a fission bomb makes this method impractical for power generation. Not only would the fission triggers be expensive to produce, but the minimum size of such a bomb is large, defined roughly by the critical mass of the plutonium fuel used. Generally, it seems difficult to build efficient nuclear fission devices much smaller than about 1 kiloton in yield, and the fusion secondary would add to this yield. This makes it a difficult engineering problem to extract power from the resulting explosions. Project PACER studied solutions to the engineering issues, but also demonstrated that it was not economically feasible. The cost of the bombs was far greater than the value of the resulting electricity.
Mechanism of action The energy needed to overcome the Coulomb barrier corresponds to the energy of the average particle in a gas heated to 100 million K. The specific heat of hydrogen is about 14 Joule per gram-K, so considering a 1 milligram fuel pellet, the energy needed to raise the mass as a whole to this temperature is 1.4 megajoules (MJ).
In the more widely developed magnetic fusion energy (MFE) approach, confinement times are on the order of one second. However, plasmas can be sustained for minutes. In this case the confinement time represents the amount of time it takes for the energy from the reaction to be lost to the environment - through a variety of mechanisms. For a one-second confinement, the density needed to meet the Lawson criterion is about 1014 particles per cubic centimetre (cc). For comparison, air at sea level has about 2.7 × 1019 particles/cc, so the MFE approach has been described as "a good vacuum".
Considering a 1 milligram drop of D-T fuel in liquid form, the size is about 1 mm and the density is about 4 × 1020/cc. Nothing holds the fuel together. Heat created by fusion events causes it to expand at the speed of sound, which leads to a confinement time around 2 × 10−10 seconds. At liquid density the required confinement time is about 2 × 10−7s. In this case only about 0.1 percent of the fuel fuses before the drop blows apart.
The rate of fusion reactions is a function of density, and density can be improved through compression. If the drop is compressed from 1 mm to 0.1 mm in diameter, the confinement time drops by the same factor of 10, because the particles have less distance to travel before they escape. However, the density, which is the cube of the dimensions, increases by 1,000 times. This means the overall rate of fusion increases 1,000 times while the confinement drops by 10 times, a 100-fold improvement. In this case 10% of the fuel undergoes fusion; 10% of 1 mg of fuel produces about 30 MJ of energy, 30 times the amount needed to compress it to that density.
The other key concept in ICF is that the entire fuel mass does not have to be raised to 100 million K. In a fusion bomb the reaction continues because the alpha particles released in the interior heat the fuel around it. At liquid density the alphas travel about 10 mm and thus their energy escapes the fuel. In the 0.1 mm compressed fuel, the alphas have a range of about 0.016 mm, meaning that they will stop within the fuel and heat it. In this case a "propagating burn" can be caused by heating only the center of the fuel to the needed temperature. This requires far less energy; calculations suggested 1 kJ is enough to reach the compression goal.
Some method is needed to heat the interior to fusion temperatures, and do so while when the fuel is compressed and the density is high enough. In modern ICF devices, the density of the compressed fuel mixture is as much as one-thousand times the density of water, or one-hundred times that of lead, around 1000 g/cm3. Much of the work since the 1970s has been on ways to create the central hot-spot that starts off the burning, and dealing with the many practical problems in reaching the desired density.
Heating concepts Early calculations suggested that the amount of energy needed to ignite the fuel was very small, but this does not match subsequent experience.
Hot spot ignition
The initial solution to the heating problem involved deliberate "shaping" of the energy delivery. The idea was to use an initial lower-energy pulse to vaporize the capsule and cause compression, and then a very short, very powerful pulse near the end of the compression cycle. The goal is to launch shock waves into the compressed fuel that travel inward to the center. When they reach the center they meet the waves coming in from other sides. This causes a brief period where the density in the center reaches much higher values, over 800 g/cm3.
The central hot spot ignition concept was the first to suggest ICF was not only a practical route to fusion, but relatively simple. This led to numerous efforts to build working systems in the early 1970s. These experiments revealed unexpected loss mechanisms. Early calculations suggested about 4.5 × 107 J/g would be needed, but modern calculations place it closer to 108 J/g. Greater understanding led to complex shaping of the pulse into multiple time intervals.
Direct vs. indirect drive In the simplest method of inertial confinement, the fuel is arranged as a sphere. This allows it to be compressed uniformly from all sides. To produce the inward force, the fuel is placed within a thin capsule that absorbs energy from the driver beams, causing the capsule shell to explode outward. The capsule shell is usually made of a lightweight plastic fabricated using plasma polymerization, and the fuel is deposited as a layer on the inside by injecting or diffusing the gaseous fuel into the shell and then freezing it.Morse, Samuel F. B., editor. "Direct-Drive Target Designs for the National Ignition Facility." LLE Review 79: Quarterly Report, vol. 79, Apr. 1999, pp. 121–130.
Shining the driver beams directly onto the fuel capsule is known as "direct drive". The implosion process must be extremely uniform in order to avoid asymmetry due to Rayleigh–Taylor instability and similar effects. For a beam energy of 1 MJ, the fuel capsule cannot be larger than about 2 mm before these effects disrupt the implosion symmetry. This limits the size of the laser beams to a diameter so narrow that it is difficult to achieve in practice.
On the other hand, "indirect drive" illuminates a small cylinder of heavy metal, often gold or lead, known as a hohlraum. The beam energy heats the hohlraum until it emits X-rays. These X-rays fill the interior of the hohlraum and heat the capsule. The advantage of indirect drive is that the beams can be larger and less accurate. The disadvantage is that much of the delivered energy is used to heat the hohlraum until it is "X-ray hot", so the end-to-end energy efficiency is much lower than the direct drive method.