A power-generation technology that employs various means, including high-intensity lasers, magnetic direct drive, and heavy-ion beams, to heat and compress fuel capsules sufficiently to drive controlled nuclear fusion reactions among contained deuterium and tritium nuclei. The basic idea of inertial confinement fusion (ICF) is to assemble a 1 to 10 mg capsule of highly compressed, minimally heated, fusion main fuel (a mixture of deuterium and tritium) around a very hot igniter plasma so that the energy released in the central igniter drives a burn wave into the surrounding main fuel to ignite it before it can expand significantly. The confinement is, therefore, accomplished by the inertia of the fuel and any surrounding tamper mass. ICF is distinct from other approaches to nuclear-fusion energy production, such as magnetic confinement and muon-catalyzed fusion. See also: Deuterium; Magnetic confinement fusion; Muon-catalyzed fusion; Nuclear fusion; Plasma (physics); Tritium
Brief history of inertial confinement fusion
ICF was conceived in the nuclear weapons program shortly after lasers were invented. ICF was declassified and announced in 1972 with the first publication by John Nuckolls at Lawrence Livermore National Laboratory (LLNL) in the United States. In the subsequent four decades, the basic science of high-energy-density laboratory plasmas was developed, and many approaches to ICF were proposed, researched, and evolved through international efforts in the United States, the Soviet Union, the United Kingdom, France, Japan, Germany, and China. Development of increasingly sophisticated computational simulations and experimental diagnostics permitted steady progress. In the second decade of the twenty-first century, scientists at the National Ignition Facility (NIF) at LLNL are attempting to ignite thermonuclear fuel in the laboratory for the first time. Ignition-class magnetic direct-drive experiments are being readied for the Z Machine at Sandia National Laboratories in the United States. The Laser Mégajoule (LMJ) in France, a second ignition-class laser that is similar to the NIF, began operations in 2014 and is driving ICF experiments for French scientists and colleagues. The High Power laser Energy Research (HiPER) facility is being considered for a European inertial fusion facility to study direct drive. A newly proposed target has reinvigorated the heavy-ion fusion approach. This may be the long-awaited decade for fusion to be realized. See also: Laser
Hydrogen bombs are ICF devices with 1015 J yields, so the process works at a huge scale; the challenge is to produce a thermonuclear yield of 108–1010 J with a minimal 106–107 J energy input. A yield of 200 MJ is equivalent to about 50 kg (100 lb) of TNT, or about 4 liters (1 gallon) of oil. This amount of energy can be contained in a facility and is potentially useful for basic and applied science, research on weapons physics and weapons effects, energy production, and production of special nuclear materials and nuclear fuels.
Assembling the laboratory ICF fuel configuration inside the ICF capsule requires a driver to provide a carefully programmed pressure source, so that a sequence of small shock waves compresses the main fuel to a 10–100 g/cm3-density shell. This shell is then accelerated to a high velocity, while the separate shocks combine into a very strong shock wave ahead of the main fuel to strongly heat the igniter plasma. The igniter plasma is then driven to a temperature of about 4–13 keV by (1) simple compression as the main fuel implodes in hot-spot ignition, (2) a final strong shock in shock ignition, or (3) particle beams in fast ignition. Although the igniter starts self-heating at 4 keV (the minimal ignition temperature in steady state), alpha heating, which is supplementary heating from energy deposition by the helium nuclei (called alpha particles) from the deuterium–tritium (D–T) fusion reaction, is required to get the fuel to 10–13 keV before the fuel disassembles. A strong burn wave propagates into the main fuel, and high yields can be obtained before disassembly of the compressed fuel. See also: Alpha particles; Shock wave
A laser, pulsed-power generator, or particle beam can provide the precisely programmed power source—either directly to the capsule in ablative, magnetic, or mechanical direct drive or through conversion to x-rays that subsequently drive the capsule in ablative indirect drive (Fig. 1). Magnetic and mechanical direct drives are less well developed and will be discussed in a later section.
The events leading to a capsule burn with ablative drive are as follows. The laser light or x-rays deposit energy into the outer layer of the fuel-containing capsule (Fig. 1 b). This energy deposition ablates the surface of the capsule, and the ablation acts as a rocket exhaust to generate pressures from 1 Mbar (1 million atmospheres or 100 GPa) to about 100 Mbar (10 TPa) over about 10 ns. The pressure accelerates the material of the capsule inward to velocities of approximately 2 × 105 to 4 × 105 m/s. As shown in Fig. 2, the inwardly directed momentum of the fuel and pusher continue to compress the fuel until it stagnates in the configuration shown in Fig. 3 for a hot-spot-ignition capsule.
The product of fuel density ρ and fuel radius r is an important parameter, analogous to the nτE product in magnetic fusion. The hot-spot value of ρr must exceed the alpha-particle range (expressed as product of linear range and fuel density) for effective self-heating in the igniter plasma. The burn efficiency η for inertial confinement fusion depends on ρr according to the following equation:
is expressed in g/cm2. A ρr value of 3 g/cm2 using liquid-density D–T gives 33% burn efficiency but would require about 3 kg of uncompressed D–T in a sphere. The fusion yield of that much D–T, however, is about 70 kilotons, which cannot be contained in a practical way. If the D–T fuel is compressed to 1000 times liquid density, then about 5 mg of D–T is sufficient to obtain the same ρr, and the yield is then about 125 kg (250 lb) of TNT, which can be readily contained. Therefore, ICF necessarily requires compression to high density, which is achieved by carefully shaping the applied pressure pulse.
High compression also requires keeping x-ray and electron preheat of the fuel less than a few electronvolts and requires managing hydrodynamic instabilities. Success in ICF requires nearly perfect capsules. The many advances in capsule fabrication have been an essential element in the progress of ICF. For example, capsules must be built to very tight tolerances with nearly perfect finishes (roughness of less than 50 nm) to avoid mixing the outer portion of the shell (the ablator) with the fuel, mixing the inner portion of the shell (the pusher) with the fuel, and degrading the burn efficiency. In addition to the fabrication tolerances, pressures generated by the ablation process must be uniform to about 2% to maintain a nearly spherical implosion.
If the yield is at least 14 times the driver's stored energy, then inertial fusion energy with ICF might well be economically viable. Depending on which of the drivers, capsule drive modes, and ignition approaches are ultimately successful, the driver has to deliver sufficient energy, power, and intensity (106–107 J, 1015 W in about 8–10 ns, and 1014–1015 W/cm2) to a capsule several millimeters in diameter.
Flash-lamp pumped and laser-diode pumped neodymium:glass lasers, pulsed-power-driven current sources, electron-beam pumped krypton fluoride (KrF) lasers, and induction-linac-driven heavy-ion accelerators are being developed for driving ICF capsules. Although they are at widely different stages of development, all of these technologies could probably provide a driver capable of delivering 5–10 MJ. It is not clear, however, that they can all deliver this energy while meeting the additional requirements of flexible temporal pulse shape, 1015 W peak power, intensity greater than 1014 W/cm2, and protecting the driver and target chamber from the explosion products. See also: Particle accelerator
National Ignition Facility (NIF)
Flash-lamp pumped, neodymium:glass laser technology is the most developed of the four and is the driver for the 1.8-MJ NIF, which is the first facility with the mission of igniting an ICF capsule in the laboratory. Ablative indirect (x-ray–driven) ICF is being explored at the NIF (Fig. 4).
After the laser and core diagnostics were brought to full operational status and the hohlraum conditions were optimized by beam balancing and pointing, ignition experiments began in late 2011. Dramatic progress toward ignition by the LLNL-led team is illustrated in Fig. 5.
In spite of the complexities and some surprises, the team made rapid progress in the first year of ignition experiments. The integrated measure of the probability of ignition (ITFX) improved by a factor of 50 during the year (Fig. 5). At the beginning of 2012, the team needed a factor of 6 improvement in yield and a factor of 1.5 improvement in the fuel ρr to get into the regime of strong alpha heating and ignition. The goal of ignition by the end of September 2012 was not reached even though the laser exceeded its ambitious goals. Priorities were shifted to weapon physics and ICF experiments designed to identify the physics that prevented ignition. See also: Optical pulses
The only other facility that has comparable energy and power output is Z, a 2-MJ pulsed-power-current source, which is being used to explore magnetic direct drive at Sandia National Laboratories. A cutaway illustration of Z with its companion laser Z-Beamlet and the Magnetic Liner Inertial Fusion (MagLIF) capsule are shown in Fig. 6.
In computer simulations of the MagLIF capsule, a 30-tesla axial magnetic field (not shown) is generated by field coils above and below the MagLIF capsule. Then the 10-kJ laser pulse preheats the igniter D–T fuel to 250 eV as the 27-MA current from Z produces a magnetic field B outside the liner; the associated radial pressure P drives the cylindrical implosion. The pressure P for magnetic drive at a radius R in millimeters (mm) is given by the following equation:
For comparison, the ablation pressure associated with a 300-eV hohlraum is about 140 Mbar (140 million atmospheres or 14 TPa). The trapped axial magnetic field increases to 13,500 T in the simulation, reduces thermal conduction losses so that an implosion velocity of only 1.2 × 105 m/s is sufficient, and provides some magnetic trapping of the fusion α particles to reduce the required ρr product to 0.03–0.08 g/cm2, depending on the density of the liner. Finally, detailed experiments on the growth of the Rayleigh-Taylor instability in magnetically driven liners on Z show that the computer simulations are highly predictive and the MagLIF design in Fig. 6 should not be disrupted by this instability if the root-mean-square surface roughness of the liner is less than 20 nm. One-dimensional (1D) computer simulations of only the principal (radial) dynamics predict the yield (thermonuclear energy produced) to be 500 kJ for Z. Similar simulations for future pulsed power drivers predict approximately 3-GJ/cm yield per unit axial length and target gain (yield divided by the sum of kinetic plus internal energy absorbed by the liner and the magnetized fuel during the implosion) of 400 for a 60-MA driver and 10-GJ/cm yield and target gain of 1000 for 70 MA. See also: Magnetism
Connecting the driver to the capsule is the major challenge for magnetic drive. A reactor must use a recyclable transmission line for coupling the current source with the fusion capsule and must replace it for every shot at a rate of once every 10 s. The cost of these recyclable lines drives the system design to large (∼1010 J) yields for economic fusion. Curved transmission lines avoid a direct path for neutrons and debris flowing into the driver and are replaced every shot, so the long-lasting walls of the reactor chamber can be protected by a flowing liquid barrier that absorbs the neutrons and debris. The recyclable transmission lines also simplify targeting because the capsule is secured to the transmission lines instead of free-falling into the target zone.
Use of KrF laser drivers
Once ignition occurs by any means, a substantial program will be needed to determine the optimum capsule and driver for single-shot, high yields (200–20,000 MJ) and then for repetitive yields (200 MJ at approximately 10 pulses per second to 20,000 MJ at 0.1 pulse per second) for inertial fusion energy. Ablative indirect drive with the krypton-fluoride (KrF) laser is a potential candidate for these missions and is being developed at the Naval Research Laboratory with shock ignition, which was first developed at the LLE. Krypton fluoride has advantages in that its shorter-wavelength light (248 nm) penetrates deeper into the ablator to increase the coupling efficiency and target gain (Fig. 7), and is less susceptible to laser–plasma instabilities.
Krypton fluoride also has a larger bandwidth than neodymium:glass, so the beams can be more easily conditioned to provide uniform illumination. Results of the Electra KrF high-repetition-rate laser (Fig. 8), and a system study of a future KrF reactor, indicate that the cost per joule of laser energy, repetitive operability, and efficiency may also be advantageous for a KrF driver. The next step for developing a KrF driver is to design and build a full module to validate its merits for inertial fusion.
Use of heavy-ion drivers
For several decades, heavy ions were examined as a reactor driver for indirect-drive, hot-spot-ignition capsules. The relative ease of repetitive operation and standoff motivated the studies. In the first decade of the twenty-first century, the tilted X-target configuration, shown in Fig. 9, was invented at Lawrence Berkeley National Laboratory to make full use of the advantages of heavy ions. The X-target is a mechanical direct-drive approach in which contained materials are heated with heavy-ion beams that have a range (expressed as the product of linear range and fuel density) of 2 g/cm2; the resulting material pressure drives the implosion. The parameters of the three annular beams are shown in Fig. 9. Two annular ion beams compress the fuel and a third ion beam heats the assembled fuel for fast ignition, a concept that was invented at LLNL and demonstrated for laser-driven fusion with Gekko XII at Osaka University in Japan.
The diversity of capsule designs, ignition mechanisms, and fusion drivers mitigates the remaining risk of failure. After 40 years of work, inertial confinement fusion is on the threshold of ignition.