Heavy ion fusion is a fusion energy concept that uses a stream of high-energy ions from a particle accelerator to rapidly heat and compress a small pellet of fusion fuel. It is a subclass of the larger inertial confinement fusion (ICF) approach, replacing the more typical laser systems with an accelerator.
Accelerators have the potential to be much more efficient in terms of delivering energy to the fuel pellet; typical laser-based "drivers" have overall efficiency on the order of 1%, while heavy-ion systems aim for 30% or more. Additionally, they can produce pulses of energy many times a second, while existing high-energy laser systems require lengthy cooling periods between "shots". These advantages would be useful in a commercial setting, as they would greatly lower the cost of operation, and somewhat lower the cost of building the plant compared to a laser system.
The basic concept of replacing photon beams with particle beams had been suggested on occasion before 1970, using either electrons or protons. Fundamental limits on the beam focusing and stopping distances using pulsed power acceleration of electrons or protons led to the concept of using heavy ions, whose higher atomic number causes each ion to stop more rapidly in fusion target material, which allows them convey thousands of times electrons or protons with the same stopping range.
The high energy of each ion means a given beam power level will be achieved with less current. Conventional accelerator beams focus to small spots. For example, CERN's Large Hadron Collider started out to hit beams head-on with 250nm focal spot radius, and in routine operation has achieved much better.
The high energy ions need to be in a low charge state to handle space charge forces. Singly ionized ions are strongly favored. The ions have substantial rigidity and do not scatter easily in modest vacuum, allowing the high quality beams focus to small radius spots in inertial fusion reaxtion chambers.
A major meeting in 1976 led to the rapid uptake of the concept through the late 1970s and early 1980s. In the late 1970s, heavy ion fusion (HIF) was described as "the conservative approach" to a working fusion reactor. In the early 1970s, definitive studies showed that inerital fusion chambers can avoid the damage of chamber materials by high energy neutrons from the deuterium-tritium reaction.
Lawrence Livermore National Laboratory (LLNL) confirmed with the 8-yr HYLIFE study (1977-85) that inertial fusion chambers can, because of the neutron protection from injected lithium, be built with "common steels." HYLIFE's highly desirable conclusion required that each pellet yield the energy equivalent of a ton of high explosives, 4 GJ, or more.
Results from the NOVA laser at LLNL in the late 1980s and early 1990s indicated that a laser would not be able to ignite such a large energy release. Livermore conceived an innovative pattern of jets to create the thick liquid wall to work with smaller fusion yields and higher repetition rates, upending HYLIFE's clean design by substituting FLIBE for pure lithium in the 1992-94 HYLIFE-II design [citation].
Since that time, in spite of continued interest, no large-scale experimental device using the approach has been built. It has the disadvantage that accelerators with the required energies can only be built in a large size, on the order of kilometres, which makes it difficult to test with low-cost systems. In contrast, even small lasers can reach the desired conditions, which is why they remain the focus of the ICF approach.
Confidence in ion acclerator system design has historically seen multiple occasions where large, new machines use innovations to increase performance parameters by factors of ten. For this reason, large new machines can be built based on the precedents that reduced new ideas to practice in industrial as well as research accelerator systems.
Background Fusion basics
Fusion takes place when atoms come into close proximity and the nuclear force pulls them together to form a single larger nucleus. Counteracting this process is the positive charge of the nuclei, which repel each other due to the electrostatic force. In order for fusion to occur, the nuclei must have enough energy to overcome this coulomb barrier. The barrier is lowered for atoms with less positive charge, those with the fewest protons. The nuclear force is increased with additional nucleons, the total number of protons and neutrons. This means that a combination of deuterium and tritium has the lowest coulomb barrier, at about 100 keV (see requirements for fusion), as they contain a single proton and one or two neutrons.
When the fuel is heated to high energies, the electrons separate from the nuclei, leaving individual ions and electrons mixed in a gas-like plasma. Particles in a gas are distributed across a wide range of energies known as the Maxwell–Boltzmann distribution. At any given temperature the majority of the particles are at lower energies, with a "long tail" containing smaller numbers of particles at much higher energies. So while the 100 keV threshold represents a temperature of over one billion degrees, in order to produce fusion events the fuel does not have to be heated to this temperature as a whole; some reactions will occur at lower bulk temperatures due to the small number of high-energy particles in the mix.
The fusion reactions give off large amounts of energy, and some of that energy will be deposited back in the fuel, heating it. There is a critical temperature at which the rate of reactions, and thus the energy deposited in the fuel, balances losses to the environment through escaping particles and radiation. At this point the reaction becomes self-sustaining, a point known as ignition. For D-T fuel, the self-heating is primary through alpha particles and the corresponding temperature is between 50 and 100 million degrees. The overall rate of fusion is dependent on the combination of temperature, density and energy confinement time, known as the fusion triple product.
Fusion reactors Two primary approaches have developed for the fusion energy problem. The most widely studied approach is magnetic confinement. Since the plasma is electrically charged, it will follow magnetic lines of force and a suitable arrangement of fields can keep the fuel away from the container walls. The fuel is then heated over an extended period. At the densities that are possible using magnets, the fusion process is fairly slow, so this approach requires long confinement times on the order of tens of seconds. Confining a gas at millions of degrees for this sort of time scale has proven difficult, although modern experimental machines are approaching the conditions needed for net power production, or breakeven.
The second major approach is inertial confinement. The alphas from the fusion reactions travel a distance that is dependent on the density of the fuel. In the low-densities of a magnetic reactor, often described as "a good vacuum", this may be on the order of many meters, but at very high densities this is greatly reduced, down to microns. The inertial approach takes advantage of this effect by compressing the fuel to extremely high density, at which point a tiny drop of fuel on the order of milligrams will be enough to allow ignition. Additionally, the collapse causes the temperature of the fuel to rise through the adiabatic process, providing two of the three inputs of the triple product.
There is no attempt to maintain these conditions for any significant period of time, the fuel explodes outward soon after the driver pulse ends, slowed only by the inertia of the particles. The confinement time is on the order of microseconds, so the temperatures and density have to be very high in order for any appreciable amount of the fuel to undergo fusion. This approach has been successful in producing fusion reactions, but to date, the devices that can provide the compression, typically lasers, require much more energy than the reactions produce.
ICF details There are two separate effects in the ICF process, one is to compress the fuel so the alphas are captured within it, and the second is to heat the fuel to the temperatures needed for the reactions to begin. In order to efficiently capture the alphas, a density of about 1000 times that of water is desired, which requires a beam energy of about 107 Joules per gram (J/g) of target mass. In contrast, heating the fuel to fusion temperatures requires about 109 J/g. For this reason, effort has been put into ways to separately heat the fuel; the typical solution is to shape the energy delivery to create a brief period of higher energy, creating a shock wave that travels into the compressing fuel. This is known as "hot spot ignition".
Most ICF systems to date have used lasers as the "driver". In the simple case when the laser is shone directly on the fuel target, known as "direct drive", the heat created by the laser causes the outer layer of the plastic capsule to explode outward. Due to Newton's third law, this causes the interior of the capsule to be driven inward. Direct drive places very strong limits on the focussing and delivery time, and is difficult to achieve. For this reason, most large ICF devices use the "indirect drive" process, in which the driver heats a metal cylinder known as a "hohlraum" so hot that it begins to release X-rays, which in turn shine on a capsule suspended inside. This allows the heating process to take place over a longer period and reduces the need to focus as tightly but has the disadvantage that much of the original beam energy is used to heat the cylinder, and does not contribute to the implosion.
Delivering a pulse with the required energy levels and timing is a significant challenge. To date, the energy requirements have normally been met using complex neodymium-doped phosphate glass lasers that have an overall efficiency around 1%. Optical systems needed to focus and control the beam remove an additional 50% of the energy, and in the case of indirect drive, significant amounts of what remain are lost heating the metal cylinder. For the National Ignition Facility, the largest and most powerful ICF system to date, only about 10 to 14 kJ of the original 4 MJ of the original laser energy reaches the target, which required 422 MJ of electrical power to generate.
In order for the fusion reactions to produce enough energy to match the original energy of the laser, it will have to produce at least 4 MJ, and for practical reasons, at least three times that, implying the ratio of input laser energy to output fusion energy, or gain, has to be on the order of hundreds or thousands. To date, the record on NIF is 1.3 MJ of fusion from 2 MJ of laser output, from 422 MJ of electricity, so it is extremely unlikely the current approach could ever be used for power production.
Alternate drivers In 1963, Friedwardt Winterberg introduced the concept of igniting fusion using small groups of particles that have been accelerated to about 200 km/s, a concept that is now known as cluster impact fusion. This concept is not similar to modern ICF, as the accelerated particles are intended to undergo fusion directly, and are not being used solely a driver as in the ICF concept. The publication of several ICF-related papers in the late 1960s prompted Winterberg to publish a 1968 paper outlining the use of accelerated electrons or ions in place of the laser systems in an ICF arrangement.