The “Advanced Divertor eXperiment”, a proposed new experiment at MIT that would develop solutions for taming the interface between the extremely hot plasma and ordinary matter — a key challenge for the development of practical fusion energy. The experiment would also explore advanced techniques for steady-state plasma sustainment by driving currents using RF waves.
MIT's recent tokamak fusion experiment. The third in a series of compact high-magnetic-field devices at MIT, Alcator C-Mod holds the record for the highest plasma pressure achieved in any tokamak.
A conceptual fusion pilot power plant design from MIT using demountable, high-field, high-temperature superconducting magnets. The design concept includes innovative features like a molten salt blanket and an advanced, long-legged divertor.
The symbol used to denote the strength of the magnetic field, typically given in units of “Tesla”. The Earth's magnetic field is 0.00005 Tesla. Fusion experiments at MIT have had fields in the range of 8-12 Tesla, the world’s highest.
In a fusion power plant, the plasma will be surrounded by a blanket which serves three crucial functions — breeding tritium fuel, extracting heat from the fusion reactions and shielding the magnet and other components from fusion neutrons.
To be useful for energy production, a plasma must produce more fusion power output than the power required to heat it to fusion-relevant temperatures. This condition is termed “Breakeven” and has been the goal of research since 1955, when John Lawson derived a simple formula that translated the goal into quantitative requirements for plasma density, temperature and confinement.
Fusion reactions only occur at tens or hundreds of millions of degrees, a state of matter called a plasma. Since these temperatures are inconsistent with ordinary states of matter, the plasma must be well insulated — that is confined or isolated from any material apparatus that surrounds it. While the plasma in stars is confined gravitationally by their enormous mass, on earth strong magnetic fields are the most successful method.The transport of heat across magnetic fields by collisions and turbulence in the plasma was one of the most important and most challenging scientific problems facing practical fusion.
Cryogenics is the production and study of very low temperatures. Exploitation of cryogenic materials, often in the form of liquefied gases, is a key technology for fusion experiments and fusion power.
The mechanism by which electrical currents are maintained in a magnetically confined plasma. Because the tokamak concept requires a substantial current to function, current drive mechanisms are a subject of intense study. The most promising approach uses microwave RF waves.
The spiraling motion of charged particles in a magnetic field. Also called gyration or gyro-motion. Cyclotron motion is the basis for magnetic confinement, particle accelerators and methods for heating plasmas.
Deuterium and tritium are the two forms of hydrogen which are used as fusion fuel. While the nucleus of ordinary hydrogen is a single proton, deuterium nuclei contain a neutron as well and tritium nuclei contain a proton and two neutrons. The deuterium-tritium reaction is the easiest fusion reaction to use for energy production. Deuterium occurs naturally and is plentiful as a fuel for fusion. Tritium does not occur naturally and must be produced in the blanket surrounding a fusion plasma.
The magnetic “exhaust pipe” of a tokamak. The interaction between the hot plasma and ordinary matter is controlled by the divertor.
The light, negatively charged particles in a plasma. At ordinary temperatures, electrons are tightly bound into atoms and molecules, but at fusion-relevant temperatures, the atoms are pulled apart into their constituents.
Heated to its melting point, a salt called FLiBe is a promising blanket material, containing the elements Fluorine (F), Lithium (Li), and Beryllium (Be). FLiBe becomes a liquid at 459°C and remains liquid up to 1427°C.
Fusion is the process that powers the sun and the stars and produces all of the elements of the periodic table. In a fusion reaction, two light nuclei combine to make heavier nuclei. For practical fusion, we fuse deuterium and tritium in a plasma, forming helium and a neutron. The difference in mass is released in the form of energy as a result of E=mc2.
High-Energy-Density Physics (HEDP) is the study of matter under extreme states of pressure, typically one million to one trillion times the atmospheric pressure on the surface of the Earth. At these pressures, which occur naturally in the core of stars and planets, matter exhibits new properties. HEDP is studied in the lab in Inertial-Confinement-Fusion (ICF) implosions providing better understanding of how stars form and how elements are made
These are materials that can become superconducting at temperatures significantly above absolute zero The most promising HTS materials for fusion are made of YBCO – Yttrium Barium Copper Oxide, which will remain superconducting even at the temperatures of liquid nitrogen temperature at 77 °K (equal to -196 °C).
ICF is a method of inducing fusion by extreme compression of matter. In the lab, ICF is typically driven by powerful lasers, which are focused on tiny fuel capsules. NIF and OMEGA are two large ICF facilities in the U.S.
The heavy, positively charged particles in a plasma. Ions are created when electrons are stripped away from atoms in hot plasmas.
A very large tokamak experiment under construction in France by a consortium of nations including the European Union, China, India, Japan, Korea, Russia and the U.S. The ITER magnets are made with conventional superconductors (LTS), providing a moderate magnetic field and necessitating its very large size. ITER is expected to begin research operations in 2030.
Lithium, the third lightest element in the periodic table, and deuterium will provide the fuel for fusion power plants. While the burning plasma is made of deuterium and tritium, the tritium is recycled through a breeding process in the blanket. The net reaction is lithium and deuterium combining to make helium. Enough lithium exists on earth for millions of years of fusion energy.
LTS is an older generation of superconductors that require cooling to extremely low temperatures, often using liquefied helium at 4 °K. A tokamak made with LTS cannot achieve a magnetic field in the plasma any higher than 6 Tesla. (With HTS, 15 Tesla should be possible.)
An electromagnet in which a loop of electrical current produces a magnetic field. The strength of the magnetic field is set by the amount of electrical current in the loop and its geometry as defined by Ampere’s law.
The property of magnetic fields, imbedded in hot plasmas, which slows the transport of heat. Magnetic confinement is critical to insulate the plasma, at temperatures up to 200,000,000 °C from ordinary matter.
A fundamental field in nature, produced by moving electrical charges, i.e. electrical currents. Magnetic fields have a magnitude and direction, leading to the concept of the magnetic field line, which is aligned with the local direction of the field. Magnetic fields exert a force on charged particles that causes them to spiral around the magnetic field lines in what is called cyclotron or gyro-motion. Since it is composed of electrically charged particles, magnetic fields have very strong effects on plasmas on earth and in space.
Denotes half the distance – the long way - across a toroid, usually referred to with the symbol R. A related quantity is the minor radius, a, which is half the distance the short way. For illustration a car tire and a bicycle tire (both toroids) have roughly the same major radius, but the car tire has a much larger minor radius. Fusion experiments have been built with major radii from about 0.5 to 5.5 meters.
A unit of power, useful to describe how much electricity a power plant can produce, with one MW equal to a million watts. For comparison, the average microwave oven uses 1,000 watts, or 0.001 MW. Each MW of electrical power produced serves approximately 160 American households (a figure which includes their use at home and their share of commercial and industrial electrical consumption). A compact fusion power plant, based on HTS technology, might produce 100-500 MW.
MHD is a theory of plasma dynamics in which the plasma is treated as a fluid. While not a complete description, it is extremely useful in establishing the basic equilibrium and stability of magnetically confined plasmas.
An ICF facility sited at the Lawrence Livermore National Laboratory (LLNL) in California, which houses the most powerful laser system on Earth. The lasers produce almost 2 million Joules of ultraviolet light in a pulse lasting less than 10 nanoseconds. While constructed for national security missions, NIF is also used to push the frontiers of HEDP science.
An ICF facility sited at the University of Rochester in New York. Capable of producing 30 thousand Joules per pulse, OMEGA is one of the largest laser systems in the world and by far the largest on a university campus. As such, it has a major role in basic HEDP, laser and optics research and in the training of students.
Machines used to raise the energy of elementary particles to enormous levels. The largest, such as the LHC at CERN, are used for fundamental studies of matter, but smaller particle accelerators have found a broad range of applications in materials research, industry and medicine. Because the physics in accelerators involve the interactions of charged particles with electrical and magnetic fields, it has always had a close relation to the field of plasma physics.
A gas which is so hot that electrons have been stripped from its constituent atoms. Almost all of the visible universe is made of plasmas — that is all of the stars and galaxies that we can see in the sky. On earth, lightning and the auroras are examples of naturally formed plasmas. Plasma can rightfully be called the “fourth state of matter” along with solid, liquid and gas. It is formed when ordinary matter is heated above 10,000 to 20,000°C
A technique for accelerating charged particles to very high velocities utilizing the electric fields created in plasma waves. Plasma acceleration has the potential to shrink the size of particle accelerators.
As an excellent conductor of electricity, large currents can be driven in plasmas, creating, at the same time, an embedded magnetic field. Plasma current is a key element in the operation of tokamaks, the most common magnetic confinement configuration.
The number of plasma particles in a given volume (per cubic meter in standard units). Normal air has a molecular density of 2x1025m-3 while the density of a fusion plasma is 2x1020m-3, 100,000 times less. This is one of the reasons why fusion is so safe—there is an extremely low amount of fuel in the core at any one time. Since the fusion rate in a plasma is proportional to the square of its density, it is a critical parameter for power generation.
Significant amounts of heat must be added to a plasma for it to attain the range of temperatures required for fusion. A number of methods have been tested in experiments over the years, but two have emerged as the most effective – that is injection of high energy neutral beams and application of RF waves at frequencies where they resonate with gyrating plasma ions. In a burning deuterium-tritium plasma most of this “extra” heating can be turned off and the plasma is heated instead by the energetic helium ions produced by fusion.
The interface between the hot plasma and ordinary matter presents one of the greatest challenges for fusion energy. The power load on the wall of a fusion device can rival what is found in a rocket nozzle — but the wall of a fusion device must survive for years, not minutes. The solution requires both innovation in engineering and plasma physics.
Like the pressure of ordinary gases, plasma pressure is proportional to the product of temperature times density. It is also a measure of the plasma energy density, the energy contained per unit volume. At fusion relevant temperatures, the fusion rate is proportional to the pressure squared while plasma stability limits the pressure by a factor proportional to the magnetic field squared. Thus all other things being equal, fusion power density increases as the fourth power of the magnetic field.
The average energy of the plasma's particles. Compared to air, which has a typical temperature of 20 °C, plasmas occur when temperatures exceed 10,000 °C and fusion energy requires temperatures well above 50,000,000 °C.
Just as an ordinary fluid medium like air carries sound waves, plasmas support a rich array of waves. These plasma waves have a distinct and complicated set of properties owing to the low density and high temperature of the medium, the influence of electric and magnetic field on plasma motion and because the plasma motion itself can generate electric and magnetic fields. Plasma waves can be used to heat plasmas, drive current or to make measurements.
Many of the plasma waves of interest are in parts of the frequency spectrum designated as radio frequencies — roughly 20 kHz to around 300 GHz. As a result, electromagnetic waves in this frequency range are useful for heating, driving current or making measurements of the plasma.
In many fields such as plasma physics and materials science, the underlying equations are known, but calculating the solutions is far beyond what can be done with pencil and paper. For these problems, computers can be used to derive approximate solutions — a process called simulation. Fusion research has always been a leader in simulation, using the most advanced computers available at any given time. Its requirements drove the founding of the first non-classified supercomputer center in 1974.
A proposed high-field, compact fusion experiment which will burn deuterium-tritium fuel and is aiming to be the first to exceed energy breakeven. Using the new high-temperature superconductors (HTS), SPARC would have a magnetic field at the plasma center of 12 Tesla and a major radius, R, of about 1.6 meters. It is predicted that this device would produce 50-100 MW of fusion power.
A toroidal magnetic confinement device where the magnetic field lines are twisted through the use of coils with three-dimensional shapes. Though more complicated to build, stellarators unlike tokamaks, do not require any external means to sustain their equilibrium.
A material that can conduct electricity without any resistance. Many materials have been identified that become superconducting when very cold, typically a few degrees above absolute zero (defined as 0 °K = -273 °C). Only a handful of these have any practical or commercial use however, but those that do have become essential in many applications. In recent years, so-called high temperature superconductors have been developed which can operate up to about 90 °K.
A toroidal magnetic confinement device where an essential part of the magnetic configuration is generated by a large current flowing in the plasma itself. For this reason, steady-state operation requires an external means of driving the current — for example through the application of microwaves. Tokamaks have demonstrated the highest performance levels of any controlled fusion approach.
An object with a hole in the middle like a doughnut or inner tube. Because magnetic fields only confine plasma in the direction perpendicular to a magnetic field and not in the direction parallel to the field, a successful magnetic confinement concept must have field lines without beginnings or ends. This is accomplished by wrapping the field around on themselves, covering the surface of a torus.
Plasma Science and Fusion Center
Massachusetts Institute of Technology
77 Massachusetts Avenue, NW17
Cambridge, MA 02139
info@psfc.mit.edu
Accessibility
