DIII-D is a tokamak that has been operated since the late 1980s by General Atomics (GA) in San Diego , California, for the United States Department of Energy . The DIII-D National Fusion Facility is part of the ongoing effort to achieve magnetically confined fusion . The mission of the DIII-D Research Program is to establish the scientific basis for the optimization of the tokamak approach to fusion energy production.
41-404: DIII-D was built on the basis of the earlier Doublet III, the third in a series of machines built at GA to experiment with tokamaks having non-circular plasma cross sections. This work demonstrated that certain shapes strongly suppressed a variety of instabilities in the plasma, which led to much higher plasma pressure and performance. DIII-D is so-named because the plasma is shaped like the letter D,
82-537: A β max {\displaystyle \beta _{\text{max}}} around 5%, the Troyon limit was a serious concern when it was introduced. However, it was found that β N {\displaystyle \beta _{N}} changed dramatically with the shape of the plasma, and non-circular systems would have much better performance. Experiments on the DIII-D machine (the second D referring to
123-487: A plasma , symbolized by β , is the ratio of the plasma pressure ( p = n k B T ) to the magnetic pressure ( p mag = B /2 μ 0 ). The term is commonly used in studies of the Sun and Earth's magnetic field , and in the field of fusion power designs. In the fusion power field, plasma is often confined using strong magnets. Since the temperature of the fuel scales with pressure, reactors attempt to reach
164-424: A combination of electromagnets and electrical currents running through the plasma itself. Systems using only magnets are generally built using the stellarator approach, while those using current only are the pinch machines. The most studied approach since the 1970s is the tokamak , where the fields generated by the external magnets and internal current are roughly equal in magnitude. In all of these machines,
205-400: A critical field other "high-n instabilities" will invariably appear, notably the ballooning mode . For any given fusion reactor design, there is a limit to the beta it can sustain. As beta is a measure of economic merit, a practical tokamak based fusion reactor must be able to sustain a beta above some critical value, which is calculated to be around 5%. Through the 1980s the understanding of
246-639: A self-sustained burning plasma that will produce 10 times as much energy from fusion reactions as it requires for heating. The DIII-D research program is a large international collaboration, with over 600 users participating from more than 100 institutions. General Atomics operates the San Diego–based facility for the Department of Energy through the Office of Fusion Energy Sciences. Research in DIII-D aims to elucidate
287-441: A shaping that is now widely used on modern designs, and has led to the class of machines known as "advanced tokamaks." Advanced tokamaks are characterized by operation at high plasma β through strong plasma shaping , active control of various plasma instabilities, and achievement of steady-state current and pressure profiles that produce high energy confinement for high fusion gain (ratio of fusion power to heating power). DIII-D
328-414: A useful figure of merit when comparing MCF designs. Plainly, the higher the beta value, the more economically viable the design is and further the higher Q value the design possibly has. In effect, the ratio illustrates how effectively a design confines its plasma. This ratio, beta, is widely used in the fusion field: β {\displaystyle \beta } is normally measured in terms of
369-463: Is also used as a test bed to investigate innovative mechanisms for plasma heating, fueling and current drive. In May 1974, AEC selected General Atomics to build the Doublet III magnetic fusion experiment based on the success of earlier Doublet I and II magnetic confinement experiments. In Feb 1978, the Doublet III fusion experiment achieved its first operation with plasma at General Atomics. The machine
410-495: Is created by applying a voltage to generate a large electric current (more than one million amperes) in the chamber. The plasma is heated to temperatures ten times hotter than that of the sun by a combination of high-power neutral beams and microwaves. The plasma conditions are measured using instrumentation based on intense lasers, microwaves, and other precision plasma diagnostics. Experiments explore such topics as confinement, transient events, and power and particle exhaust. DIII-D
451-403: Is normally given as 0.028 if I is measured in megaamperes. However, it is also common to use 2.8 if β max {\displaystyle \beta _{\text{max}}} is expressed as a percentage. Given that the Troyon limit suggested a β max {\displaystyle \beta _{\text{max}}} around 2.5 to 4%, and a practical reactor had to have
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#1732883797148492-417: Is often known simply as the beta limit in tokamaks. The Troyon limit is given as: where I is the plasma current, B 0 {\displaystyle B_{0}} is the external magnetic field, and a is the minor radius of the tokamak (see torus for an explanation of the directions). β N {\displaystyle \beta _{N}} was determined numerically, and
533-500: Is one of two large magnetic fusion experiments in the U.S. (the other being NSTX-U at Princeton Plasma Physics Laboratory ) supported by the U.S. Department of Energy Office of Science. The program is focusing on R&D for pursuing steady-state advanced tokamak operation and supporting design and operation of the ITER experiment now under construction in France. ITER is designed to demonstrate
574-404: Is reached, the gas will be constantly losing energy to its surroundings (cooling off). This gives rise to the concept of the "confinement time", the amount of time the plasma is maintained at the required temperature. However, the fusion reactions might deposit their energy back into the plasma, heating it back up, which is a function of the density of the plasma. These considerations are combined in
615-404: Is the vacuum permeability . Any magnetic field has an associated magnetic pressure contained by the boundary conditions on the field. It is identical to any other physical pressure except that it is carried by the magnetic field rather than (in the case of a gas ) by the kinetic energy of gas molecules. A gradient in field strength causes a force due to the magnetic pressure gradient called
656-523: The Cauchy momentum equation : where the first term on the right hand side represents the Lorentz force and the second term represents pressure gradient forces. The Lorentz force can be expanded using Ampère's law , μ 0 J = ∇ × B {\displaystyle \mu _{0}\mathbf {J} =\nabla \times \mathbf {B} } , and the vector identity to give where
697-485: The Lawson criterion , or its modern form, the fusion triple product. In order to be efficient, the rate of fusion energy being deposited into the reactor would ideally be greater than the rate of loss to the surroundings, a condition known as "ignition". In magnetic confinement fusion (MCF) reactor designs, the plasma is confined within a vacuum chamber using a series of magnetic fields. These fields are normally created using
738-462: The magnetic pressure force . In SI units, the magnetic pressure P B {\displaystyle P_{B}} in a magnetic field of strength B {\displaystyle B} is where μ 0 {\displaystyle \mu _{0}} is the vacuum permeability and P B {\displaystyle P_{B}} has units of energy density . In ideal magnetohydrodynamics (MHD)
779-417: The tensile strength of the wire, causing it to fracture, or even explosively fragment. Thus, management of magnetic pressure is a significant challenge in the design of ultrastrong electromagnets. The force (in cgs ) F exerted on a coil by its own current is where Y is the internal inductance of the coil, defined by the distribution of current. Y is 0 for high frequency currents carried mostly by
820-506: The 1990s as well as both active and passive edge localized mode suppression mechanisms in the 2000s. In 2021, the program announced an improved boundary cooling approach, replacing a gaseous solution with a boron , boron nitride , lithium powder mixture. This dissipated the plasma's heat and protected the reactor walls. 32°53′36.46″N 117°14′4.40″W / 32.8934611°N 117.2345556°W / 32.8934611; -117.2345556 Beta (plasma physics) The beta of
861-526: The Sun's corona has a beta around 1%. Active regions have much higher beta, over 1 in some cases, which makes the area unstable. Magnetic pressure In physics , magnetic pressure is an energy density associated with a magnetic field . In SI units, the energy density P B {\displaystyle P_{B}} of a magnetic field with strength B {\displaystyle B} can be expressed as where μ 0 {\displaystyle \mu _{0}}
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#1732883797148902-406: The basic physics processes that govern the behavior of a hot magnetized plasma, and to establish a scientific basis for future burning plasma devices such as ITER. Ultimately, the goal is to use this understanding to develop an economically attractive fusion power plant. The tokamak consists of a toroidal vacuum chamber surrounded by magnetic field coils which contain and shape the plasma. The plasma
943-540: The cross-sectional shape of the plasma) demonstrated higher performance, and the spherical tokamak design outperformed the Troyon limit by about 10 times. Beta is also sometimes used when discussing the interaction of plasma in space with different magnetic fields. A common example is the interaction of the solar wind with the magnetic fields of the Sun or Earth . In this case, the betas of these natural phenomena are generally much smaller than those seen in reactor designs;
984-502: The density of the particles in the plasma is very low, often described as a "poor vacuum". This limits its approach to the triple product along the temperature and time axis. This requires magnetic fields on the order of tens of Teslas , currents in the megaampere, and confinement times on the order of tens of seconds. Generating currents of this magnitude is relatively simple, and a number of devices from large banks of capacitors to homopolar generators have been used. However, generating
1025-457: The external magnetic field is the driver of reactor cost, "beta external" is used to consider just this contribution. In a tokamak , for a stable plasma, β {\displaystyle \beta } is always much smaller than 1 (otherwise thermal pressure would cause the plasma to grow and move in the vacuum chamber until confinement is lost). Ideally, a MCF device would want to have as high beta as possible, as this would imply
1066-536: The first term on the right hand side is the magnetic tension and the second term is the magnetic pressure force. Magnetic tension and pressure are both implicitly included in the Maxwell stress tensor . Terms representing these two forces are present along the main diagonal where they act on differential area elements normal to the corresponding axis. The magnetic pressure force is readily observed in an unsupported loop of wire . If an electric current passes through
1107-417: The high-n instabilities grew considerably. Shafranov and Yurchenko first published on the issue in 1971 in a general discussion of tokamak design, but it was the work by Wesson and Sykes in 1983 and Francis Troyon in 1984 that developed these concepts fully. Troyon's considerations, or the "Troyon limit", closely matched the real-world performance of existing machines. It has since become so widely used that it
1148-419: The highest pressures possible. The costs of large magnets roughly scales like β . Therefore, beta can be thought of as a ratio of money out to money in for a reactor, and beta can be thought of (very approximately) as an economic indicator of reactor efficiency. For tokamaks , betas of larger than 0.05 or 5% are desired for economically viable electrical production. The same term is also used when discussing
1189-409: The interactions of the solar wind with various magnetic fields. For example, beta in the corona of the Sun is about 0.01. Nuclear fusion occurs when the nuclei of two atoms approach closely enough for the nuclear force to pull them together into a single larger nucleus. The strong force is opposed by the electrostatic force created by the positive charge of the nuclei's protons , pushing
1230-458: The loop, the wire serves as an electromagnet , such that the magnetic field strength inside the loop is much greater than the field strength just outside the loop. This gradient in field strength gives rise to a magnetic pressure force that tends to stretch the wire uniformly outward. If enough current travels through the wire, the loop of wire will form a circle . At even higher currents, the magnetic pressure can create tensile stress that exceeds
1271-480: The magnetic pressure force in an electrically conducting fluid with a bulk plasma velocity field v {\displaystyle \mathbf {v} } , current density J {\displaystyle \mathbf {J} } , mass density ρ {\displaystyle \rho } , magnetic field B {\displaystyle \mathbf {B} } , and plasma pressure p {\displaystyle p} can be derived from
DIII-D (tokamak) - Misplaced Pages Continue
1312-449: The minimum amount of magnetic force needed for confinement. In practice, most tokamaks operate at beta of order 0.01, or 1%. Spherical tokamaks typically operate at beta values an order of magnitude higher. The record was set by the START device at 0.4, or 40%. These low achievable betas are due to instabilities in the plasma generated through the interaction of the fields and the motion of
1353-449: The nature of the fuel at high temperatures. When the fusion fuel gasses are heated to the temperatures required for rapid fusion, they will be completely ionized into a plasma, a mixture of electrons and nuclei forming a globally neutral gas. As the particles within the gas are charged, this allows them to be manipulated by electric or magnetic fields. This gives rise to the majority of controlled fusion concepts. Even if this temperature
1394-547: The nuclei apart. The amount of energy that is needed to overcome this repulsion is known as the Coulomb barrier . The amount of energy released by the fusion reaction when it occurs may be greater or less than the Coulomb barrier. Generally, lighter nuclei with a smaller number of protons and greater number of neutrons will have the greatest ratio of energy released to energy required, and the majority of fusion power research focusses on
1435-419: The outer surface of the conductor, and 0.25 for DC currents distributed evenly throughout the conductor. See inductance for more information. Interplay between magnetic pressure and ordinary gas pressure is important to magnetohydrodynamics and plasma physics . Magnetic pressure can also be used to propel projectiles ; this is the operating principle of a railgun . When all electric currents present in
1476-456: The particles due to the induced current. As the amount of current is increased in relation to the external field, these instabilities become uncontrollable. In early pinch experiments the current dominated the field components and the kink and sausage instabilities were common, today collectively referred to as "low-n instabilities". As the relative strength of the external magnetic field is increased, these simple instabilities are damped out, but at
1517-478: The required energy even when the gas as a whole is relatively "cool" compared to the Coulomb barrier energy. In the case of the D-T mixture, rapid fusion will occur when the gas is heated to about 100 million degrees. This temperature is well beyond the physical limits of any material container that might contain the gases, which has led to a number of different approaches to solving this problem. The main approach relies on
1558-412: The required magnetic fields is another issue, generally requiring expensive superconducting magnets . For any given reactor design, the cost is generally dominated by the cost of the magnets. Given that the magnets are a dominant factor in reactor design, and that density and temperature combine to produce pressure, the ratio of the pressure of the plasma to the magnetic energy density naturally becomes
1599-432: The total magnetic field. However, in any real-world design, the strength of the field varies over the volume of the plasma, so to be specific, the average beta is sometimes referred to as the "beta toroidal". In the tokamak design the total field is a combination of the external toroidal field and the current-induced poloidal one, so the "beta poloidal" is sometimes used to compare the relative strengths of these fields. And as
1640-463: The use of deuterium and tritium , two isotopes of hydrogen . Even using these isotopes, the Coulomb barrier is large enough that the nuclei must be given great amounts of energy before they will fuse. Although there are a number of ways to do this, the simplest is to heat the gas mixture, which, according to the Maxwell–Boltzmann distribution , will result in a small number of particles with
1681-422: Was later upgraded and renamed DIII-D in 1986. The DIII-D program achieved several milestones in fusion development, including the highest plasma β (ratio of plasma pressure to magnetic pressure) ever achieved at the time (early 1980s) and the highest neutron flux (fusion rate) ever achieved at the time (early 1990s). Major scientific discoveries include the validation of sheared flow suppression of turbulence in