Magnetic Confinement Reactors

The electrical conductivity of plasma allows it to be shaped and controlled by magnetic fields. At reactor start-up the D and T are heated to a temperature hot enough to strip the electrons from the atoms and create plasma. A temperature of only 0.015 keV is all that is required to create plasma. This relatively low temperature can be produced by simple high voltage discharges. The low temperature plasma of D and T is captured and held in position by strong magnetic fields. To start the thermonuclear reaction the D-T plasma is heated by external sources until a temperature equivalent to 10 keV is achieved. At this temperature, and above, with long confinement times and adequate density, the fusion reaction becomes self-sustaining.

The product of the D + T reaction is a helium nucleus, He4, or alpha particle, and a neutron (n). The average kinetic energy of the He is 3,500 keV and the n, 14,100 keV. The He4 has a positive charge of two. This charge forces it to strongly interact with the plasma. This interaction adds energy to the plasma. Some helium ions produced by the reaction are lost from the plasma into the surrounding vacuum and others lose energy by contact with the walls of the reactor. If the losses are not too great, the interaction of the He4 ions with the plasma can provide sufficient energy to keep the plasma at the fusion temperature. The neutron has no charge and passes through the plasma with only slight interaction. The occasional interactions of the neutrons add a small but valuable bit of heat to the plasma.

After leaving the plasma, the neutrons interact with the reactor inner wall or the cooling materials behind it. The inner wall and the coolant are termed the blanket. When the neutrons react with the blanket, their energy is deposited as heat. The resulting heat is used in conventional steam generation to provide process heat for the generation of electricity.

Molten lithium metal is a potential candidate for the coolant to be circulated through the blanket. Lithium is a light metal with a low melting point (186 degrees Celsius). In the liquid state, it has a high specific heat and thermal conductivity. These properties make it an excellent heat transfer material and thus, a good choice as a means of removing heat from the reactor. When lithium is used in the blanket for heat transfer it also serves as the primary absorber of the 14,100 keV neutrons from the D + T reaction.

131 Soures, John M., McCrory, Robert L. and Craxton, R. Stephen, "Progress in Laser Fusion", Scientific American, Vol. 255, No. 2, August 1986, Page 68

Lithium is comprised of two isotopes in a ratio of 7.4% lithium 6 to 92.6% lithium 7. Some of the fast neutrons from the D + T fusion react with a lithium 7 atom and split it into tritium and helium. The neutron is slowed down by this collision. Some of the slow neutrons then react with lithium 6 to produce another tritium and helium atom and 4,800 keV more energy. The tritium is extracted from the liquid lithium blanket for use as fuel.

It is feasible to breed more tritium in a lithium cooled reactor than is used in the reaction. The excess tritium can be used to start other reactors or in a reactor using some coolant other than lithium that prevents it from breeding its own tritium. Nature has been kind with the properties of lithium. It is an excellent choice for transferring heat from the reactor and it is the raw material needed for the continual production of more fuel. Both these functions can be provided by the use of liquid lithium as the blanket material. 132 The isotopic composition of the lithium may be adjusted to provide the proper balance of lithium 6 and lithium 7 to optimum heat transfer and production of tritium. The lithium can also be diluted with metallic sodium or potassium to aid in adjusting the tritium production rate.

Achieving a fusion reaction requires control of the 10 keV (100 million Kelvin) plasma. The plasma is electrically conductive and can be shaped and confined by strong magnetic fields. The ion and electrons tend to spiral around the magnetic lines of force, but do not move across them. In a linear machine, magnetic coils around the cylindrical body generate a uniform field parallel to the axis of the cylinder. The uniform field prevents plasma leakage perpendicular to the axis of the cylinder. With a uniform axial field, plasma can still be lost through the ends. The end loss allows a portion of the plasma to leak from the reaction zone. If the leakage through the ends is large when compared to the total plasma, it has the effect of cooling the plasma below the reaction temperature. Two techniques are used to prevent leakage from the ends.

LINEAR MAGNETIC CONFINEMENT REACTORS

One method of reducing the effects of end leakage is to make the plasma confinement cylinder so long the total volume is large compared to the leakage at the ends. Unfortunately, to confine the plasma in a simple magnetic cylinder for sufficient time to generate power, the reactor may need to be so long it is impractical.

Greatly increasing and shaping the magnetic field strength at the ends of the cylinder reduces the plasma loss. If the magnetic field is increased to a high value, the ions tend to be reflected from the high field strength area back toward the relatively lower field strength near the center of the cylindrical chamber. Because the ions are reflected from the high field strength regions, this type of confinement is called magnetic mirror confinement. Even with extremely strong fields at the ends, some plasma still leaks from the reactor.

Part of the leakage from the ends is the result of ions striking the field at an angle too great to be reflected. Other leakage results from the large difference in mass between the negatively charged electrons and the positively charged helium, tritium and deuterium ions. The electrons and the ions have the same thermal temperature, and thus energy. The velocity of the electrons is much higher because they weigh only 0.0027 as much as a deuterium ion. In a machine with the ends terminating in an increased strength magnetic field, with the same cylindrical cross section as the main body of the reactor, the electrons penetrate much deeper into the pinched portion of the magnetic field. The greater

132 Coppi, Bruno and Rem, Jan, "The Tokamak Approach in Fusion", Scientific American, Vol. 227, No. 1, July 1972, Page 65

penetration of the electrons produces a charge separation and the plasma is no longer neutral. A charged increment of plasma at the end plug creates an instability causing the mirror effect to fail. The charge induced mirror failure allows the remainder of the plasma to leak from confinement.

Several schemes for improving the simple pinched magnetic field plug at the ends of the reactor section are under test. These involve the careful shaping of the cross section and linear shape of the magnetic field at the ends of the cylindrical section. These concepts have been tested and they show dramatic improvement in the confinement. Researchers working on the linear machines believe it is possible to build a fusion power reactor today using the cylindrical section with shaped magnetic field end plugs. The general outline of the machine would be as follows.

The central cylindrical section surrounded by magnetic field coils will be long, possibly as long as 100 meters. The surface of the cylinder will be fabricated with channels to carry the liquid metal coolant. Each end of the cylindrical section would be fitted with an electrostatic/ magnetic end cap to prevent excess leakage of the plasma from the ends. The magnetic field used to confine the plasma would rely on superconducting coils to minimize the power required to sustain the confinement fields.133,134'135

A rudimentary diagram is of a linear machine shown in Figure 3.1. It is shown using superconducting magnets to confine the plasma. Superconductivity is a property shown by many metals and alloys. Among the most useful for the fabrication of powerful magnets are alloys of niobium, titanium, vanadium and tin. When these alloys are cooled to the temperature of liquid helium, 4.2 Kelvin, they lose all resistance to electrical current flow. If an electric current is started in a super conductive coil, the coil acts much like a permanent magnet. The current flow generates a magnetic field. Since there is no resistance to the current flow, the current continues and sustains the magnetic field indefinitely. Super conducting coils are used to generate powerful magnetic fields without the continuous consumption of energy. 136'137

The requirement for exotic alloys and liquid helium coolant makes superconducting magnets expensive to build and complex to operate. In applications where strong magnetic fields are required for sustained times, such as fusion reactors, their cost is low when compared to the cost of continuously supplying electric power to room temperature magnets. There is currently much progress in this area magnet technology. A class of ceramic superconductors has been discovered that Operate at much higher temperatures. Some of these ceramic superconductors are making it into the demonstration market. They are being used in short underground runs to supply power where overhead lines are not possible.138

New types of permanent magnets are available that produced very strong stable magnetic fields without expending any power. These magnets are an alloy of iron, neodymium and boron. They are currently applied to replace the superconducting magnets in nuclear Magnetic Resonance Imaging machine (MRI) used in medical diagnosis. This technology may allow fusion reactors to be developed without recourse to liquid helium cooled magnets.

133 Furth, Harold, "Progress Toward Tokamak Fusion Reactor", Scientific American, Vol. 241, No. 2, August 1970, Page 50

134 Conn, Robert W., "The Engineering of Magnetic Fusion Reactors", Scientific American, Vol. 249, No. 4, October 1983, Page 60

135 "Magnetic Fusion Development", Proceedings of the Institute of Electrical And Electronic Engineer, Special Issue, 12 articles, Vol. 69, Number 9, August 1981

136 Bryant, Lynwood, "Advances in Superconducting Magnets", Scientific American, Vol. 216, No. 3, March 1967, Page 114

137 Wolsky, Alan M., Giese, Robert F., and Daniels, Edward J., "The New Super Conductors: Prospects for Applications", Scientific American, Vol. 260, No. 2 February 1989, Page 60

138 Editors, "Superconducting power Cables, at last!", The Economist, Vol. 360, No. 8230, July 14 2001, Page 75

Liquid lithium will be pumped through the reactor walls where it would absorb the thermal energy and neutrons generated by the plasma. The neutrons from the reaction will react with the lithium to provide more tritium fuel. After passing through, and cooling the walls of the reactor, the hot lithium will be conducted to heat exchangers to generate steam. The steam will be used to generate electric power by use of conventional steam turbine generators. After it passes through the steam generators, the lithium will be pumped back through the reactor. At some point in the heat exchange loop, a portion of the lithium will be diverted through a tritium separation process where the tritium will be recovered for

Figure 3.1 A Linear Fusion Reactor

Author's notional drawing of a linear reactor later use in the reactor.

Figure 3.1 A Linear Fusion Reactor

Author's notional drawing of a linear reactor

The plasma is confined within the reactor by magnetic mirrors at each end of the cylindrical section. The mirror concept relies on charged particles of the plasma being reflected or turned back on themselves by magnetic field gradients at the ends of the cylindrical section. In the seventies research at Lawrence Livermore National Laboratory, in Livermore, California, investigated a variety of mirror magnetic shapes. One example was the tandem mirror machine. It uses linear cylindrical plasma, plugged at the ends by magnetic mirrors to prevent the plasma leakage. A mirror fusion facility was built to test this concept. Unfortunately, the facility was mothballed before testing due to a shortage of funds. If and/or when these tests are performed, it will establish the laws for scaling the mirror confinement concept and provide a basis for comparing mirrors with other types of plasma confinement schemes. Tandem mirror systems were also under investigation at the University of Wisconsin, Madison, Wisconsin and Cornell University, Ithaca, New York.

TOROIDAL MAGNETIC CONFINEMENT REACTOR (TOKAMAK)

The second method for the prevention of leakage of ions through the ends of the magnetic confinement is to eliminate the ends. This is achieved by wrapping the confining magnetic fields up to form a torus

(doughnut) shape. This eliminates the ends, but presents a new problem. It is difficult to stabilize a magnetic field bent into a toroidal shape.

Many of the fusion research machines employ the torus shaped vacuum chamber and magnetic fields. External magnets are used to generate part of the field that confines the plasma, but much of the confinement is produced by the magnetic fields induced by electric currents in the continuous ring of plasma. This ring current not only helps generate the confinement magnetic field but it supplies part of the energy necessary to heat the plasma. These machines are called Tokamaks, a name given to them by Russian researchers who were the first to use them in fusion experiments. Fusion research machines of this type are operating in the United States at Princeton University, Oak Ridge National Laboratory, General Atomics Corporation and the Massachusetts Institute of Technology.139

The toroidal machines have provided a solution for the leakage of the plasma from the ends of the reaction volume at the cost of greater instability of the fields and complex physical access problems to the reaction zone. In the early toroidal machines, powerful electric currents flowing in the plasma ring generated the magnetic field. This generated a uniform field that wraps around the small diameter of the plasma like hoops around a barrel. The magnetic field lines were closer together (the field was stronger) on the inside of the torus than on the outside. This resulted in non-uniform heating of the plasma and allowed it to leak out from the outer surface of the torus at a high rate. With great difficulty, external coils were added to produce a twist in the magnetic field. The twisted toroidal fields improved the plasma confinement. Unfortunately, a combination of complex magnetic field coils and the piping for heat removal had to be squeezed into the narrow bore in the center of the toroidal machines. The complexity and lack of space made construction exceedingly difficult.

In the toroidal machines, significant heating can be achieved by generating a large current flow in the plasma. This technique is simple and efficient for initial heating. Before the plasma reaches fusion temperature, the electrical resistance becomes so low heating stops. Final ignition temperatures must be produced by some other method of heating. 140 Figure 3.2 shows the International Toroidal Experimental Reactor (ITER).

A number of methods of heating both linear and toroidal plasma are in test. Energy can be pumped into the plasma by means of microwave or laser beams. The wavelength of the energy beams must be carefully selected to achieve good absorption of the energy by the plasma. Additionally, as the plasma gets hotter the absorption factors change and the beams must be altered to achieve good efficiency. High intensity sources with the proper wavelength require further development.

Encouraging results have been obtained by a technique called neutral beam heating. This is shown in the diagram of the linear machine, Figure 3.1. In this technique, a separate particle accelerator produces an intense beam of deuterium ions. The velocity and thus energy of the deuterium ions in the beam is equivalent to a temperature much higher than needed for ignition of the D + T reaction. The beam of charged particles cannot penetrate the magnetic fields confining the plasma because the charged ions would be scattered by the powerful magnetic fields. To achieve penetration, electrons are added to the beam to neutralize the charge of the ions. The neutral atoms formed by the combination of the ion beam and the electrons still have the same effective temperature but can now penetrate the magnetic field. The neutral beam enters the plasma and its particles collide with the plasma particles.

139 Furth, Harold P. "Reaching Ignition in the Tokamak", Physics Today, Vol. 38, No. 3, March 1985, Page 52

140 Davidson, D. C. and 7 others, "Soviet Magnetic Confinement Fusion Research", Science Applications International Corporation, October 1987

The collision transfers the energy of the beam to the plasma increasing its temperature. The collision also ionizes the neutral beam particles that then become part of the plasma. Neutral beam heating has been tested in both linear and toroidal machines. This method appears to provide a method of heating the plasma to the ignition temperature.

With both linear and toroidal machines, it is necessary to keep the reacting plasma extremely pure. Electrons in the plasma are moving at high velocities. As they pass through the plasma, they encounter the positive charged nuclei repeatedly. Each time the electron passes a nucleus its path is changed and it releases part of its energy as radiation. This radiation travels to the wall of the reactor where it is adsorbed. This process tends to cool the bulk plasma. The amount the electron's path is changed is controlled by the strength of the electric charge on the nuclei. Atoms of high nuclear charge produce far more radiation heat loss than atoms of small nuclear charge. The losses from the interaction of the electrons with the D, T, and He nuclei are acceptable, but the loss from impurity atoms such as carbon, nitrogen, and oxygen are unacceptably high. Atoms of metals used in construction, such as iron, cause catastrophic radiation heat loss. The initial vacuum in the reaction chamber must be of extremely high quality. Impurities must be continually removed to keep the energy loss from the plasma at an acceptably low level. This removal process must also remove the helium that is produced by the reaction. Excess helium can lead to excessive radiation losses.

Much progress has been made in developing methods of maintaining the high purity levels in the plasma. It is now possible to produce plasma with minimum necessary purity and maintain it for the duration (a few seconds) of current experiments. On November 7, 1991 near Oxford in the United Kingdom, the Joint European Torus experiment generated 2 million watts of power in two seconds. 141 The demonstration used a blend of 14% tritium with 86% deuterium as fuel. This success clearly demonstrated the scientific feasibility of the generation of energy from fusion reactions. 142 Refinement and extension of the techniques used in the research machines will lead to the techniques and hardware suitable for continuous operations of a base load power plant. 143' 144 As early as 1976 fusion power plant demonstrations were seriously considered to be feasible. 145

There are a number researchers working to produce or promote magnetically confined fusion reactors. Those currently active are:

CRPP EPFL Lausanne, Switzerland http://crppwww.epfl.ch/

General Atomics, San Diego USA http://fusioned.gat.com/

Jet Joint Undertaking, United Kingdom http://www.jet.uk/tour.html

Fusion Power Associates, Maryland USA http://www.fusionpower.org/

Georgia Tech Fusion Research Center, Georgia USA http://fusion.gat.edu/

Max-Planck-Institut fur Plasmaphysik, Garching Ger. http://www.ipp.mpg.de/

141 http://www.fusion.org.uk/

142 Hamilton, David P, EDITOR "A Fusion First", Science Scope, Science, Vol. 254, No. 5034, November 15,1991, Page 927

143 Sweet, William, "Super Powers Promote Design Effort for Fusion Demonstration Reactor", Physics Today, January 1988, Page 75

144 Thomsen, D. E., "Charging Their Way Toward Fusion", Science News, December 21, 1985, Page 389

"Fusion Power by Magnetic Confinement", Prepared by the Division of Magnetic Fusion Energy of the U. S. Energy Research and Development administration (ERDA-76/110/1 UC-20)

NIFS, Toki, Japan http://nifs.ac.jp/

UKAEA Culham Lab. United Kingdom http://www.fusion.org.uk/

University of Wisconsin-Madison http://fti.neep.wisc.edu/

MIT Plasma Science & Fusion Center Massachusetts, USA http://www.psfc.mit.edu

University of California at Berkeley USA

These studies are examining various aspects of producing fusion energy. They cover a host of approaches. One interesting alternative uses magnetic fields that are produced only out side the reactors. 146 This reactor is termed the CFBR and is discussed below in Other Magnetic Confinement Techniques.

Ongoing research is producing new and useful results. Recently the United States National Fusion Facility in San Diego (Operated by General Atomics Corporation) announced that they had quadrupled the rate of fusion in deuterium plasma. 147 They attempted to increase the stability and lifetime of the plasma by causing it to spin around the axis of the Tokamak reactor. They found that they could spin the plasma and that spinning increased stability. They also found that the plasma spin tended to slow down at an undesirable rate. Further analysis showed that small variations in the smoothness of the magnetic fields were the cause of the slowing. By detecting these small variations and correcting them in real time, the plasma spin did not slow down. The long duration spin stabilized the plasma and allowed a significantly increase the plasma pressure and temperature. The smoothing of the magnetic field required very little power because the variations were small. There is good reason to expect this technique will be useful in all magnetic confinement fusion reactors.

Power plant size will be a major factor in the implementation of fusion energy. Very large machines can function with less intense magnetic fields. Low intensity fields can be operated with less stringent control. Large size provides more room for the placement and access of support equipment such as heat exchangers, magnetic field coils, impurity removal equipment, external heating equipment, vacuum pumps and reactor support structures. The workers involved with both the linear and toroidal machines agree, using today's knowledge it is possible to design a large reactor that will produce power at practical efficiencies. The sizes projected for the operational reactors are in the range of 30,000 to 50,000 Megawatts electrical (MWe), much larger than the 300 to 2,000 MWe of current power stations. The developers of the fusion reactors have seen this large size as a barrier to the near-term implementation of fusion power. In Chapter 6, we will show this large size is not a barrier, but is highly advantageous and will allow these large reactors to serve as the backbone of the future power system.

Princeton Plasma Physics Lab New Jersey USA http://www.pppl.gov/

University of Texas, USA http://w3fusion.ph.utexas.edu/

Los Alamos National Laboratory, New Mexico USA http://fusionenergy.lanl.gov/

Oak Ridge National Laboratory, Tennessee USA

146 http://fusion.ps.uci.edu/beam/introb.html

147 Samuel, Engenie, "Here Comes The Sun", New Scientist, Vol. 171, No. 2299, July 14, 2001, Page 4

Figure 3.2 The ITER Tokamak Reactor

Picture Down-Loaded from ITER Web Site

Figure 3.2 The ITER Tokamak Reactor

Picture Down-Loaded from ITER Web Site

OTHER MAGNETIC CONFINEMENT TECHNIQUES

A spherical configuration of a tokomak has shown significant advantages over the design used in ITER. It is not clear if the advantages will remain when this type of reactor is scaled to larger sizes. Another alternate magnetic confinement scheme is under investigation with funding from the Office of Naval Research. The University of California, Irvine, the University of Florida, Gainesville and the National High Magnetic Field Laboratory, Tallahassee are members of the team. They hope to demonstrate a "Reverse Field" confinement that will permit the use of the boron + proton reaction. They call it the Colliding Beam Fusion Reactor (CBFR). Such a reactor would be a very desirable break through because the boron-proton reaction produces no radioactivity. These reactors also use a linear configuration. They would superficially be similar to the drawing shown in Figure 3.1. During the initial research phase of the program suggested in Chapter 9, the CBFR should be given much attention. It would offer a fusion reactor with very low radiation hazard. The presence of side reactions may produce a small number of neutrons but the promise is a reactor that will be radiation free and easy and low cost to decommission.

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Responses

  • brhane
    Does the heat of plasma leak in magnetic confinement?
    2 years ago
  • Elina
    Does magnetic confinement stop radiation?
    3 months ago

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