|Fig. 1: Schematic diagram of the tokamak reactor. (Source: J. Li, after Baker et al. )|
Among a variety of elements that can undergo fusion reactions, the deuterium-tritium (D-T) fuel cycle has the highest reaction rate and largest power density (17.6 MeV per reaction) at a specific amount of plasma density.  The D-T fusion reaction is
Thus, the D-T fusion cycle has been paid special attention by the researchers during the past decades, and it is thought that the first commercial fusion reactor may be a D-T fusion reactor. This report will introduce the concepts of design on Tokamak, Mirror, and Elmo Bumpy Torus magnetic confinement fusion reactors.
The tokamak reactor was initiated in the Soviet Union in the mid 1950s, and now becomes the mainstream device being studied in the field of magnetic confinement fusion research in the world.  The basic tokamak configuration is shown in Fig. 1. The tokamak topology is an axisymmetric, toroidal (closed) configuration in which the plasma particles are confined by nested surfaces composed of helical magnetic field lines. There are three parts of magnetic fields in the device. The strongest one is the toroidal field, BT, created by a toroidal solenoid magnet. The poloidal field, BP, is created by the toroidal plasma current and additional poloidal coils surrounding the plasma. Also, a weak vertical magnetic field, BV, is provided. The ordering of the magnitude of these magnetic fields is generally BT > BP > BV.  The tokamak reactor is the most promising device for magnetic confined fusion reaction so far, and is expected to be the vehicle by which energy breakeven will first be demonstrated for fusion energy in the future.
|Fig. 2: Evolution of mirror reactors. (Source: J. Li, after Baker et al. )|
Magnetic fusion devices of the mirror type are characterized by open magnetic field line geometries (magnetic flux passes through a mirror-type device and intersects material walls outside the reaction chamber), as shown in Fig. 2. In order for such a device to be an adequate container of fusion plasma, it is essential that end leakage of the plasma be strongly inhibited. In a simple mirror device, a charged particle travels in a helical orbit around an axially directed magnetic field line. When traveling into a region of increasing magnetic field strength, the particle's rotational energy is increased at the expense of its axial energy. Depending on the strength of the mirror field and the particle's initial energy distribution, the particle will be reflected. 
|Fig. 3: Simplified schematic of the Elmo Bumpy Torus reactor. (Source: J. Li, after Baker et al. )|
The EBT concept is a toroidal array of simple magnetic mirrors, shown as Fig. 3. The promise of a steady- state, high-beta reactor that operates at or near D-T ignition emerges from this combination of simple mirrors and toroidal geometry. The creation of an rf-generated, low-density, and energetic electron ring at each position between mirror coils is needed to stabilize the bulk, toroidal plasma against well-known instabilities associated with simple mirror confinement.  The EBT configuration of fusion reactor is described with the following attractions: steady-state operation in an ignited or a high-Q mode; a potential for high-beta operation with the attendant efficient utilization of magnetic field energy; large aspect ratio to give an open and accessible geometry; an engineering-assembly that is comprised of relatively simple and compact modules; ease of maintenance, modular construction, and a relatively simple magnet system. 
© Jiachen Li. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 J. D. Lawson, "Some Criteria for a Useful Thermonuclear Reactor," UK Atomic Energy Resarch Establishment, AERE-GP/R-1807, December 1955.
 L. A. Artsimovich, "Tokamak Devices," Nucl. Fusion 2, 215 (1972).
 C. C. Baker, G. A. Carlson, and R. A. Krakowski, "Trends and Developments in Magnetic Confinement Fusion Reactor Concepts," Fusion Sci. Technol. 1, 5 (1981).
 D. J. Bender et al., "Reference Design for the Standard Mirror Hybrid Reactor," Lawrence Livermore National Laboratory, UCRL-52478, May 1978.
 D. G. McAlees et al., "The Elmo Bumpy Torus Reactor (EBTR) Reference Design," Oak Ridge National Laboratory, ORNL/TM-5669, November 1976.