Field-reversed configuration

Field-reversed configuration: a toroidal electric current is induced inside a cylindrical plasma, creating a poloidal magnetic field, reversed in respect to the direction of an externally applied magnetic field. The resultant high-beta axisymmetric compact toroid is self-confined.

A field-reversed configuration (FRC) is a device developed for magnetic confinement fusion research that confines a plasma on closed magnetic field lines without a central penetration.[1]

History

The FRC was first observed in laboratories in the late 1950s during theta pinch experiments with a reversed background magnetic field.[2] The first studies of the effect started at the United States Naval Research Laboratory (NRL) in the 1960s. Considerable data has been collected since then, with over 600 published papers.[3] Almost all research was conducted during Project Sherwood at Los Alamos National Laboratory (LANL) from 1975 to 1990,[4] and during 18 years at the Redmond Plasma Physics Laboratory of the University of Washington,[5] with the large s experiment (LSX).[6] More recently some research has been done at the Air Force Research Laboratory (AFRL),[7] the Fusion Technology Institute (FTI) of the University of Wisconsin-Madison[8] and the University of California, Irvine.[9] Some private companies now theoretically and experimentally study FRCs in order to use this configuration in future fusion power plants they try to build, like General Fusion, Tri-Alpha Energy, Inc., and Helion Energy.[10]

The FRC is also considered for deep space exploration, not only as a possible nuclear energy source, but as means of accelerating a propellant to very high levels of specific impulse (Isp) for electrically powered spaceships and fusion rockets, with interest expressed by NASA[11][12][13][14][15][16] and the media.[17][18]

Operation

One approach to producing fusion power is to confine the plasma with magnetic fields. This is most effective if the field lines do not penetrate solid surfaces but close on themselves into circles or toroidal surfaces. The mainline confinement concepts of tokamak and stellarator do this in a toroidal chamber, which allows a great deal of control over the magnetic configuration, but requires a very complex construction. The field-reversed configuration offers an alternative in that the field lines are closed, providing good confinement, but the chamber is cylindrical, allowing simpler, easier construction and maintenance.[19]

Field-reversed configurations and spheromaks are together known as compact toroids. Unlike the spheromak, where the strength of the toroidal magnetic field is similar to that of the poloidal field, the FRC has little to no toroidal field component and is confined solely by a poloidal field. The lack of a toroidal field means that the FRC has no magnetic helicity and that it has a high beta. The high beta makes the FRC attractive as a fusion reactor and uniquely suited to aneutronic fuels because of the low required magnetic field. Spheromaks have β  0.1 whereas a typical FRC has β  1.[20][21]

Formation

In modern FRC experiments, the plasma current that reverses the magnetic field can be induced in a variety of ways.

When a field-reversed configuration is formed using the theta-pinch (or inductive electric field) method, a cylindrical coil first produces an axial magnetic field. Then the gas is pre-ionized, which "freezes in" the bias field from a magnetohydrodynamic standpoint, finally the axial field is reversed, hence "field-reversed configuration." At the ends, reconnection of the bias field and the main field occurs, producing closed field lines. The main field is raised further, compressing and heating the plasma and providing a vacuum field between the plasma and the wall.[22]

Neutral beams are known to drive current in Tokamaks[23] by directly injecting charged particles. FRCs can also be formed, sustained, and heated by application of neutral beams.[21][24] In such experiments, as above, a cylindrical coil produces a uniform axial magnetic field and gas is introduced and ionized, creating a background plasma. Neutral particles are then injected into the plasma. They ionize and the heavier, positively-charged particles form a current ring which reverses the magnetic field.

Spheromaks are FRC-like configurations with finite toroidal magnetic field. FRCs have been formed through the merging of spheromaks of opposite and canceling toroidal field.[25]

Rotating magnetic fields have also been used to drive current.[26] In such experiments, as above, gas is ionized and an axial magnetic field is produced. A rotating magnetic field is produced by external magnetic coils perpendicular to the axis of the machine, and the direction of this field is rotated about the axis. When the rotation frequency is between the ion and electron gyro-frequencies, the electrons in the plasma co-rotate with the magnetic field (are "dragged"), producing current and reversing the magnetic field. More recently, so-called odd parity rotating magnetic fields[27] have been used to preserve the closed topology of the FRC.

Plasma stability

Charged particles inside an FRC follow large betatron orbits (their average gyroradius is about half the size of the plasma) which are typical in accelerator physics rather than plasma physics. FRCs are very stable because the plasma is not dominated by usual small gyroradius particles like other thermodynamic equilibrium or nonthermal plasmas. Its behavior is not described by classical magnetohydrodynamics, hence no Alfvén waves and almost no MHD instabilities despite their theoretical prediction,[28] and it avoids the typical "anomalous transport", i.e. all processes in which loss of particles or energy occurs.[29][30][31]

As of 2000, several remaining instabilities are being studied:

Experiments

Selected field reverse experiments, pre-1988[3]
Year Device Location Device length Device diameter B-field Fill pressure Confinement Studied
MeterMeterTeslaPascalSeconds
1959-NRL0.100.0610.0013.332.E-06Annihilation
1961Scylla ILASL0.110.055.5011.333.E-06Annihilation
1962Scylla IIILASL0.190.0812.5011.334.E-06Rotation
1962ThetatronCulham0.210.058.6013.333.E-06Contraction
1962Julich0.100.046.0030.661.E-06Formation, tearing
1963Culham0.300.105.006.676.E-06Contraction
19640-PIIGarching0.300.055.3013.331.E-06Tearing, contraction
1965PharosNRL1.800.173.008.003.E-05Confinement, rotation
1967CentaurCulham0.500.192.102.672.E-05Confinement, rotation
1967JuliettaJulich1.280.112.706.672.E-05Tearing
1971E-GGarching0.700.112.806.673.E-05Tearing, rotation
1975BNKurchatov0.900.210.450.27 - 1.075.E-05Formation
1979TORKurchatov1.500.301.000.27 - 0.671.E-04Formation
1979FRX-ALASL1.000.250.600.53 - 0.933.E-05Confinement
1981FRX-BLANL1.000.251.301.20 - 6.536.E-05Confinement
1982STP-LNagoya1.500.121.001.203.E-05Rotation
1982NUCTENihon2.000.161.006.E-05Confinement, rotation
1982PIACEOsaka1.000.151.406.E-05Rotation
1983FRX-CLANL2.000.500.800.67 - 2.673.E-04Confinement
1984TRX-1MSNW1.000.251.000.67 -2.002.E-04Formation, confinement
1984CTTXPenn S U0.500.120.4013.334.E-05Confinement
1985HBQMU Wash3.000.220.500.53 - 0.933.E-05Formation
1986OCTOsaka0.600.221.001.E-04Confinement
1986TRX-2STI1.000.241.300.40 - 2.671.E-04Formation, confinement
1987CSSU Wash1.000.450.301.33 - 8.006.E-05Slow formation
1988FRXC/LSMLANL2.000.700.600.27 - 1.335.E-04Formation, confinement
1990LSXSTI/MSNW5.000.900.800.27 - 0.67Stability, confinement
Selected field reverse configurations, 1988 - 2011[40]
Device Institution Device type Electron density Max ion or electron FRC diameter Length/diameter
10E19 / Meter^3Temperature [eV][Meter]
Spheromak-3Tokyo UniversityMerging spheromak 5.0 – 10.020 – 100 0.401.0
Spheromak-4Tokyo UniversityMerging spheromak 10 – 40 1.20 - 1.400.5 – 0.7
Compact Torus Exp-IIINihon UniversityTheta-pinch 5.0 – 400.0 200 – 300 0.10 - 0.40 5.0 – 10.0
Field-Reversed Exp LinerLos AlamosTheta-pinch 1,500.0 – 2,500.0 200 – 700 0.03 - 0.05 7.0 – 10.0
FRC Injection ExpOsaka UniversityTranslation trapping3.0 – 5.0 200 – 300 0.30 - 0.407.0 – 15.0
Swarthmore Spheromak ExpSwarthmoreMerging spheromak 10020 – 40 0.401.5
Magnetic Reconnection ExpPrinceton (PPPL)Merging spheromak 5.0 – 20.0 301.000.3 – 0.7
Princeton Field Reverse Exp (PFRC)Princeton (PPPL)Rotating B-field0.05 – 0.3 200 – 300 0.06
Translation Confinement SustainmentUniversity of WashingtonRotating B-field0.1 – 2.5 25 – 50 0.70 - 0.74
Translation Confinement Sustainment-UpgradeUniversity of WashingtonRotating B-field 0.4 – 1.5 50 – 200 0.70 - 0.741.5 – 3.0
Plasma Liner CompressionMSNWTranslation trapping0.20
Inductive Plasma AcceleratorMSNWMerging collision23.0 – 26.0 3500.20
Inductive Plasma Accelerator-CMSNWMerging compression300.01200 - 20000.210.0
Colorado FRCUniversity of ColoradoMerging spheromak
Irvine Field Reverse ConfigurationUC IrvineCoaxial source150.0100.60
C-2Tri Alpha Energy, Inc.Merging collision5.0 – 10.0200 – 500 0.60 - 0.803.0 – 5.0

See also

External links

References

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