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A reversed-field pinch (RFP) is a device used to produce and contain near-thermonuclear plasmas. It is a toroidal pinch which uses a unique magnetic field configuration as a scheme to magnetically confine a plasma, primarily to study magnetic fusion energy. Its magnetic geometry is somewhat different from that of the more common tokamak. As one moves out radially, the portion of the magnetic field pointing toroidally (see inset) reverses its direction, giving rise to the term "reversed field". This configuration can be sustained with comparatively lower fields than that of a tokamak of similar power density. One of the disadvantages of this configuration is that it tends to be more susceptible to non-linear effects and turbulence. This makes it a perfect laboratory for non-ideal (resistive) magnetohydrodynamics. RFPs are also used in the study of astrophysical plasmas as they share many features.

The largest Reversed Field Pinch device presently in operation is called the Reversed-Field eXperiment (RFX) in Padua, Italy. Others include the Madison Symmetric Torus, EXTRAP T2R in Sweden, and TPE-RX in Japan.

A toroidal coordinate system in common use in plasma physics. The red arrow indicates the poloidal direction (θ) and the blue arrow indicates the toroidal direction (φ)

Unlike the Tokamak, which has a much larger magnetic field in the toroidal direction than the poloidal direction, an RFP has a comparable field strength in both directions (though the sign of the toroidal field reverses). Moreover, a typical RFP has a field strength approximately one half to one tenth that of a comparable Tokamak. The RFP also relies on driving current in the plasma to reinforce the field from the magnets through the dynamo effect.

There are several potential advantages to an RFP. One such advantage is that there is no established β limit. β is a parameter which relates the average pressure gradient to the magnetic field strength. In practical terms β can be thought of as a term of relative efficiency (this is not precisely true, but works as a rough guide).

• Natural plasma
• Economical
• Compact
• High power density (minimum energy, stable)
• Ignition without the need of auxiliary heating
• Internal dynamos
• Turbulence driven
• Established scalings

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The reversed-field pinch works towards a state of minimum energy.

The magnetic field lines coil loosely around a center torus. They coil outwards. Near the plasma edge, the toroidal magnetic field reverses and the field lines coil in the reverse direction.

Internal fields are bigger than the fields at the magnets.

• Self-organized plasmas [PDF]
• RFX: Reversed-Field eXperiment
• Measurement of superthermal electron flow and temperature in a reversed-field pinch experiment by an electrostatic electron energy analyser

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