Reversed field pinch

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 the RFX (R/a = 2/0.46) in Padua, Italy. Others include the MST (R/a = 1.5/0.5) in the United States, EXTRAP T2R (R/a = 1.24/0.18) in Sweden, TPE-RX (R = 0.51/0.25) in Japan, and KTX (R/a = 1.4/0.4) in China.

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.

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.

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