A reversed-field pinch (RFP) is a device used to produce and contain near-thermonuclear plasmas. It is a toroidal pinch that uses a unique magnetic field configuration as a scheme to magnetically confine a plasma, primarily to study magnetic confinement fusion. Its magnetic geometry is somewhat different from that of a tokamak. As one moves out radially, the portion of the magnetic field pointing toroidally 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 useful system for studying non-ideal (resistive) magnetohydrodynamics. RFPs are also used in studying astrophysical plasmas, which share many common 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, RELAX (R/a = 0.51/0.25) in Japan, and KTX (R/a = 1.4/0.4) in China.

Characteristics

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.

Magnetic topology

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.

RFP in Fusion Research: comparison with other confinement configurations

The RFP has many features that make it a promising configuration for a potential fusion reactor.

Advantages

Disadvantages

Several key areas present challenges in the development of RFP reactors. Researchers are actively working on solutions for these issues:

Plasma Physics Research

The Reversed Field Pinch is also interesting from a physics standpoint. RFP dynamics are highly turbulent. RFPs also exhibit a strong plasma dynamo, similar to many astrophysical bodies. Basic plasma science is another important aspect of Reversed Field Pinch research.