Magnetic and thermal relaxation in the reversed field pinch

The paper is devoted to a tutorial review of the relaxation process in reversed field pinch (RFP) plasmas. Both magnetic and thermal aspects of the relaxation process are discussed and the main models proposed to explain it in the framework of nonlinear magnetohydrodynamics are reviewed. Particular attention is paid to the bifurcation between single helicity and multiple helicity states which is present in the phase space of a quasirelaxed RFP. Experimental evidence of transitions between bifurcated states, observed in many experiments and leading to enhanced confinement regimes, are discussed and a possible explanation for them is proposed. The role of Prandtl number in controlling transitions is discussed.