Experimental scaling of fluctuations and confinement with Lundquist number in the reversed-field pinch

The scaling of the magnetic and velocity fluctuations with Lundquist number (S) is examined experimentally over a range of values from 7 x 10(4) to 10(6) in a reversed-field pinch (RFP) plasma. Magnetic fluctuations do not scale uniquely with the Lundquist number. At high (relative) density, fluctuations scale as (b) over tilde alpha S-0.18, in agreement with recent numerical results. Fluctuations are almost independent of S at low (relative) density, (b) over tilde alpha S-0.07. The range of measured exponents is narrow and is in clear disagreement with theories predicting (b) over tilde alpha S-1/2. At high relative density, the scaling of the energy confinement time follows expectations for transport in a stochastic magnetic field. A confinement scaling law (nT(E) alpha beta(4/5).T-7/10.a(-3/5).I-phi(2)) is derived, assuming the persistent dominance of stochastic magnetic diffusion in the RFP and employing the measured scaling of magnetic fluctuations, The peak velocity fluctuations during a sawtooth cycle scale marginally stronger than magnetic fluctuations but weaker than a simple Ohm's law prediction. The sawtooth period is determined by a resistive-Alfvenic hybrid time (T(saw)alpha root tau(R) tau(A)) rather than a purely resistive time. (C) 1998 American Institute of Physics.