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

• Published in: Physics of plasmas 1998-04, Vol.5 (4), p.1004-1014
• Main Authors: Stoneking, M. R., Chapman, J. T., Den Hartog, D. J., Prager, S. C., Sarff, J. S.
• Format: Article
• Language: English
• ISSN: 1070-664X; 1089-7674
• DOI: 10.1063/1.872670

The scaling of the magnetic and velocity fluctuations with Lundquist number (S) is examined experimentally over a range of values from 7×10 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 b̃∝S −0.18 , in agreement with recent numerical results. Fluctuations are almost independent of S at low (relative) density, b̃∝S −0.07 . The range of measured exponents is narrow and is in clear disagreement with theories predicting b̃∝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 (nτ E ∝β 4/5 ⋅T −7/10 ⋅a −3/5 ⋅I φ 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-Alfvénic hybrid time (T saw ∝ τ R τ A ) rather than a purely resistive time.