Measurement of internal magnetic field fluctuations in a reversed-field pinch by Faraday rotation.

Magnetic field fluctuations (and the associated current perturbation) have been measured in the core of a high-temperature reversed-field pinch using a newly developed fast-polarimetry system. Radial magnetic field fluctuation levels of � 1% are measured in standard-reversed-field pinch discharges which increase to � 4% during the sawtooth crash (enhanced dynamo). The fluctuation level is reduced fourfold for high-confinement plasmas where the core-resonant tearing modes are suppressed. In many laboratory and natural plasmas, the magnetic field fluctuates spontaneously in space and time. Magnetic fluctuations can have a large effect on the macroscopic behavior of plasmas. Two effects are of particular importance. First, magnetic fluctuations can cause magnetic field lines to wander stochastically. Energy and particle transport can then arise from particles streaming along the stochastic magnetic field lines. Second, fluctuations can generate a large-scale magnetic field through current driven by a dynamo effect. Energy transport and dynamo effects from magnetic fluctuations are especially crucial to the reversed-field pinch (RFP) laboratory plasma configuration. It has long been considered that the transport arises from stochasticity induced by overlapping magnetic islands, and that the current density profile is partly determined by the dynamo [1]. In addition, improved confinement in the RFP, induced by control of the current density profile, has been conjectured to be related to the reduction of magnetic fluctuations within the plasma [2 ‐ 4]. Understanding transport and dynamo physics requires measurement of the fluctuating magnetic field inside the high-temperature plasma. Magnetic probes have been used to measure the edge magnetic field fluctuations, as well as the internal fluctuation radial profiles in smaller, colder RFP plasmas [5,6]. Until now, however, direct nonperturbing measurement of the magnetic fluctuations in the hot plasma core has been lacking in the RFP, as well as in other toroidal configurations. In this Letter, we report the first measurement of magnetic field fluctuations (and the related current density fluctuations) in the core of a high-temperature RFP. Their relation to transport and dynamo in the RFP is investigated. This is accomplished through measurement of the Faraday rotation of an injected far-infrared laser beam using a new high-speed polarimeter system. We report three new results. First, we find that broadband internal magnetic fluctuations grow rapidly at the sawtooth crash, indicating their relation to the dynamo. The change in fluctuation amplitude correlates with the relaxation of the core current density profile and is consistent with the standard model of MHD relaxation and dynamo in the RFP. Second, fluctuating current associated with the dominant magnetic islands is measured and found to have a spatial extent � 8c mabout the rational surface. The localization of the current, while the magnetic fluctuation is radially global, is consistent with the theoretical expectation for tearing modes. Third, we find that in plasmas with improved confinement the long-wavelength core magnetic fluctuation amplitude decreases fourfold. The decrease in core magnetic fluctuations is larger than has been previously inferred from measurements at the plasma surface.