Study of the scaling of magnetic fluctuations in the RFX reversed field pinch¹

The scaling of magnetic fluctuations with the Lundquist number ( S ) is studied in the RFX reversed field pinch (RFP) device for different operational regimes. For the first time in a RFP experiment a complete set of diagnostics to measure all the quantities needed to estimate S is available. Due to the high plasma currents (up to 1.1 MA) obtained in RFX, a broad range of S values is available for scaling. The results are compared with numerical MHD simulations (SPECYL code) providing a reasonable agreement (α exp = -0.18±0.02, α SPECYL = -0.22). The modification to the expected scaling law due to viscous effects is also discussed. We also perform a comparison of the amplitude of magnetic fluctuations in discharges with different characteristics: standard, pulsed poloidal current drive (PPCD), quasi single helicity and rotating locked mode induced by an external rotating toroidal field modulation. A reduced level of fluctuations is obtained during PPCD operations, although the dependence on S is the same. Concerning confinement time, at fixed I / N ≈3 × 10 14 A m, a favourable scaling with S is found. Its interpretation in terms of the simple Rechester-Rosenbluth model is briefly discussed.