The Lundquist number scaling of nonlinear MHD fluctuations and transition from quasi-continuous to discrete reconnection activity in MST RFP plasmas*

Nonlinear MHD fluctuations appear in both natural and magnetic confinement settings, such as the solar wind, self-organization dynamics in the RFP and spheromak, and current disruptions in tokamak plasmas. Here we describe parameter scaling experiments oriented toward nonlinear MHD dynamics in RFP plasmas. Experimental data are gathered spanning a wide range of Lundquist number, S ∼ 10⁴ −10⁷ by varying the plasma current and density, n_e/n_G , where n_G is the empirical density limit. Magnetic fluctuation amplitudes, b_n, for toroidal modes n= 7-10 resonant at mid-radius are scaled as b_n ∝ S^-α, with α=0.3  for <!--[if gte msEquation 12]> style='font-family:"Cambria Math",serif;mso-ascii-font-family:"Cambria Math";
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normal'>n _{style='mso-bidi-font-style:normal'>e} n_e/n_G =0.23 and α= 0.4<!--[if gte msEquation 12]> style='font-size:12.0pt;font-family:"Cambria Math",serif;mso-fareast-font-family:
"Times New Roman";mso-bidi-font-family:"Times New Roman";mso-ansi-language:
EN-US;mso-fareast-language:EN-US;mso-bidi-language:AR-SA'> m:val="roman"/>α= 0.4 for n_e/n_G =0.34. A new programmable power supply enabled low-current, low-S operation, which overlaps parameters available in numerical MHD modeling. Experimental S scalings for b_n agree well with those from the DEBS and NIMROD codes. A transition from bursty relaxation with discrete sawteeth at high current to quasi-continuous relaxation at low current is observed at threshold S ∼ 10⁵. The spectral properties of the magnetic fluctuations change, including a reduction in phase velocity that suggests plasma flow and/or flow profile changes. Work supported by the U.S. DOE and an M. Hildred Blewett Fellowship.