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Unified parametric dependence, reconstruction, and control of 3D equilibria in the RFP

Author: Brett E. Chapman
Requested Type: Consider for Invited
Submitted: 2012-12-07 14:51:08

Co-authors: Representing the MST group, Auburn University, Consorzio RFX, and UCLA

Contact Info:
UW-Madison
1150 University Avenue
Madison, WI   53706
USA

Abstract Text:
A helical (stellarator-like) equilibrium can occur in the core of nominally axisymmetric tokamak and RFP plasmas. Examples include the "snake" in tokamaks and the "single-helical-axis" (SHAx) state in RFP's. Such an equilibrium is also predicted to emerge in planned hybrid-scenario plasmas in ITER. The SHAx state emerges in both the MST and RFX-mod RFP's when the m = 1 tearing mode spectrum spontaneously condenses -- the innermost resonant mode grows to large amplitude while the other, secondary mode amplitudes are reduced. In MST, the amplitude of the dominant mode reaches 8% of the equilibrium field strength. In both devices, the likelihood and duration of these peaked spectra increase strongly with the toroidal plasma current, Ip. However, the Ip at which these spectra emerge in MST is nearly three times smaller than in RFX-mod. But due to additional differences in temperature, effective ionic charge, majority ion mass, and density, the two devices share a similar range of Lundquist number, S = tau_resistive/tau_Alfven ~ Ip*(Te^3/2)/Z/sqrt(m*n) > 6 x 10^5, an important dimensionless parameter in resistive MHD. This unification suggests S as a predictive parameter for the onset of the peaked spectra and helical equilibria. The evolution of the central magnetic topology during the emergence of SHAx was measured directly in MST with a combined Faraday rotation-interferometry diagnostic. This was the first such measurement in a tokamak or RFP, as previous observations relied largely on x-ray and electron temperature data. Given the parallels between SHAx and stellarator plasmas, the 3D equilibrium reconstruction code V3FIT is now being applied to both MST and RFX-mod. And given the success of Faraday rotation on MST, this diagnostic is now being incorporated in V3FIT. SHAx plasmas in RFX-mod exhibit improved global energy confinement, due both to the reduction of the secondary modes and the formation of an electron thermal transport barrier in the core. This is in otherwise typical RFP plasmas with a steady Ip waveform. In such plasmas in MST, there is a similar reduction in the secondary modes, which provides a modestly improved global energy confinement time > 1 ms, but these MST plasmas do not exhibit a central transport barrier. In MST we have now applied inductive current profile control to these 3D plasmas. Such control has long been utilized as a robust means of reducing current-driven tearing instability. By slowly ramping down Ip, the inductive parallel electric field profile is broadened, which further reduces the secondary mode amplitudes. This produces a larger central Te > 1 keV, an energy confinement time of about 3 ms, and a 6 keV/m Te gradient in the core, similar to what is observed with the transport barrier in RFX-mod. Work supported by USDOE.

Characterization: 1.4

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