Tomographic imaging of resistive mode dynamics in the Madison Symmetric Torus reversed-field pinch

Abstract

A detailed study of the dynamics and magnetic topological effects of resistive-tearing modes is presented for different operational regimes in the Madison Symmetric Torus reversed-field pinch [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)]. Soft-x-ray tomography and magnetic measurements, along with numerical reconstruction of magnetic-field lines with the ORBIT code [R. B. White and M. S. Chance, Phys. Fluids 27, 2455 (1984)], have been employed. Magnetic-mode dynamics has been investigated in standard plasmas during the transition to the quasi-single helicity state, in which a single mode dominates the mode spectrum. Single helical soft-x-ray structures are studied with tomography in these cases. These structures are associated with magnetic islands, indicating that helical flux surfaces appear in the plasma. Mode dynamics has also been examined during auxiliary inductive current drive, the goal of which is to reduce the tearing-mode amplitudes. In this case the phenomenology of the soft-x-ray structures appearing in the plasma is more complex. In fact, when a quasi-single helicity spectrum occurs, a single island bigger than in the standard case is usually found. On the other hand, when all modes decrease, two helical soft-x-ray structures are observed, with the same helicity as the two innermost resonant modes. This constitutes the first direct evidence of magnetic-chaos reduction during auxiliary inductive current drive, which is responsible for the achievement of the best confinement in the reversed-field pinch configuration up to now.

What is a Reversed Field Pinch?

“…Therefore the magnetic field displays two magnetic axes: the unperturbed axisymmetric one and the one related to the island O-point. For such states, termed Double Axis (DAx) states, a thermal helical structure winding around the unperturbed magnetic axis is observed [91], characterized by an electron internal transport barrier [92,93], and the maximum electron temperature (up to 1.2 keV in 2011) is tightly correlated with the location of the magnetic island as reconstructed by external measurements. This internal transport barrier is the strongest in the rising phase of the quasi single helicity state [94].…”

Vanishing Magnetic Shear And Electron Transport Barriers In The RFX-Mod Reversed Field Pinch

“…Therefore the magnetic field displays two magnetic axes: the unperturbed axisymmetric one and the one related to the island O point. For such states, termed double axis (DAx), a magnetic island with electron ITB [8] winds around the magnetic axis [9] and is characterized by a maximum of the electron temperature tightly correlated to the location of the O point as reconstructed by external measurements. The second bifurcation changes the topology of the magnetic field through the coalescence of the axisymmetric O point with the island X point.…”

High-performance double-filter soft x-ray diagnostic for measurement of electron temperature structure and dynamics

“…[1][2][3] Moreover, the double-foil ratio technique can be applied to brightness or emissivity measurements to calculate electron temperature (T e ). 4 The double-foil technique has been used on RFX and NSTX for single-point and radial T e profile measurements.…”