Non-axisymmetric equilibrium reconstruction and suppression of density limit disruptions in a current-carrying stellarator

Abstract

Reconstruction of non-axisymmetric, three-dimensional (3D) plasma equilibria is important for understanding 3D confinement and stability in stellarators as well as in nominally axisymmetric plasmas in tokamaks and reversed-field pinches (RFP). Tests of non-axisymmetric free-boundary equilibrium reconstruction are performed on the Compact Toroidal Hybrid (CTH) experiment, a small torsatron/tokamak hybrid device. The flexible CTH magnetic configuration allows varying the amount of 3-D shaping by modifying the rotational transform of the magnetic field. 3D equilibrium reconstruction on this device is used to determine the effect of the internal current on the 3D shaping of the MHD equilibrium. The results are used to interpret the stability, and disruptive characteristics of hybrid stellarator/tokamak plasmas.

These studies were performed using the 3D equilibrium reconstruction code V3FIT with experimental measurements from external magnetics and soft X-ray (SXR) cameras. It was verified that external magnetic diagnostics have limited sensitivity to accurately reconstruct the internal experimental current distribution. Instead the current distribution was reconstructed by two methods using soft X-ray measurements. The location of the sawtooth inversion radius can be identified by soft X-ray analysis to infer the radial location of the q=1 surface, which is used to fit parameters of plasma current profile with greater precision than with magnetic signals alone. Secondly, SXR emissivity multi-channel measurements are used to reconstruct the shape and position of flux surfaces, and infer the current distribution within the plasma. The reconstruction results are consistent with those using external magnetic data and the constraint of the location of q=1 surfaces determined from inversion surface radii extracted from SXR emission data.

Improved reconstructions of current and q profiles provide insight into understanding the physics of density limit disruptions observed in current-carrying discharges in CTH. The phenomenology of hybrid discharge terminations is similar to tokamak disruptions. As a result of the ability to adjust the external vacuum rotational transform in CTH, we have found the density limit at a given current increases linearly with the addition of vacuum transform. Consequently, plasmas with densities up to two times the Greenwald limit are attained at the maximum vacuum transform of 0.22. Equilibrium reconstructions show that addition of 3D fields effectively moves resonance surfaces towards the edge of the plasma where the current profile gradient is lower, providing a stabilizing effect to the growth of resistive rearing modes.