Low edge safety factor operation and passive disruption avoidance in current carrying plasmas by the addition of stellarator rotational transform

Abstract

Low edge safety factor operation at a value less than two (q(a)=1/ι̷tot(a)<2) is routine on the Compact Toroidal Hybrid device with the addition of sufficient external rotational transform. Presently, the operational space of this current carrying stellarator extends down to q(a)=1.2 without significant n = 1 kink mode activity after the initial plasma current rise phase of the discharge. The disruption dynamics of these low edge safety factor plasmas depend upon the fraction of helical field rotational transform from external stellarator coils to that generated by the plasma current. We observe that with approximately 10% of the total rotational transform supplied by the stellarator coils, low edge q disruptions are passively suppressed and avoided even though q(a) < 2. When the plasma does disrupt, the instability precursors measured and implicated as the cause are internal tearing modes with poloidal, m, and toroidal, n, helical mode numbers of m/n=3/2 and 4/3 observed on external magnetic sensors and m/n=1/1 activity observed on core soft x-ray emissivity measurements. Even though the edge safety factor passes through and becomes much less than q(a) < 2, external n = 1 kink mode activity does not appear to play a significant role in the disruption phenomenology observed.

Perturbing an axisymmetric magnetic equilibrium to obtain a quasi-axisymmetric stellarator

“…The idea of passively stabilizing a tokamak by non-axisymmetric perturbations is also supported by a number of experimental results, when the perturbation generates a sufficient ‘external’ boost in rotational transform (W VIIA Team 1980; Pandya et al. 2015). Solving the magnetohydrodynamic (MHD) equilibrium problem for optimal stellarator equilibria, without the use of numerical optimization algorithms (i.e.…”

Properties of a new quasi-axisymmetric configuration

“…These configurations are capable of being compact thanks to their relatively large bootstrap current which provides a source of rotational transform, in addition to that from the coils. On the other hand, the similarity to stellarators provides potential benefits: QA configurations can run in steady state potentially without any current drive and there is evidence from other types of stellarators that disruptions can be avoided if the vacuum rotational transform, created solely by the coils, is sufficiently large [2][3][4]. The related concept of quasi-helical symmetry was numerically proven in 1988 [5] and experimentally confirmed in a series of experiments on the HSX stellarator [6,7].…”

Stellarator Research Opportunities: A Report of the National Stellarator Coordinating Committee

“…A common metric employed to predict the disruptivity is the fraction of transform produced from external coils with typical values of order 10% distinguishing disruption from nondisrupting shots. The CTH program has dedicated a substantial portion of its research activities to addressing this question [79]. If QS stellarators are susceptible to current driven instabilities, methods for profile control may need to be developed.…”