The Search for Reconnection and Helicity During Formation of a Bounded Spheromak

Romero-Talamás, C.A.; McLean, H. S.; Hooper, E. B.; Wood, R. D.; LoDestro, L.L.; Moller, J.M.

doi:10.1007/s10894-007-9110-5

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“…Our study is concerned with linear current-driven instabilities, which in their nonlinear phase cause the velocity and magnetic field perturbations responsible for driving the closed flux current in HICD toroidal confinement. The instability driving this process in laboratory spheromak experiments can be identified as a finite amplitude kink instability of the open flux tube which has a helical structure and rotates [8,10,11,20,[32][33][34]. The perturbation of the closed flux by the rotating mode is responsible for the current drive which sustains it against resistive decay.…”

Section: Introductionmentioning

confidence: 99%

Helicity-injected current drive and open flux instabilities in spherical tokamaks

Brennan¹,

Browning²,

Gates³

et al. 2009

Plasma Phys. Control. Fusion

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The toroidal current driven by relaxation processes in a cylindrical spherical tokamak (ST) geometry with coaxial injected flux is estimated by use of the linear ideal stability boundary of equilibria with a high current on the open driven flux and a lower current on the closed flux. Instabilities with toroidal mode number n = 1 have been shown to play a vital role in the helicity injection current drive, being closely associated with the relaxation process which distributes current from a directly driven open flux to a closed flux. Previous results for spheromaks (1D and 2D equilibria) and STs (1D equilibria) have predicted stabilization, for a given open flux current, if the closed flux plasma current is sufficiently large, suggesting that the current drive mechanism is self-limiting. New results presented here for 2D ST equilibria are consistent with the 1D results, but new features appear in the stability maps as the axial length and toroidal field (TF) strength are varied in the equilibria. These include changes in the shape of stability boundaries and the estimated driven current, changes in the mode structure due to equilibrium changes and resonance effects which extend stability boundaries into the stable region. As the minimum and maximum of the safety factor q profile cross integer rational values, the resonant mode is destabilized, causing regions of enhanced instability in the current profile parameter space. The results show the effects on the stability of varying the geometric length ratio R/L and provide driven current estimates with varying imposed TF strengths; these results have implications both for existing STs and for the design of future devices.

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