Relaxation of spheromak configurations with open flux

García-Martínez, Pablo Luis; Farengo, Ricardo
November 2009
Physics of Plasmas;Nov2009, Vol. 16 Issue 11, p112508
Academic Journal
The relaxation of several kink unstable equilibria with open flux representative of spheromaks sustained by dc helicity injection is studied by means of three-dimensional, resistive magnetohydrodynamic simulations. No external driving is applied, but the initial conditions are chosen to reproduce the current profiles existing in a gun driven spheromak, which has a high current density in the open flux region and a low current density in the closed flux region. The growth and nonlinear saturation of various unstable modes, the dynamo action which converts toroidal flux into poloidal flux, and the evolution of the λ profile (λ=μ0J·B/B2) are studied. An initial condition is found which results in a dynamo that produces enough poloidal flux to compensate the resistive losses occurred during a characteristic time of the instability. The flux amplification factor around which this case oscillates is consistent with existing experimental data. During the relaxation, the central open flux tube develops a helical distortion and the closed flux surfaces are destroyed. After the relaxation event, close flux surfaces form again but the final profiles are not fully relaxed and the central open flux tube remains distorted. The effect of the Lundquist number on the evolution and its impact on the required level of fluctuations are evaluated. Finally, the dynamics of the system for different current levels in the open flux region is studied.


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