The destruction of magnetic surfaces in tokamaks by current perturbations

Investigations of the feasibility of the hypothesis that the disruptive instability in tokamaks is caused by the break-up of magnetic surfaces are presented. The envisioned break-up is caused by the interaction of m=2, n=1 and m=3, n=1 helical perturbations or by the interaction of the m=2, n=1 perturbation and the m=1, n=0 toroidal field (with R/a=5). Both numerical and analytic work has been done for two MHD-equilibrium current profiles: parabolic [q approximately (1-r2/2a2)-1] and the peaked model (q approximately 1+1lambda.r2/a2). For the parabolic profile, perturbation strengths of 1.5% are required to break up surfaces when m=2 and m=3 perturbations are present. When the m=2 perturbation alone interacts with the toroidal field, perturbation strengths greater then 4% are required. For the peaked model with q(0)=1.1, q(a)=3.7, perturbation strengths of 0.5% were found to be sufficient to break the surfaces when both m=2 and m=3 perturbations are present, whereas strengths of 1.5% are required when only the m=2 perturbation interacts with the toroidal field. These perturbation amplitudes are in reasonable agreement with experimental measurements. The peaked model allows break-up of surfaces for smaller perturbation strengths, primarily because of its greater shear. This dependence on shear permits speculation on a possible mechanism for the disruptive instability. (author)