Plasma shape stabilisation of current rise MHD instabilities in TCV

Abstract. The well-known and potentially disruptive plasma “current rise” instabilities are studied as a function of the plasma shape in the tokamak à Configuration Variable (TCV). Disruptions typically occur in quasi-circular plasmas at qa ∼ 3 in both non-sawtoothing and sawtoothing discharges with peaked current profiles. The perturbations in the plasma parameters before disruption are characterised, and the main unstable modes identified as coupled m/n=2/1 and 3/2 rotating tearing modes. In the early phase, coupling between 3/1 and 2/1 modes is found to play a major role in determining whether or not the disruption will occur. Plasma cross section shaping is observed to reduce or to completely stabilise the disruptive mode and is regularly used in TCV operation as a tool for safe initial current ramp-up. Plasma elongation, positive and negative triangularity prevent the growth of a large 2/1 mode at qa ∼ 3, thus reducing or even suppressing the disruptions. We also attempt an interpretation of the experimental results. Calculations of the tearing mode stability parameter ∆ ′ using the experimental plasma equilibria suggest the dominant role of toroidal mode coupling in the destabilisation of the m/n = 2/1 mod