For current profiles with a slope discontinuity at ∊2 close to ∊1[q(∊1) = 1] and a small shear 1(∊1), the jump of the derivative of the magnetohydrodynamic (MHD) solution across the singular q = 1 layer is of the form ∊ is the finite inverse aspect ratio, μ ≡ 1∊1/8(∊2–∊1) measures the ratio of the two small parameters. The growth rate of the sawtooth trigger can accordingly pass from resistive values either negative and finite or positive) to MHD values for slight variations of either 1 or (∊2–∊1)/∊1. The above profile properties have been observed in sawtoothing TEXTOR discharges.
Improved criterion for sawtooth trigger and modelling