Impact of the α parameter on the microstability of internal transport barriers

In plasmas exhibiting an internal transport barrier (ITB), locally very high pressure gradient (∇P) is obtained. It induces high values of the magnetohydrodynamic α parameter (α = −q ² βR∇P/P, with R the major radius, q the safety factor, P the pressure, ∇ the radial gradient and β the ratio between kinetic and magnetic pressure). Similarly to low or negative magnetic shear (s), high α reduces the curvature and ∇ B drifts driving curvature-type microinstabilities. Therefore, high values of α can stabilize part of the microturbulence, which leads to higher pressure gradient and to even higher α. This possibility for entering a positive feedback loop is very attractive to sustain ITBs in high performance plasmas. Indeed, α scales favourably with higher pressure and does not require any external momentum input. In this paper, after having discussed the α stabilization mechanism in detail, we report the experimental microstability analyses of ITBs from an international multi-machine database—the International Tokamak Physics Activity database, accessible on the web^b. We show that α is indeed a relevant parameter of ITB physics that should be taken into account in interpretative and predictive one-dimensional transport codes.