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 ? parameter (? = ?q 2 ?R?P/P, with R the major radius, q the safety factor, P the pressure, ? the radial gradient and ? the ratio between kinetic and magnetic pressure). Similarly to low or negative magnetic shear (s), high ? reduces the curvature and ? B drifts driving curvature-type microinstabilities. Therefore, high values of ? can stabilize part of the microturbulence, which leads to higher pressure gradient and to even higher ?. This possibility for entering a positive feedback loop is very attractive to sustain ITBs in high performance plasmas. Indeed, ? scales favourably with higher pressure and does not require any external momentum input. In this paper, after having discussed the ? 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. We show that ? is indeed a relevant parameter of ITB physics that should be taken into account in interpretative and predictive one-dimensional transport codes.