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