Interpretation of negative central shear electron internal transport barriers

Box-type electron temperature profiles observed in negative central shear plasmas with electron internal transport barriers are explained with the help of a novel instability mechanism that is predicted when keeping account of the radial component of the trapped electron gradB and curvature drifts in the theory of the trapped electron mode. These radial velocity components lead to a new term in the quasi-slab radial eigenvalue equation which modifies the asymptotic behaviour in such a way that growing–decaying pairs of bounded solutions are obtained (instead of the usual magnetic shear damped solutions) if ŝLTe > 0. There are no bounded solutions if ŝLTe < 0. As LTe = Te/∂rTe is usually negative, instability requires negative magnetic shear ŝ. The driving mechanism is largest for quasi-slab modes in view of the poloidal angle dependence of the radial drift components. The ratio Qe/Te e∼(qRω∗ e,l/cs), where Qe is the anomalous electron energy flux and e the particle flux, is quite large owing to the non-adiabatic trapped electron response and the condition (qRω∗ e,l/cs) 2 1 that must be satisfied to avoid ion Landau damping on the side-bands (ω∗ e,l is the electron diamagnetic frequency, l the toroidal mode number, 1/qR the parallel wave vector of the side-bands and cs = √ Te/mi the sound speed).