Turbulence and the formation of transport barriers in finite β tokamaks

The paper presents the results of 3-D nonlinear simulations of drift-ballooning modes in a torus, including electromagnetic effects, the self-consistent modification of the local magnetic shear due to the finite β shift of the flux surfaces and velocity shear. In the electrostatic limit the simulation reveal that a transport barrier forms spontaneously when the shift of the surfaces reverses the local magnetic shear on the outside midplane. Simulations with electromagnetic effects reveal that in plasmas with circular cross sections a strong increase of the transport occurs well below both the ideal β limit and the point where local shear reversal occurs. This strong enhancement of electromagnetic turbulence prevents the formation of the transport barrier unless the local sheared flow is sufficiently large. (author). 20 refs, 4 figs