The density limit as a consequence of the theory of sawtoothing and the drift wave transport code

At any β and any q0 < 1, the jump of the derivative of the MHD solution across the singular q=1 layer (ρ=ρ1) is sensitive to a rapid variation of the slope of the current density occurring at some ρ2 if the distances ratio (ρ2-ρ1)/ρ1 and the shear parameter s-circumflex(ρ1)=s-circumflex1 are small (∼ aρ1/R). It results that the instability growth rate can pass from small resistive to large MHD values - so that sawtooth relaxations can be triggered - if s-circumflex1 and/or (ρ2-ρ1)/ρ1 is allowed to vary over a certain, correspondingly small, range. Dissipative trapped electron transport in the 'confining' zone beyond the q=1 surface is compatible with these requirements for the occurrence of sawteeth at sufficiently low densities but tends to prevent, for a fixed amplitude of the sawteeth, the MHD phase of the precursors from developing at higher densities. There are thus two possible scenarios for rising density: either the amplitude of the sawteeth increases or the MHD instability is quenched; the sawteeth would then disappear, the current sweep into the core and the plasma contract violently and disrupt. Dissipative trapped electron transport otherwise reproduces extremely well the Neo-Alcator scaling law in Ohmic discharges and the confinement degradation when auxiliary heating is turned on. (author). 9 refs, 5 figs