The possibility of geodesic acoustic mode-initiated LH-transition in the TUMAN-3M tokamak

Abstract

A simple numerical model is proposed for modeling the possibility of the LH-transition (transition to improved plasma confinement state) being triggered by a burst of radial electric field oscillations in a tokamak. One-dimensional density evolution is considered to be governed by an anomalous diffusion coefficient dependent on radial electric field shear (spatial inhomogeneity measure). The radial electric field is taken as the sum of oscillating term and quasi-stationary one determined by density and ion temperature gradients through neoclassical formula. If oscillating field parameters (amplitude, frequency etc) are properly adjusted, a transport barrier forms at the plasma periphery and sustains after oscillations are switched off, manifesting a transition into high confinement mode with strong inhomogeneous radial electric field and suppressed transport at the plasma edge. Electric field oscillation parameters required for LH-transition triggering are compared with geodesic acoustic mode parameters observed at the TUMAN-3N tokamak in the ohmic LH-transition discharges. It is concluded on the basis of this comparison that the geodesic acoustic mode may act as a trigger for the LH transition.