High density experiments in TCV ohmically heated and L-mode plasmas
Recent experiments have been performed on the Tokamak a configuration variable (TCV) to investigate the confinement properties of high density plasmas and the mechanism behind the density limit. In a limiter configuration with plasma elongation kappa = 1.3-1.4 and triangularity delta = 0.2-0.3 the operational density range has been extended up to 0.65 of the Greenwald density at I-p = 200 kA (q(95) = 3.7) and even to the Greenwald value at low plasma current I-p = 110 kA (q(95) = 7). A transition from the linear to the saturated ohmic confinement regime is observed at high density similar to 0.4n(GW). A further density increase leads to sawtooth stabilization and is accompanied by a decrease of the energy and particle confinement times. The development of the disruption at the density limit was preceded by sawtooth stabilization. It is shown that electron cyclotron heating leads to the prevention of sawtooth stabilization and then to the increase of the density limit value.