Improved confinement regime in the current ramp down phase of Frascati Tokamak Upgrade plasmas

The current ramp down phase of the Frascati Tokamak Upgrade plasma has been found to exhibit very different particle transport than that which is observed in the Ohmically heated steady state phase. The current ramp down phase can be characterized by a central (r/a<0.5) region that has anomalous transport similar to that seen in the steady state phase and an outer region (r/a>0.5) that has transport two to three times less. The temperature in the inner region had a Gaussian-like shape as a function of minor radius. In the outer region, the temperature had roughly a single value (∼100 eV) with no sharp gradients which implied no gradient driven transport. To study the particle transport, iron was introduced into the ramp down phase by using the laser blow off technique. The time behaviour of several charge states of iron have been measured spectroscopically to determine the transport in the different spatial locations in the plasma. The time histories of the charge state line emission in the ramp down phase have been observed to decay much more slowly than in the steady state phase which implied the existence of a transport barrier at the half radius. The behaviour of the impurity charge states has been modelled with the MIST transport code using up to date atomic physics rates to infer the particle transport.