Chapter 4: High-Density Regimes in the FTU

High-density plasmas (n_o ~ 8 × 10²⁰ m^-3) achieving steady improved core-confinement have been obtained in the Frascati Tokamak Upgrade (FTU) up to the maximum nominal toroidal field (8 T) by deep multiple pellet injection. These plasmas exhibit also high purity, efficient electron-ion coupling, and peaked density profiles sustained for several energy confinement times. Neutron yields in excess of 1 × 10¹³ n/s are measured, consistent with the reduction of the ion transport to neoclassical levels. Improved performance is associated with sawtooth stabilization that occurs when the pellet penetrates close to the q = 1 surface. In this regime, impurity accumulation can be prevented if a slow sawtooth activity is maintained. Experiments aimed at obtaining radiation-improved modes at high field have also been carried out using neon injection. The observed increase of the average density, with respect to the reference discharge, is significantly larger than the contribution of Ne. The neutron yield increases also by a factor of 3 to 6, and the energy confinement time increases by a factor up to 1.4.