Fully Off-Axis ECCD Driven Plasmas in TCV

摘要

Fully non-inductive plasma discharges sustained by off-axis Electron Cyclotron Current Drive (ECCD) in the TCV tokamak are reported. Steady state discharges with very good confinement properties and a large bootstrap current fraction can be created in the absence of an Ohmic field. The total current density profile is hollow owing to the bootstrap current. A steady state electron Internal Transport Barrier (eITB) is observed inside the ECCD deposition radius. Once fully formed, the eITB is efficient in that the addition of 0.45 MW central EC power causes no degradation in the confinement time. Best results are obtained with a small central counter current drive component. In this plasma H_(RLW) = 4.5 is reported (H_(TLW) =τEe/τRLW) with bootstrap current fraction I_(BS)/I_P = 50%. The current profile can be finely tuned at zero loop voltage, allowing operation just below the MHD instability limit. Current density profile modeling is performed with a Fokker-Planck code using a model for radial transport. The bootstrap current is calculated from Thomson profiles.