Electron Thermal Transport Barrier and Magnetohydrodynamic Activity Observed in Tokamak Plasmas with Negative Central Shear

In the Rijnhuizen Tokamak Project, plasmas with steady-state negative central shear (NCS) are made with off-axis electron cyclotron heating. Shifting the power deposition by 2 mm results in a sharp transition of confinement. The good confinement branch features a transport barrier at the off-axis minimum of the safety factor $(q)$, where q≤3, and two magnetohydrodynamic (MHD) instabilities, where one is localized at the off-axis minimum of q and one covers the entire NCS region. The low confinement branch has $q>3$ everywhere, no transport barrier, and no MHD activity.