Description
Stationary, long-lived doublet plasmas lasting several current redistribution times have been obtained for the first time ever at TCV. Doublets are tokamak equilibria with two current maxima creating two ‘lobes’ delimited by a figure-8 separatrix and surrounded by a ‘mantle’ region of closed field lines. Doublets feature two unstable n=0 modes and have proven challenging to stabilize in past efforts, both in a series of dedicated ‘Doublet’ devices (1969-1983) and in previous experiments on TCV. Developments of a multi-domain capable free-boundary Grad-Shafranov equilibrium evolution solver and the implementation of a new advanced magnetic control system at TCV enabled accurate feedback control and stabilization of the doublet configuration in a recent breakthrough. Doublets lasting up to 2s, or approximately 10 current redistribution times, are routinely obtained and permit novel and exciting opportunities to study various aspects of these configurations. For the first time in history, doublets are analyzed under the effects of different heating and current drive systems and using modern diagnostics. We present results for various operational regimes, exploring the behaviour at different densities, shapes, and plasma currents. We experimentally validate model-based estimates of axisymmetric stability and explore the effects of mantle size on MHD activity and operational limits. Finally, we explore the role of the mantle for particle and heat exhaust through impurity seeding experiments.