First Achievement of Stationary Doublet Plasmas in the TCV Tokamak

Stationary, long-lived doublet plasmas lasting several current redistribution times have been obtained for the first time ever. Doublets are tokamak equilibria with two distinct current maxima creating two 'lobes' delimited by a figure-8 separatrix and surrounded by a 'mantle' region of closed field lines. Doublets require simultaneous feedback control of two unstable n=0 modes and have proven challenging to stabilize in past efforts in a series of dedicated 'Doublet' devices (1969-1983) as well as in previous experiments on TCV. The recent breakthrough was enabled by the development of a multi-domain capable free-boundary Grad-Shafranov equilibrium evolution solver coupled with a toroidal current diffusion equation, which permitted extensive pre-shot closed-loop control simulations. Accurate control of the magnetic configuration is achieved by an advanced multivariable feedback controller acting directly on poloidal field coil voltages based on estimated last closed flux surface position errors at a set of control points. Doublets lasting up to 2s, or approximately 10 current redistribution times, are now routinely obtained. The plasmas have an elongation of kappa=3.1 using up the entire volume of the TCV vessel and exceeding TCV's record elongation of conventional, single-axis plasmas, without using in-vessel stabilizing coils. First diagnostic measurements and observations of these fully stationary doublet configurations will be presented, as well as first exploration and phenomenology of plasma current and density limits in heated and non-heated doublet scenarios. This novel development offers new and exciting opportunities to study the doublet configuration in detail, in particular the region of natural reversed magnetic shear in the mantle, with a modern complement of diagnostics and under the effect of various heating and current drive systems.