Simulation and Control of Doublet Plasmas in TCV

The doublet plasma configuration in tokamaks is characterized by two plasma cores connected by an internal separatrix surrounded by a further `mantle' region of confined plasma. Axisymmetric control of doublets is notoriously difficult due to the presence of two vertically unstable modes, one symmetric and one antisymmetric with respect to the midplane. The latter of which is coupled to changes in current distribution and plasma shape, necessitating a virtual testbed for controller design.

To enable model-based doublet controller design, the FGE free-boundary Grad-Shafranov evolution code has been extended to include an integrated 1D current diffusion model crucial to replicate the experimental behavior of doublets in simulation. FGE enables rapid in-the-loop testing of controllers and additionally provides a linearized model of plasma dynamics for growth rate estimation and controller tuning.

A first-of-its-kind doublet shape controller has been developed and tested in closed-loop simulations with FGE.

The controller is designed to provide direct feedback control on mantle shape which was found to be an important parameter when tackling the vertical instability.

We present results from the doublet campaign at TCV including progress on controller development, demonstrations of the FGE-based doublet simulations, and experimental outcomes.