Plasma control system design and disruption forces modelling with the open-source platform ERMES

Plasma control is essential for the correct operation of a magnetic confinement nuclear fusion reactor. The plasma must be properly shaped and positioned to make possible the generation of fusion energy. Control systems are composed of a set of coils interacting with this plasma through magnetic fields. These fields can be distorted by the different components of the reactor. These perturbative effects must be considered during the design phase and be adequately compensated to guarantee the successful performance of the machine. Also, in the event of losing control of the plasma, we need to calculate the induced forces to design the supporting structures accordingly and avoid damage.

For the modelling of those problems, we have developed the C++ open-source finite element code ERMES. Thanks to its stabilized formulations in the frequency domain, it can solve realistic geometries with low memory requirements. Also, it saves computational time by treating each frequency independently and in parallel. Once the frequencies of interest are calculated, we generate a transfer function characterizing the fusion machine. This transfer function can be used in a real time controller. Alternatively, we can generate the time evolution of the fields and forces via Laplace or Fourier transforms. We can even reproduce the movement of the plasma and the halo currents. Complex frequencies and plasma modes have been implemented in a new version of ERMES to increase its modelling capabilities. The inclusion of non-linear materials using frequency domain techniques is the next stage of development.