Section: New Results

Optimization of tokamak breakdown scenarios

Participants : Holger Heumann, Eric Nardon.

The standard method to initiate a plasma in a tokamak is to realize a so called Townsend avalanche by applying a high enough toroidal electric field (i.e. loop voltage) by means of a fast variation of the current in the poloidal field coils (in particular the central solenoid). For the avalanche to take place, the electrons need to be able to travel along the field lines over a long enough distance, so that they can gain an energy significantly larger than the ionization energy of the atoms. An empirical criterion for a successful breakdown is thus $E{L}_c>70V$, where $E$ is the toroidal electric field and $L_c$ the connection length of the field lines. Hence, it is highly desirable to create a configuration in which the field length is as large as possible, or equivalently, in which the poloidal component of the field is as small as possible. We reformulated this task as a constrained optimization problem and used an implementation in FEEQS.M to find in an automated fashion such optimal configurations. Publication is in preparation.