First principles simulation of early stage plasma initiation process in ITER-scale tokamak

A first principles 6D kinetic model is developed to study the earliest times of unassisted plasma breakdown in an International Thermonuclear Experimental Reactor (ITER)-scale tokamak. This is then used for a comparative study of the predicted ionisation rate and the electron parallel velocity between the standard model for tokamak breakdown, assuming a zero-D (OD) Townsend avalanche, and the new kinetic model. The detailed model allows us to study the influence of the magnetic field configurations on the formation of plasma while explicitly resolving the electron trajectories. We introduce a 'back-traced' connection length Lbt as a useful predictive tool for the spatial distribution of charged particles during the breakdown process. It is also found that the ionisation rate and the mean electron parallel velocity from the kinetic model generally exceed the 0D model predictions, demonstrating a growth in the total electron population from 103 to the order of 108 in approximately 1 ms. This implies that the 0D model can still serve as a conservative prediction for the first plasma campaign on ITER.