ITER breakdown and plasma initiation revisited

This paper revisits a number of key aspects of plasma initiation, aiming to clarify concepts, provide definitions and improve understanding, in view of ITER First Plasma operation. It shows that size matters and that breakdown and plasma initiation differ in larger devices. The large thick and conductive ITER vessel slows down the prefill process, the development of the toroidal electric field and affects the dynamics of the poloidal magnetic field at breakdown. The large vacuum vessel requires more than 1 s for the prefill gas to spread over the vessel. It slows down the development of the toroidal electric field, applied to ionize the prefill gas, by about 1 s and complicates the control of the magnetic configuration required for plasma initiation. It is shown that the avalanche process that provides the initial ionization slows down towards the end, if the cross-section of the toroidal discharge is larger. On the other hand, a larger plasma volume is beneficial for the subsequent burn-through of main-species ionization and impurity line-radiation. ITER First Plasma operation aims to reach a minimum plasma current of 100 kA, which will require a full avalanche and a partial burn-through to reach electron temperatures of at least 10-20 eV. The burn-through limits the ITER prefill pressure upper range of about 1 mPa. These predictions are based on OD models of plasma initiation that furthermore assume the plasma volume remains constant. As burn-through is a critical process, models that consider radial profiles and/or volume dynamics, could provide further insight in the constraints of ITER plasma initiation. Electron cyclotron heating (ECH) high-power microwaves can be used to assist the ITER plasma initiation process. Although the ECH burn-through assist is reasonably well understood, no model exist that can predict the behaviour of ECH pre-ionization on ITER plasma initiation. ECH burn-through assist is shown to be ineffective for ITER First Plasma operations. Low prefill pressures are thought to increase the likelihood to develop a highly energetic supra-thermal electron discharge during the plasma initiation process. Considering supra-thermal electrons could affect the assumptions for the plasma resistance or even the ionization rate coefficient in plasma initiation models, hence altering the expected dynamics. Furthermore, only a qualitative picture of the processes that cause such supra-thermal discharges exists and a dedicated model is needed to improve ITER predictions.