Electromagnetic Analysis of Breakdown Conditions and Experimental Validation of 0d Townsend Breakdown Theory on the Tcv Tokamak

A better understanding of the physics of tokamak breakdown has the potential to ensure reliable operation and increase confidence in the startup scenarios of future tokamak devices. This article presents an experimental validation of 0D Townsend breakdown theory in a toroidal geometry using the TCV tokamak and two separate time-dependent 2D torodally-symmetric electromagnetic field reconstruction techniques. Breakdown parameters have been scanned along three experimental axes: neutral pressure, toroidal loop voltage, and-for the first time-poloidal magnetic field null gradient. The magnetic null gradient is observed to have a direct influence on the effective connection length: a smaller magnetic null gradient leads to a larger effective connection length and hence a lower loop voltage required for breakdown. 0D Townsend theory successfully predicts whether a breakdown would occur within 1 sigma error bars in over 96% of cases only when a minimum number of ionizing collisions per unit connection length between 8 and 13 is considered. Secondary electron emission from the high-field-side (HFS) wall is hypothesized to explain both the observed proximity of early radiation to the HFS wall and the empirically inferred minimum number of ionizing collisions required per connection length to achieve the onset of breakdown. There could be, however, additional physics at play.