Study of the relation between density and temperature fall-off lengths in a detached divertor plasma

Theory has shown that completely detached gas targets are completely controlled by the transverse ion–neutral collisionality. In order to transform this information to a format of practical relevance, knowledge of the plasma density width Δ*n at the gas target entrance (GTE) is required. In a previous work, Δ*n was determined with the relation Δ*n∝ΔT, where ΔT is the upstream temperature fall-off length, which can be reliably estimated. This paper provides evidence for this relation, which is not a priori obvious. In the absence of empirical data, one has to rely on a computational (B2-EIRENE) database. Proportionality between Δ*n and ΔT is confirmed. Complementary analytical considerations are presented to elucidate the underlying physics.