Empirical study of gradient lengths ratio η _e in the near SOL region in ASDEX Upgrade tokamak

A survey of the correlation between density and temperature gradient lengths (&lamba;_ne and λ_Te) has been performed for the near SOL regions in the ASDEX Upgrade tokamak. In common with previous studies, the ratio of the near SOL density and temperature gradient lengths, is found to be close to unity across a wide range of plasmas. However, it is found that it is possible for both the density and temperature gradients to vary separately without varying the other greatly. The empirical results show that the correlation between density and temperature profiles in the near SOL is related to the divertor regimes. In the linear/sheath limited regime, the density and temperature profiles are closely correlated, η_e is observed to be close but slightly less than unity. In the high recycling/conduction limited regimes, which is the most common case for ASDEX Upgrade plasma, &lamba;_ne and λ_Te are correlated due to a common dependence on q₉₅ or, equally, I_p A weak dependence of η_e on separatrix temperature is observed in this regime. Since the separatrix temperature depends only weakly on the power density flowing through the SOL and the variation is very small, a small variation of η_e is observed. For ASDEX Upgrade, it lies between 1 and 2 for most of discharges in the conduction limited regime. In the detached regime very high η_e of up to 6, are observed. This is associated with the transition to the high density limit where convective transport due to filaments dominates particle transport resulting in very flat density profiles. At the same time, temperature profiles are observed to be unchanged.