The multiple facets of Ohmic confinement in ASDEX

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Abstract
The scaling of the energy confinement time with plasma density and current has been investigated for ohmically heated tokamak discharges in ASDEX. The linear dependence τE e is maintained in the high density improved Ohmic confinement (IOC) regime with peaked density profiles. The peaking of the radial density profile can be brought about by reducing the net power flow through the plasma surface, thereby leading to a reduction of the edge density. Tailoring of the radiation profile with the addition of low-Z impurities, for example neon, gives access to the IOC regime under conditions where otherwise the degraded saturated Ohmic confinement (SOC) behaviour prevails. The energy confinement time τE increases with current and decreases with heating power also in Ohmic discharges, as is shown by a statistical analysis. However, with the intrinsic coupling between power and current, the two relationships cancel and τE becomes independent of POH and Ip. The two most prominent features of Ohmic confinement can therefore be explained on the basis of simple physical models.