Characterizing the edge plasma of different Ohmic confinement regimes in ASDEX

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Abstract
To compare different Ohmic confinement regimes in ASDEX, the edge conditions are analyzed in detail. The results show that the improved Ohmic confinement coincides with a drop of the separatrix density. This drop allows the density profile to peak and seems to be the trigger of a change in the transport. A universal scaling between the electron temperature and the electron density at the separatrix prevails for all Ohmic scenarios. In addition, the total particle flux across the separatrix is evaluated and found to be strongly correlated to the separatrix density. Thus, the convective energy loss contributes less to the total energy losses when the confinement is improved. Since the correlations between the edge parameters do not change in different Ohmic confinement regimes of ASDEX, the edge physics appears to remain the same. Improved Ohmic confinement is characterized by an optimum separatrix density which provides a sufficiently high edge temperature together with low particle fluxes. These optimum conditions yield the maximum particle confinement.