Dynamics of the inboard and outboard density profile at ASDEX Upgrade

We present our studies of the scrape-off layer (SOL) at ASDEX Upgrade, focusing on the behavior of the inboard SOL in different magnetic configurations and the dynamics of the high-density region near the inboard plasma edge. Understanding SOL behavior is essential to manage heat exhaust and achieve a detached operation in current and future reactors.

We investigate the existence of hollow density profiles at the inboard midplane SOL by combining several diagnostics and using a novel interpretation of reflectometry data that relies on analyzing the spectral signatures of the beat signal. We show that the poloidal density asymmetry can influence line-integrated measurements, such as interferometry, and should be included in an integrated data analysis.

We investigate the role of the magnetic configuration in shaping the inboard SOL profile, particularly as the plasma approaches a double-null configuration typical of many reactor designs. Our investigations reveal a narrowing of the inboard SOL profile, driven by the magnetic disconnection from the outboard SOL, underscoring the importance of magnetic topology and parallel heat conduction in determining the inboard SOL properties.

We see a steep density gradient at the secondary separatrix in both the midplane and divertor regions, which we attribute to the presence of the inboard high-density region, observable using reflectometry and Thomson scattering diagnostics. Near double-null, a drastic reduction in conducted heat to the inboard SOL leads to decreased electron and neutral density in the inboard divertor.

Lastly, we observe that the high-density region is significantly suppressed in unfavorable field configurations. We use these scenarios to measure the edge density decay length at the inboard and outboard midplane. We estimate a consistently larger outboard decay length than the inboard side, with an average ratio of 1.7, corroborating the expected ballooning nature of radial transport in tokamaks.