Particle flux profile widths and broadening in the divertor region of ASDEX Upgrade

In future divertor tokamaks, power exhaust remains one of the critical challenges to ensure long-term operation. The divertor heat flux onto the target plate depends crucially on the heat flux profile width, since a broad profile spreads the heat flux onto a larger area and increases the radiated power. The total heat flux profile width is determined by the SOL width, and the additional broadening across the divertor leg into the Private Flux Region (PFR). While the SOL has been studied extensively [1], comparatively little is known about the PFR transport.

This contribution presents key experimental findings on profile widths measured by Langmuir probes in ASDEX Upgrade (AUG). Analysis of L- and H-mode datasets reveals that the outer target ion flux extends 5-10 mm into the PFR in attached conditions. This width agrees quantitatively with the results of an analytical model, based on the balance between parallel and the non-divergence-free part of the diamagnetic flow in the PFR. According to the model, the PFR gradient length scales inversely with poloidal magnetic field Bp and increases with distance ∆𝑅 between outer target and X-point major radii. Although the variation of B_p and within the investigated AUG datasets is too small to test their dependencies, future dedicated experiments in AUG and comparison to other machines will allow for such testing. For ITER and SPARC the model predicts an order of magnitude lower widths than for AUG, due to the larger B_p, i.e. no significant broadening is expected.