This paper addresses two key issues regarding plasma exhaust in future fusion reactors. Firstly, using newly developed spectroscopic models to measure the divertor concentration of Ne and Ar, it is shown that the experimental detachment threshold on ASDEX Upgrade with Ar-only, an Ar+N mixture, and a Ne+N mixture scales as expected in comparison with an analytical equation derived by Kallenbach et al [1]. However, it is found that Ar radiates more efficiently and Ne less efficiently in the scrape-off layer (SOL) than the model predicts. The second issue concerns the divertor reattachment timescales following transient changes to the upstream SOL conditions. By separately increasing the neutral beam injection power and cutting the impurity gas flow, it is shown that the partially detached scenario reattaches in ~100 ms while the strongly detached X-point radiator (XPR) scenario takes ~250 ms. The former timescale is determined by the core energy confinement time, whereas the latter also has an additional delay caused by the time required for the detachment front to move from the X-point to the target. A simple equation with scalable geometric terms to predict the front movement time in future machines is proposed.