Importance of the divertor configuration for attaining the H-regime in ASDEX

Confinement in ASDEX with neutral beam and RF heating

“…Recent experiments with high-power ICRH in ASDEX, however, afford strong evidence that the L and H-regimes are universal confinement modes of auxiliary-heated tokamak plasmas in the sense that they are not dependent on a specific heating method (heating-related features such as power deposition or impurity production can, however, indirectly influence the confinement). They further support our previous findings (Wagner and Keilhacker, 1984; that it is the near-edge electron temperature (or its gradient) that determines whether a discharge develops into an L or an Ii-mode. This latter fact is further verified by recent experiments utilizing carbonized walls which yield lower edge temperatures (owing to an impurities) and are thus less susceptible to transition into the H-mode.…”

Neutral recirculation—the key to control of divertor operation

“…The divertor is intended to shift the plasma-wall contact area further away from the core plasma in the hope of improving the screening efficiency of the impurities produced by sputtering, and improving the pumping efficiency by compressing the neutrals in front of the pump duct entrance. Furthermore, experiments show that the magnetic configuration with an x-point inside the plasma volume can be beneficial for core confinement [1]. However, there is no free lunch: compression of the flux surfaces in the divertor leads to concentration of the power flux on the divertor targets and this results in severe power loads, which can become a major operational constraint in a fusion reactor [2].…”