A highly radiating zone (MARFE) just above the divertor X-point has been used to access the marginal transition regime to study the existence of a critical point for the L to H transition. Phase transition models predict that at the critical point the transition duration increases and the plasma parameters vary continuously between the L- and H-mode. In these experiments, the L to H transition duration increased 50-100 times over fast transitions. However, the evolution of shear, the edge density gradient, the H-mode pedestal, and fluctuations is essentially unchanged from that in fast transitions. The only difference is in the speed with which and the degree to which the fluctuation amplitudes are transiently reduced. This difference is understandable in terms of the time scales for fluctuation amplitude reduction and edge pressure gradient increase (several ms), provided the edge fluctuations are pressure-gradient driven.
Electron density evolution after L–H transitions and the L–H/H–L cycle in ASDEX Upgrade