Study of the phase transition dynamics of the L to H transition

A highly radiating zone (MARFE) just above the divertor X-point has been us ed to access the marginal transition regime P-sep approximate to P-thres to study the existence of a critical point for the L to H transition. Phase t ransition 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 50-10 0 times over fast transitions. However, the evolution of E-r shear, the edg e density gradient, the H-mode pedestal, and fluctuations is essentially un changed from that in fast transitions. The only difference is in the speed with which and the degree to which the fluctuation amplitudes are transient ly reduced. This difference is understandable in terms of the time scales f or fluctuation amplitude reduction (less than or equal to 100 mu s) and edg e pressure gradient increase (several ms), provided the edge fluctuations a re pressure-gradient driven.