DYNAMICS OF SPATIOTEMPORALLY PROPAGATING TRANSPORT BARRIERS

A simple dynamic model of spatiotemporally propagating transport barri ers and transition fronts from low (L) to high (H) confinement regimes is presented. The model introduces spatial coupling (via transport) i nto the coupled evolution equations for flow shear and fluctuation int ensity, thus coupling the supercritical L to H bifurcation instability to turbulent transport. Hence; fast spatiotemporal front propagation and evolutionary behavior result. The theory yields expressions for th e propagation velocity and termination point of an L-H transition fron t and transport barrier When the evolution of the pressure gradient, d el P-i, and the contribution of del P-i to sheared electric field, E'( r), is included, the ambient pretransition pressure gradient acts as a local source term that drives the evolution of the poloidal velocity shear. The transition may then evolve either as a spatiotemporally pro pagating front or as a uniform (i.e., nonlocal) fluctuation reduction or quench. The precise route to transition adopted depends on the rela tive magnitudes of the front transit time, tau(T), and the fluctuation reduction time, tau(f), respectively. The relevance of spatiotemporal ly propagating L-H transition fronts to the very high confinement regi me (VH mode) evolution in DIII-D [R. I. Pinsker and the DIII-D;Team, P lasma Physics and Controlled Nuclear Fusion Research 1992 (Internation al Atomic Energy Agency, Vienna, 1993), Vol, 1, p. 683] and in the Joi nt European Torus (JET) [Plasma Physics and Controlled Nuclear Fusion Research 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 27] is discussed. (C) 1995 American Institute of Physics.