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.