Measurements in the DIII-D tokamak [J. L. Luxon , Plasma Physics and Contro
lled Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 198
7), Vol. I, p. 159] show that edge gradients of ion and electron temperatur
e and pressure, delT(i), delT(e), delP(i), and delP(e), are good candidates
for parameters which respond to the heating power to control the fundament
al physics of the spontaneous Low-mode (L-mode) to High-mode (H-mode) trans
ition. These gradients are measured in the region where the H-mode transpor
t barrier forms and they are found to consistently increase in time during
the L-phase of discharges which make a transition to H-mode. Moreover, for
a fixed magnetic configuration, there is a well-defined boundary between th
e L-mode and H-mode states in the delT(e)-delP(e) operational space. The va
lues of T-i, T-e, P-i, and P-e, measured at this location, show smaller rel
ative changes during the L-phase of these discharges, indicating that the s
cale lengths of these quantities are decreasing as the L-H transition is ap
proached in time. These results are consistent with several theoretical mod
els in which gradients of pressure or temperature cause the transition. (C)
2001 American Institute of Physics.