H-MODE CONFINEMENT IN TOKAMAKS
The paper reviews present understanding of H-mode physics by summarising relevant experimental observations and discussing possible interpretation. The most important features of the H-mode are a minimum threshold edge temperature (threshold input power) required to achieve the H-mode; the bifurcation nature of the H- transition with instantaneous changes at the plasma edge; and the formation of a transport barrier at the plasma edge leading to pedestals in the density and temperature profiles. Global energy confinement times, tau-"rm E", are typically 2x to 3x longer in H- than in L-mode plasmas, reaching, for example, almost 1 s in 3 MA JET X- point discharges. tau-"rm E" is found somewhat contradictory: no power dependence is found in ASDEX and DIII-D, whereas a degradation with power is indicated in JET and in JFT-2M limiter H-modes. Correspondingly, predictions for full power (40 MW) 6 MA X-point discharges in JET range from 0.6 s to > 1 s, depending upon which scaling is adopted. Two main theoretical models have been proposed to explain the H-mode with its heat barrier at the plasma edge. Such a barrier is predicted by the stability properties of ballooning modes close to a magnetic separatrix, corresponding to the transition to a second stable region above a certain threshold power. It could also arise from a critical temperature gradient model based on self-consistent stochasticity.
Bibliographic Reference: REPORT JET-P(87)36, 1987, AVAILABILITY: JET, ABINGDON, UK
Record Number: 1989126082500 / Last updated on: 1989-05-01
Available languages: en