Theoretical approaches to turbulent transport near marginal stability
- Princeton Univ., NJ (United States)
The presence of large microturbulent transport, such as caused by ion-temperature-gradient-driven (ITG) fluctuations, tends to drive temperature profiles toward marginal (linear) stability over much of the minor cross-section. The possibility that some profiles may be submarginal (linearly stable) yet carry substantial turbulent flux is of particular interest: it affects the interpretation of experimental data, and may imply that linear analysis is inadequate for the accurate determination of stable (e.g., enhanced-reversed-shear) operating regimes. Submarginal profiles are intimately related to nonlinear instability mechanisms for the self-sustainment of turbulence even in the presence of eigenmodes that axe linearly stable. Such self-sustainment has been observed in a variety of computer simulations. In the present work, the following approaches to the analysis of submarginal transport and nonlinear self-sustainment are conceptually linked and exploited: exactly solvable statistical model problems; discrete {open_quotes}sand-pile{close_quotes} dynamics and self-organized criticality (SOC); bifurcation theory; and {open_quotes}nonlinear instability{close_quotes} mechanisms. A nontrivial yet solvable statistical advection model is constructed that emphasizes the importance of subcritical bifurcations to submarginal turbulent profiles. The SOC of discrete lattice automata is interpreted as a consequence of a kind of subcritical bifurcation, and the submarginal profiles of certain SOC models axe related to the subcritical dynamics of the solvable model. Drake`s recent reduced model for the nonlinear instability of collisional drift waves is shown to exhibit a subcritical Hopf bifurcation, lending support to the interpretation of the mechanism as a driver for self-sustained turbulence. An analogous bifurcation is sought for a simple model of ITG turbulence, and the universality of the nonlinear instability is addressed.
- DOE Contract Number:
- AC02-76CH03073
- OSTI ID:
- 489365
- Report Number(s):
- CONF-960354-; TRN: 97:011506
- Resource Relation:
- Conference: International Sherwood fusion theory conference, Philadelphia, PA (United States), 18-20 Mar 1996; Other Information: PBD: 1996; Related Information: Is Part Of 1996 international Sherwood fusion theory conference; PB: 244 p.
- Country of Publication:
- United States
- Language:
- English
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