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Tokamak plasma is characterized by the presence of multiple time scales for transport processes. Two possible mechanisms for very fast propagation of the heat pulses have been proposed. One is based on an assumption that plasma turbulence due to toroidal or nonlinear effects generates very long correlated structures across the magnetic field. In this case the information about a sudden change of the turbulence induced transport coefficients propagates along the radius with the group velocity of the unstable oscillations, which is usally very large. The second possiblity is the idea of critical marginality which implies the existence of a finite threshold for the excitation of the turbulence. If above the threshold the thermal conductivity is sufficiently large, the temperature profile will adjust in such a way to be almost everywhere close to marginal stability. Thus at steady state the value of the thermal conductivity will be much smaller than the value well above threshold. However, if the system is driven far from marginality by some perturbation, it will react on the fast time scale associated to the diffusivity above the threshold. Both these ideas are examined, paying particular attention to the numerical analysis of the different transient phenomena recently observed.

Additional information

Authors: CHERUBINI A ET AL, JET Joint Undertaking, Abingdon, Oxon. (GB);GALLI P, Association EURATOM-ENEA-VNR, Istituto di Fisica del Plasma, Milano (IT);ROMANELLI F, ENEA, Centro Richerche Energia Frascati, Roma (IT)
Bibliographic Reference: Report: JET-P(96)08 EN (1996) 15pp.
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon, OX14 3EA (GB)
Record Number: 199610691 / Last updated on: 1996-06-21
Original language: en
Available languages: en