Speaker
Description
Even though the importance of the mesoscale structures (e.g. “avalanches”, “blobs”, vortices) in
the magnetized plasma transport was recognized a long time ago (e.g. see [1-4]), still the physics
of such objects has many open issues and the extensive studies of these phenomena, both
theoretical and experimental, are continue (e.g. see [5] and the references herein).
The avalanche is usually portrait as a “front” of enhanced plasma (particle/energy) flux
propagating “ballistically” in radial direction. As a “proof” of such phenomenon following from
the simulations, the researchers present the poloidally averaged flux, j(t,x), as the function of
time t and the radial coordinate x. For the case of the radial ballistic propagation of the flux
enhancement, δj, the function δj(t,x) produces a straight “stripe” on (t, x) plane (e.g. see Fig. 2
from [6]).
Our simulations of the resistive-drift-wave (RDW) turbulence with the modified
Hasegawa-Wakatani (mHW) equations [7] for a small electron adiabaticity parameter α also
show similar “stripes” of δj(t,x) on the (t, x) plane (see Fig. 4 from [6]). However, it is well
known that for α <1 the RDW turbulence becomes rather similar to the 2D fluid turbulence [8-
10]. It does not exhibit any front-like phenomena and is dominated by a long leaving mesoscale
vortices (e.g. see Fig. 1 from 2D fluid turbulence modeling [8] and Fig. 3 from the plasma RDW
turbulence modeling with the mHW equation [10]).
Further examination of the results of our RDW turbulence modeling reveal that the origin
of the “stripes” on the function δj(t,x), is a “pairing” of the different sign but similar magnitude
vortices forming poloidally directed “dipoles”, which can propagate ballistically in radial
direction on a large distance before the pair is disintegrated due to the interactions with other
vortices. In addition to that, such a “dipole” results in “dragging” radially the whole plasma and
enhances the perturbation of plasma density/energy in between the vortices, which increases the
flux even more. We notice that whereas the positive perturbation is advected by the dipole
“down the hill” of plasma density (they can be the “seeds” of the blobs in the SOL!), the
negative one moves “up the hill”. Finally, no proximity to “marginal stability” is needed for the
advection mechanism described here. Whether such a turbulent mesoscale advection mechanism
works for the turbulence driven by other plasma instabilities relevant to fusion devices is a
matter for our further studies.
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