379 research outputs found
Dynamics of plasma blobs in a shear flow
The global dynamic of plasma blobs in a shear flow is investigated in a simple magnetized torus using
the spatial Fourier harmonics (k-space) framework. Direct experimental evidence of a linear drift in
k space of the density fluctuation energy synchronized with blob events is presented. During this drift, an
increase of the fluctuation energy and a production of the kinetic energy associated with blobs are observed.
The energy source of the blob is analyzed using an advection-dissipation-type equation that includes
blob-flow exchange energy, linear drift in k space, nonlinear processes, and viscous dissipations.
We show that blobs tap their energy from the dominant E B vertical background flow during the linear
drift stage. The exchange of energy is unidirectional as there is no evidence that blobs return energy to the
flow
Fluctuation characteristics of the TCV snowflake divertor measured with high speed visible imaging
Tangentially viewing fast camera footage of the low-field side snowflake
minus divertor in TCV is analysed across a four point scan in which the
proximity of the two X-points is varied systematically. The motion of
structures observed in the post- processed movie shows two distinct regions of
the camera frame exhibiting differing patterns. One type of motion in the outer
scrape-off layer remains present throughout the scan whilst the other, apparent
in the inner scrape-off layer between the two nulls, becomes increasingly
significant as the X-points contract towards one another. The spatial structure
of the fluctuations in both regions is shown to conform to the equilibrium
magnetic field. When the X-point gap is wide the fluctuations measured in the
region between the X-points show a similar structure to the fluctuations
observed above the null region, remaining coherent for multiple toroidal turns
of the magnetic field and indicating a physical connectivity of the
fluctuations between the upstream and downstream regions. When the X-point gap
is small the fluctuations in the inner scrape-off layer between the nulls are
decorrelated from fluctuations upstream, indicating local production of
filamentary structures. The motion of filaments in the inter-null region
differs, with filaments showing a dominantly poloidal motion along magnetic
flux surfaces when the X-point gap is large, compared to a dominantly radial
motion across flux-surfaces when the gap is small. This demonstrates an
enhancement to cross-field tranport between the nulls of the TCV low-field-side
snowflake minus when the gap between the nulls is small.Comment: Accepted for publication in Plasma Physics and Controlled Fusio
Dependence on plasma shape and plasma fueling for small edge-localized mode regimes in TCV and ASDEX Upgrade
Within the EUROfusion MST1 work package, a series of experiments has been conducted on AUG and TCV devices to disentangle the role of plasma fueling and plasma shape for the onset of small ELM regimes. On both devices, small ELM regimes with high confinement are achieved if and only if two conditions are fulfilled at the same time. Firstly, the plasma density at the separatrix must be large enough (n e,sep/n G ⌠0.3), leading to a pressure profile flattening at the separatrix, which stabilizes type-I ELMs. Secondly, the magnetic configuration has to be close to a double null (DN), leading to a reduction of the magnetic shear in the extreme vicinity of the separatrix. As a consequence, its stabilizing effect on ballooning modes is weakened
Blob-induced toroidal momentum transport in simple magnetized plasmas
The link between toroidal flows and density blobs is experimentally demonstrated in TORPEX simple magnetized plasmas: momentum is transferred from an ideal-interchange mode to density blobs. The phase shift between the toroidal flow and the density perturbations observed in the interchange mode where the blob is born is conserved along the blob radial trajectory. This leads to dipolar structures of the blob-induced flow or to monopolar perturbations, so large that the toroidal flow gets transiently reversed. The turbulent toroidal momentum flux is dominated either by the nonlinear flux or by the convective part but not by the Reynolds stress componen
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