Current filaments in turbulent magnetized plasmas. E. Martines. Introduction. Turbulent transport in fusion plasmas is intermittent , because it is dominated by the contribution of coherent structures .
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turbulent magnetized plasmas
Turbulent transport in fusion plasmas is intermittent, because it is dominated by the contribution of coherent structures.
Coherent structures (blobs) are usually identified through their electrostatic features (potential or density structures).
However, at the values found in the edge of fusion devices, electromagnetic features are expected.
This is the main (but not exclusive) motivation for looking for current filaments.
A good ground for comparison with other plasma physics branches.
... and certainly many others!!
U-probe, a complex probe equipped with triple probes (n, Te, p) and magnetic probes (Br, B, B).
In particular, we can compute
The magnetic field and EB hodograms in the perpendicular plane display closed patterns, corresponding to the effect of current density and vorticity filaments,
The vEB perturbation matches the Alfvén velocity one.
The current density filament is associated to a parallel vorticity peak, i.e. the fluid rotates in the perpendicular plane.
All these results, taken together,
allow to identify the detected structures as
Drift Alfvén vortices
M. Spolaore et al., “Direct measurement of current filament structures in a magnetic confinement fusion device”, submitted to Physical Review Letters.
N. Vianello et al., “Observation of drift-Alfvén vortices in a laboratory plasma”, submitted to Nature Physics.
Drift-Alfvén vortices have been observed in the magnetosphere by the 4-spaceship Cluster mission.
D. Sundkvist et al., Nature 436, 825 (2005).
Sawtooth crashes in RFPs are accompanied by a growth of m=1 modes, with an energy cascade towards higher n.
Subsequently, the m=1 amplitude drops and energy is transferred to a localized m=0 magnetic perturbation, which is formed at the locking position and then starts to rotate.
Using the U-probe, the passage of the m=0 perturbation has been associated to a parallel current density perturbation, which can be identified as the current sheet associated to the reconnection event.
The perturbation amplitude decays exponentially as it moves toroidally, with a time constant of about 400 s.
Considering the magnetic field diffusion equation
and neglecting the convection term, this yields an upper limit to the radial size of the structure of about 6 cm.
The toroidal dimension of the current sheet is of the order of 2 m.
M. Zuin et al., Plasma Phys. Control. Fusion 51, 035012 (2009).
Current sheets have been measured by the Cluster mission in reconnection events occurring in the bow-shock region of the magnetosphere.
The sheet thickness is ~ i, i.e. 100 km.
A. Retinò, D. Sundkvist et al., Nature Physics 3, 235 (2007).
Plasma filamentation during ELMs is observed in many tokamaks.
Three components of B measured by an insertable probe during type I ELMs in ASDEX-Upgrade.
Low-frequency (< 20 kHz) fluctuations are analysed using the Degree of Polarization (DOP) technique looking for coherent structures.
ELMs exhibit multiple peaks in Isat (density) and increased magnetic activity, with a drop in DOP indicating the presence of coherent structures.
3D hodograms (Br- B-B plots)display elliptic closed trajectories, lying in a plane which is found to be perpendicular to the average magnetic field.
This is the signature of field-aligned current filaments.
N. Vianello, R. Schrittwieser, V. Naulin et al., “Direct observation of current in ELM filaments on ASDEX Upgrade”, to be submitted to Physical Review Letters.
Through the use of insertable probes:
RFX-mod can contribute to the study of current filaments through: