RFX-mod Program Workshop, Padova, 20-22 January 2009 1 Current filaments in turbulent magnetized plasmas E. Martines

1RFX-mod Program Workshop, Padova, 20-22 January 2009

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.

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.


Phenomena where current filaments appear

Coherent structures in RFP edge microturbulence (small scale)

Coherent structures in magnetospheric turbulence (small scale)

Reconnection events associated to RFP sawteeth (large scale)

Reconnection events in the magnetosphere (large scale)

ELMs in tokamaks (large scale)

... and certainly many others!!


Current filaments in RFP microturbulence (1)

Data analysis technique: • wavelet transform to select structures at a given scale;• conditional averaging.

Diagnostic:

U-probe, a complex probe equipped

with triple probes (n, Te, p) and

magnetic probes (Br, B, B).

In particular, we can compute

vvB

1v

B

1v

BB1

j

rr

prpr

rr0



Structures at = 3.3 s (L~ 5 cm):

• Electron pressure/density peak• Potential well on density peak• Parallel current density peak

The pressure structure has a

radial extension of 2-4 s (1-2 cm).

The current density structure

has an amplitude of a few %

of the average j, and a toroidal

size of 100 s (30-40 cm) due to

the stretching effect of the velocity

shear.



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.

Alfvénic structures



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.

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


Drift-Alfvén vortices in the magnetosphere

Drift-Alfvén vortices have been

observed in the magnetosphere by the

4-spaceship Cluster mission.

D. Sundkvist et al., Nature 436, 825 (2005).


Current sheets in RFP sawtooth crashes (1)

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 in magnetosheath reconnection

A. Retinò, D. Sundkvist et al., Nature Physics 3, 235 (2007).

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.


Current filaments during ELMs in tokamaks (1)

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.


Summary

Through the use of insertable probes:

Turbulent structures in the RFP outer region have been identified as

drift-Alfvén vortices, similar to those observed in the magnetosphere.

The formation of large scale current structures during reconnection

events in RFPs has been clearly measured.

Current filaments in tokamak ELMs, possibly caused by the peeling-

ballooning mechnism, have been measured for the first time.


Outlook

RFX-mod can contribute to the study of current filaments through:

Further studies with probes at low current in RFP discharges.

Studies with probes at higher current (requires fast manipulator).

Studies of turbulent structures in tokamak and ULQ discharges.

Documents

RFX-mod Program Workshop, Padova, 20-22 January 2009 1 Current filaments in turbulent magnetized plasmas E. Martines