Current filaments in turbulent magnetized plasmas

1. Current filaments in turbulent magnetized plasmas E. Martines

2. 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.

3. 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!!

4. Current filaments in RFP microturbulence (1) Diagnostic: U-probe, a complex probe equipped with triple probes (n, Te, p) and magnetic probes (Br, B, B). In particular, we can compute • Data analysis technique: • wavelet transform to select structures at a given scale; • conditional averaging.

5. Current filaments in RFP microturbulence (2) • 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.

6. Current filaments in RFP microturbulence (3) The magnetic field and EB hodograms in the perpendicular plane display closed patterns, corresponding to the effect of current density and vorticity filaments, The vEB perturbation matches the Alfvén velocity one.  Alfvénic structures

7. Current filaments in RFP microturbulence (4) 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.

8. 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).

9. 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.

10. Current sheets in RFP sawtooth crashes (2) 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.

11. Current sheets in RFP sawtooth crashes (3) 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).

12. Current sheets in magnetosheath reconnection 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).

13. Current filaments during ELMs in tokamaks (1) Plasma filamentation during ELMs is observed in many tokamaks.

14. Current filaments during ELMs in tokamaks (2) 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.

15. Current filaments during ELMs in tokamaks (3) 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.

16. 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.

17. 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.