Intermittency and blob dynamics in the TORPEX device

1. Intermittency and blob dynamics in the TORPEX device Ivo Furno, A. Fasoli, D. Iraji, B. Labit, P. Ricci, C. Theiler Centre de Recherches en Physique des Plasmas École Polytechnique Fédérale de Lausanne, Switzerland M. Spolaore, N. Vianello Consorzio RFX, Associazione Euratom-ENEA sulla Fusione Corso Stati Uniti 4, 35127 Padova, Italy 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting Córdoba, Spain, September 7 - 10, 2010

2. Outline • Mechanism for blob generation from interchange waves • Detailed study of blob propagation • First measurements of 2D parallel current inside a blob • Conclusions and outlook I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 2010

3. The TORPEX device Source (EC and UH resonance) Parallel losses 2.45GHz, <20kW Bz Plasma gradients B, curvature Btor • Plasma produced by EC-waves • Open field lines - no plasma current • Extensive diagnostic coverage for turbulence and plasma response(electrost., magnetic probes) • R = 1 m, a = 0.2 m • H2, He, Ne, Ar plasmas • Btor ≤100 mT; Bz ≤10 mT; rs/a~0.02 • Te = 2 – 20 eV; ne = 0.1–5x1016 m-3  Complete characterization of turbulence in conditions relevant to tokamak SOL A. Fasoli et al., PoP (2006) I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 2010

4. Instability diagram ktoroidal =0, λz =ND k||≠0 due to resistivity Resistive Interchange Drift wave N ~ 2 Ideal interchange k|| =0, λz =D Ideal Interchange k|| =0, λz =D N  1/Bz P. Ricci, et al., PRL 104, 145001 (2010) I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 2010

5. Ideal interchange regime: waves and blobs Blob region Interchange wave I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 2010

6. Ideal interchange regime: waves and blobs I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 2010

7. Blob generation mechanism • Time resolved 2D profiles of ne, Te, fpl from conditional sampling • Interchange wave moves with vExB • Radially elongated structures form from positive cells • ExB flow shear breaks off the structures and forms blobs • Structures form in ~100 μs ~ estimated shearing time • H. Biglari et al., PF B (1990) • Energy is transferred from shear flow to blobs I. Furno et al., PRL 100, 055004 (2008); C.Theiler et al., PoP (2008); A.Diallo et al., PRL (2008) I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 2010

8. Blobs identified by pattern • recognition, providing • Radial velocity  v • Vertical size a • Density dne, ne e e z [m] e r [m] Motion of filaments/blobs in simple geometry limiter • Steel limiter on low-field side, defining region with • Constant curvature along field lines and connection length (~2pR) • Near-perpendicular incidence of B-field lines, no magnetic shear z r z r limiter z (m) S. H. Müller et al., PoP 2006; C. Theiler et al., PRL 103, 065001 (2009). I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 2010

9. Joint probability of blob velocity – size • Similar sizes in all gases • Similar values of dn/n • Mean velocity of blobs over their entire trajectory • Significant differences in the typical velocity, ranging from 500 m/s (Ar) to 2000 m/s (He) Range of blob sizes below diagnostic resolution I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 2010

10. Vorticity equation Generalization of 2D blob models and scaling laws Key question to understand blob motion is which mechanism compensates curvature-driven charge separation Fig. from Krasheninnikov et al. C. Theiler et al., PRL 103, 065001 (2009). I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 2010

11. Comparison with 2D scaling laws Experimental data in normalized units Damping due to ion-polarization currents S. I. Krasheninnikov, PLA 2001 Ne H 2 He Ar Damping due to parallel currents O. E. Garcia et al., PoP 2005J. R. Myra et al., PoP 2005 I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 2010

12. Bz Bt Br trigger trigger • Flat one-directional LP (8 mm diam.)at limiter potential  J// is measured • 3 cm away from the limiter • Current probeBr,z,t/t J=xB/0 M. Spolaore, N. Vianello et al, PRL 102, 165001 (2009) Two methods to measure J// inside a blob Conditional sampling time resolved2D profiles of J// I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 2010

13. He: J// from flat LP A / m2 J// to the limiter H2: J// from flat LP from current probe A / m2 • Dipolar structure of J// is observed inside a blob in H2 and He • The dipole is not perfectly balanced I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 2010

14. H2 gas Vfl [V] How important is J// for the blob dynamics? Vorticity equation Charge separation IS MOSTLY damped by parallel currents to the sheath Charge separation IS NOT damped by parallel currents to the sheath I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 2010

15. Outlook: blob control through boundary conditions Conclusions • In TORPEX, blobs form from an interchange wave crest that radially elongates and is eventualy sheared off by the E x B flow. • Agreement between data and a generalized 2D model for blob speed and size. • Parallel currents to the limiter, cross-field ion polarization currents, and ion-neutral collisions • First detailed 2D measurements of parallel current inside a blob have been obtained. • Parallel currents to the limiter are important in balancing charge separation in hydrogen, but not He Limiter à configuration variable I. Furno, 13th EU-US TTF Workshop - 3rd EFDA TTG Meeting, 2010