No rotational transform Electrostatic confinement Coherent structures in turbulence Turbulence-induced transport

1. Simple Magnetized Torus as a Model System for Basic Investigation of Edge-Plasma TransportM. E. KoepkeDepartment of Physics, West Virginia UniversityMorgantown, WV 26506-6315 USA No rotational transform Electrostatic confinement Coherent structures in turbulence Turbulence-induced transport In-depth discussions with K. Rypdal, V. Demidov, T. Brundtland, S. Mueller, A. Fasoli, and K. Gentle are gratefully acknowledged

2. International Simple-Magnetized-Torus Community Princeton – ACT-1 India – BETA Norway → West Virginia – Blaamann Italy – THORELLO Germany – TEDDI Switzerland – TORPEX Texas – Helimak

3. Talk Outline Summary of Device Parameters Objectives and Long-Term Goals Theoretical Models for Interpreting Experiments Scientific Progress: (Blaamann, TORPEX, Helimak) Summary of Conclusions *Investigations are being performed in the outer-boundary layer of toroidal plasma experiments where interpretation-convenient geometry is prioritized over fusion-reactor scaling accuracy. *The turbulence-induced transport of particles and energy through this outer-boundary layer has been found to be crucial to optimizing fusion-relevant plasma conditions in the plasma core. *Techniques/interpretations from basic experiments are applicable to optimizing performance in devices with fusion-reactor scaling.

4. Simple Magnetized Torus has no poloidal magnetic field Compared to plasma in linear machines, toroidal plasma is sustainable at low neutral-gas fill pressures, reducing charge-exchange cooling of ions by neutrals. Thus, ambient temperature of ions (typically protons) is at approx. 1 eV, improving fusion relevance. Unwanted end effects are eliminated, but no rotational transform exists. Plasma produced by hot cathode filament, electron-cyclotron waves, lower-hybrid waves Negative poloidally symmetric potential profile responsible for poloidal rotation of plasma Helimak configuration has small vertical magnetic field Equipotential surfaces are vertically oriented, so that all but radial axis can be ignored. Filament-emitted electrons travel hundreds of meters before being lost to chamber wall. Equilibrium sustained by vertical plasma currents being shorted through chamber wall.

5. Device Parameters ACT BETA Thorello Blaamann TORPEX TEDDI Helimak Major R (m) 0.59 0.45 0.40 0.65 1 0.3 1.1 Minor W (m) 0.10 0.15 0.08 0.13 0.2 0.1 1.0 Minor H (m) 0.10 0.15 0.08 0.13 0.2 0.1 2.0 Lifetime ∞ 1.2s ∞ ∞ ∞ ∞ 60s Source wire wire wire wire, 5 kW wire 8 kW ECH,LH helicon ECH helicon ECH log([Ne (m-3)] < 17 16 17 17 < 17 16 17 Te (eV) 2-30 10 3 5 5 4 10 Btoroidal (T) 0.5 0.1 0.2 0.3 0.1 0.15 0.1 Bvertical (mT) 0 0 0 < 3 < 5 0 < 1 *Helimak Config. * * *

6. Helimak (Zimmerman&Luckhardt, 1993), Univ. Texas, AustinSimple toroidal configurations should lack a stable equilibrium, but the field lines terminate on the top and bottom of the vessel, allowing currents from top to bottom to stabilize the plasma. Images from Helimak website

7. Modeling the Helimak Configuration Dahlburg, Horton, Perez, Gentle, APS-DPP 2004, poster CP1.050 MHD and electrostatic drift wave simulations are carried out for the Helimak. Biased end plates control the radial electric field that drives sheared axial ExB flow. Objective: classify the turbulence and control the turbulent transport. Viscous-resistive MHD simulations as well as 2D and 3D drift wave simulations are being carried out to predict the fluctuation spectrum. The viscous-resistive MHD code has sheared flows and sheared magnetic fields and uses a radial Chebyshev-tau expansion that resolves steep radial gradients. Miracle, Felkl, Gentle, Lee, Wiley, APS-DPP 2006, poster UP1.076 Observed structures compared to simulations using Kotschenreuther’s modification of the D'Ippolito-Krasheninikov equations. Turbulence is a combination of Raleigh-Taylor and Kelvin-Helmholtz instabilities. Changes in the two-dimensional structures versus driven flows that shear the turbulence and stabilize the fluctuations in the Helimak plasma.

8. Objectives ACT-1: RF wave heating, current generation BETA: Turbulence suppression by edge Er(r) Blaamann (Norway): Coherent structures Blaamann (West Virginia): ExB shear, intermittency Thorello: Coherence, wave-wave interactions TEDDI: Spatiotemporal structures of turbulence TORPEX: Characterization of turbulence Helimak: Drift-wave plasma turbulence

9. Long-Term Goals Edge-Plasma Boundary-Layer Dynamics Control of Radial Profiles and Gradients Plasma Wave Localization and Competition Fluctuation-Induced Particle and Energy Loss Coherent Structures within Turbulence Spatial and Temporal Correlations Transport Suppression Modelling

10. Modeling the TORPEX Configuration V. Naulin, Global 2-Dim, drift fluid code ESEL V. Naulin, Global 3-Dim, two-fluid simulation on the basis of the ESEL code Müller, EPS 2006

11. Scientific Progress Focus on Blaamann, TORPEX, and Helimak Understanding the physics of stable confinement in a simple-magnetized torus Development of diagnostic techniques and analysis Unfiltered density measurement Fourier-space analysis of fluctuations Real-space analysis of fluctuations Probabilistic properties of structure motion (Pilot Chart) Strong fluctuation levels in toroidal plasmas Local correlation and dispersion properties help identify waves Nonlocal spatiotemporal representation for structure classification Formation mechanism for waves and structures (blobs) Propagation of structures (blobs) and spectral broadening due to ExB convection Particle transport induced by both structure-driven and local-fluctuation mechanisms

12. Blaamann moved in 2006 from Univ. of Tromsø to West Virginia Univ. Collaboration based on cooperative effort on probe diagnostic development WVU emphasis is on velocity-shear effects on low-frequency instabilities Cooperation with Univ. Texas, Austin, on velocity-shear controlling plates Cooperation with Univ. Texas, Austin, on turbulence characterization Future: Personnel exchanges, joint experiments, cooperation in research

13. WVU Univ. of Tromsø

14. Relocation of Blaamann in 2006 • Loaded into shipping container • (b) Leaving Auroral Observatory, UiT • (c) Arriving at Physics Dept., WVU

17. Blaamann Electrons are emitted from a negatively biased hot cathode. Main emission takes place near minor axis of the torus. Radial discharge wall current compensates injected charge. This current requires a radial E field, giving rise to a poloidal ExB convection along the circular contours of potential. By adding an internal anode with adjustable voltage bias, the poloidal rotation can be controlled in direction (clockwise or counter-clockwise) and in magnitude of shear. Waves driven unstable by the density gradient are convected along the circular flow surfaces. Rypdal and Ratynskaia, Phys. Scr. T122, 52, 2006

18. Poloidal ExB Convection in Blaamann One direction of rotation Rypdal & Ratynskaia, Phys. Scr. T122, 52, 2006

19. Poloidal ExB Convection in Blaamann Opposite direction of rotation Rypdal & Ratynskaia, Phys. Scr. T122, 52, 2006

20. Blaamannvertical (slab-like) contours of potential and pressure for Bz = 0.01 B, B = 18 mT Rypdal & Ratynskaia, PRL 95, 225002, 2005

21. Plug Probe – Langmuir Probe Array Blaamann Ratynskaia, Demidov, Rypdal, Phys. Rev. E, 65, 066403, 2002 WVU Q Baffled Probe Koepke et al., 2006

22. Coherent structures in Blaamann Fredriksen et al., PPCF 45, 721, 2003

23. CRPP at EPFL, Lausanne, Switzerland

24. TORPEX, CRPP at EPFL, Switzerland Page from Müller’s TORPEX EPS 2006 poster

25. Bvert optimizes TORPEX confinement Page from Müller’s EPS 2006 poster

26. Page from Müller’s TORPEX EPS 2006 poster

27. Page taken from Fasoli’s talk at Transport Task Force, Marseille, 2006

28. Page from Müller’s TORPEX EPS 2006 poster

29. ←Mach Probe Langmuir Probe Array→ TORPEX Images from TORPEX website

30. Hi-Freq Probe Array TORPEX Images from TORPEX website

31. Hexagonal Turbulence Imaging Probe TORPEX Müller et al., Rev. Sci. Instrum., 2005 Images from TORPEX website

32. Important Answers Found Presently, there are several Simple Magnetized Torus investigations underway New diagnostic techniques and analysis methods enable lab results to be subjected to detailed quantitative comparison with predictions from recent theoretical models Unfiltered density measurement Fourier-space analysis of fluctuations Real-space analysis of fluctuations Probabilistic properties of structure motion (Pilot Chart) Local correlation and dispersion properties identify drift waves and interchange modes Nonlocal spatiotemporal representation establishes classification of structures Insight into formation mechanism for waves and structures (blobs) Insight into structure propagation and ExB-convection-induced spectral broadening Particle transport induced by both structure-driven and local-fluctuation mechanisms

33. Summary of Conclusions Statistical analysis of turbulence reveals transport-relevant correlations in time & space Measurements of anomalous particle and energy fluxes in a magnetized plasma, Ratynskaia, Demidov, and Rypdal, Phys. Rev. E 65, 066403, 2002 Probability distributions of sizes and velocities reveal confinement-relevant interrelationships between formation and propagation Probabilistic analysis of turbulent structures from 2-dimensional plasma imaging, Müller, Diallo, Fasoli, Furno, Labit, Plyushchev, Podesta, Poli, Phys. Plasmas 13, 100701, 2006 Different plasma sources can create edge-plasma-layer conditions with different spatiotemporal transport characteristics Comparative experimental study of coherent structures in a simple magnetized torus, Grulke, Greiner, Klinger, and Piel, Plasma Phys. Control. Fusion 43, 525, 2001

34. Wong, K.L., M. Ono, and G.A. Wurden, ACT-1: A steady-state torus for basic plasma physics research, Rev. Sci. Instrum. 53, 409, 1982. • Bora, D., V.N. Rai, and P.K. Kaw, Observations on period-doubling phenomena in a basic plasma experiment, Phys. Lett. A 119, 411, 1987. • Jain, K.K., Observation of improved behavior by electrode biasing of a toroidal plasma having no poloidal magnetic field, Phys. Rev. Lett. 70, 806, 1993. • Alba, S., M. Fontanesi, A. Galassi, C. Riccardi, and E. Sindoni, Plasma-wave interaction in a toroidal steady-state device, Phys. Essays 6, 225, 1993. • Zimmerman, E.D., and S. C. Luckhardt, J. Fusion Energy 12, 289, 1993. • Rypdal, Grønvoll, Øynes, Fredriksen, Armstrong, Trulsen, Pécseli, Confinement and turbulent transport in a plasma torus with no rotational transform, PPCF 36, 1099, 1994. • Riccardi, C., D. Xuantong, M. Salierno, L. Gamberale, M. Fontanesi, Experimental analysis of drift wave destabilization in a toroidal plasma, Phys. Plasmas 4, 3749, 1997. • Grulke, O., F. Greiner, T. Klinger, and A. Piel, Comparative experimental study of coherent structures in a simple magnetized torus, Plasma Phys. Control. Fusion 43, 525, 2001 • Müller., Fasoli, Labit, McGrath, Podestà, Poli, Effects of a vertical magnetic field on particle confinement in a magnetized plasma torus, Phys. Rev. Lett. 93, 165003, 2004. • Gentle, K.W., J. Felkl, K. Lee, D. Miracle, Controlled shear flow stabilization of turbulence, U.S. Transport Task Force, April 4 - 7, 2006, Myrtle Beach, SC, USA.