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Magnetic Reconnection: Progress and Status of Lab Experiments

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  1. Magnetic Reconnection: • Progress and Status of Lab Experiments Masaaki Yamada SLAC, April 28th 2011 In collaboration with members of MRX group and NSF-DoE Center of Magnetic Self-organization

  2. Outline • Basic physics issues on magnetic reconnection • Reconnection rate is faster than the classical MHD rate • Fast reconnection <=> Resistivity enhancement • But lower collisionality induces faster reconnection • Two-fluid physics analysis in a local reconnection layer • X-shaped neutral sheet • Physics of Hall effects and experimental verification • Identification of e-diffusion region • Observation of fluctuations (EM-LHDW) • A scaling in transition from MHD to 2-fluid regime • Global reconnection issues; • Reconnection in fusion plasmas • High energy particles • Impulsive reconnection • M. Yamada, R. Kulsrud, H.Ji, Rev. Mod. Phys. v.82, 603 (2010) E. Zweibel & M. Yamada, Ann. Rev.AA, AA47-8, 291 (2009)

  3. Magnetic Reconnection • Topological rearrangement of magnetic field lines • Magnetic energy => Kinetic energy • Key to stellar flares, coronal heating, particle acceleration, star formation, self-organization of fusion plasmas Before reconnection After reconnection

  4. Reconnection always occurs very fast (reconn << SP) after build-up phase of flux X-ray intensity Solar flare time(hour) Magnetic Field strength Magnetospheric Aurora-substorm time(hour) Stellar flare X-ray intensity Tokamak Sawtooth disruption Electron temperature time(sec)

  5. Magnetic Reconnection in the Sun • Flux freezing (Ideal MHD) makes storage (flux build up) of magnetic energy easy at the photo surface • Magnetic reconnection occurs when flux freezing breaks • Magnetic reconnection causes conversion of magnetic energy =>radiation, particle acceleration, the kinetic energy of the solar wind.

  6. A. Local Reconnection Physics MHD analysis Two-fluid analysis

  7. The Sweet-Parker 2-D Model for Magnetic Reconnection • Assumptions: • 2D • Steady-state • Incompressibility • Classical Spitzer resistivity Vin Vout B is resistively annihilated in the sheet reconn << SP~ 6−9 months Mass conservation: Pressure balance: S=Lundquist number

  8. Dedicated Laboratory Experiments on Reconnection

  9. MRX: Dedicated reconnection experimentGoal: Provide fundamental data on reconnection, by creating proto-typical reconnection phenomena, in a controlled setting Local physics problems addressed in collaboration with numerical simulations The primary issues; • Study non-MHD effects in the reconnection layer; [two-fluid physics, turbulence] • How magnetic energy is converted to plasma flows and thermal energy, • How local reconnection determine global phenomena - Effects of external forcing and boundary

  10. Pull Reconnection in MRX IPF

  11. Pull Reconnection in MRX IPF IPF

  12. Experimental Setup and Formation of Current Sheet Experimentally measured flux evolution ne= 1-10 x1013 cm-3, Te~5-15 eV, B~100-500 G,

  13. Resistivity increases as collisionality is reduced in MRX Effective resistivity But the cause of enhanced  was unknown

  14. Local Reconnection Physics MHD analysis Two-fluid analysis

  15. Extensive simulation work on two-fluid physics carried out in past 10 years Sheath width ~ c/wpi ~ i P. L. Pritchett, J.G.R 2001 Out of plane magnetic field is generated during reconnection

  16. The Hall Effect Facilitates Fast Reconnection Normalized with Hall term Electron inertia term Electron pressure term Ideal MHD region Vin -jin Ion diffusion region Vout~ VA Electron diffusion region • The width of the ion diffusion region is c/pi • The width of the electron diffusion region is c/pe ?

  17. MRX with fine probe arrays Linear probe arrays • Five fine structure probe arrays with resolution up to ∆x= 2.5 mm in radial direction are placed with separation of ∆z= 2-3 cm

  18. Evolution of magnetic field lines during reconnection in MRX e Measured region Electrons pull field lines as they flow in the neutral sheet

  19. Rectangular shape Collisional regime:mfp < Slow reconnection No Q-P field Neutral sheet Shape in MRX Changes from “Rectangular S-P” type to “Double edge X” shape as collisionality is reduced X-type shape Collisionless regime: mfp > Fast reconnection Q-P field present

  20. Two-scale Diffusion Region measured in MRX Ion Diffusion region measured:i > c/pi Electron Diffusion region newly identified:6-8 c/pe < e Electron jetting measured in both z and y direction : ve > 3-6 VAi Presence of B fluctuations Y. Ren et al, PRL 2008

  21. First Detection of Electron Diffusion Layer Made in MRX: Comparison with 2D PIC Simulations MRX: e = 8 c/pe 2D PIC Sim: e = 1.6 c/pe All ion-scale features reproduced; but electron-layer is 5 times thicker in MRX Þ importance of 3D effects

  22. Measured electron diffusion layer ismuch broader than 2-D simulation results => MMS program (Ji, et. al. Sub. GRL 2008)

  23. Recent (2D) Simulations Find New Large S Phenomena Bhattacharjee et al. (2009):MHD Daughton et al. (2009): PIC Sweet-Parker layers break up to form plasmoids when S > ~104 Impulsive fast reconnection with multiple X points 23

  24. In a large high S (>104) system, flux ropes can be generated => Impulsive fast reconnection Daughton et al, Nature Phys.2011

  25. Fast Reconnection <=> Two-fluid Physics • Hall MHD Effects create a large E field (no dissipation) • Electrostatic Turbulence • Electromagnetic Fluctuations (EM-LHW) • All Observed in space and laboratory plasmas

  26. Magnetic Reconnection in the Magnetosphere A reconnection layer has been documented in the magnetopause d ~ c/wpi Mozer et al., PRL 2002 POLAR satellite

  27. Similar Observations in Magnetopause and Lab Plasma MRX EM waves (a) (b) (Space:Bale et al. ‘04) EM ES (c) ES waves low b high b low  high  low  EM waves correlate with 

  28. MRX Scaling:* vs (c/i)/ sp A linkage between space and lab on reconnection Breslau 2 Fluid simulation (c/pi)/ sp ~ 5( mfp/L)1/2 Nomalized by Spitz Yamada et al, PoP, 2006 MRX scaling shows a transition from the MHD to 2 fluid regime based on (c/pi)/ sp

  29. Linkages between space and lab on reconnection di/ dsp ~ 5( mfp/L)1/2

  30. Global study of magnetic reconnectionHow is reconnection rate determined by global boundary conditions? Flux build up phase Magnetic self-organization External forcing: Vrec vs. f Impulsiveness

  31. Sawtooth relaxation; reconnection in a tokamak H. Park et al (PRL-06) on Textor 2-D Te profiles obtained by measuring ECE (electron cyclotron emission) represent magnetic fluxes

  32. Sawtooth crash (reconnection) occurs after a long flux build up phase tH ~ 200msec tre ~ 0.2 msec

  33. reconn << SP

  34. Generation of high energy electron during reconnection Suvrukhin, 2002

  35. Ion Temperature increases during RFP sawtooth reconnection Ti (eV) Emag (kJ)

  36. Summary • Progress has been made in reconnection research both in laboratory and space astrophysical observations => collaboration started in study of magnetic reconnection/self-organization • Transition from collisional to collisionless regime documented • A scaling found on reconnection rate • Notable progress made for identifying causes of fast reconnection • Two fluid MHD physics plays dominant role in the collisionless regime. Hall effects have been verified through a quadrupole field • Electron diffusion identified • Impulsive reconnection coincides with disruption of formed current sheet • Causal relationship between these processes for fast reconnection is yet to be determined • Guiding principles to be found for 3-D global reconnection phenomena • Magnetic self-organization • Global forcing • Impulsive reconnection after flux build-up

  37. Reconnection research will build a new bridge between lab and astrophysical scientists Global Plasma in Equilibrium State Self-organization Processes -Magnetic reconnection -Dynamos -Magnetic chaos & waves -Angular momentum transport Energy Source Unstable Plasma State Physics Frontier Center for Magnetic Self-organization in Laboratory and Astrophysical plasmas [Sept.03-]U. Wisconsin[PI], U. Chicago, Princeton U., SAIC, and Swarthmore

  38. 2D Reconnection “Phase Diagram” for MRX-U Study 14 12 10 Assume Np=S/Sc Hybrid Collisionless 8 Collisional MHD with Plasmoids 6 4 2 Collisional MHD (Sweet-Parker) 2 4 6 8 10

  39. Petschek model Shock