1 Hantao Ji Princeton Plasma Physics Laboratory Experimentalist Laboratory astrophysics –Reconnection, angular momentum transport, dynamo effect… –Center for Magnetic Self-organization (CMSO, a NSF Physics Frontier Center) Current main research projects (>~10%) –Magnetic Reconnection Experiment (MRX) –Magnetorotational Instability (MRI) liquid gallium experiment –Plasma MRI experiment –Free-surface liquid gallium experiment –Madison Symmetric Torus (MST) experiment –Field Reversed Configuration (FRC) experiment –National Spherical Torus experiment (NSTX) Interests: –Collisionless shocks

2 Laboratory plasmas Solar plasma Magnetospheric plasma More distant astrophysical plasmas Magnetic Reconnection

3 Fundamental Physics Questions for Magnetic Reconnection How does reconnection start? (The trigger problem) Why reconnection is fast compared to classical theory? How ions and electrons are heated or accelerated? How to apply local reconnection physics to a large system? …

4 Magnetic Reconnection Experiment (MRX)

5 Experimental Setup in MRX Well-controlled and diagnosed experiment

6 Realization of Stable Current Sheet and Quasi-steady Reconnection

7 Reconnection Rates Agree with a Generalized Sweet-Parker Model The model modified to take into account of –Measured enhanced resistivity –Compressibility –Higher pressure in downstream than upstream (Ji et al. PRL ‘98) model

8 Why Dissipation is Enhanced at Low Collisionalities? Turbulence or Hall effect Ji et al. PRL (‘04)Ren et al. PRL (‘05)

9 Both Observed in Magnetospheric Reconnection ES EM (Bale et al. ‘04) (Mozer et al., PRL 2002) Q’field Polar Satellite

10 Reconnection Rate Also Affected by System Size

11 Angular Momentum Transport in Accretion Disks Many important processes happen in accretion disks: –Formation of stars and planets in proto-star systems –Mass transfer and energetic activity in binary stars –Release of energy (as luminous as of Sun) in quasars and AGNs The Problem: why accretion is fast? Or equivalently why angular momentum outward transport is fast? Two Candidate Mechanisms to Generate Turbulence –Hot disks: highly electrically conducting  Magnetorotational Instability (MRI) –Cold disks: perhaps insufficiently conducting for MRI, but essentially inviscid  nonlinear instability at large Reynolds #s HH30 By HST

12 Basic Idea: Magnetized Taylor- Couette Flow of Liquid Gallium Centrifugal force balanced by pressure force from the outer wall MRI destabilized with appropriate  1,  2 and Bz in a table-top size. Identical dispersion relation as in accretion disks in incompressible limit Bz < 1T Ga Not to simulate accretion disks, but to study basic physics

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14 Water Results: Negligible Transport Found in Quasi-Keplerian Flows Re based on outer cylinder Re based on inner cylinder

15 No Signs of Turbulence up to Re=2  10^6 RZ99 Large Reynolds stress detected if –Boundary conditions not optimum, or –Even with optimum boundary conditions, but at smaller Re’s  =(0.72  2.7)  10 -6, or <6.2  with 98% confidence, as compared to required ~10 3  unlikely larger at even larger Re’s, as in pipe flows and also by theoretical arguments (Lesur & Longaretti, 2005) Split-ring cases Ji et al, Nature 444, 343 (2006)

16 Summary Mechanism (parameter) MRI (  ) Nonlinear Hydro (  ) Observational requirements* e.g e.g. 2   Theoretical arguments No predictions? Inward transport if any (  <0)** Simulations None-existing for Keplerian flows Previous lab exp’ts None***  =(1-2)  based on Wendt(‘33), Taylor (‘36). Qualitative exp by Richard (‘01) Princeton MRI Exp (Re  2  10 6 ) in transition from water to liquid metal  <6.2  (98% conf.)  =  q