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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

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Laboratory plasmas

Solar plasma

Magnetospheric plasma

More distant astrophysical plasmas

Magnetic Reconnection

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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?

• …

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Magnetic Reconnection Experiment (MRX)

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Experimental Setup in MRX

Well-controlled and diagnosed experiment

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Realization of Stable Current Sheet and Quasi-steady Reconnection

€

BZ = B0 tanhR − R0

δ

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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

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Why Dissipation is Enhanced at Low Collisionalities? Turbulence or Hall effect

Ji et al. PRL (‘04) Ren et al. PRL (‘05)

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Both Observed in Magnetospheric Reconnection

ES

EM

(Bale et al. ‘04)

(Mozer et al., PRL 2002)

Q’field

PolarSatellite

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Reconnection Rate Also Affected by System Size

0 1

(1 / )out out

in

A Aeff

z

V VS

Lnn VnV

L

n

μη

= ⋅ ⋅+ &

1R

A eff

V

V S=

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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 1015 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

HH30By HST

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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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Water Results: Negligible Transport Found in Quasi-Keplerian Flows

Re based on outer cylinder

Re

base

d on

inne

r cy

linde

r

€

ˆ β ≡˜ V r ˜ V θ

Vθ2

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No Signs of Turbulence up to

Re=210^6

RZ99

• Large Reynolds stress detected if– Boundary conditions not

optimum, or– Even with optimum boundary

conditions, but at smaller Re’s =(0.722.7)10-6, or

<6.210-6 with 98% confidence, as compared to required ~103

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)

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Summary

Mechanism

(parameter)

MRI

()

Nonlinear Hydro

()

Observational requirements*

e.g.

10-3-10-1

e.g.

210-5-410-4

Theoretical arguments

No predictions? Inward transport if any (<0)**

Simulations 10-3-10-1 None-existing for Keplerian flows

Previous lab exp’ts

None*** =(1-2)10-5 based on Wendt(‘33), Taylor (‘36).

Qualitative exp by Richard (‘01)

Princeton MRI Exp

(Re2106)

in transition from water to liquid metal

<6.210-6 (98% conf.)

= q