hantao ji princeton plasma physics laboratory
DESCRIPTION
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%) - PowerPoint PPT PresentationTRANSCRIPT
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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