Transcript Magnetized Laboratory Plasmas and Astrophysical Jets …And Space Physics Dr. Robert Sheldon October 10, 2003

Magnetized Laboratory
Plasmas
and Astrophysical Jets
…And Space Physics
Dr. Robert Sheldon
October 10, 2003
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Abstract
• Despite the high conductivity of laboratory and space
plasmas, which cause most theorists to treat them as
quasi-neutral fluids, inhomogeneous magnetic fields can
produce and support quite large potential differences,
even along field lines. The theory of field-aligned
potentials is several decades old, but remains a neglected
part of plasma theory and experiment. We present some
in situ spacecraft measurements and laboratory results
suggesting the presence of these parallel electric fields.
Should these fields scale to stellar sizes, they could easily
exceed the 1.1MV threshold for pair production, and
generate positron jets. We model the astrophysical system
with a laboratory angular magnet, and demonstrate the
dual jet-like features of this steady state system. We argue
that this electric quadrupole, far from being a peculiar
laboratory curiosity, is the energetically favored, first
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Talk Outline
I.
Astrophysical Jets
A. Apologies
B. Characteristics of Astrophysical
Jets
II.
Space Plasma LINACs
A. Resonant vs Non-resonant
B. The spinning magnet linac
III. Laboratory Plasma Physics
A. Field-aligned voltages
B. Dusty plasmas
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Apologies
• I’m a space plasma guy, so please
excuse my astrophysical chutzpah as
enthusiasm for cross-disciplinary
studies.
• Much of the jet research reflects a
research proposal compiled 6 years ago.
Feel free to correct my outdated or
misinformed data.
• This presentation falls between
disciplines—it has too much data & not
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Astrophysical
Jets
I.
Herbig-Haro Objects: YSO
Stars with Accretion Disks
HH30
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The Crab
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Some jet theories…
Necessary Conditions for Jets
(Proceedings of 1993 Astrophysical Jets Symposium, ed.
J.Pringle)
• Strong Magnetic field
• Determined by synchrotron radiation
• Accretion disk
• Large angular momentum
• Central attractor—BH, neutron star star
• Compact (esp. compared to jets)
• 15 minute variation in AGN variability
• Spinning?
• Hot? (nonthermal emission)
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Why would I study jets?
• The usual suspects: cool, mystery,
funding…
• Cool: It sure beats space physics in
photogenicity.
• Mystery: No accepted explanation in 40
years.
• Funding: n/a
• Some unusual aspects of jets:
Practicality.
• Its huge cosmic scaling (km parsecs)
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Jets, Entropy, and Heat Engines
• X-rays are usually non-thermal.
• if thermal they would cool too fast
• Synchrotron is by definition non-thermal
• Non-thermal=low entropy (S)
=acceleration
• Acceleration is either 1-step, multi-step
• Multistep is either resonant or stochastic
• Most efficient is 1-step, but with smallest
entropy
• Ultimate energy source that drives jet is
either nuclear (stars) or gravitational (BH) =
high-S
Visible vs Xray: HST deep
field
12/27
Deep field image
taken by HST,
showing galaxies
as far as the eye
can see. Some
percentage of
these are x-ray
emitters. This
suggests that the
Xray continuum
is really discrete
Xray objects in
the sky.
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God’s Heat Engine
• If all those discrete x-ray sources are
associated with jets, then it makes this
jet mechanism the most ubiquitous heat
engine in the universe.
• In some sense then, it is the most
efficient heat engine possible, or else it
wouldn’t be so common.
• We might be able to domesticate it for
terrestrial use—e.g. better than Carnot
II. Space Plasma Linacs
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Trapped H+
43keV O+ Beams!
B-field aligned
“beams”
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Beams as 1-step
accelerators
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• Are the beams stochastically
accelerated? No.
• Little evidence of pitch-angle scattering
• O+ dominates BELOW H+ in the ionosphere.
In order to produce O+, it must be rapidly
extracted from D-region without
equilibriating with H+.
• Are the beams multi-step (Fermi)
accelerated?
• Fermi acceleration aka ionospheric pressure
separation: Parallel Electric
Field Theory
Whipple, JGR 1977. Ne = Ni, quasi-neutrality
(Wheaton grad 1953?)
Different pitchangles for
n
Ions and electrons
kTe
F
E ||
Wouldn’t E-field bring ions back to electrons?
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Formation: Bouncing keeps
e- apart.
H+
Bouncing motion of ion in a magnetic mirror B-field
(dipole) looks like marble rolling in a bowl.
E-field
&
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Necessary Conditions for E|| in
Space
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• Inhomogeneous strong B-field such that
grad-B drifts dominate over ExB. (Dipole
field)
• Source of hot plasma
• Injected directly (accretion disks)
• Convected from elsewhere (plasmasheet)
• Spinning central magnet?
• Result:
• Rim feed, axial exit accelerator. Efficient
• Hot, non-thermal Xray source.
• This matches all the criteria for AP
Rough Theory of the
Mechanism
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• Hot plasma trapped in an inhomogeneous
field will produce E//, i.e. quadrupolar space
charge.
• Size of E// depends on several factors:
•
•
•
•
Driving plasma source rate—accretion rate
Temperature of the source
Loss rate due to scattering, radiation, etc.
Strength of the inhomogeneous B-field gradient
• When the E-field > 1.11 MeV, pairproduction begins, and positrons are
Quadrupole Electric Field: 1st
Excited State of a Dipole B-field
21/27
-
- -
-
+ +
+ +
-
-
-
-
Some thoughts on the
driver
22/27
• In our Earth observations, the driver was
ExB drift of warm (10keV) plasmasheet
plasma, which sets up an instability with
the ionosphere.
• In astrophysical jets, much hotter plasma
from an accretion disk is available. See, for
example, Jovian plasma torus. In this case,
synchrotron cooling of hot electrons leads
to the quadrupolar space charge
distribution.
Why is the jet so
collimated?
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1) It comes from a very small source
2) Global magnetic fields further
collimate it upon exiting the core
dipole
3) Current carried by the jet (it’s nonneutral after all) does some selfcollimation, and may balance selfrepulsion.
4) We really need some relativistic
MHD simulations to do this self-
Can plasma power blazar jets?
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(not to mention stars, neutron stars,
quasars…)
• The maximum E// of a plasma is limited by
2nd order forces ((F x B) x B) that short out
E. Using typical numbers for YSO fields, we
get limiting energies of keV MeV.(Rothwell97) Applying same formula to
blazar jets, we get ~1 GeV. Precisely the
value that explains observations!
• Objection: black holes power blazars. How
does plasma affect BH gravitational
equilibrium? Theorists don’t know yet. GR
+E&M = ad hoc.
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Jet Scaling
• If Blazars have 2AU sized dipoles,
and microquasars are 10’s of km,
then this mechanism scales by
factors of 10 million.
• Can we build one in the laboratory?
• High strength, inhomogeneous
magnet
• Hot plasma source at equator
• Spinning?
• The Spinning Terrella Experiment
III. Lab Plasma
Accelerators
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1st Experimental Setup
w/electrode
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• Bell jar, oil roughing pump, HV
power supply, Nd-B ceramic
magnet (low Curie temp!)
• Needle valve used to control
the pressure from 10-400
mTorr
• Simple
• Cheap
Arcs and Sparks=> Equator
Potential
40s exposure
28/27
Arc completely around!
Arcs follow B-field lines
Electrode
spinning
stationary
2nd Lab Setup w/Biassed
Magnet
1) N & e
2) Saturated
3) -400VDC
4) 0.5Tesla
5)10-200mT
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Characteristics of
Discharge
•
•
•
•
KeV of Voltage
Discharge lasts 30 microseconds
Calculated milliCoulombs of charge
Estimated nF capacitance of magnetic
field
• In better vacuum (or collisionless
plasma) potentials are limited by 2nd
order plasma drifts
• Result: Space charge accelerator
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3rd
Lab Setup w/ Pyrex Bell
Jar
Laser
Plasma
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Saturn’s Rings in the Lab
Dust Ring
3 SiO2
dust
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Model for Jets
• So far, we have modelled the dipole field,
but our “central attractor” is very spacefilling. In astrophysical jets, the central
attractor is much smaller than the
accretion ring. Thus trapped plasma does
not precipitate on the central attractor—
the loss cone is small.
• We repeat the above experiments with an
annular magnet, which simulates the
astrophysical system. Same magnetic
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The 4th Wheaton Bell Jar Setup
• Built in Experimental
Physics class by
Geoff Poore & Ben
Noonan [2002]
• Moderate vacuum
(10mTorr) oilroughing pumped
Pyrex bell jar
• Exploring toroidal
magnetized DC glow
discharge plasma
geometry
Toroidal DC-glow discharge
2/17/03
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• Annular disk
forming at dipole
minimum
• Central jet
forming at toroidal
minimum
• Density contours
perpendicular to
B
• Asymmetric jet
possibly due to
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Conclusions
• We have some pretty pictures that look
remarkably like astrophysical jets
• We have demonstrated a novel spacecharge plasma configuration which is not
very well described by current plasma
theory
• We have an experimental system which we
are still exploring with novel plasma
diagnostics (dust tracers)
• We have interested several amateurs in
building it—high school, undergrads…
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Some References
• Sheldon & Spurrier, "The Spinning Terrella
Experiment", Phys. Plasmas, 8, 1111-1118,
2001.
• Sheldon, "The Bimodal Magnetosphere",
Adv. Sp. Res., 25, 2347-2356, 2000.
• Sheldon, Spence & Fennel, "Observation
of 40keV field-aligned beams", Geophys.
Res. Lett. 25, 1617-1620, 1998.
• Several PowerPoint presentations
• All at: http://bex.nsstc.uah.edu/RbS/
The Magnetosphere in 1990
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Better than Carnot