Transcript Rockets, Stars & Jars Hot on the trail of cosmic plasma accelerators Dr.

Rockets, Stars & Jars
Hot on the trail of cosmic
plasma accelerators
Dr. Robert Sheldon
February 19, 2003
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Talk Outline
I.
Mass Spectrometers
A. How I became a space physicist
B. Two mass spectrometers
II.
Space Plasma Physics
A. The stochastic dipole cyclotron
B. The spinning magnet linac
III. Laboratory Plasma Physics
A. Field-aligned voltages
B. Dusty plasmas
Why Space Plasma
Physics?
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I started out as an experimentalist in high school
(with the box of disassembled Timex watches…)
I switched from pre-med to physics at Wheaton
College.
In grad school, the prof with the MBE (molecular
beam epitaxy) machine had too many students,
so instead I worked on a Time-Of-Flight Mass
Spectrometer for space.
Space MS led to launching on a rocket & an Earth
orbiting satellite, NASA/AMPTE/CHEM.
Plasma physics theory was needed to explain the
data.
A lab experiment was needed to explain the
I. Mass Spectrometers
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VMASS: The 1st TOF MS [1987]
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HELIX: The 2nd TOF MS [1999]
[2003]
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Mass Market Mass Spec?
Mass Spectrometers are so versatile, they can be
used in place of other medical diagnostics.
Nature (Feb 2003) reports using MS for malaria
diagnosis. (diabetes, etc...) But the scientists
agree it needs to be cheaper.
HELIX is simple enough to be built by
undergraduates!
My own goal is to put a M/dM=10,000 Dalton
resolution MS into a cell-phone. Already the cell
phone is a digital ear and digital eye. Soon it will
be a digital nose.
Applications: Bomb detection, anti-terrorism,
home safety, drug testing, medical tests, or
II. Space Plasma Physics
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Space Physics
• Unfortunately, one cannot get a physics
PhD by designing instruments, so I needed
data to complete my PhD in 1987. This was
provided by an earlier instrument flown by
the UMd group, based on a much simpler
MS.
• This data analyzed the 1-300 keV/q
energetic ions trapped in the
magnetosphere. So perhaps a little
introduction to this region is in order.
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The Magnetosphere in 1965
Trapped
energetic
particles
have 3
conserved
“adiabatic”
motions:
1) Gyration
(t=msecond)
2) Bounce
(t=second)
3) Drift
(t=ksecond)
NASA/POLAR satellite orbit
[1996]
Many satellites had
explored the radiation
belts building up this
picture: Trapped particles
on inner & outer belts;
untrapped outside.
POLAR had cameras to
take pictures while over
the polar caps. It also had
an energetic particle
instrument vintage 1970.
It wasn’t supposed to find
anything new...
UNTRAPPED
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M’sphere
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POLAR
Energy-time
color
intensity
spectrograms
15-1500keV
protons.
Radiation
belts get
hotter as s/c
flies closer.
Except on the
2nd pass...
Trapped H+
43keV O+ Beams!
B-field aligned
“beams”
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MeV electrons in the wrong
place!
Outer
Radiation
Belt
Electrons
MeV
electrons in
the Cusp!
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Where are these energetic
particles coming from?
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• Not the solar wind! (too energetic)
• Can they be trapped particles? (wrong
pitchangles, wrong place)
• Are they just locally accelerated? W/O
drift?
• What are the mechanisms for accelerating
particles to high energy very quickly?
• How do we do it on Earth? Cyclotron &
Linear accelerators or stochastic vs.
resonant. Can the magnetosphere be
doing the same thing?
A. Space Plasma
Cyclotrons
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The Synchro-Cyclotron
• A cyclotron traps the
electron, then accelerates
it. It operates at resonance
and therefore needs to be
synchronized.
• The maximum energy is
determined by maximum
gyroradius allowed by the
pole magnet. It is efficient,
center fed, rim exit.
•The Earth is an inside-out
cyclotron. But how does it trap?
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Drift Motion in B-field Gradients
x
Trap ions w/ gyro-orbit
Two other ways to trap ions –using a
B-gradient:
Gradient IN
Dipole
or
Gradient OUT
CUSP
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1 MeV electrons in Cusp
Trapped, but how does it accelerate?
Stochastic Cyclotron
Acceleration
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• The ions are trapped in a gradient B trap.
(discovered theoretically by Singer in
1957.)
• Waves that compress the trap with the
same frequency as ion drift will accelerate
the ions by betatron acceleration (1st
order).
• But solar wind fluctuations are thought to
be random—thus the ions diffuse through
energy space—stochastic 2nd order accel.
But SDC doesn’t explain
Earth
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Although the radiation belts of the earth have
10’s MeV particles, either GeV’s precipitate
into the center, or keV’s adiabatically
escape, cooling off.
From a Mars vantage point, the Earth dipole
is a weak source of keV particles and
atoms.
Nor does adiabatic heating explain power law
tails.
The Dipole is a better trap than
accelerator.This has been known for 30
years at Earth, but doesn’t explain the
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But The Quadrupole Cusp...
• 2-Dipole interactions = Quadrupole. A
Dipole embedded in flowing plasma
creates a quadrupole cusp trap.
• How likely? About like binary stars.
• Quadrupole is both a drift+bounce+gyro
trap.
• Q is center feed, rim exit. Hi E escape.
Efficient!
• Q has no center magnet permitting higher
maximum energies.
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Quadrupole Cosmic Scales
• Planetary
• keV (Mercury) to
Magnetospheres
MeV (Jupiter)
• Stellar
• 10 - 100 MeV as
Heliospheres
observed at Sun
• Binary stars
• 1-10 GeV
• Galactic magnetic • 10-100 GeV ?
fields
• TeV?
• Galaxy
This
range clusters
of energies may explain Fermi’s
question about the origin of cosmic rays.
B. Space Plasma Linacs
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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! Ubiquitous
• 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.
Herbig-Haro Objects: YSO
Stars with Accretion Disks
HH30
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Blazar Galaxies and Schematic
Jet
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Visible vs Xray: HST deep field
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.
Can SLINAC power blazar
jets?
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• The maximum electric field of such a
system is limited by 2nd order forces ((F x
B) x B). Using some typical numbers for
YSO for magnetic field strength, we get
limiting energies of keV - MeV.
• Applying same formula to blazar jets, we
get ~1 GeV. Precisely the value that
explains observations! But black holes
power blazars.
• Q: What does a black hole magnetosphere
look like? How does plasma affect
Stochastic Cyclotron & SLINAC
Accelerator Conclusions
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• Both mechanisms are topological
• Ubiquitous. We should see them
everywhere
• Scale to all sizes.
• Quadrupole cyclotrons = 2 dipoles
• Planets embedded in flowing plasma
• Opposing magnetic fields, e.g. binary
stars
• Stars (galaxies) moving through a plasma
background
• Jets =accretion disks + spinning Bfields.
III. Laboratory 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
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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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The 4th Wheaton Belljar 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
• Asymmetric jet
possibly due to
spontaneous
symmetry
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Conclusions
• Space MS have applications to Earth as
well
• Novel acceleration mechanisms are found
in space corresponding to cyclotrons &
linacs
• These acceleration mechanisms may solve
outstanding questions from astrophysics.
• Several of these mechanisms can be
demonstrated in the laboratory with DC
glow discharges in the presence of strong
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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.
• All at: http://bex.nsstc.uah.edu/RbS/
The Magnetosphere in 1990
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