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Rockets, Stars & Jars

Hot on the trail of cosmic plasma accelerators

Dr. Robert SheldonFebruary 19, 2003

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Talk Outline

I. Mass SpectrometersA. How I became a space physicistB. Two mass spectrometers

II. Space Plasma PhysicsA. The stochastic dipole cyclotronB. The spinning magnet linac

III. Laboratory Plasma PhysicsA. Field-aligned voltagesB. Dusty plasmas

3/27Why Space Plasma Physics?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 theory.

Moral: Follow the Physics, not the method

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I. Mass Spectrometers

5/27VMASS: The 1st TOF MS [1987]

6/27HELIX: The 2nd TOF MS [1999]

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NASA patent disclosure [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 everything a german shepard could tell you if he could talk.

(During Spring Break I will be attending PittCon`03 )

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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.

• The magnetosphere was discovered in 1959 by James VanAllen...

11/27The Magnetosphere in 1965

Trapped energetic particles have 3 conserved “adiabatic” motions:

1) Gyration (t=msecond)

2) Bounce (t=second)

3) Drift (t=ksecond)

12/27NASA/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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A keV detector’s view of M’sphere

POLAR Energy-time color intensity spectrograms

15-1500keV protons.

Radiation belts get hotter as s/c flies closer.

Except on the 2nd pass...

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B-field aligned “beams”

Trapped H+

43keV O+ Beams!

15/27MeV 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?

• 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?

• Cyclotrons & Linacs

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A. Space Plasma Cyclotrons

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• A cyclotron traps the electron, then accelerates it. It operates at resonanceand 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 Synchro-Cyclotron

•The Earth is an inside-out cyclotron. But how does it trap?

19/27Drift Motion in B-field Gradients

x

Two other ways to trap ions –using a B-gradient:

Gradient IN or Gradient OUT

Dipole CUSP

Trap ions w/ gyro-orbit

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1 MeV electrons in Cusp

Trapped, but how does it accelerate?

21/27Stochastic Cyclotron Acceleration

• 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.

• Perhaps not as fast as 1st, but accel. nonetheless

22/27But SDC doesn’t explain Earth

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 origin of the Earth’s radiation belts, especially the outer electrons.

What then is the origin of the Earth’s MeV e- ?

23/27But 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.• Q is NOT adiabatic==> chaotic (fast) accel!

24/27Quadrupole Cosmic Scales

• Planetary Magnetospheres

• Stellar Heliospheres

• Binary stars• Galactic

magnetic fields• Galaxy clusters

• keV (Mercury) to MeV (Jupiter)

• 10 - 100 MeV as observed at Sun

• 1-10 GeV• 10-100 GeV ?• TeV?This range of energies may explain Fermi’s

question about the origin of cosmic rays.

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B. Space Plasma Linacs

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Nature abhors charge separation: Parallel Electric Field Theory

Whipple, JGR 1977. Ne = Ni, quasi-neutrality

(Wheaton grad 1953?)

Different pitchangles for

Ions and electrons

n

kTe ||E

Wouldn’t E-field bring ions back to electrons?

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Heuristics for Parallel-E Formation: Bouncing keeps H+ & e- apart.

E-field

Bouncing motion of ion in a magnetic mirror B-field (dipole) looks like marble rolling in a bowl.

28/27Necessary Conditions for E||

in Space• 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.

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Herbig-Haro Objects: YSO Stars with Accretion Disks

HH30

30/27Blazar 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.

32/27Can SLINAC power blazar jets?

• 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 equilibrium? General Relativity theorists don’t know yet.

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Stochastic Cyclotron & SLINAC Accelerator Conclusions• 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 B-

fields. • YSO, blazar, micro-quasars,Herbig-Haro• Earth has half an accretion

disk=plasmasheet

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III. Laboratory Plasma Accelerators

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1st Experimental Setup w/electrode

• 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

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Arcs and Sparks=> Equator Potential

40s exposure

Arcs follow B-field lines

Arc completely around!

Electrode

spinning stationary

37/272nd Lab Setup w/Biassed Magnet

1) N & e2) Saturated3) -400VDC4) 0.5Tesla5)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• (How do I know for sure? Dust tracer...)

39/273rd Lab Setup w/ Pyrex Bell Jar

Laser

Plasma

40/27Saturn’s Rings in the Lab?

3 SiO2

dust

Dust Ring

41/27The 4th Wheaton Belljar Setup

• Built in Experimental Physics class by Geoff Poore & Ben Noonan [2002]

• Moderate vacuum (10mTorr) oil-roughing pumped Pyrex bell jar

• Exploring toroidal magnetized DC glow discharge plasma geometry

42/27Toroidal DC-glow discharge 2/17/03

• Annular disk forming at dipole minimum

• Central jet forming at toroidal minimum

• Asymmetric jet possibly due to spontaneous symmetry breaking

• Stellar jets?

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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 B-fields.

• Dust provides a novel tracer of plasma E-fields

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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/

45/27The Magnetosphere in 1990

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