1

Magnetized Laboratory Plasmas

and Astrophysical Jets

…And Space Physics

Dr. Robert SheldonOctober 10, 2003

2/27

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 excited state of a magnetic dipole+plasma. If so, it may explain the ubiquity of non-thermal, energetic jets in astrophysical plasmas.

3/27

Talk Outline

I. Astrophysical JetsA. ApologiesB. Characteristics of

Astrophysical Jets

II. Space Plasma LINACsA. Resonant vs Non-resonantB. The spinning magnet linac

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

4/27

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 Feel free to correct my outdated or misinformed data.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 enough theory for an astrophysicist, too little data & too much theory for space plasma physics.

5/27

I. Astrophysical Jets

6/27

Herbig-Haro Objects: YSO Stars with Accretion Disks

HH30

7/27

The Crab

8/27

Some jet theories…

9/27Necessary 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)

10/27

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) suggests

a robust mechanism that has wide application.• It may solve the energy problem here on Earth.• I may be able to make a cheap ion source for my

lab.

11/27Jets, 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

• High-S HeatLow-S Acceleration = Heat Engine. (think: jet engine)

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.

Visible vs Xray: HST deep field

13/27

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

14

II. Space Plasma Linacs

15/27

B-field aligned “beams”

Trapped H+

43keV O+ Beams!

16/27Beams as 1-step accelerators• 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

cooker, needs a parallel E-field too. In which case, 1-step is faster than multi-step.

• 1-step=40kV E// is simplest theory

17/27

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?

18/27

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.

19/27Necessary Conditions for E||

in Space• 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 jets!

20/27Rough Theory of the Mechanism

• 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, pair-production begins, and positrons are accelerated into a beam. (YSO’s may be accelerating H+)

21/27

Quadrupole Electric Field: 1st Excited State of a Dipole B-field

+++

+

- - ---

- -- -

22/27Some thoughts on the driver

• 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.

• Thus, multiple drivers cause an energy density increase, expressed as quadrupolar E-field.

23/27Why is the jet so collimated?1) It comes from a very small source2) Global magnetic fields further

collimate it upon exiting the core dipole

3) Current carried by the jet (it’s non-neutral after all) does some self-collimation, and may balance self-repulsion.

4) We really need some relativistic MHD simulations to do this self-consistently….

24/27

Can plasma power blazar jets?(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.

• A single plasma mechanism may explain all AP jets.

25/27

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

26

III. Lab Plasma Accelerators

27/27

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

28/27

Arcs and Sparks=> Equator Potential

40s exposure

Arcs follow B-field lines

Arc completely around!

Electrode

spinning stationary

29/272nd Lab Setup w/Biassed Magnet

1) N & e2) Saturated3) -400VDC4) 0.5Tesla5)10-200mT

30/27

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...)

31/273rd Lab Setup w/ Pyrex Bell Jar

Laser

Plasma

32/27Saturn’s Rings in the Lab

3 SiO2

dust

Dust Ring

33/27

Model for Jets

• So far, we have modelled the dipole field, but our “central attractor” is very space-filling. 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 field polarity, same –400VDC applied voltage.

34/27The 4th Wheaton Bell Jar 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

35/27Toroidal DC-glow discharge 2/17/03

• Annular disk forming at dipole minimum

• Central jet forming at toroidal minimum

• Density contours perpendicular to B

• Asymmetric jet possibly due to spontaneous symmetry breaking

36/27

37/27

38/27

39/27

Conclusions• We have some pretty pictures that

look remarkably like astrophysical jets• We have demonstrated a novel space-

charge 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…

40/27

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/

41/27The Magnetosphere in 1990

42/27

Better than Carnot