Star Trek plasma shields: Measurements and modelling of a diamagnetic cavity

Ruth Bamford1,

K. J. Gibson2, A. J. Thornton2, J. Bradford1, R. Bingham1,6, L. Gargate1,3, L.O. Silva3, R.A. Fonseca3, M. Hapgood1, C. Norberg4, T. Todd5, R. Stamper1

1Space Plasmas, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK 2 Department of Physics, University of York, Heslington, York,YO10 5DD, UK 3Centro de Física dos Plasmas, Inst Superior Técnico, 1049-001 Lisboa, PORTUGAL 4 Umea University, Box 812, 981 28 Kiruna, SWEDEN. 5 UKAEA, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK 6Physics Department, University of Strathclyde, Glasgow G4 0NG, UK Motivation: Can humans survive the radiation of space?

NASA’s GOALS: 1. Complete the Space Station. 2. Develop and fly the “Crew Exploration Vehicle” now called, ORION, no later than 2014. 3. Return to the Moon no later than 2020. 4. Extend human presence across the solar system and beyond.

5. Initial budget ~$16.2B, 6. Total budget > $100B NASA Authorization Act of 2005 Most of the technical problems have envisaged solutions. Except for the susceptibility of living tissue to the increased radiation bombardment of interplanetary space. This talk concerns a possible means to solve this problem.

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 2 Introduction

• Background to spacecraft protection

• Results from the LinX Linear plasma eXperiment – Interaction of plasma beam with static dipole field

• Comparison with results from computer simulations • Summary and conclusions

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 3 The radiation hazard to astronauts is much greater outside the Earth’s magnetosphere

Earth Moon

~ Orbit of International Approximate edge Space of the Earth’s Station magnetosphere

A photograph from the Odyssey space craft looking back at the Earth-Moon system when the spacecraft was on route to Mars

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 4 The Apollo astronauts were lucky not to be killed by radiation

The radiation levels of Solar Proton Events that occurred during the Apollo Era

Apollo Mission No.

Fatal

Short term radiation sickness

Annual exposure for a radiation worker

Annual natural exposure year

• Need to bring the dose levels down to acceptable risk level • Trip to Mars 18 months • Radiation hazard of energetic protons in particular is Mission 1 critical – described by NASA as the only possible “show-stopper” 1 Frank Cucinotta, Chief Scientist for NASA's Radiation Research Program at the Johnson Space Center 5 EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, Limited number of options available for spacecraft protection

1. Build a wall – material shield. 2. Help biology cope better – Biochemical. 3. Active shield (magnetic, plasma, etc) cf Star Trek deflector shield….

All approaches are likely to be needed since no individual method is likely to reduce the risk to an acceptable level.

Active shielding is the subject of this talk.

Could an active shield possibly work from a physics point of view? If they can, would it be practical from an engineering point of view?

In this work, we will begin to answer the first question in order to start to address the second question…..

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 6 Active shields offer the potential to mimic the shielding effect of the Earth’s magnetosphere

P Need to examine the micro physics of the boundary Critical criteria is SIZE of “bubble”

Supersonic flow Dictates the power needed to create shield B~ 5- 50nT, n~0.1-2x106m-3, Is it practical for a

T~10 to 300eV, β <<1, space craft? Mass: 90% H+, 9% He2+

Vel~100-1000km/s, Mach~1 to 20 Proton energy: 100’s keV to 100MeV 1 AU

Coulombic mean free path ~ 1AU thus collisionless Collective effects dominate

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 7 The large dimensions of space have to be compressed self consistently into the laboratory scale

C om a pre bso ss lute sc Space ale

Laboratory BIG dimensions tain ain nless M nsio dime Direct ma plas ters simulation ame par of lab conditions Validate code Computer simulation of Space

3D, PIC, hybrid code Simulation dHybrid Heritage: ICE code on JET, AMPTE

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 8 LinX provides a excellent platform for laboratory astrophysics

Magnetic field coil Visible imaging camera Plasma source Target plate

• Linear plasma eXperiment Upstream chamber Target chamber (LinX )

• Magnetically confined plasma beam propagates along device to a target chamber: Axially mounted Tapered Dipole field Plasma beam Orifice source Reciprocating Radial Langmuir Probe

1.5 m

Beam: 10-20 cm diameter, B ~ 0.07T axial , T~ 5-15 eV, n~ 1016 to 1019 m-3, hydrogen, MACH> 3

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 9 Impinging plasma beam has Mach number ~3

Ion density and temperature radial profiles • Upstream plasma density profile reveals beam width 1.20E+17 at axial position=0mm 10 of the order of 10-20mm

Ion density Temperature • Electron temperature profile rather flat with peak 1.00E+17 8 temperature of ~2.5eV

• In the absence of direct measurements of the parallel 8.00E+16 6 ion energy, we infer flow velocity from the sheath coefficient, alpha, defined by φp = φf + αTe 6.00E+16 4 • For isotropic, subsonic hydrogen plasma we expect Temp (eV) Temp

Number densityNumber α = 3.3, but the upstream plasma in LinX has α = 1.7

4.00E+16 2 • A full treatment including effects of probe collection areas for ions and electrons and secondary emission

predicts a form for α 0 1 2.00E+16 α (πμ M 22 +Ψ−= γ 2 )1(2ln ) 2 s • From which we infer that the plasma in LinX has 0.00E+00 -2 Mach number M~3 010203040 Radial position /mm

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 10 Plasma stream is deflected in an apparently stable narrow layer around the magnet

Thickness of barrier sheet 3- 4mm

Plasma flow radius Approx size of the ion Larmor 0.5T

S Side view

Permanent magnet EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 11 Spatial profiles can be created by systematically scanning a reciprocating probe and the dipole source Visible camera magnet Steady state operation allows detailed measurements of plasma 0 – dipole field interaction

Stable Plasma flow 0 Axial direction

Initial experiments studied using spatially

resolved Langmuir probe Radial direction measurements and visible light imaging Reciprocating Langmuir probe

Amplifier and data acquisition

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 12 2-D maps of floating potential and ion density indicate that the width of the plasma layer is much less than ion Larmor radius

Dipole Dipole magnetic magnetic axis axis

Direction of LinX Direction of LinX B field B field

Top of cylindrical magnet Floating potential Ion density

• 2-D maps obtained by radial scanning Langmuir probe and axially scanning dipole field source • Structure of potential and density confirm visible imaging: plasma is deflected into a narrow layer of scale length << ion Larmor radius

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 13 Ions are confined in local potential well

60mm upstream 20mm downstream 60mm downstream

1.20E+17 10 3.00E+16 8 1.20E+16 8

7 7 1.00E+17 8 2.50E+16 1.00E+16

) 6 )

) 6 -3 -3 -3

8.00E+16 6 2.00E+16 8.00E+15 5 5 Density (m Density Density (m Density 6.00E+16 4 1.50E+16 4 (m Density 6.00E+15 4 Floating potential (V) potential Floating Floating potential (V) potential Floating Floating potential (V) potential Floating

3 3 4.00E+16 2 1.00E+16 4.00E+15

2 2

2.00E+16 0 5.00E+15 2.00E+15 1 1

0.00E+00 -2 0.00E+00 0 0.00E+00 0

Radial position (mm) Radial position /mm Radial position (mm) Detailed radial profiles reveal local potential well, “depth” is well matched to expected

perpendicular0 10203040 ion energy 0 10203040 0 10203040

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 14 Axial profiles reveal potential barrier in dipole field region and cavity devoid of plasma Region above the magnet

1.4E+17 K.E. ~ 9eV 14

12 1.2E+17 10 Potential barrier ~9V 1.0E+17 8 6 8.0E+16 4

2 6.0E+16 Density (m-3) Density 0 Floating potential (V) potential Floating 4.0E+16 -2

-4 2.0E+16 Essentially no plasma -6

0.0E+00 -8 Approximate Plane of probe measurements

magnet -80 -60 -40 -20 0 20 40 60 80

\Week 14\Setup12_09_07\DSC_0166.JPG Axial position (mm) Form of potential above magnetic pole may be associated with local plasma structure as indicated in visible imaging

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 15 Plasma is slowed to subsonic velocity

1.4E+17 4.5 4 1.2E+17 3.5 1.0E+17 isotropic 3 8.0E+16 M= 1 2.5

6.0E+16 2

Density (m-3) M= 3 1.5 4.0E+16 sheath coefficient alpha sheath coefficient supersonic 1 2.0E+16 0.5

0.0E+00 0 -80 -60 -40 -20 0 20 40 60 80 Axial position (mm) • reduction in plasma flow in dipole field region is consistent with potential barrier • Mach number indicates substantial reduction in flow velocity

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 16 Hybrid code modelling most appropriate for modelling laboratory mini-magnetospheres

• Plasma physics of the intermediate scale – Size of the spacecraft is << ion Larmor radii • Using particle-in-cell hybrid simulations using the code dHybrid1. Fluid electrons, kinetic ions. • Full kinetic not feasible as yet, due to the time and scale lengths • Visualization performed with the osiris2 framework • Used to understand and to optimize present day lab experiments but also to link the simulation and experimental work to the space plasma environment

• It was found that the magnetopause distance to the dipole origin increases with increasing magnetic field intensity of the dipole and with decreasing kinetic pressure of the flowing plasma, as expected from theory.

1 L. Gargate, et al Comp. Phys. Commun. 176, 419 (2007)

2 R. A. Fonseca, et al, Lecture Notes on Computer Science 2331, 342, Springer-Verlag, Heidelberg (2002)

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 17 dHybrid simulation results show narrow boundary and plasma cavity devoid of incident plasma Modelled ion density of the laboratory experiment 0.4 60

40 0.3 magnet 20 ) -3

0

-20 void 0.2

-60 Ion density (m Horizontal radial (mm) -80 0.1

-40 -20 0 20 40 60 80 100 x1016 m-3 Axial direction (mm)

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 18 The intuitive analogy is with Rutherford Scattering on a “macro particle”

Ions from the Sun of 100’s keV to 10’s MeV

No effect on GeV “cosmic rays” θ + E b + + Cavity In a Coulombically collision less plasma the inactions tend to be collective effects

The dipole field therefore is acting so as to create a “macro” particle with an electrostatic field pointing outwards in the frame of reference of the incoming particle

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 19 Summary & Conclusions part 1 of 2

• It has been demonstrated in simulation and laboratory experiment that can create an effective transport barrier to supersonic plasma flow. • Interior of “hole” in plasma stream is essentially devoid of plasma. • Boundary scale length is much less than the ion Larmor radii scale ~ 1/10th. • Thus the magnetic “bubble” needed to act as a shield can be much smaller ~100m compared to the > 20km considered previously. • Fundamentally this demonstrates the critical need to consider plasma particle kinetics at boundaries. • The LinX experiment offers an ideal platform to study a range of fundamental plasma physics phenomenon that are of relevance to astrophysical contexts.

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 20 Summary & Conclusions part 1 of 2. Future work

• So far have established proof-of-principle only. • Need to do systematic measurements in many regions and compare these with simulations. • We have initial data on impulse response showing some very interesting physicss also showed that particularly with the addition of the time dimension and reactive response, great care in the interpretation is needed.

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 21 Thank you, and live long and prosper…

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 22 spares

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 23 There are conceptual solution to dealing with the radiation from the poles

The action of the dipole field Possible solutions include: does not just deflect particles but also funnels some particles Formation flying preferentially into the magnetic poles, just like that which creates the Earth’s Aurora.

Torus space craft

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 24 The intuitive analogy is with Rutherford Scattering on a “macro particle”

Ions from the Sun of 100’s keV to 10’s MeV

θ + E b + + Cavity In a Coulombically collision less plasma the inactions tend to be collective effects

The dipole field therefore is acting so as to create a “macro” particle with an electrostatic field pointing outwards in the frame of reference of the incoming particle

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 25 Approximate Plane of probe measurements

\Week 15\Setup 17_09_07\DSC_0184.JPG EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 26 Vacuum field simulation

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 27 Ions confined electrostatically

The target Dipole magnet magnetic field Ions confined by S electrostatic field

Axial B magnetic field of the LinX device external coils Electrons confined by magnetic field

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 28 Plasma stream is deflected in an apparently stable narrow layer around the magnet

Approximate Plane of probe measurements

magnet

\Week 14\Setup12_09_07\DSC_0166.JPG

EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 29 The experimental apparatus

LinX : Side View Orifice

Plasma source Diagnostic ports

magnet 10 axial Magnetic field magnetic field coils Plasma stream Tip of Langmuir probe Equivalent plane of probe measurement (actually magnet was moved) EPS Conference on Plasma Physics, Crete, 9th-13th June 2008, 30