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 critical – described by NASA as the only possible “show-stopper”1 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 Space C o a m b p BIG dimensions so re lu s te s s ca le Maintain dimensionless Laboratory plasma Computer simulation of Space parameters Direct simulation 3D, PIC, hybrid code of lab Heritage: ICE code on JET, AMPTE dHybrid conditions Validate code Simulation 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 at axial position=0mm • Upstream plasma density profile reveals beam width 1.20E+17 10 of the order of 10-20mm Ion density Temperature • Electron temperature profile rather flat with peak temperature of ~2.5eV 1.00E+17 8 • In the absence of direct measurements of the parallel ion energy, we infer flow velocity from the sheath 8.00E+16 6 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 2.00E+16 0 1 22 2 α ( πμ M +Ψ−= γ s )1(2ln ) 2 • 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 8.00E+16 6 2.00E+16 8.00E+15 5 5 ) ) ) -3 -3 -3 6.00E+16 4 1.50E+16 4 6.00E+15 4 Density (m Density Density (m Density Density (m Density 3 3 Floating potential (V) potential Floating Floating potential (V) potential Floating Floating potential (V) potential Floating 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 0 10203040 0 10203040 0 10203040 Radial position (mm) Radial position /mm Radial position (mm) Detailed radial profiles reveal local potential well, “depth” is well matched to expected perpendicular ion energy 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 14 K.E. ~ 9eV 12 1.2E+17 10 1.0E+17 Potential barrier ~9V 8 6 8.0E+16 4 2 6.0E+16 Density (m-3) Density 0 4.0E+16 -2 (V) potential Floating -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 Axial position (mm) \Week 14\Setup12_09_07\DSC_0166.JPG 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.