NASA Institute for Advanced Concepts. Phase I Fellows Meeting. The Plasma Magnet. John Slough. University of Washington.

Transcription

1 NASA Institute for Advanced Concepts Phase I Fellows Meeting The Plasma Magnet John Slough University of Washington March 23-24, 24

2 Development of the Plasma Magnet for Deep Space Exploration John Slough, Samuel Andreason, and Louis Giersch Research Institute for Space Exploration University of Washington

3 The jet power scales with the size of the magnetic bubble. A 2 kw rotating magnetic field (RMF) can produce a magnetic bubble intercepting up to 1 MW of thrust power. The effective thruster efficiency can be greater than 1 and as large as η ~ 5 The Plasma Magnet The singularly most important aspect is the possibility of achieving multi-megawatt thrust power from the solar wind. The ultimate spacecraft speed powered by the plasma magnet is that of the solar wind (35 to 8 km/s). As opposed to the solar sails, the dynamic nature of the plasma magnet provides a constant thrust regardless of distance from the sun.

4 The Magnetic Sail (Zubrin and Andrews,1991) The major difficulty in the original concept of course was the magnet mass. The mass problem is solved by having the coil currents conducted in a plasma rather than a superconducting coil. In this way the mass of the sail is reduced by orders of magnitude for the same thrust power. The question now becomes how to generate and sustain the currents

5 ow are the plasma currents generated and sustained? Plasma Magnet from Rotating Magnetic Fields RMF driven currents No copper/superconducting magnet required Results from Phase I Have demonstrated generation and sustainment of high β dipole plasma equilibria with 1 ka of azimuthal plasma currents

6 Illustration of the Generation and Self-Inflation of the Plasma Magnet Rotating Magnetic Dipole field lines Steady dipole Field generated by Electrons moving Synchronously with RMF Plasma electrons

7 Physics of RMF Current Drive Electron equation of motion in rotating field, B ω : e ( E+ v B) ν eiv= ωv where, m B r = B ω cos( ωt), B θ = B ωsin( ωt), E = E z = ωrb ω cos( ωt) Assuming ω << ν ei, one obtains the following Ohm s law: 1 mνei E = ηj+ j B η = 2 j= nev ne ne The Hall term dominates when, B eb ω ce ei ne >> η ϖ ω >> υ m In a metal conductor the Hall term is small (large ν ei ), and one has the rotating field penetrating only the classical skin depth δ = (2η/µω) 1/2. The θ component: 1 E θ = ηj θ ( urbz jzbr ) ne The Hall term has a pondermotive component, the <j z B r > term acting in the θ direction. It is the steady component of this electric field that drives the magnet current and provides for a steadily increasing

8 Plasma Magnet: Sailing the Solar Wind Two polyphase magnetic coils (stator) are used to drive steady ring currents in the local plasma (rotor) creating an expanding magnetized bubble. Expansion is halted by solar wind pressure is in balance with the magnetic pressure from the driven currents (R 1 km) Applications: Multi-MW thruster leveraged from multi-kw RF power Magneto-braking in magnetosphere of outer planets Electrical power generation from back emf on RF field coils from solar plasma flow (solar windmill) Magnetic shielding of spacecraft from high energy solar particles

9 Terrestrial Current Ring Currents driven by the RMF find an analogous representation in the ring currents formed when the magnetosphere is compressed by the solar wind. The process of course is reversed for the plasma magnet where the driven ring current acts to expand the magnetic bubble pushing off the solar wind.

10 FUNDAMENTALS OF THE PLASMA SAIL CONCEPT: Once inflated, the interaction of the plasma magnet with the solar wind should be similar to that of the planetary magnetosphere or plasma sail. Mini-Magnetospheric Plasma Propulsion For strong interaction with the solar wind the magnetic bubble must be on the order of the solar wind proton Larmor radius (~ 1 km) and the ion inertial length (~7 km). Sufficiently large initial plasma Bubble assures the both inflation and strong interaction (plasma magnet) From R.M. Winglee, J. Slough, T. Ziemba, and A. Goodson. J. Geophys. Res.,15, 9, 21,77, 2

11 NDAMENTALS OF THE PLASMA SAIL CONCEPT MHD AND KINETIC STUDIES The density structure from MHD simulations (a) on a global scale and (b) near the region of the source From G. Khazanov et al. AIAA JPC 23 With v sw = 5 km/s, n sw = 6 cm -3 2 km radius barrier receives a force of 4 Newtons Total thrust power ~ 2 MW

12 The Magnetic Field Fall-off in the Sub-solar Direction for a 4 km Plasma Sail magnetopause bow shock From G. Khazanov et al. AIAA JPC 23 A dipole field of B z = 6 G (6x1 5 nt) at 1 m was assumed Size of plasma sail scales with magnitude of dipole field. By employin the RMF, the size of the plasma sail is limited only by the power to overcome dissipation In the plasma: µ 8η 2 P sw ~ vswr PRMF = 6x1 ed line - MHD simulation with solar wind lack line without solar wind ight Blue line - solar wind with increased dynamic pressure. hin dashed lines show 1/r and 1/r 2 dependences for comparison. 3 R P RMF

13 Plasma Magnetic Sail MS) could scale in rinciple to even higher owers with small nuclear ource 1 8 Comparison of Propulsion Systems ower and exhaust elocity sufficient for apid manned outer lanetary missions Jet Power (W) PMS urrent propulsion systems Specific Impulse (s)

14 Comparison of Kinetic and Fluid Treatment of the Solar Wind in the Plasma Sail Interaction F x ~ 3.4 N F y ~ 1.4 N Contours show the density of the solar wind particles Source particles are indicated in red. In the kinetic case (a), the source particles are lost from the bubble in the transverse direction. In the fluid case (b), the source particles are lost predominantly in the downstream direction hanging relative plasma sail size allows for thrust vectoring

15 With magnetic bubble size determined by ambient solar wind pressure, force on plasma sail remains constant as spacecraft moves outward (or inward). From this scaling, one could imagine moving in toward sun until 2 km plasma sail is reduced to.2 m (1,:1). The scale of the sail is now On the order of the laboratory experiment. The solar wind pressure is ~ 2 np. The required pressure to compress down to the laboratory size is thus (15)2 2x1-9 or ~ 2 Pascals. The radial magnetic pressure from a 1 G magnetic field is ~ 4 Pascals.

16 Phase I Feasibility Experiments at the University of Washington ith the external magnetic field parallel to the polar axis, a radially inward ressure will oppose the plasma expansion, much like the solar wind will do space. The plasma magnet will thus remain compressed at the meter cale from the kilometer scale. Helmholtz pair produces external magnetic field

17 he rotating magnetic field (RMF) that sustains the large-scale dipole urrents can be constructed out of copper tubing, and driven by a tuned scillator (1 1 khz) with very high efficiency Even though the plasma may be more resistive than the superconducting ires of the MagSail, the huge difference in cross sectional area that the lasma subtends (km 2 vs. cm 2 ) minimizes the additional power requiremen

18 Two-Phase Oscillator Driver Circuits Used to Obtain Rotating Magnetic Field 5 I_South I_North Tuning Capacitors Current monitors I ant (A) Energy Storage Caps IGBT Switching supplies Time (msec)

19 In the Experiment the Initial Plasma was Provided by a Magnetized Cascaded Arc Source (MCAS) MCAS D 2 Discharge Only (no RMF) MCAS Guide field magnet

20 Magnetized Cascaded Arc Source Gas Feed V d Cathode Boron Nitride Washers Molybdenum Washers Solenoidal Coil Plasma Discharge Anode Directed force from plasma gun can be substantial. From probe measurements: (I dis ~ 2 ka), v s ~ 35 km/s (H) dn Fz = mivs = dt N time (1-4 s) dn/dt (1 21 ) 5 x 1-4 r= Normalized nv i angular position from source centerline (rad) Time (1-4 s)

21 Plasma torus Experimental Verification of the Plasma Magnet Langmuir Probe (n, T) Up to 1 ka of plasma current have been generated and sustained Ring expansion pressure ~ 4 Pa sufficient for 1:1, expansion against the solar wind Plasma remains linked to RMF antenna in the presence of large

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23 Visible Spectra for Ar Plasma Magnet with Deuterium Solar Wind Ar II C II March12. 4.Master.scope D(H) Ar I? Arb He Ar I = Red, Ar II = blue From pictures and radiation Plasma is fully ionized

24 Electron Temperature and Density from Double Langmuir Probe Current Probe (.5 V/A) nt 1/2 /1e T e ~ 18 ev 45 V 36 V 27 V 18 V 9 V V -9 V.1 2 mm x.5 mm diam Tungsten wire TIME (msec) Probe current for symmetric double probe: ev B 1 k(te + T i) I = i+ tanh, where i+ = n Ap kte 2 Mi 1 2

25 Magnetic Fields on Axis of the Plasma Magnet B-dot loop tilted at 45 to pick up both B z and B RMF simultaneously B z B RMF (mt) Increasing gas pressure TIME (msec)

26 Plasma Magnet Density and Current Profile N e (1 19 m -3 ) 1 z Obtain J θ from Amperes Law: 1 db Jθ = dr µ Synchronous electrons J θ =neωr Inferred rigid rotor density profile 8 6 J θ (ka/m 2 ).4.2 Measured density Radius (m) Total PM current ~ 8-1 ka

27 uration of RMF Measurement of the Rotating Magnetic inside the Plasma Magnet as a Function of Radius Int (db/dt).5 BRAW BRAW BRAW BRAW BRAW BRAW BRAW BRAW (From B-dot probe array) BRAW TIME ( ) Time (ms) R (cm)

28 Particle confinement in high beta dipolar field τ N = D n dvol dn dr da ~ η 2µ β(3m m 2 ) r R m 3 1 r where m is the power by which the density decreases (i.e. 1/r m ) from a characteristic surface that intercepts the equatorial plane at R. With a density fall-off of 1/r 3 (not critical but less than constant source fed) 2 3 N ~ 4πR n ln r R τ N 2µ 3η β r R ~ ln r 2 At high β (~1) and T e ~ 2 ev (η ~ 1.5x1-3 T e (ev) -3/2 17 µω-m) For an R = 1 m RMF antenna with sufficient current (density n ~ 1 17 m -3 ) to inflate to a r=1 km bubble: m H N = 1.8 mg τ N = 4.5x1 7 s ~ 1.5 years

29 Motion of Electrons in Rotating Dipole Field 3-D Calculation of electron motion in rotating magnetic field Present numerical work is aimed at self consistent motion with Self-generated hall currents

30 Phase I Results: Summary A plasma magnet was generated and sustained in a space-like environment with a rotating magnetic field Sufficient current (~ 1 ka) was produced for inflation of plasma to 1s of km. Intercepted significant momentum pressure from an external plasma source without loss of equilibrium Plasma and magnetic pressure forces observed to be reacted on to rotating field coils through electromagnetic interaction

31 Further Work on the Plasma Magnet Sail With successful Phase I testing, it is possible to perform the critical experiments necessary for concept validation in phase II. Construct a sufficiently large dielectric vacuum chamber and install a plasma magnet as well as an intensified solar wind surrogate source. Perform a scaled test of the PM with and without the solar wind source, and measure the thrust imparted to the PM. Measure all relevant plasma and field parameters for extrapolation to larger scale testing. Develop 2 and 3D numerical model for benchmarking against experimental results.

32 ~1 m Phase II: Concept Validation: Thrust Measurement And Scalability of Plasma Magnetic Sail Fused Silica Tubes Dump Chamber Ti Getter Pumped ~5 m Plasma magnet (pendulum suspended) Lab Solar Wind Source (LSW) (array of MCAS) Want to maintain plasma magnet size with smaller antenna to observe expansion Against Laboratory Solar Wind (LSW) D Antenna ~ 1/4 (.4 m).1 m With Constant force expansion/contraction need P LW = P SW = 2 MW Want ρ i /R to scale for kinetic effects V LSW ~ V SW ~ 4 km/s