Experimental Investigations of Magnetic Reconnection. J Egedal. MIT, PSFC, Cambridge, MA

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1 Experimental Investigations of Magnetic Reconnection J Egedal MIT, PSFC, Cambridge, MA

2 Coronal Mass Ejections Movie from NASA s Solar Dynamics Observatory (SDO)

3 Space Weather The Solar Wind affects the Earth s environment

4 The Earth s Magnetic Shield During Before reconnection

5 The Daily-Show

6 International Thermonuclear Experimental Reactor Magnetic Fusion Devices

7 Magnetic Fusion Devices International Thermonuclear Experimental Reactor Reconnection in ITER could destroy the device:

8 The Tokamak Device Best known confinement device on Earth Shaping and Control Coils Solenoid Toroidal Field Coils Plasma

9 Shaping and Control Coils Z [m] Sawtooth reconnection in Tokamaks Neutron yield in a tokamak R [m] H Park, PRL 2005: localized reconnection

10 Electromagnetism 101 Faraday s law: EMF Area db dt Faraday s law for a conducting ring: EMF=0. The magnetic flux through the ring is trapped This also holds if the ring is made of plasma plasma frozen in condition

11 Reconnection: A Long Standing Problem Simplest model for reconnection: E + v B = j [Sweet-Parker (1957)] t X E X j X

12 Reconnection: A Long Standing Problem Simplest model for reconnection: E + v B = j [Sweet-Parker (1957)] L Outflow speed: (Alfven speed) Sweet-Parker: L >> : t sp t R t A 2 0L L v A Unfavorable for fast reconnection Two months for a coronal mass ejections

13 Outline The MRX experiment at PPPL 2D reconnection in VTF open configuration Oscillatory reconnection response 3D reconnection in VTF closed configuration Explosive reconnection response Conclusions

14 Family of Reconnection Experiments (H.Ji, PPPL) 2D w/ walls 2D Symmetry local steady state collisionless boundary process collisionality global transient collisional

15 The MRX experiment at PPPL M. Yamada, H Ji, et al.

16 The MRX experiment at PPPL

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21 Outline The MRX experiment at PPPL 2D reconnection in VTF open configuration Oscillatory reconnection response 3D reconnection in VTF closed configuration Explosive reconnection response Conclusions

22 The Versatile Toroidal Facility (VTF)

23 The Versatile Toroidal Facility (VTF) External Coils Vacuum Vessel

24 The Versatile Toroidal Facility (VTF) Diagnostics External Coils Vacuum Vessel TF Coils RF-Power

25 The Versatile Toroidal Facility (VTF)

26 Magnetic Diagnostics Voltage in loops ~ (db/dt) A Assuming toroidal symmetry we can use B pol = (A e ) and build an array to integrate up A Magnetic flux array Kesich et al., RSI 79, (2008)

27 Magnetic Array Measured current & magnetic field

28 Rogowski Array Construction: copper wire wound on teflon tube Measures current through each opening

29 Two different magnetic configurations An open cusp magnetic field. Fast reconnection by trapped electrons. A closed cusp by internal coil. Passing electrons & spontaneous reconnection events.

30 Plasma response to driven reconnection

31 Kinetic modeling Why is the experimental current density so small? Liouville/Vlasov s equation: df/dt=0 For a given (x 0,v 0 ), follow the orbit back in time to x 1 Particle orbits calculated using electrostatic and magnetic fields consistent with the experiment. Massively parallel code evaluates f(x 0,v 0 ) = f ( v 1 ). J. Egedal et al., Computer Physics Communications, (2004)

32 Kinetic modeling The current is calculated 3 as j v f dv Theory consistent with measurements (B-probe resolution: 1.5cm) Experimental scaling j~nl 0 E z is reproduced Theory ka/(vm 2 ) Experiment

33 Temporal evolution of the current channel Time response of the toroidal current Eigen response, f= khz

34 The electrostatic potential +70 V Experimental potential, -70 V Electron flow: v E B - B g B 2 Ideal Plasma: E B B E0 External fields pol B cusp 2mT 0 10V/m ~500V/m 87mT

35 The electrostatic potential Ideal Plasma: E B Epol B cusp 0 Frozen in law is broken where E B 0 E B

36 Plasma response to driven reconnection

37 Ion polarization currents due to d /dt Ion polarization current: j nm 2 B d dt J Quasi neutrality: ( j j ) ll 0 J J J d j nm 2 ll 2 ll B dl d dt Measured Model

38 Model for dynamical response J + A ext Φ E B 0 A φ A = 1 J + A ext, J = 2 d /dt, = - 3 da /dt, A = - d 2 A / dt 2 + A ext, = > 0 Oscillating solutions Egedal et al., PRL (2003)

39 Outline The MRX experiment at PPPL 2D reconnection in VTF open configuration Oscillatory reconnection response 3D reconnection in VTF closed configuration Explosive reconnection response Conclusions

40 Spontaneous Reconnection Coronal mass ejections: - The most powerful explosions in our solar system

41 Other Outstanding Problems Arcade Arcade as seen from above Heating 3D effects Trigger

42 Closed Magnetic Configuration

43 Plasma in the VTF Closed Configuration Visible light of an Argon discharge n e ~ m -3, T e ~ ev, e ~ 10 m. B g ~ 50 mt. B p ~ 5 mt

44 Spontaneous reconnection observed J. Egedal et al., (2007) Phys. Rev. Lett. 98,

45 No simple resistivity, E *j!

46 Plasma outflows

47 Toroidal Asymmetry: Delayed Onset Toroidal localized onset 5 μs delay

48 3D A φ / t Data Confirms Asymmetry Use 2 fixed arrays & variable onset location to construct full dataset Shift onset angle to φ=0, record relative angle of arrays Onset

49 3D reconnection (Cartoon)

50 3D reconnection (Measurements)

51 Total E-Field is Localized Strong toroidal electrostatic E Φ x measured at x-line, keeps total E localized;

52 Spontaneous reconnection only for rational q q = toroidal winding # poloidal winding # q-profile of VTF q=2

53 Large q=2 and q=3 modes observed during reconnection V float for q=2 V float for q=3

54 Potential maintains E B~0 away from X-line (ohm s law) E φ B φ E pol B pol (from V f ) E tot B tot TV/m + = Experiments show 20 - A / t rms

55 Exponential Growth in the Reconnection Rate Growth rate γ~1/(20μs ± 6μs) at onset location Model for Onset J Φ A φ Ohm s Law

56 E rec A 4 Current Continuity: Cartoon A t, 0 3 x j ( x') d x x' '

57 Current Continuity: Cartoon J 0 J J 0 nm B J 2 de dt Ion polarization current

58 q=2 electrostatic mode Mode amplitude increases during reconnection onset

59 Growing q=2 Potential Ion polarization currents maintain : J J B J =0

60 Model for dynamical response J + A ext Φ E B 0 A φ A = 1 J + A ext, J = - 2 d /dt, = - 3 da /dt, A = d 2 A / dt 2 + A ext, = > 0 Exponentially growing solution! N. Katz et al., (2010) Phys. Rev. Lett.

61 Conclusions Collisional reconnection model and the Hall effect have been verified in MRX In the collisionless regime of VTF important collisionless effects become evident Experiment offers the opportunity to address the trigger problem 3D effects are important!