Laboratory Observations of Fast Collisionless Magnetic Reconnection

1. Laboratory Observations of Fast Collisionless Magnetic Reconnection J. Egedal, J. Dorris, W. Fox, E. Ptacek, J. Nazemi, M. Porkolab and A. Fasoli MIT Plasma Science and Fusion Center Physics Department

2. Outline • Driven reconnection in the VTF open cusp • Size of diffusion region (where EB0) scales with the electron drift orbit width • j||and dE/dt linked through ion-polarization • LCR-circuit model for j|| • Drift kinetic model for VTF applied to new Wind satellite data • Closed cusp configuration: • the effect of passing particles • Conclusions and future work

3. Magnetic Reconnection on VTF The VTF device 2 m

4. Primary Diagnostics 45 heads L-probe 40 Channels B-probe

5. Plasma response to driven reconnection

6. - Ez Experimental potential E= Ezz -  The size of the diffusion region Frozen in law is broken where EB0 EB=(E -) B Theory: EB=0 =½Ezl0log(|x/y|)

7. Neon Nitrogen Krypton Xenon • The size depends • on [cm] The size of the diffusion region • The size of the • diffusion region is • independent of ion • mass and number • density.

8. Plasma response to an oscillating drive

9. In phase withVloop 900 ahead of Vloop 0 – 1.2 kA/(Vm2) 0 – 20 mAs/(Vm2) Ion polarization current: Quasi neutrality: Plasma response to an oscillating drive • The current profile • can be separated in • two parts:

10. Theory Empirical Cj2Vloop Cj2Vloop [As] [As] Temporal evolution of the current channel Time response of the toroidal current modeled by LCR-circuit Eigen response, f= 10-30 kHz

11. Drift kinetic modeling(3) Density p||/p • Recent Wind satellite observation: • Strong anisotropy in f near X-line • This anisotropy was predicted in the • drift kinetic modeling J.Egedal, PoP 9, (2002) • Good agreement with observation • when including pitch angle diffusion • Wind data consistent with an • open cusp configuration M. Øieroset et al. PRL 89, (2002) J.Egedal, PoP 9, (2002)

12. Boundary conditions New configuration: Closed cusp  passing particles

13. Boundary conditions Open cusp: Closed cusp:

14. Future research in VTF • The closed configuration provides boundary conditions and • plasma parameters ideal for our future research on reconnection VTF closed cusp

15. Conclusions • Fast, collisionless driven reconnection observed in the open cusp configuration • Dynamic evolution of the current profile and the self-consistent plasma potential observed for the first time during reconnection • LCR circuit accounts for the time behavior of the toroidal plasma current • Classical collisions are not important • The width of the diffusion region scales with cusp • Toroidal current limited due to trapped orbits • Open cusp drift kinetic model consistent with Wind satellite data • New closed cusp configuration, suitable for future research • Strong current throughout the cross section. • The closed geometry will be used in our future research on collisionless reconnection

16. Thank you for your attention

17. Plasma response to driven reconnection

18. What breaks the frozen in condition? Anomalous resistivity, , is incompatible with observations j|| E||

19. Future developments • Full high resolution reconstruction of j(x,t), Y(x,t), E(x,t), n(x,t) and v(x,t) in different Bcusp/Bf regimes (probe upgrades) • Scaling of the size of the diffusion region with Bcusp,, Bf • Direct measurements of off diagonal elements in pe • Direct measurements of ion heating • Machine upgrades • Increase strength of reconnection drive • Reinstallation of in-vessel coils

20. ½Ezl0log(x/y) Experiment Theory Ez 0 Poloidal Drift w/o e.s. potential: charge separation No charge separation if EtotB =½Ezl0log(x/y) Drift kinetic modeling J.Egedal and A.Fasoli, PRL 86, 5047 (2001)

21. Drift kinetic calculation (1) Linear cusp, B=b0(xx-yy+l0z), Az=b0xy-Ezt,  =½Ezl0log(x/y) • Conserved • quantities:  = mv2/(2B) J =  v dl (here =v||/v)

22. Drift kinetic calculation (2) Characteristics in (x,)-space ~ ~ ~ •  = v 2 g1; g1 (x,)=(1-2)/(2B) • J = v g2; g2 (x,) =   dl • /J2 = g1 (x,)/[g2 (x,)] 2 • is a COM  contours are • characteristics through the • (x,)-plane • Consider two points on a characteristic: • (v1,x1,1) and (v2,x2,2 ): • v12g1 (x1,1) = v22g1(x2,2)  • v1/v2 = [g1(x2,2) / g1(x1,1)]½ • = h(x1,1,x2) h(x1,1), for x2=6l0

23. Drift kinetic modeling(2) Density p||/p Example of distribution: Assume a Maxwell distribution: n = n0 and p= p0 for x=6l0 After integration over v: n = ½n0-11 h3 d p= ½p0-11 (1-2) h5 d p||= ½p0-11 2 h5 d

24. Drift kinetic calculation (4) • Flows: • v= E  B / B2 • obtain v|| • from div(nv)=0

25. Electron Hybrid Probe: • Invented by Khash Shadman. • A probe compatible with VTF plasmas • was implemented by James Dorris. • The probe provides information about • the rotational energy of the electron as • a function of v||. I [A] t [s]

26. Sketch of poloidal flux during reconnection drive No reconnection as in ideal MHD Fast reconnection as in vacuum

27. Itot E J(r) n(r) Current sheet, electric field, density, and potential evolution for weak Ip Vfloat(r) plasma slows down reconnection

28. Two fluid model for VTF (J.Ramos, F.Porcelli) • Strong Bguide, low b, boundaries at , T=const, p isotropic • Grad(p) (rS) and /tJ (de) terms are important • Current decay: transition from collisionless to collisional • Agrees with • Self-consistent potential away from separatrix • Absence of steady-state current layer. • Disagrees with • Time evolution of E, E hJ even for t   • Large diffusion region observed in experiment

29. VTF Diagnostics: LIFE.g. planar set-up: fi(vklaser,x,y) • Pulsed dye laser (Lambda Physik Scanmate pumped by Nd:Yag) pumps 611.5 nm line Elaser ~ 20 mJ in 10 ns • LIF detected at 461 nm (intensified CCD?) 2 1 3