1 / 32

Dynamical plasma response during driven magnetic reconnection in the laboratory

Dynamical plasma response during driven magnetic reconnection in the laboratory. Ambrogio Fasoli * Jan Egedal MIT Physics D pt & Plasma Science and Fusion Center Ackn.: W.Fox, J.Nazemi, M.Porkolab *Now at EPFL Physics D pt & Centre de Recherche en Physique des Plasmas.

zanna
Download Presentation

Dynamical plasma response during driven magnetic reconnection in the laboratory

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Dynamical plasma response during driven magnetic reconnection in the laboratory Ambrogio Fasoli* Jan Egedal MIT Physics Dpt & Plasma Science and Fusion Center Ackn.: W.Fox, J.Nazemi, M.Porkolab *Now at EPFL Physics Dpt &Centre de Recherche en Physique des Plasmas

  2. Our definition of magnetic reconnection • Change of magnetic field topology in the presence of plasma • Reconnection rate: value of E-field along X-line, perpendicular to plane over which flux annihilates

  3. Outline • Driven reconnection in the VTF open cusp • Conditions to create a current channel • Dynamical evolution of j and E • j||and dE/dt linked through ionpolarization current • Size of diffusion region (EB0) • Orbit effects • Future work on VTF • New diagnostics • Closed cusp configuration

  4. Family of Reconnection Experiments(from H.Ji, PPPL)

  5. Magnetic Reconnection on VTF The VTF device 2 m • Origin of fast time scale for reconnection, mechanisms behind breaking of frozen-in flux • Particle orbits ? • Instabilities / waves ? • ….

  6. Diagnostic ‘workhorses’ on VTF 45 heads L-probe 40 Channels B-probe

  7. Ex. of target plasma profiles Bcusp = 50mT, Bguide= 87mT; PECRH ~ 30 kW VTF configuration • lmfp>>L, tcoll>torbit,tA;ri<<L • S = m0LvA/h>>1 • Plasma production by ECRH separate from reconnection drive J.Egedal, A.Fasoli et al., RSI 71, 3351 (2000)

  8. Reconnection drive • Ohmic coils driven by LC resonant circuit • Flux swing ~ 0.2 V-s, duration ~ 6 ms (>>treconnection) • Vloop ~ 100 V, vExB ~ 2km/s ~ vA/10

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

  10. Plasma response to driven reconnection(1)

  11. Plasma response to driven reconnection(3) • Current layers develop for l0=Bguide/Bcusp~3m • No steady-state • Questions: • How much max current / min anomalous resistivity (though ratio E/J is not constant!)? • What determines the size and time evolution of the diffusion region where ideal MHD breaks?

  12. Ipmax [kA] l0=Bguide/Bcusp [m] Anomalous resistivity - Current sustained in plasma determines reconnection rate - For l0=Bguide/Bcusp<3m Ip ~ 0  reconnection rate is same as in vacuum hmeas/hSpitzervs le/l0 First observation of strongly anomalous parallel resistivity (hmeas = Ef/Jfmax)

  13. Ideal region: E + v×B= 0 E · B = 0 - Ez -l0 l0 2x 2y Electrostatic potential(away from the X-line) E= Ezz -  B=b0 ( x x – y y + l0 z) =½Ezl0log(|x/y|) E = Ez ( x + y + z)

  14. 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) Observation of self-consistent e.s. potential J.Egedal and A.Fasoli, PRL 86, 5047 (2001) Deviations from EB=0 are observed close to separatrix  diffusion region

  15. The size of the diffusion region(1) Experimental measurement Frozen in law is broken where EB0 EB=(E -) B Fit extending form valid in ideal region d = 3.5 cm

  16. Neon Nitrogen Krypton Xenon The size of the diffusion region(2) • The size of thediffusion region is clearly independent of ionmass and ne • It cannot be related to c/wpi,e or ri,s

  17. The size depends • on [cm] The size of the diffusion region(3)

  18. Temporal evolution of the current channel Time in steps of 12 ms Time response of the toroidal current H2 f ~20-30 kHz Ar

  19. Plasma response to an oscillating drive(1)

  20. In phase withVloop 900 ahead of Vloop 0 – 1.2 kA/(Vm2) 0 – 20 mAs/(Vm2) Plasma response to an oscillating drive(2) • The current profile • can be separated in • two parts: What causes the out of phase current?

  21. Ion polarization currents due to d/dt Ion polarization current: Quasi neutrality:  fits exp. Data with  = 3.5 cm

  22. Interpretation with polarisation current predicts time evolution and shape of current channel MEASURED change in f and  j dt between t=0 and 30ms THEORY / FIT

  23. , ; Circuit model for VTF plasmas(1)

  24. Deviation? As the observed dependence implies Cj2Vloop Cj2Vloop [As] [As] Circuit model for VTF plasmas(2) • Total current is measured in each • shot by a Rogowsky coil • Values of R j1 and Cj2 obtained • by curve fitting

  25. Would give c/wpe too small (<103) & can’t explain phase far too small with strong guide field, would give c/wpi Only off-diagonal terms (toroidal symmetry) What breaks the frozen in condition? The plasma frozen in condition is violated where: Generalized Ohms law:

  26. [cm] Breaking the frozen in law • All electrons are trapped  • limiting macroscopic current channel • Electrons short circuit electric • fields along their orbits • The frozen in law is broken in areas • where the orbits do not follow the • field lines: E•B0 Orbit width, cusp= (g l0)0.5 J.Egedal, PoP 9, 1095 (2002)

  27. Conclusions • Fast, collisionless driven reconnection directly observed • Bguide <~ Bcusp • No current channel, trapped orbits; self-consistent plasma potential • Bguide>>Bcusp • Dynamic evolution of current profile and self-consistent potential • Classical collisions: not important • Ion polarisation current (analogy with RLC circuit) explains observed reconnection dynamics • Key parameter is • Diffusion region does not scale with el./ion Larmor radius, el./ion skin depth, but with characteristic particle orbit size • Direct measurements of different terms in generalised Ohm’s law suggest that pe (off-diagonal) and/or dJ/dtterms are needed (kinetic effect)

  28. Future developments • Energy and velocity distributions • Laser Induced Fluorescence • fi(v) at one position; planar LIF fi(v, x) with intensified CCD • E.s. energy analyzer • Systematic analysis of e.s. and e.m. fluctuations • Spatial and temporal correlations; effect on plasma effective resistivity • Combined reconstruction of Y(x,t) and fe,i(v,x) around X-point  particle energization mechanism • Machine upgrades • Increase strength of reconnection drive (reduce ECRH frequency) • Installation of in-vessel coils • Reduction in direct plasma losses: from collisionless to collisional regime

  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

  30. PMT IF Present set-up for LIF on VTF laser beam

  31. First observation of LIF on VTF ArII plasma Broad band, Elaser ~ 5 mJ/pulse, Dtlaser~ 15 ns

  32. Plasma edge time (10-5 s) 250 250 Freq (MHz) Freq (MHz) Increased turbulence close to current channel, where gradients are large f < 100 MHz << fpe 0 0 E.S. fluctuations during reconnection, weak Ip J(r) vs .time

More Related