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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

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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 Our definition of magnetic reconnection

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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 ion polarization current –Size of diffusion region (E B 0) –Orbit effects Future work on VTF –New diagnostics –Closed cusp configuration Outline

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

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

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

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VTF configuration mfp >>L, coll > orbit,t A ; i <

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Reconnection drive –Ohmic coils driven by LC resonant circuit –Flux swing ~ 0.2 V-s, duration ~ 6 ms (>>t reconnection ) –V loop ~ 100 V, v ExB ~ 2km/s ~ v A /10

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

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Plasma response to driven reconnection (1)

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Plasma response to driven reconnection (3) Current layers develop for l 0 =B guide / B cusp ~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?

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Anomalous resistivity First observation of strongly anomalous parallel resistivity ( meas = E /J max ) - Current sustained in plasma determines reconnection rate - For l 0 =B guide / B cusp <3m I p ~ 0 reconnection rate is same as in vacuum I p max [kA] l 0 =B guide / B cusp [m] meas / Spitzer vs e /l 0

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Electrostatic potential (away from the X-line) B=b 0 ( x x – y y + l 0 z) E = E z ( x + y + z) -l 0 l 0 2x 2y -- EzEz =½E z l 0 log(|x/y|) Ideal region : E + v×B = 0 E · B = 0 E= E z z -

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Observation of self-consistent e.s. potential E z 0 Poloidal Drift w/o e.s. potential: charge separation No charge separation if E tot B =½E z l 0 log(x/y) ½E z l 0 log(x/y) ExperimentTheory Deviations from E B=0 are observed close to separatrix diffusion region J.Egedal and A.Fasoli, PRL 86, 5047 (2001)

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The size of the diffusion region (1) Frozen in law is broken where E B 0 Experimental measurement = 3.5 cm E B=(E - ) B Fit extending form valid in ideal region

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The size of the diffusion region (2) NeonNitrogen KryptonXenon The size of the diffusion region is clearly independent of ion mass and n e It cannot be related to c/ pi,e or i,s

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

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Temporal evolution of the current channel Time response of the toroidal current Time in steps of 12 s f ~20-30 kHz H2H2 Ar

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Plasma response to an oscillating drive (1)

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0 – 1.2 kA/(Vm 2 ) In phase withV loop 0 – 20 mAs/(Vm 2 ) 90 0 ahead of V loop Plasma response to an oscillating drive (2) The current profile can be separated in two parts: What causes the out of phase current?

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

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

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Circuit model for VTF plasmas (1), ;

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Circuit model for VTF plasmas (2) Total current is measured in each shot by a Rogowsky coil Values of R j1 and C j2 obtained by curve fitting Deviation? As the observed dependence implies C j2 V loop [As]

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What breaks the frozen in condition? The plasma frozen in condition is violated where: Generalized Ohms law: too small (<10 3 ) & can’t explain phase far too small with strong guide field, would give c/ pi Would give c/ pe Only off-diagonal terms (toroidal symmetry)

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Breaking the frozen in law Orbit width, cusp = ( g l 0 ) 0.5 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: EB 0 J.Egedal, PoP 9, 1095 (2002) [cm]

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Conclusions –Fast, collisionless driven reconnection directly observed B guide <~ B cusp –No current channel, trapped orbits; self-consistent plasma potential B guide >> B cusp –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 p e (off-diagonal) and/or dJ/dt terms are needed (kinetic effect)

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Future developments Energy and velocity distributions –Laser Induced Fluorescence »f i (v) at one position; planar LIF f i (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 (x,t) and f e,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

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VTF Diagnostics: LIF E.g. planar set-up: f i (v klaser,x,y) –Pulsed dye laser (Lambda Physik Scanmate pumped by Nd:Yag) pumps 611.5 nm line E laser ~ 20 mJ in 10 ns –LIF detected at 461 nm (intensified CCD?) 1 3 2

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

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First observation of LIF on VTF ArII plasma Broad band, E laser ~ 5 mJ/pulse, t laser ~ 15 ns

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

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