ET Troy A. Carter Dept. of Physics and Astronomy, …tcarter/tac_ttf03.pdfINTERMITTENT TURBULENCE...
Transcript of ET Troy A. Carter Dept. of Physics and Astronomy, …tcarter/tac_ttf03.pdfINTERMITTENT TURBULENCE...
INTERMITTENT TURBULENCE AND COHERENTSTRUCTURES IN LAPD AND ET
Troy A. CarterDept. of Physics and Astronomy, UCLA
acknowledgments:D. Pace, B. Brugman, R. Taylor, L. Schmitz, G. Morales, J. Maggs,
M. Vanzeeland, P. Gourdain, J.-L. Gauvreau (UCLA); D. Rudakov, R. Moyer,S. Krasheninikov (UCSD)
Transport Task Force Meeting, Madison, WisconsinApril 3, 2003
What is intermittent turbulence?
B
+++
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EvExB
� “Bursty” or “spiky” signals, non-Gaussian fluctuation amplitude PDF
� Can be a signature of significant convective transport (E× B convectionof “blobs” [Boedo, Antar, et. al.])
� Found in a wide range of magnetic confinement devices (linear, tokamak,stellarators, etc) [Antar, et. al.]; May play a role in the density limit intokamaks [Greenwald, et. al.]
Goals/results of this study
� Origin of coherent structures: by what mechanism are “blobs”generated?
� Blob transport: what sets blob size, velocity, frequency of generation?
. Theory [Krasheninnikov]: Blob polarization through drift-currentcharging (∇B or rotation required)
� Results of this study
. Detailed statistical study of properties of intermittently generatedcoherent structures in LAPD
. Initial measurements of intermittency on the inboard and outboardmidplane of ET
. Measurements suggest drift-current charging is unlikely to be theexplanation for blob convection.
The LArge Plasma Device (LAPD) at UCLA
� Emissive Barium Oxide cathode source (50V, 10kA in source region)
� 0.5 < B < 2.5 (kG), ne < 5× 1012 cm−3, Te . 10eV, Ti � Te(∼ 1eV)
� 1 m diameter, 20 m long, floating plate located 10 m from the source
� He, Ne, Ar plasmas
� 1 Hz rep rate, ∼ 10 ms pulse length
� National user facility (http://plasma.physics.ucla.edu/bapsf )
Limiter produced density gradients in LAPD
� Floating plate located 10m fromsource is partially closed
� Profile behind the plate and as-sociated turbulence is studied
� Measurements with tripleLangmuir probe and radialLangmuir array (0.5 cm spac-ing, 6 tips)
Large amplitude fluctuations localized to limiter producedgradient
� Density gradient scale length ∼ 2− 4cm (∼ 10− 20ρi)
� Large amplitude fluctuations localized to the edge, Broadband frequencyspectrum
� Substantial density behind floating plate (no parallel source)
Example signals behind floating plate: Strong intermittencyand evidence for both “blobs” and “holes”
Fluctuation amplitude PDF: highly non-Gaussian
holesblobs
PDF (log)
Conditionally averaged events (B = 1.5kG)
� Blob event is asymmetric in time, hole exhibits inverse asymmetry
� Cross-conditional average on V f indicates charged objects (rotating)
Blob width (∆tFWHM) increases with decreasing field
� ∆tFWHM computed for each event to construct PDF
� Change in event asymmetry for low field?
Blobs propagate into the SOL; holes back into the core plasma
� v ∼ vs/10, holes a bit faster than blobs; no dependence on field strengthobserved
� Vertical flow (or rotation?) does not seem to be significant
Mean blob size scales with the ion sound gyroradius
� Varied field, fill pressure, working gas (He, Ne)
� For He data, 〈ρb〉 ∼ 10ρs
� Mean size is comparable to density gradient scale length
Waiting time: PDF depends primarily on B
� twait: time between successive events
� Broadband creation (wide range of timescales); reflects frequencyspectrum
� Mean timescale larger as field is lowered; qualitative differences atlowest field (more coherent?)
Passive imaging shows structure in emission behind floatingplate
� Intensified CCD images show large scale structure (& 10cm) (imagestaken by R. Taylor)
Edge measurements in the Electric Tokamak
� Fixed radial arrays of Langmuir probes (0.3,0.5 cm spacing) in theinboard and outboard midplane edge
� Measurements in inboard and outboard scrape-off-layer (SOL) as well asinside LCFS
� Differences from LAPD study: magnetic topology, ∇B, rotation,temperature
Outboard measurements: intermittency in the SOL
� Fixed probe, plasma is pushed off outer wall to place probe in SOL
� Inside LCFS, PDF of fluctuation amplitude is closer to Gaussian
Inboard measurements: No poloidal asymmetry inintermittency
� Rules out ∇B-drift polarization as primary polarization mechanism?
Measurements: summary
� LAPD. Observed blobs propagating into limiter shadow, holes
propagating back into core plasma (generation localized togradient region)
. Mean size of these objects scales with ρs, velocity scales withvs
. Holes and blobs have similar statistics: Pairwise produc-tion?
. Observations in LAPD do not seem to support the drift-current charging model (no∇B, no strong evidence for rota-tion).
� ET. No poloidal asymmetry observed in intermittency. Rules
out ∇B as primary driver for blob convection? Rotation isstill a possibility?
Speculation/Future work
� Blob/hole polarization: diamagnetic polarization (nonlineardrift wave turbulence, neutral wind [Krasheninnikov])?
� Polarization could be an effect rather than a cause:polarization response to initial cross-field velocity[Borovsky]
� Future work:. Polarization measurements: compare E× B to measured
velocity in LAPD. Study of blob/hole production: e.g. correlations between
blobs/holes. Further imaging measurements (LIF, GPI). Repeat statistical study in ET for cross-comparison
(relevance of results to tokamaks?). Density limit study in ET (LAPD?)