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?)