INTERMITTENT TURBULENCE AND COHERENT STRUCTURES IN LAPD AND ET Troy A. Carter Dept. 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, Wisconsin April 3, 2003
What is intermittent turbulence? B +++ E ---- v ExB Bursty or spiky signals, non-gaussian fluctuation amplitude PDF Can be a signature of significant convective transport (E B convection of 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 in tokamaks [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-current charging ( B or rotation required) Results of this study Detailed statistical study of properties of intermittently generated coherent structures in LAPD Initial measurements of intermittency on the inboard and outboard midplane of ET Measurements suggest drift-current charging is unlikely to be the explanation 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), n e < 5 10 12 cm 3, T e 10eV, T i T e ( 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 from source is partially closed Profile behind the plate and associated turbulence is studied Measurements with triple Langmuir probe and radial Langmuir array (0.5 cm spacing, 6 tips)
Large amplitude fluctuations localized to limiter produced gradient Density gradient scale length 2 4cm ( 10 20ρ i ) Large amplitude fluctuations localized to the edge, Broadband frequency spectrum Substantial density behind floating plate (no parallel source)
Example signals behind floating plate: Strong intermittency and evidence for both blobs and holes
Fluctuation amplitude PDF: highly non-gaussian PDF (log) holes blobs
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 ( t FWHM ) increases with decreasing field t FWHM 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 v s /10, holes a bit faster than blobs; no dependence on field strength observed 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, ρ b 10ρ s Mean size is comparable to density gradient scale length
Waiting time: PDF depends primarily on B t wait : time between successive events Broadband creation (wide range of timescales); reflects frequency spectrum Mean timescale larger as field is lowered; qualitative differences at lowest field (more coherent?)
Passive imaging shows structure in emission behind floating plate Intensified CCD images show large scale structure ( 10cm) (images taken by R. Taylor)
Edge measurements in the Electric Tokamak Fixed radial arrays of Langmuir probes (0.3,0.5 cm spacing) in the inboard and outboard midplane edge Measurements in inboard and outboard scrape-off-layer (SOL) as well as inside 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 in intermittency Rules out B-drift polarization as primary polarization mechanism?
Measurements: summary LAPD ET Observed blobs propagating into limiter shadow, holes propagating back into core plasma (generation localized to gradient region) Mean size of these objects scales with ρ s, velocity scales with v s Holes and blobs have similar statistics: Pairwise production? Observations in LAPD do not seem to support the driftcurrent charging model (no B, no strong evidence for rotation). No poloidal asymmetry observed in intermittency. Rules out B as primary driver for blob convection? Rotation is still a possibility?
Speculation/Future work Blob/hole polarization: diamagnetic polarization (nonlinear drift 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?)