Presented at PPPL, Princeton, NJ May 20, 2008 ~ Measurements of Core Electron Temperature Fluctuations A. E. White University of California-Los Angeles, Los Angeles, California, USA L. Schmitz, a) W.A. Peebles, a) T.A. Carter, a) G.R. McKee, b) C. Holland, c M.E. Austin, d) K.H. Burrell, e) J. Candy, e) J.C. DeBoo, e) E.J. Doyle, a) M.A. Makowski, f) R. Prater, e) T.L. Rhodes, a) M.W. Shafer, b) G.M. Staebler, e) G.R. Tynan, c) R.E. Waltz, e) G. Wang a) and the DIII-D Team, e) a) University of California-Los Angeles, Los Angeles, California, USA b) University of Wisconsin-Madison, Madison, Wisconsin, USA c) University of California-San Diego, La Jolla, California, USA d) University of Texas, Austin, Texas, USA e) General Atomics, P.O. Box 85608, San Diego, California, USA f) Lawrence Livermore National Laboratory, Livermore, California, USA 1 Both Electron Temperature and Density Fluctuations Provide Information about Physics of Turbulence and Transport Several types of instabilities may contribute to electron heat and particle transport in the tokamak Ion temperature gradient (ITG) mode ( 2 ) Core electron temperature and density fluctuations both contribute to energy transport flux (Liewer 1985, Wootton 1990, Ross 1992) Measurements of T e probe physics of non-Boltzmann electron response, in particular, trapped electrons Turbulence models: electron heat and particle transport result from non Boltzmann (non-adiabatic) electrons Trapped electrons destabilize ITG mode, drive TEM unstable ~ 2 Time history of T e /T e during single discharge reveals changes in amplitude in L-mode, H-mode and Ohmic plasmas Electron temperature fluctuations, T e /T e, and density fluctuations, /n, have similar spectra, amplitudes and increase with radius GYRO predicts T e /T e ~ e /n e, consistent with observations. GYRO/synthetic diagnostics do not fully reproduce increase in fluctuation level with radius. Electron Cyclotron Heating (ECH) during beam heated L-mode plasmas results in increased T e /T e, but not /n Summary of Results ~ ~ ~ ~ 3 SSB receiver with two channel filter bank 4 Emission in non-overlapping frequency bands Separated by less than turbulence correlation length Cross-correlate signals to measure RMS amplitude and spectrum f 1 f 2 Correlation Electron Cyclotron Emission (CECE) Diagnostic Measures Local, Low-k Electron Temperature Fluctuations f 2 f MHz 250 MHz The thermal noise feature is broadband in frequency The temperature fluctuation feature can be measured (~ 100 ms average) in cases of moderate filter overlap when B sig < B vid MHD modes (B sig 15% Sensitivity improves T e /T e > 0.2% 36 Correlation Radiometry Needed for Measurements of Turbulent Temperature Fluctuations from ECE Past Work: TEXT (Cima 1995, Deng 1998 ), W7-AS (Sattler 1994, Hartfuss 1996, Watts 2004), RTP (Deng 2001), DIII-D (Rettig 1997, Schmitz 2008) ~ ~ 37 CECE Gaussian Optics Provide Small Spot-Size Needed for Turbulence Measurements CECE is sensitive to long wavelength fluctuations of electron temperature Laboratory tests: 94 GHz incident beam focused using parabolic mirror The beam agrees well with a Gaussian spatial profile. Small spot-size makes turbulence measurements possible 1/e 2 power diameter Radial Extent of CECE Sample Volume is Determined by the Natural Linewidth, with Small Corrections From Filter Width 38 Natural linewidth of the emission layer is given by the emissivity (ECESIM DIII-D IDL-based code) Separation of sample volumes determines radial wavenumber resolution, k r < 4 cm -1 Radial sample size (r ~ 1 cm) for a single IF filter is slightly wider than the natural linewidth Linewidth ( r ~ 0.8 cm) is determined by the relativistic broadening and re- absorption in the plasma amplitude Calibrated fixed filter CECE signals give T e Relatively calibrated signals give T e /T e From the correlation coefficient function Calibrated tunable YIG CECE signals give T e from correlation function Calibrated YIG CECE signals normalized to local T e give T e /T e from the correlation function T e and T e /T e can also be calculated by integrating the cross-power spectrum, P xy, over frequency range, [f 1, f N ] of interest The CECE radiometer is calibrated to measure T e and T e, No calibration needed for T e /T e measurements ~ ~ ~ ~~ ~ ~~ Thermal noise fluctuations decorrelate when f ~ B if independent of radiation source, or sample volume in plasma (c) (a) 2-18 GHz noise source (input to first amplifier in radiometer) (b) W-band noise source (input at antenna) (c) L-mode and H-mode plasmas In optically grey plasma the density fluctuations can contribute to signal, leading to apparent temperature fluctuations [Rempel RSI 1994] Contribution from Density Fluctuations to Signal Due to Low Optical Depth are Negligible Ray-tracing code GENRAY is used to estimate the effects of refraction on the CECE sample volume size and location Ray-tracing (disk-to-disk) 25.4 cm diameter (at mirror) to 3.8 cm diameter (in plasma) Low-density plasmas n 0 ~3.5x10 19 m -3 Refractive effects: Sample volume location Vertical up-shift < 0.5 cm Sample volume diameter Spot size changes < 0.2 cm * * Comparable to measurement uncertainty of spot-size in lab (a) Case with density, n e = 4.3x m -3 only 80 % of cut-off density for 2f c 93 GHz. n e, cut-off (93 GHz) ~5.35 x m -3. (b) Case with density > 100 % of cut-off density. Substantial refractive effects obvious for 15 degree mirror, no signal is seen for 7 degree mirror. Modulations of the index of refraction along the line of sight will not cause apparent T e if ECE is far from cut-off Refractive effects are negligible for plasmas under consideration: n e < 0.8 n e cut-off ~ Profile Comparison from ECH Experiment 44 CERFIT Analysis Indicates Slight Reduction In E r for ECH Case - Expect Narrowing of Turbulent Spectra 45 Temperature Fluctuations Increase Across Radius with ECH 46 TGLF Results from ECH Experiment: TEM Linear Growth Rate Increases with ECH 47 Gamma/(Cs/a) Beam Emission Spectroscopy (BES) measures spatially localized, long-wavelength density fluctuations