Burrell, K.H.

Kee, G.R.

Moyer, R.A.

Porkolab, M.

Rhodes, T.L.

Rost, J.C.

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Comparison of Edge Fluctuation Measurements fromPCI, BES, Langmuir Probes and Reflectometry onDIII–DJ.C. Rost,1 M. Porkolab,1 T.L. Rhodes,2 R.A. Moyer,3G.R. McKee,4 K.H. BurrellGeneral Atomics, P.O. Box 85608, San Diego, California 92186-56081Massachusetts Institute of Technology, Cambridge, Massachusetts2University of California, Los Angeles, California3University of California, San Diego, California4University of Wisconsin, Madison, Wisconsin.AbstractPhase contrast imaging (PCI), beam emission spectroscopy (BES), reflec-tometry, and Langmuir probes provide different but complementary mea-surements of the density fluctuations in the edge of the DIII–D tokamakplasma. In particular, the PCI is sensitive to modes with kθ  kr, while theBES and probe response depends less on the angle of ~k⊥.On plasma discharges in which these diagnostics collect data from thesame location at the same time, comparison of the signals gives more in-formation about the turbulence than a single measurement. Comparingmeasurements from L- and H-mode plasma phases, we found that the L-mode frequency spectra (10–250 kHz) from BES, PCI, and reflectometryhave identical shapes. However, the H-mode spectra differ significantly, withthe kθ  kr modes suppressed the most. This suggests that the edge fluctua-tions are nearly isotropic in L-mode but highly anisotropic in H-mode. Othercomparisons at the last closed flux surface in two similar low power plasmadischarges differing by a factor of four in density showed power spectra witha more complicated structure.I. IntroductionUnderstanding heat and particle transport via density fluctuations may bekey in designing a plasma fusion reactor. Any single diagnostic can only mea-sure a restricted set of the characteristics of the turbulence, including limitsin frequency and wave number range and measurement location. Conse-quently, employing several of these diagnostics simultaneously gives a betterpicture of the turbulence than any of these alone.1The advances in computer simulations of plasma transport are leading tomature models describing plasma fluctuations[1], including models specificto the plasma edge[2]. The type of comparisons presented here are needed toassess the validity of the modeling results. Furthermore, the general resultsof the computer models can improve our understanding of the diagnosticmeasurements and thus allow us to better interpret the observations.In this work, we compare measurements of density fluctuations in theplasma edge acquired with the phase contrast imaging (PCI) interferometer[3],the beam emission spectroscopy (BES) diagnostic[4], the reflectometer system[5],and fast scanning Langmuir probes[6] on the DIII–D tokamak[7]. While thestrengths and limitations of each diagnostic may be found in the references,the most important difference for this study is that BES and probes areequally sensitive to all ~k⊥ wave number directions, while PCI responds tothe kθ  kr range of wave number space, and reflectometry responds to krcomponents but more so to kθ components. Note that this implies that thePCI data is minimally affected by Doppler shifts caused by plasma rotation.We note that acquiring data for these comparisons is not a trivial exercise.The PCI and probe measurements are not available for all plasma shapes.The BES requires that a particular neutral beam heating source be used,which precludes use of the fast scanning probe. The position of fluctuationmeasurements by the reflectometer depends on the density profile, and maynot be specified in advance.In order that the features compared be well-defined, we examine turbu-lence frequency power spectra of the various diagnostics. We interpret theresults in terms of the different k-space response of the diagnostics.II. L–H TransitionValuable data for this comparison was acquired on a DIII–D experimental runday dedicated to studying the L–H transition. The plasmas were sawtooth-free, double null diverted discharges, with ne = 4× 1019 m−3 and 7.5 MW ofneutral beam power.Comparisons of data from PCI, BES, and reflectometry made 100 msbefore the transition and 140 ms after the transition are shown in Fig. 1. Noabsolute calibration of the signals was used. Multiplication factors were usedto align the L-mode signals, and the same multiplicative factors were usedfor the H-mode data.The L-mode spectra are identical in shape. The minimal hypothesis isthat the edge turbulence at this time is isotropic.210 kHz 100 kHz1010101010100-1-2-3-4-5(L-mode level)L-mode H-mode100167BESRefl.PCIBESRefl.PCI100 kHz10 kHzPower Spectra [AU]Figure 1: PCI, BES, and reflectometer fluctuation power spectra in L- andH-modeComparing the H-mode data to that from L-mode shows that the com-ponents to which the PCI is sensitive are most strongly suppressed. Thismay be a direct effect of the increased edge velocity shear in the H-modeedge[8]. Velocity shear breaking up the edge turbulence can move energyinto higher kr, above the PCI limit (in the current configuration) of approxi-mately 6 cm−1. Another possible interpretation is that the instability drivesthe finite kθ modes which couple nonlinearly to modes at kθ ∼ 0 (this behav-ior is predicted by models of core fluctuations[9]). Hence a decrease in thefinite kθ modes results in a larger decrease in the nonlinearly driven kθ ∼ 0modes.III. Density ScanData for comparison of diagnostics was also obtained during a DIII–D ex-periment studying ITG turbulence[10]. Calculations for these limited, nearlycircular plasmas predicted that the ITG instability was stable in the coreat the low density, but unstable at higher density. PCI, reflectometer, andLangmuir probe data from the last closed flux surface (there may be nodirect correspondence between the edge turbulence and the instabilities inthe core plasma) at two very different densities, ne0 = 0.9 × 1019 m−3 andne0 = 4× 1019 m−3, is shown in Fig. 2.In contrast to the L-mode plasma shown in Fig. 1, the spectra from thethree diagnostics do not have the same shape; the relative amplitude ofthe spectra from the different diagnostics varies by two orders of magnitudebetween 10 kHz and 100kHz. However, all three follow the same trend: the310 kHz 100 kHz 1 MHz 10 kHz 100 kHz 1 MHzHigh density Low density10101010101010 99805 99815, 99819probePCIRefl.probePCIRefl.-2-3-4-5-101Power Spectra [AU]Figure 2: PCI, reflectometer, and Langmuir probe fluctuation power spectraat the LCFS, with arbitrary normalization. Right-hand plot includes datafrom two identical shots. Spectra are more heavily smoothed at high frequen-cies for clarity.fall off of the power spectrum becomes steeper at higher density. Assuminga functional form of S ∝ 1/f γ near 100 kHz, the spectra varied as γ = 2in the low density shot, but obeyed γ of 3–4 at higher density (PCI datathroughout the experiment showed that γ increased smoothly with plasmadensity). The Langmuir probe data, absolutely calibrated, shows that thehigh density discharge had a larger RMS amplitude of fluctuations, but thatthe lower density plasma had more power in fluctuations above 100 kHz.While the spectra are more complicated than in the previous section,the evidence does suggest the following interpretation. In the high densitycase, the kθ ∼ 0 components as seen by the PCI dominate the turbulence,and so the probe spectrum follows the PCI spectrum. However, at higherfrequencies the turbulence is dominated by the poloidal components, and thereflectometer and probe spectra are similar. The low frequency componentsmay be due to small radial oscillations in the plasma position, which wouldshow up in the PCI and probe data.In the lower density discharge, the reflectometer and probe spectra havesimilar shapes, while the PCI spectrum is steeper at low frequencies. Thereare several features apparent in the graph, however, which defy simple ex-planation.4IV. ConclusionsThe comparisons of measurements made by several edge fluctuation diagnos-tics show that the wave number spectra have a complex structure. Betterknowledge of structure of the spectra can help in understanding phenomenasuch as H-mode transport barriers. The description of the variation of thespectra in different plasma conditions is a necessary part of benchmarkingthe fluctuation details of turbulence models, analytic or computational. Onegoal of this work is to document the fluctuations in a way appropriate forthese detailed comparisons.This work also highlights difficulties in the interpretation of fluctuationdata. While the turbulent transport generally decreases with decreased fluc-tuation level, we have seen that different spectral components of the fluc-tuations increase or decrease by different amounts as the overall fluctuationamplitude changes. Thus a more detailed knowledge of which spectral com-ponents generate the transport in a given regime is needed to correlate quan-titatively changes in transport and fluctuation amplitude. Similar problemsarise with another parameter of the turbulence used in transport studies,namely the autocorrelation time, which is roughly the inverse of the widthof the fluctuation spectrum[11]. In the data presented, the autocorrelationtime inferred from different diagnostics varies significantly.We believe that the work presented here shows the importance of em-ploying a complementary set of diagnostics to fully characterize plasma tur-bulence.AcknowledgmentsThis is a report of work supported by the U.S. Department of Energy underContract No. DE-AC03-99ER54463 and Grant Nos. DE-FG02-94ER54235,DE-FG03-86ER53225, DE-FG03-95ER54294, and DE-FG02-92ER54139. Theauthors would like to thank the DIII–D team for their contributions to thiswork.References[1] Dimits, A. M., Bateman, G., Beer, M. A., et al., Phys. Plasmas 7 (2000)969.[2] Xu, X. Q. and Cohen, R. H., Contrib. Plasma Phys. 36 (1998) 158.[3] Coda, S., Porkolab, M., and Carlstrom, T. N., Rev. Sci. Instrum. 63(1992) 4974.5[4] McKee, G., Ashley, R., Durst, R., et al., Rev. Sci. Instrum. 70 (1999)913.[5] Rhodes, T. L., Peebles, W. A., Doyle, E. J., et al., Plasma Phys. Control.Fusion 40 (1998) 493.[6] Watkins, J. G., Salmonsen, J., Moyer, R., et al., Rev. Sci. Instrum. 63(1992) 4728.[7] Luxon, J. L. and Davis, L. G., Fusion Technol. 8 (1985) 441.[8] Burrell, K. H. et al., Phys. Plasmas 1 (1994).[9] Waltz, R. E., Kerbel, G. D., and Milovich, J., Phys. Plasmas 1 (1994)2238.[10] Rettig, C. L., Rhodes, T. L., Peebles, W. A., et al., Bull. Am. Phys.Soc. 44 (1999) 74.[11] Bendat, J. S. and Piersol, A. G., Random Data: Analysis and Measure-ment Procedures, John Wiley and Sons, 1986.6

Comparison of Edge Fluctuation Measurements from PCI, BES, Langmuir Probes and Reflectometry on DIII-D

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Comparison of Edge Fluctuation Measurements from PCI, BES, Langmuir Probes and Reflectometry on DIII-D

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