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Plasma and Fusion Research: Regular Articles Volume 2, S1025 (2007)

Plasma Turbulence Imaging via Beam Emission Spectroscopy inthe Core of the DIII-D Tokamak

George R. McKEE, Raymond J. FONCK, Deepak K. GUPTA, David J. SCHLOSSBERG,Morgan W. SHAFER, R´ejean L. BOIVIN1) and Wayne SOLOMON2)

University of Wisconsin-Madison, Madison, Wisconsin, 53706, USA1)General Atomics, San Diego, California, 92121, USA

2)Princeton Plasma Physics Laboratory, Princeton, New Jersey, USA

(Received 2 December 2006/ Accepted 6 April 2007)

Beam Emission Spectroscopy (BES), a high-sensitivity, good spatial resolution imaging diagnostic system,has been deployed and recently upgraded and expandedat the DIII-D tokamak to better understand densityfluctuations arising from plasma turbulence. The currently deployed system images density fluctuations over anapproximately 5× 7 cm region at theplasma mid-plane (radially scannable over 0.2 < r/a ≤ 1) with a 5× 6(radial× poloidal) grid of rectangular detection channels, with one microsecond time resolution. BES observescollisionally-induced, Doppler-shifted Dα fluorescence (λ = 652-655 nm) of injected deuterium neutral beamatoms. The diagnostic wavenumber sensitivity is approximatelyk⊥ < 2.5 cm−1, allowing measurement of long-wavelength (k⊥ρI < 1) density fluctuations. The recent upgrade includes expanded fiber optics bundles, custom-designed high-transmission, sharp-edge interference filters, ultra fast collection optics, and enlarged photodiodedetectors that together provide nearly an order of magnitudeincrease in sensitivity relative to an earlier generationBES system. The high sensitivity allows visualization of turbulence at normalized density fluctuation amplitudesof n̄/n < 1%, typical of fluctuation levels in the core region. The imaging array allows for sampling over 2-3turbulent eddy scale lengths, which captures the essential dynamics of eddy evolution, interaction and shearing.

c© 2007 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: turbulence, plasma diagnostic, imaging, beam emission spectroscopy, density fluctuation, anomaloustransport

DOI: 10.1585/pfr.2.S1025

1. IntroductionPlasma turbulence plays acritical role in the confine-

ment of energy, particles and momentum in magnetically-confined fusion plasmas. Pressure gradients inherent tosuch plasmas are thought to drive drift-wave turbulence[1], which in turn drives cross-field transport at levels farexceeding the irreducible minimum of collisionally-driven“neoclassical” transport. Plasma turbulence is thought tobe dominantly electrostatic in nature [2], and so radialtransport arises from correlated fluctuations in the den-sity, radial E×B velocity, temperature and parallel andpoloidal velocities [3]. At higher normalized plasma pres-sure(beta), electromagnetic effects may also become im-portant. Because of the centralrole that turbulent-driventransport plays in the confinement of fusion plasmas andtherefore performance of fusion systems, it is necessary tothoroughly understand plasma turbulence from both a the-oretical and experimental perspective. Because turbulenceis manifest as fluctuations in the main plasma parametersof density, temperature, and velocity, it has been neces-sary andfruitful to deploy numerous fluctuation diagnos-tics on a variety of experiments to measure and charac-terize such turbulence so that a deeper understanding and,

author’s e-mail: [email protected]

ultimately, a predictive capability can be developed. Suchcapability will require an aggressive program to validatenonlinear simulations of plasma turbulence and transportwith detailed measurements of fluctuations, and their scal-ing properties, in well-characterized experiments.

Turbulence diagnostics have advanced significantly incapability and performance over the last couple of decadesand are now allowing detailed measurements of fluctuationproperties in high-performance fusion-grade plasmas overa wide range of wavenumbers, with good spatial localiza-tion for several fields (e.g., density and temperature).

Turbulence in magnetically-confined plasmas is alargely two-dimensional phenomenon, since the magneticfield defines an ignorable coordinate. Turbulent eddy struc-tures havek|| << k⊥, and so the most important turbulencedynamics take place in the plane perpendicular to the mag-netic field. As such, it has been widely recognized thatcharacterization of turbulence in the 2D radial-poloidalplane is of special importance and has motivated the de-velopment of 2D fluctuation diagnostics. Despite the sig-nificant development of fluctuation diagnostics, there areremarkably few that can measure fluctuations in 2D. Suchdiagnostics have included probe arrays [4], gas puff imag-ing systems [5–7] and an earlier generation beam emis-

c© 2007 The Japan Society of PlasmaScience and Nuclear Fusion Research

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sion spectroscopy system [8–10]. These diagnostics werelimited to the edge, separatrix, and scrape off regions orcold plasmas due to the various diagnostic limitations. 2DECE Imaging systems are now providing electron tempera-ture fluctuation measurements in the radial-poloidal planein the core of the TEXTOR tokamak [11], though gener-ally not with the time-resolution and sensitivity requiredfor time resolved turbulence measurements.

The beam emission spectroscopy system at the DIII-Dtokamak has been recently upgraded to substantially highersensitivity. This dramatically enhanced signal-to-noise ra-tio of the upgraded system now allows for time-resolvedimaging of turbulent density fluctuations into the core re-gions of low-confinement mode (L-mode) discharges. Thefirst imaging measurements of core (r/a < 0.8) den-sity fluctuations in a tokamak plasma were obtained re-cently [12]. This BES system has recently been expandedto nearly double the array size of this first phase. This isproviding turbulence imaging measurements over a widerarea, now covering a few turbulence correlation lengths inthe radial and poloidal directions.

This paper presents and discusses these spatially ex-panded high-sensitivity 2D measurements of density fluc-tuations. Section 2 reviews the principles of the BES di-agnostic and the main elements of the expanded, high-sensitivity system. The characteristics of plasma turbu-lence in a set of typical low-confinement mode dischargesis presented in Section 3, and imaging methods and mea-surements are presented in Section 4. The paper concludeswith a discussion of applications of such measurements,future directions and summary in Section 5.

2. Overview of the Beam EmissionSpectroscopy DiagnosticBeam Emission Spectroscopy measures long-

wavelength (k⊥ρI < 1) density fluctuations by observingcollisionally-induced Doppler-shifted Dα emission frominjected neutral beam atoms (n =3-2,λo = 656.1 nm) [13].The optical and detection systems allow for multi-channelmeasurements and good localization in the radial-poloidalplane. For the system discussed here, 32 channels aredeployed in a 5 (radial)× 6 (poloidal) 2D grid, plus twocommon-mode rejection channels [14]. Each channel im-ages an approximately 0.9 cm (radial)× 1.2 cm (poloidal)region with channels located immediately adjacent toeach other in each direction, for a total sampling area ofapproximately 5× 7 cm. The spatial point spread functionis radially dependent and is calculated using the opticalsightline and neutral beam viewing geometry, magneticfield geometry, and optical ray tracing, and also includesfinite excited state lifetimes [15], The actual sampled areafor a given channel thus extends modestly beyond the di-rectly imaged region. The 2D grid can be radially scannedon a shot-to-shot basis across the outboard midplane ofthe plasma (nearZ = 0) to provide measurements over

Fig. 1 Equilibrium reconstruction, and positions of 5× 6 BESgrid detection array for the four discharges presented.

the radial range 0.2 < r/a < 1 (plasmaminor radius, a),as well as into the scrape-off-layer region. The channelconfiguration for the discharges used for these imagingmeasurements isoverlaid on a reconstruction of theequilibrium magnetic field shown in Fig. 1. Four repeatdischarges were utilized with the radial position scannedfrom shot to shot betweenr/a = 0.4 andr/a = 0.9.

Details of the diagnostic system are discussed else-where [16,17] and an overview is provided here. The ob-served emission results from an injected deuterium neutralbeam with an energy of 70-80 keV (v = 2.6 × 106 m/s),and Pinj ≈ 2.5 MW. Electron and ion collisions withbeam atoms result in excitation and Dα emission fromthe three beam-energy components. Emission is Doppler-shifted over approximatelyλ ≈ 652-655 nm. A high-throughput objective lens (f = 40 cm, f /2) images lightfrom the beam onto a set of fiber optic bundles with a mag-nification, M ≈ 3. The sightline is approximately tangentto flux surfaces and is angled to match the dominant mag-netic field pitch angle to provide good spatial resolutionperpendicular to the field lines. The fibers are located ona motorized mount that can be remotely scanned betweenshots. Each detection channel consists of 11 1-mm plastic-clad-silica fibers, 40 m in length, arranged in a 4:3:4 con-figuration, that convey the light to a remotely located spec-troscopy lab.

Each detection channel consists of a collimating lens,

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interference filter, high-speed focusing lens and PIN pho-todiode. The interference filter is a custom-designed, hightransmission filter that transmits light in the spectral rangeλ = 652-655.5 nm, cutting off near Dα,o. Specialized ultralow noise cryogenically-cooled transimpedance preampli-fiers [18] convert the photodiode current to a voltage sig-nal. Signal conditioning electronics then frequency filterand further amplify the signal. The signal is digitized us-ing multichannel simultaneous digitizers (D-tAcq SolutionInc.) Two 14-bit, 16-channel digitizer boards (ACQ16PCI)utilize synchronized external clocks and triggers to insurethat all 32 channels are sampled at 1 MHz on a commontime-base, crucial for cross correlation and cross phasemeasurements. Each board has 128 MB of on-board mem-ory, allowing for 64 MS for up to 4.2 sec of synchronizeddata acquisition at 1 MHz, the typical sampling rate.

The newly implemented BES system has been sub-stantially upgraded from an earlier generation system atDIII-D [10, 16], and relocated to a larger laboratory to fa-cilitate further expansion. The upgraded system providessubstantially increased sensitivity to small-amplitude den-sity fluctuations. The primary elements of the upgrade arehigher throughput fibers (11 1-mm fibers/channel, com-pared to 4), higher transmission, wider-band interferencefilters, ultra high-speed collection lenses and a larger areadetector. The net result of these various improvements wasan approximately 5-10 increase in the measured signallevel, and an associated increase of 30-50 in fluctuationpower, a consequence of the relationship between elec-tronic noise, photon noise and density fluctuation signal.See Ref. 12 for a detailed discussion of the noise compo-nents and SNR improvements. The first phase of the up-graded BES system consisted of 16 channels arranged in a4× 4 grid, while the recently expanded system consists of32 high-sensitivity channels (5× 6 grid plus two common-mode channels).

Two common-mode rejection channels have been de-ployed with the recently expanded system. These channelsare located approximately five and 10 cm inboard of themain 5× 6 channel grid, respectively, and are shown inthe configuration in Fig. 1. The purpose of these channelsis to measure and isolate anyfluctuation components onthe neutral beam itself that do not represent local plasmafluctuations and thus should be subtracted from the mea-sured signals [14]. Such common-mode fluctuations arisepredominantly from fluctuations in the neutral beam ionsource [19], as well as large-amplitude edge fluctuationsthat become imprinted on the beam due to fluctuatingbeam attenuation. Such fluctuations are superimposed onthe measured local fluctuations andcan thus complicateanalysis. The common-mode channels allow for the identi-fication of spectral signals that are common to all channels.They are separated radially from the main 5× 6 array by adistance that is larger than the radial correlation length ofturbulent eddies. Therefore, any signal common betweenthese channels and the array can be rejected as common-

Fig. 2 Block diagram of the main components of the BeamEmission Spectroscopy detection and control system, in-cluding spectrometers, detectors, electronics, tempera-ture regulators, timing, and control components.

mode signal. The channels need to be accurately calibratedto apply this procedure. An example of this is presented inSection 3.

An all-new control system has been implemented tocontrol timing, detector temperature control and AC powerto various system components, and provide for safety ofthe equipment. This system incorporates several NationalInstruments compact Field Point (cFP) components (relay,thermocouple, analog input modules) and a timing board,that have been integrated with the LabView virtual instru-ment software package for interface and control running ona host PC. The control system allows for fully remote op-eration of the BES system, system protection, and record-ing of diagnostic system parameters. The system controlsLN2 flow to the detectors and preamplifiers to maintaina temperature near 140 K to minimize e-noise and photo-diode dark current (all cooled components are located ina vacuum box to prevent condensation). Power is cycledto cryogenic solenoid valves in response to real-time tem-perature measurements. Thetiming system allows for dataacquisition to commence at any point during the plasmadischarge with a precision of 1 microsec. A block diagramillustrating the detection and control system is shown inFig.2.

3. Measured Turbulence Characteris-ticsThe first core turbulence imaging measurements with

theupgraded and expanded 5× 6 channel BES array were

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obtained in a set of low-confinement (L-mode) dischargeson DIII-D. The primary goal of the experiment was to de-termine the toroidal Mach number dependence of turbu-lence and transport; the toroidal Mach number was var-ied by adjusting the torque applied to the plasma using thenewly implemented co-current and counter-current neutralbeam injection capability on DIII-D [20]. The array wasscanned shot-by-shot over four radial locations, as indi-cated in Fig. 1. The discharge parameters after current flat-top were: Ip = 1.0 MA, BT = −2.0 T, Pinj = 5.0 MW,ne,o = 2.9× 1019 m−3, Te,o ≈ 2.2 keV,Ti,o ≈ 2.7 keV.

A spectrogram of the temporally and spectrally re-solved density fluctuations is shown in Fig. 3. These fluc-tuations,obtained nearr/a = 0.65, are displayed over thefrequency range 15-450 kHz, from 0.32-3.3 sec. The den-sity fluctuations associated with turbulence are clearly ev-ident as a broadband feature that increases in frequencyas the discharge evolves. The central frequency increasesfrom less than 100 kHz in the early phase (t < 1.0 sec) toover 200kHz (t > 1.5 sec) primarily as the E×B poloidalvelocity and resulting Doppler shift increase with increas-ing plasma rotation. Thefrequency range extends fromseveral tens of kHz up to roughly 400 kHz, indicating ahighly nonlinear saturated turbulence state. Also evidentis a sharpreduction in the fluctuations near 2.55 sec whenthe plasma undergoes a transition from a low-confinementmode (L-mode) to a high-confinement mode (H-mode).While the LH transition is primarily associated with therapid development of a strongradial electric field shearlayer and fluctuation suppression near the plasma pedestal(r/a > 0.9) [21], it is clear that fluctuations in this deepercore region are also affected and suppressed on a short timescale.

Oscillations arising in the neutral beam source it-self are also observed in the frequency range 0-70 kHz

Fig. 3 Spectrogram of measured density fluctuations atr/a =0.65 showing evolution of the turbulent density fluctua-tions.

continuously throughout the discharge. The frequency-integrated amplitude of the beam oscillations is typicallyn̄beam/no < 1%, but relative to local turbulent fluctua-tions, these non-local beam oscillations can be significantin the core plasma. In cases like that shown in Fig. 3,the beam oscillations can be spectrally isolated from thehigher-frequency turbulent fluctuations, but in lower rota-tion plasmas, it becomes necessary to measure and sub-tract off the beam-source oscillation component using thecommon-mode rejection channels discussed previously.Fig. 4 demonstrates the common-mode isolation and sub-traction procedure. Fig. 4(a) shows two ensemble-averagedcross-power spectra: one from two poloidally-separatedchannels (∆Z = 1.2 cm, datafrom same channels as shownin Fig.3), and one between one of those channels and oneof the common-mode channels (∆R = 10 cm,∆Z = 0 cm).The local cross-power spectrum (red-solid) exhibits botha broadband turbulence component (80-400 kHz, domi-nantly) along with the beam oscillation component, orcommon-mode spectrum (f < 70 kHz)). The common-mode spectrum (blue-dashed) exhibits only the common-

Fig. 4 (a) Cross-power fluctuation spectrum from adjacentchannels (red) and two widely spaced channels (blue)showing the common-mode component; (b) Local fluc-tuation spectrum after subtraction of the common modecomponent.

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Fig. 5 (a) Density fluctuation power spectra at four radial loca-tions; (b) Spectrally integrated fluctuation amplitude pro-file (t = 1.0-1.5 sec).

mode beam oscillations because the two channels are sepa-rated by a distance large compared to the radial correlationlength of the turbulence. Fig. 4(b) shows the local fluctua-tion spectrum after the common-mode (dominantly beamoscillation) component is subtracted off. It can be seen thatthe broadband turbulence spectrum extends down into thelower frequency range (f < 70 kHz) where the beam os-cillations dominate, though the resulting spectrum is ofrelatively low-amplitude in this case. The resulting spec-trum is relatively noisy at this lower frequency since twolarger signals are being subtracted to obtain the residuallocal fluctuation component.

The power spectra of the broadband turbulence varystrongly with radius. Figure 5 displays the power spectraat four radii (integrated overt = 1000-1500 ms) extend-ing from mid-radius to near the plasma edge. It can bereadily observed that the fluctuation amplitude increasesstrongly towards the plasma edge, and that the spectrumis Doppler-shifted towardshigher frequency towards themid-radii of the plasma (the common-mode beam oscil-lationshave been subtracted off using the procedure dis-cussed above, where significant). The integrated fluctua-

Fig. 6 (a) Radial time-lag cross-correlation functions(r/a = 0.6); solid lines are cross-correlation func-tions, dashed lines envelope functions; (b) Poloidalcross-correlation functions; (c) 2D (radial-poloidal)spatial cross-correlation function at time lag,τ = 0.

tion amplitude is likewise shown in Fig. 5(b), showing afactor of 30 variation fromr/a = 0.4out to the edge, whichis typical of L-mode plasma conditions [22,23].

The radial and poloidal time-lag cross-correlationfunctions exhibit a strong asymmetry due primarily to thelargepoloidal E×B advection of turbulentstructures. Theradial cross correlation functions at increasing radial sep-aration nearr/a = 0.6 are shown in Fig. 6(a), and thepoloidal cross correlation functions in Fig. 6(b). The solid

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lines represent the cross-correlation functions, and the as-sociated dashed lines represent the respective envelopefunctions (defined using the Hilbert transform). The radialcross-correlation functions all peak at a time lag of zero, re-flecting zero time-averaged radial propagation of turbulenteddy structures. The radial correlation length, defined asthe 1/e decay distance, isLc,r ≈ 2.5 cm here. In contrast, thepoloidal cross-correlation functions peak at monotonicallydecreasing time-lags as a result of the strong poloidal E×Badvection. The corresponding poloidal correlation length,taken again as the 1/e decay distance at zero time lag, isLc,θ ≈3.0 cm. The decorrelation time of the turbulence isdefined as the time-lag at which the cross-correlation hasdecayed to 1/e. As a result of the strong poloidal advec-tion, thepoloidal cross-correlation amplitudes do not de-cay to 1/e over the 6 cmobservation range, but rather de-cay to only 0.5. The decorrelation time is estimated by ex-trapolation to be about τc ≈ 8µs. Theτ = 0 2D spatialcross-correlation function is shown in Fig. 6(c), showingthe expected wavelike structure in the poloidal direction,with a monotonically decaying radial structure. This graphwas constructed by computing thetime-lag cross correla-tion function between a reference channel at the corner ofthe 2D array and all other channels, evaluating atτ = 0,and smoothing the resulting 2D (5× 6) graph using a min-imum curvature spline surface routine.

The poloidal advection velocity of the turbulent ed-dies, or group velocity, can be straightforwardly deter-mined from the inverse of the slope of the peak time-lagvs. poloidal separation. The measured group velocity pro-file is shown in Fig. 7. In these co-injection plasmas, thepoloidal velocity peaks nearr/a = 0.45 at roughly 14 km/s,and decays to about 5 km/s near the edge and also showsa small by significant drop towards the center, inboardfrom the peak atr/a = 0.45. The poloidal velocity of tur-bulence is governed primarilyby the radial electric field,which naturallydrops to zero at the magnetic axis. The

Fig. 7 Poloidal turbulence velocity profile, and E×B velocitymeasured from charge exchange recombination spec-troscopy. Horizontal bars at bottom represent the radialextentof BES array for the four shots examined.

vE×B profile is simultaneously measured using charge ex-change recombination spectroscopy [24] and is overlaid inFig. 7 as a dashed line. The radial electric field is measuredfrom the carbon ion force balance equation using the di-rectly measured toroidal and poloidal rotation velocitiesand pressure gradient. The E×B velocity is then simply

calculated asvE×B,θ =�Er×�BT

B2 . The very good quantitativeagreement between these two independent measurementsdemonstrates that the turbulence advects poloidally at theE×B velocity. The fluctuations are propagating in the lab-oratory frame in the ion diamagnetic direction, which isthe same direction as the E×B velocity driven by the co-injecting neutral beams on DIII-D. The turbulenceS (k, ω)spectrum exhibits a wide range of wavenumbers and fre-quencies, as seen in the power spectra in Fig. 5 and spa-tial cross-correlation in Fig. 6(c). At a given location, allwavenumbers/frequencies are advected nearly uniformlyat the derived velocity shown. In other words, there is lit-tle dispersion over much of the core region of the plasma.This is demonstrated by a monotonically increasing lin-ear phase shift in the cross power spectra measured frompoloidally-separated channels. This is to be expected sinceany dispersive effects are predicted to be on the scale ofthe diamagnetic velocity for drift wave turbulence, thoughtto dominate the broadband fluctuations; this is typicallyfar smaller than the E×B velocity. An exception may oc-cur near the plasma edge region where the E×B velocitybecomes smaller, while diamagnetic drifts become largerand dispersive effects have been observed [25].

4. Turbulence ImagesThe general characteristics of the turbulence were pre-

sented in Section 3 and illustrated the ensemble-averageproperties of the broadband fluctuations associated withplasma turbulence. Correlation lengths are observed to bea few cm, decorrelation times near 10µs, andpoloidal ad-vection speeds of 5-10 km/s for these L-mode dischargeson DIII-D. The time-resolved dynamics of the turbulenceare necessary to identify and quantify several importantnonlinear characteristics of the turbulence. These include,for example, zonal flows and geodesic acoustic modes,energy cascades, and may facilitate direct turbulent par-ticle flux measurements. To begin to address these issues,and to illustrate the time-resolved eddy interactions suchas shearing and accretion, images and movies of the tur-bulence in the core plasma region have been developedfrom the recently upgraded and expanded BES densityfluctuation measurements. Figure 8 displays thefrequency-filtered time-series fluctuation signal over 150µs fromthree poloidally-separated channels nearr/a = 0.8. Thesignal-to-noise ratio for this time-resolved data is ap-proximately 2 across the frequency range considered (60-300 kHz, where the plasma fluctuation signal dominates),with noise composed primarily of photon noise and a smallelectronic noise component. Thus, the signal appears tur-

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Fig. 8 Time-resolved fluctuation measurements at threepoloidally-displaced channels nearr/a = 0.82.

bulent, as such, and is dominated by the local plasma den-sity fluctuations. Thehigh coherency of the signals is read-ily apparent, with structures strongly correlated from onechannel to the next. The poloidal advection is likewise ap-parent as a time-shift of the coherent structures betweeneach channel.

Images of the turbulence are constructed from the spa-tially resolved fluctuation measurements as has been dis-cussed previously [9,10,12], Each signal is first frequency-filtered over the relevant spectral range. This is identifiedby determining where the broadband turbulence dominatesthe spectrum, as measured via the respective coherencyspectra between adjacent channels. The RMS values ofsignals from the 30 channels in the 5× 6 grid are thennormalizedto eliminate channel-to-channel variations. A2D spline fit is performed with the discrete 5× 6 chan-nel grid using a minimum curvature spline surface routine(MIN CURVE SURF routine in IDL) to generate a 50×60point interpolated grid. This is accomplished by construct-ing aset of basis functions with corresponding coefficients,solving for the coefficients, and evaluating the resultingsurface at the desired points. The resulting array is thenlinearly interpolated in 2D to expand to the desired pixelsize.

A selected set of turbulence images generated withthe above method is shown in Fig. 9. Each image coversan approximately 4.5 cm× 7.2 cm (radial× poloidal) re-gion along the outboard midplane, with locations shownon the equilibrium reconstruction in Fig. 1. The aspect ra-tio is nearly preserved in these images. Four very similar(repeat) discharges were used to obtain this sequence, withthe radial position of the fiber array moved shot-to-shot tothe respective locations. Images in each vertical columnwere obtained from one discharge and each frame is sepa-rated by 2µs, with time progressingdownwards.Here, thegreen color represents an equilibrium density value, whilered represents positive density fluctuations, and blue neg-ative. The eddy structures are generally propagating in avertically upward direction (E×B direction) in each frame.The individual turbulent eddies are most easily identified

Fig. 9 Images of turbulent density fluctuations from four similardischarges at four radial locations, each frame separatedby 2 microsec (color-coding: green-neutral, red-positive,blue-negative). Frequency filter range for each set of im-ages is shown below figures.

in the radially outermost set of data, where the poloidalflow shear can be seen to be tearing the eddies apart. Theeddy structures at the inner radii undergo significant de-formation from frame-to-frame, reflecting the complex in-teractions of the eddies. The structure scale lengths andlifetimes show general consistency with the ensemble av-eraged measurements presented earlier, but here the strongtemporal dynamics and interactions can be directly ob-served. Time-resolved visualizations have been made fromthese images and canbe obtained [26].

5. Discussion and Conclusion2D turbulence imaging data allows for far more de-

tailed study of turbulence than is possible with zero or1D measurements. The Beam Emission Spectroscopy den-sity fluctuation diagnostic system at DIII-D has recentlybeen upgraded to achieve significantly higher sensitivityto small-scale fluctuations, andhas been expanded to 32high-sensitivity channels. The channels are deployed in a5× 6 channelgrid covering an approximately 4.5× 7.2 cm

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region along the outboard midplane of the tokamak thatcan be radially scanned from shot-to-shot. Two channelsare deployed at radially inboard locations to monitor andisolate common-mode fluctuations from the neutral beamsource or edge turbulence imprinted on the beam. The en-hanced sensitivitynow allows for time-resolved measure-ments of density fluctuations at normalized fluctuation am-plitudesn̄/n ≤ 1%. The2D measurement capability in theradial-poloidal plane allows for detailed characterizationof the inherently 2D turbulence in magnetically-confinedplasmas. This allows for direct measurement of such quan-tities as the equilibrium flow shear of the turbulence, 2Dcorrelation functions andS (kr, kθ) wavenumber spectra,zonal flows and geodesic acoustic modes and nonlinear en-ergy transfer.

Newly developed routines are being applied to mea-sure the time-resolved 2D velocity field of the turbulenteddy structures [27, 28]. The inferred velocity field mayallow for direct measurements of turbulent-driven particleflux, zonal flow measurement, and can provide an addi-tional field describing the turbulence for use with advancednonlinear analysis routines. Future studies will concentrateon the 2D fluctuation dynamics leading up to and acrossthe LH transition, as well as development of the H-modepedestal, zonal flow properties, turbulence dynamics dur-ing the formation of internal transport barriers, and dimen-sionless scaling properties of turbulence and transport. To-gether, these studies will playa crucial role in the valida-tion of sophisticated nonlinear turbulence simulations andshould aid the development of a predictive capability forturbulent-driven transport in fusion plasmas.

AcknowledgmentsThe authors would like to thank the DIII-D program

for its technical and engineering support for this diagnos-tic development program as well as the scientific supportfor experimental run time for these and related researchactivities. This activity supported by U.S. DOE under DE-FG02-89ER53296 and DE-FC02-04ER54698.

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plasma turbulence imaging via beam emission spectroscopy ... · plasma turbulence imaging via beam...

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