1

Initial Density Fluctuation MeasurementsUsing Fixed-Frequency Quadrature

Reflectometers on DIII-D

G. Wang, W.A. Peebles, T.L. Rhodes, E.J. Doyle, N.A. Crocker,X. Nguyen, L. Zeng, S. Kubota

Department of Electrical Engineering and PSTI,University of California, Los Angeles, California, USA

G.R. McKeeUniversity of Wisconsin, Madison, Wisconsin, USA

M. VanZeelandOak Ridge Institute for Science Education, Oak Ridge, Tennessee USA

Presented at the 7th International Reflectometry WorkshopMay 9-12, 2005

Max-Planck-Institut für PlasmaphysikGarching, Germany

UCLAUCLA UCLA

2

Overview

• Two fixed-frequency (42 and 65 GHz) quadrature reflectometerswere recently installed on the DIII-D tokamak for densityfluctuation measurements

• Coherent mode/MHD activities are identified in the reconstructedphase

- Detection of tearing modes � Density fluctuation levels inferred using 1-D phase screen model are in reasonable agreement with Beam Emission Spectroscopy (BES) measurements - Detection of compressional Alfvén eigenmodes (CAE) � Ongoing analysis for contributions to CAE studies

• Initial turbulent broadband density fluctuation measurement inedge pedestal

- Comparison of broadband frequency spectrum and density fluctuation levels with BES measurements using basic 1-D phase screen model shows reasonable agreement

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Motivation• Reflectometer density fluctuation measurements are highly

desirable due to: - highly localized measurement - highly sensitive - flexible spatial coverage - non-perturbative - ITER compatible

• Reflectometer density fluctuation diagnostic can contribute tophysics studies on DIII-D:

- Localized coherent mode/MHD activity measurement and quantitative comparison with theoretical predictions of mode structures, e.g. tearing modes, CAE,…

� Tearing modes can limit plasma confinement, and may be responsible for keeping q(0)>1 in hybrid discharges

� CAE is potential fast ion transport mechanism - Localized, sensitive turbulent density fluctuation measurements for understanding transport in the pedestal

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Principle of Density Fluctuation Measurementby Fixed-Frequency Quadrature Reflectometer

• Detects , i.e. and , where φ is the phase delay between the launched microwave and its reflection from the plasma cutoff layer

• 1-D phase screen model: R. Nazikian, et al., Phys. Plasmas 8, 1840 (2001).

Kz is the wave number of the density perturbation in the direction of wave propagation. For O-mode measurement.

• 2-D models: considering various effects in real geometry

E t( ) = A t( )eiφ t( )A t( )cos φ t( )( ) A t( )sin φ t( )( )

δφ ≈ 2(k0Ln )δnene

,

δφ ≈ 2k0

LnKz

⎛

⎝ ⎜

⎞

⎠ ⎟

1/ 2δnene

, KzLn >1

KzLn <1 Long wavelength limit

Short wavelength limit

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Fixed-Frequency Quadrature ReflectometerSystem on DIII-D

• 2 similar systems, 42 and 65 GHz• Utilize existing antennas normally used in UCLA profile reflectometer systems• Flexible polarization choices to probe different plasma regions• Launch and receive in the outboard midplane• BES data will also be shown with similar location and localization for comparison - BES measures a k range of 0-3 cm-1

• A schematic plot

CouplerDirectional

RotatableLaunch

Receive

Mixer

RFLOOscillator

Gunn

Isolator

Isolator

Isolator

A(t)cos(φ(t))

A(t)sin(φ(t))

Amplifier

255o

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Fixed-Frequency Quadrature ReflectometerSystem on DIII-D - continued

Quadrature Mixer

Isolators

I output

Q output

Isolator

Gunn Oscillator

Signal Amplifiers

Launch Receive

• Antenna array

• Gunn, mixer, etc

Q-launch Q-X receive

Q-O receive

V-launch V-X receive

V-O receive

Toroidal direction

Mac

hin

e

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Identification of Coherent mode/MHD Activitiesin Reconstructed Phase Information

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Tearing Modes Detected in Core of HybridDischarges, Frequency SpectrumConsistent With Magnetic Loops

• Density fluctuation level will be estimated using 1-D phase screen model• Mode structure could be characterized in the future from spatial fluctuation level distribution obtained using multi-channel reflectometer measurements

Contour of PhaseFluctuationSpectrumFrom Reflectometer

3800 3900 4000 4100 4200Time (ms)

Fre

qu

ency

(kH

z) 80

60

40

20F

req

uen

cy (

kHz)

80

60

40

20

31

2.8e-7

Inte

nsi

ty s

cale

5601

2.1e-5

Inte

nsi

ty s

cale

3/2 mode

2/1 mode

121772

Contour ofMagneticFluctuationSpectrumFrom MagneticLoops

• 65 GHz system, O-mode at ρ~0.15

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3/2 Tearing Mode Fluctuation Levels Inferred From Basic 1-D Phase Screen Model Are in Reasonable

Agreement with BES Measurements

• Both reflectometer and BES detection positions at ρ~0.8

• BES: average over 10 ms

Reflectometer:

average over 0.655 ms, Ln interpolated from 9 time slices of Thomson scattering data

δnene

≈δφ

2(k0Ln )Contour of PhaseFluctuation SpectrumFrom Reflectometer

Contour of MagneticFluctuation SpectrumFrom Magnetic Loops

4000 4020 4040 4060 4080Time (ms)

Fre

qu

ency

(kH

z)

80

60

40

20

4.1

1.3e-7

Inte

nsi

ty s

cale

4100

80

60

40

20

2.5

1.5

0.5

328.8

0.0

Inte

nsi

ty s

cale

Dα

3/2 tearing mode

δn

/n (

%) 0.6

0.40.20.0

BES65 GHz O-mode reflectometer

(a.

u.)

121962

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CAE Detected, Frequency Spectrum ConsistentWith Measurement by Magnetic Loops

Contour of PhaseFluctuationSpectrumFromReflectometer

Contour ofMagneticFluctuationSpectrumFrom MagneticLoops

• 65 GHz system, X-mode at ρ~0.4-0.6

880 900 920 940 960Time (ms)

Fre

qu

ency

(kH

z)F

req

uen

cy (

kHz)

3800

3400

0.0

3e-7

Inte

nsi

ty s

cale

0.772

3.5e-6

Inte

nsi

ty s

cale

3000

3800

3400

3000122298

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Ongoing Data Analysis For Quantitative CAE Studies• CAE were detected by both the 42 and 65 GHz systems

simultaneously. The relative amplitude of the density fluctuations atthe two detection positions will be compared to code predictions ofmode structures

• Comparison of the mode characteristics between DIII-D and NSTX - See example of CAE observation by similar system on NSTX on the next viewgraph

Fredrickson IAEA 2004

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CAE Observed on NSTX by Similar System

• Detected at ρ~0.77• δn/n~1.5x10-4 at 0.25 s, 1.3 - 1.37 MHz• Reflectometer measurements on DIII-D and NSTX allow quantitative comparison between the devices

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Turbulent Density Fluctuation Measurementin Edge Pedestal

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Initial Comparison of Broadband Fluctuations WithBES Shows Qualitative Similarity

• f -3.3 spectrum decay indicates non-random phase measurement

10 100 1000Frequency (kHz)

f -3.3

PhaseFluctuationFrom Reflectometer

1219624000-4002 ms

-1.0 0.0 1.0EImag (V)

-1.0

0.0

1.0

ER

eal (

V)

Den

sity

(10

19m

-3)

0

2

4

6

8

2.2 2.3 2.4Radius (m)

2.0 2.1

Thomsonscattering

65 GHzO-modeCutoff

121962, 4000 ms

10 100 1000Frequency (kHz)

Au

to-p

ow

er (

a.u

.)

10-8

10-6

10-4

100

10-2

Au

to-p

ow

er (

a.u

.)

10-8

10-6

10-4

100

10-2

BES

1219624000-4002 ms

1219624000-5000 ms

f -3.3

Beam Fluctuations

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Rough Estimate For Density Fluctuation Level in Pedestal Using Basic 1-D Phase Screen

Model Agrees With BES Measurements

δnn

=kr

1/ 2

2k0Ln1/ 2 δφ

ReflectometerShot 121962Average over 4000-4002 msIntegrated over 60-400 kHz1-D phase screen model: k0=13.6 cm-1 (65GHz system) Ln~8.56 cm (from Thomson scattering)

(estimated from radial correlation length lcr~1.2 cm measured by BES)

kr ~ 1/ lcr ~ 0.83cm−1

δφ ~ 0.21rad.

δnn

~ 0.24%

BESShot 121962Averaging over 4000-5000 ms,Integrated over 60-400 kHz

BES channels, 121962, 4000 ms

• Observed level of agreement probably fortuitous!• Next step: 2-D model calculations will be pursued using PPPL codes in collaboration with G. Kramer, R. Nazikian

δnn

~ 0.25%

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Simultaneous Phase Fluctuation and High-ResolutionDensity Profile Measurements Would Be Useful in

Characterizing Density Fluctuation Evolution in Edge Pedestal

L-H transition 1st ELM

123187

• Profile reflectometer on DIII-D provides high-resolution (≥10 µs) information of detection location and density scale length• At present we have to make choice between fluctuation/profile reflectometer measurement due to sharing of the antennas/waveguides

Contour of PhaseFluctuationSpectrumFrom 42 GHzReflectometer

Contour of PhaseFluctuationSpectrumFrom 65 GHzReflectometer

960 1000 1040 1080Time (ms)

Fre

qu

ency

(kH

z)

800600400200

10

6.4e-7

Inte

nsi

ty s

cale

800600400200

17

3e-9

Inte

nsi

ty s

cale

Dα2

1

0

(a.u

.)

8

4

8

4

0

RMS of Phase Fluctuation 65 GHz, X-mode

RMS of Phase Fluctuation 42 GHz, O-mode

(rad

.)

960 1000 1040 1080Time (ms)

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Summary• Two fixed-frequency (42 and 65 GHz) quadrature reflectometers were recently

installed on the DIII-D tokamak for density fluctuation measurements

• Coherent mode/MHD activities are identified in the reconstructed phase - Detection of tearing modes in hybrid plasmas � Density fluctuation levels inferred using 1-D phase screen model are in reasonable agreement with BES measurements - Detection of compressional Alfvén eigenmodes � Ongoing analysis for contributions to CAE studies

• Initial turbulent broadband density fluctuation measurement in edge pedestal - Comparison of broadband frequency spectrum and density fluctuation levels with BES measurements using basic 1-D phase screen model shows reasonable agreement

• Future direction - 2-D model calculations will be pursued using PPPL codes in collaboration with G. Kramer, R. Nazikian - Would like to upgrade fluctuation reflectometer system: replace homodyne system with quadrature