Coherent and turbulent density fluctuation measurements using 2D MIR - - PowerPoint PPT Presentation

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Coherent and turbulent density fluctuation measurements using 2D MIR - - PowerPoint PPT Presentation

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Supported by Coherent and turbulent density fluctuation measurements using 2D MIR system in KSTAR W. Lee, J. Leem, J. A. Lee, Y . B. Nam, M. Kim, G. S. Yun, H. K. Park (POSTECH), Y . G. Kim, H. Park, K. W. Kim (KNU), C. W. Domier, N. C.

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SLIDE 1

Coherent and turbulent density fluctuation measurements using 2D MIR system in KSTAR

  • W. Lee, J. Leem, J. A. Lee, Y

. B. Nam, M. Kim, G. S. Yun, H. K. Park (POSTECH), Y . G. Kim, H. Park, K. W. Kim (KNU), C. W. Domier, N. C. Luhmann, Jr. (UCD), and KSTAR T eam KSTAR Conference 2013 (Lotte Buyeo Resort, Chungnam, February 26-27, 2013)

Supported by

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SLIDE 2

Outline

  • Why imaging reflectometry?
  • Overview of dual frequency MIR system

– Measures coherent and turbulent density fluctuations in semi-2D (16 x 2) – Fully characterized through laboratory test (probe beam for curvature matching and diffraction test with corrugated target)

  • Verification of the MIR data (from a known coherent fluctuation)

– Reflected signals from the density fluctuation during sawtooth oscillation is used to verify the system performance

  • Beginning of turbulence study

– Turbulence measurement in the ECH assisted L-mode plasma.

  • Future plan

– Synthetic image construction from turbulence/blobs with the FWR2D/PIC simulation codes and comparison with the measured 2D MIR data

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 2

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SLIDE 3

Why imaging reflectometry?

  • Limitations of the conventional 1D reflectometry :

– long wavelength and small amplitude fluctuations only

  • Density fluctuations in the plasma are 2D/3D in nature:

– cutoff layer is like a diffraction grating – reflected beam is largely scattered – reflections from multiple points make interference – Interference get worse for a shorter wavelength and increased fluctuation level

  • An imaging optics with a large aperture is required:

– Collect the scattered beams to form an image at the detection plane – Make the measurement less sensitive to interference

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013)

1-D fluctuation 2-D fluctuation

3

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SLIDE 4

Microwave imaging reflectometry (MIR)

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013)

With the imaging optics, detector collects diffracted beams from small area and reconstruct the phase and amplitude.

Concept of MIR system on KSTAR

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SLIDE 5

Dual frequency MIR system (X-mode)

Characteristics:

  • tunable probing frequencies from 78 to 96

GHz (86 and 90 GHz in 2012 campaign)

  • detectable poloidal wave number (k

range): 0.6 ~ 2.5 cm-1

  • detection channel: poloidal 16 and radial 2
  • spatial resolution:

– response spot size: 1 cm (FWHM) poloidal and 0.3 cm (cut-off layer depth) radial – channel spacing: 0.6 cm poloidal and 0.3 ~ 15 cm radial (depends ne gradient)

  • time resolution: up to 0.5 μs (2 μs normal)

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013)

0.3~15 cm 0.6 cm

5

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SLIDE 6

Schematic of KSTAR MIR system

  • MIR and 2nd ECEI combined system.
  • Probing source, optics, and detector array are in the housing on the deck.
  • Electronics, digitizer, and data server are in the Faraday cage, which shields

electromagnetic field and RF noise.

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 6

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SLIDE 7

Laboratory test results

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 7

Corrugated reflecting target: Corrugation λ = 50 mm (k = 1.26 cm-1)

  • Corr. depth ~ 1.1 mm ~ 4 radian (for λ0

=3.3 mm)

Measurement with 16 channels :

  • Corrugation shape (depth and

wavelength) is properly imaged.

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SLIDE 8

MIR System installed on G-port in 2012

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013)

MIR and 2nd ECEI system (2012) 1st ECEI system (2010) H-port G-port

KSTAR tokamak

Faraday cage (2012)

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SLIDE 9

Verification of the MIR system in the plasma

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013)

Only sawtooth (no other MHDs)

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Real time EFIT :: Shot 7335

T = 6.007s Coherent density fluctuation during sawtooth oscillation

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SLIDE 10

R ~ 187 cm R0 ~ 175 cm r/a ~ 0.24

Different responses at two radial positions

R ~ 178 cm R0 ~ 175 cm r/a ~ 0.06

δne δTe

δφ = +/- 4 radians δφ = +/- 1 radians

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 10

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SLIDE 11

Simple modeling of 11 kHz pre-cursor oscillation

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 11

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SLIDE 12

Movie of modeling

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 12

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SLIDE 13

Turbulent fluctuation measurement

  • ECCD heated ohmic plasma
  • During ECCD modulation (170

GHz, 400 kW, 5 Hz), the ECCD – heated electron temperature more than 50 % – modified the spectrum of density fluctuation (measured by the MIR).

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 13

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SLIDE 14

Correlation techniques for turbulence analysis

  • Cross-spectral density:
  • Complex coherence function:
  • Coherence:
  • Cross phase spectrum:

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 14

) ( ) ( ) ( ) ( f P f P f P f

yy xx xy xy

 

2 2

) ( ) ( f f

xy xy

           

)) ( Re( )) ( Im( tan ) (

1

f f f

xy xy xy

  

  • Measure overlapped (close spacing)

plasma regions

  • Radiation or electrical noises are

uncorrelated.

  • Plasma fluctuations (appeared in density
  • r temperature) are correlated.
  • Correlation techniques can provide

spatial structures of fluctuations. – size of fluctuation (correlation length) – shape of fluctuation

) ( * ) ( ) ( f Y f X f P

xy

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SLIDE 15

ECCD effect on poloidal turbulence scale

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 15

ECCD OFF ECCD ON

Reference Reference

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SLIDE 16

Future plan

  • Synthetic images of reflected beam (intensity and phase) by turbulence will be

constructed with FWR2D/CodeV simulations.

  • And these synthetic intensity and phase are compared with measured MIR data.

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013)

Zoom lens 2 Zoom lens 1 H-plane focus lens Cutoff layer Zoom lens 3 E-plane focus lens

FWR2D CODE V

~ 1300 mm

1200 (mm) 560 (mm) 1000 (mm)

16

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SLIDE 17

4 frequency MIR system

  • 4 frequency system is being developed

for 2014 campaign.

  • The system deploys 4 x 16 (radial and

poloidal) detection channels for 2D imaging of coherent (MHDs) and turbulent (drift modes) density fluctuations.

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 17

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SLIDE 18

Summary

  • Dual frequency (X-mode) MIR system was successfully commissioned in 2012

KSTAR campaign.

  • MIR system performance was verified with the coherent fluctuation in the

plasma.

– A simple modeling of the sawtooth pre-cursor oscillation used for a synthetic beam phase oscillation and the synthetic signal is compared to the measured one – Multi channel phase image will be provided.

  • Turbulent fluctuation measurement was analyzed through correlation length

calculation.

– Poloidal correlation length (poloidal scale length of turbulent fluctuation) was modified (suppressed or enhanced) by ECH modulation in a low current L-mode plasma.

  • Construction of synthetic beam intensity and phase by turbulence with

FWR2D/PIC simulations has started.

– The synthetic intensity and phase will be compared with the measured one.

  • 4 frequency MIR system will be developed for 2014 campaign.

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 18

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SLIDE 19

Back up

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SLIDE 20

Beam phase oscillation due to density fluctuation

  • Beam phase oscillation can be obtain by a general integration form

where ε and ε0 are the perturbed and unperturbed permittivity, respectively. But this method requires exact density profile data (both equilibrium and perturbed profiles) near the cut-off layer.

  • We need an analytic relation to apply even to the case when the density profile

data is not available. Several authors (E. Mazzucato, N. Bretz, etc.) made a good relation under some approximation given as (for the case of ) where M is 1 for O-mode and 2 for X-mode, k0 is the probe beam wavenumber in the vacuum, Lε is the scale length of equilibrium permittivity near the cut-off, and kr is the radial wavenumber (or ~1/width) of fluctuation.

        

 

2

c c

r r

dr dr k   

(1)

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013)

                         

e e r e e r

n n k L n n k L k M    

 

64

5 . 2

(2)

L kr / 1 

20

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SLIDE 21

Plasma center position with ECE signals

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 21

Inversion radius = ch33 (R = 185.7 cm) Inversion radius ~ ch49 or 50 (R = 166.4 cm or 165.4 cm)

Center ~ ch40 or 41 (R = 176.1 cm or 174.9 cm) Cut-off layer 1 (90 GHz) Cut-off layer 2 (86 GHz)

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SLIDE 22

Time delay between MIR and ECE signals

  • Angle between port G and K = 90 degrees

– ∆L = Rθ = 2.82 m

  • Pre-cursor oscillation frequency = 11 kHz

– vt = Rω = 124 km/s – Period of one oscillation = 91 us

  • ∆t = ∆L / vt = 23 us (MIR later than ECE)

– phase delay = 23 us / 91 us = 90 degrees

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 22

G MIR K ECE

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SLIDE 23

Radial spatial resolution

  • Cut-off layer depth (radial spatial

resolution) is given by [1][2] Since the k0 = 18 cm-1, the depth is about 4 mm in the core or mid region (Lε ~ 20 cm) and about 2 mm in the H-mode pedestal (Lε ~ 2 cm). These values are comparable with the probe beam wavelength ~3.4 mm.

KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 23

Radial separation of two cut-off layers

1)

  • V. L. Ginzburg, The Propagation of Electronics

Waves in Plasmas, 2nd ed. (Pergamon Press, Oxford) 1970. 2)

  • G. R. Hanson, Ph.D Thesis, Georgia Institute of

Technology, 1991.