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

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. Luhmann, Jr. (UCD), and KSTAR T eam KSTAR Conference 2013 (Lotte Buyeo Resort, Chungnam, February 26-27, 2013)

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

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 : 1-D fluctuation – 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 2-D fluctuation • 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) 3

Microwave imaging reflectometry (MIR) Concept of MIR system on KSTAR With the imaging optics, detector collects diffracted beams from small area and reconstruct the phase and amplitude. KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 4

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  0.3~15 cm range): 0.6 ~ 2.5 cm -1 0.6 cm • 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 μ s (2 μ s normal) KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 5

Schematic of KSTAR MIR system MIR and 2 nd 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

Laboratory test results Measurement with 16 channels : Corrugated reflecting target: • Corrugation shape (depth and Corrugation λ = 50 mm (k = 1.26 cm -1 ) wavelength) is properly imaged. Corr. depth ~ 1.1 mm ~ 4 radian (for λ 0 =3.3 mm) KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 7

MIR System installed on G-port in 2012 KSTAR tokamak G-port H-port 1 st ECEI system (2010) MIR and 2 nd ECEI system (2012) Faraday cage (2012) KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 8

Verification of the MIR system in the plasma Coherent density fluctuation Real time EFIT :: Shot 7335 during sawtooth oscillation T = 6.007s Only sawtooth (no other MHDs) KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 9

Different responses at two radial positions R ~ 178 cm R ~ 187 cm R 0 ~ 175 cm R 0 ~ 175 cm r/a ~ 0.06 r/a ~ 0.24 δ Te δ ne δφ = +/- 1 radians δφ = +/- 4 radians KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 10

Simple modeling of 11 kHz pre-cursor oscillation KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 11

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

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

Correlation techniques for turbulence analysis • Cross-spectral density: • Measure overlapped (close spacing) plasma regions  P ( f ) X ( f ) Y * ( f ) xy • Complex coherence function: • Radiation or electrical noises are uncorrelated. P ( f ) xy   ( f ) xy P ( f ) P ( f ) xx yy • Plasma fluctuations (appeared in density • Coherence: or temperature) are correlated. 2    2 ( f ) ( f ) xy xy • Correlation techniques can provide • Cross phase spectrum: spatial structures of fluctuations. – size of fluctuation (correlation length)    Im( ( f ))    xy 1 ( f ) tan   – shape of fluctuation xy  Re( ( f ))     xy KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 14

ECCD effect on poloidal turbulence scale Reference Reference ECCD OFF ECCD ON KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 15

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. Zoom lens 2 E-plane focus lens ~ 1300 mm CODE V FWR2D Cutoff layer Zoom lens 3 Zoom lens 1 H-plane focus lens 1200 (mm) 1000 (mm) 560 (mm) KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 16

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

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

Back up

Beam phase oscillation due to density fluctuation • Beam phase oscillation can be obtain by a general integration form   r r      c   c 0  2 k dr dr (1)     0 0 0 0 where ε and ε 0 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 ) k r 1 / L  0 . 5         2 k L n L n             (2) M 0 e 64 e       k n k n       r e r e where M is 1 for O-mode and 2 for X-mode, k 0 is the probe beam wavenumber in the vacuum, L ε is the scale length of equilibrium permittivity near the cut-off, and k r is the radial wavenumber (or ~1/width) of fluctuation. KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 20

Plasma center position with ECE signals 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) Inversion radius = ch33 (R = 185.7 cm) KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 21

Time delay between MIR and ECE signals • Angle between port G and K = 90 degrees – ∆ L = R θ = 2.82 m • Pre-cursor oscillation frequency = 11 kHz – vt = R ω = 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 K ECE G MIR KSTAR Conference 2013 (Lotte Ruyeo Resort, Chungnam, Feb 26-27, 2013) 22