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Correlation reflectometry for pitch angle measurements on NSTX

Correlation reflectometry for pitch angle measurements on NSTX. Outline Correlation reflectometry and pitch angle measurement Operation and analysis Results Conclusion. 2005/7/20 A. Ejiri Univ. Tokyo. Various configurations of correlation reflectometry. Radial Correlation O/O-mode L R

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Correlation reflectometry for pitch angle measurements on NSTX

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  1. Correlation reflectometry for pitch angle measurements on NSTX • Outline • Correlation reflectometry and pitch angle measurement • Operation and analysis • Results • Conclusion 2005/7/20 A. Ejiri Univ. Tokyo

  2. Various configurations of correlation reflectometry • Radial Correlation • O/O-mode LR • O/X-mode |B| (Tried in 2001 firstly, and M.Gilmore wrote a paper in 2003) • Perpendicular Correlation • Multi Antenna L^ • Longitudinal Correlation • Multi AntennaB/|B| (Preliminary analysis was tried in 2003) Those two have been tried during this visit. (LR was also measured simultaneously.)

  3. Poloidal B d Ref. #2 Ref. #1 Toroidal pitch angle scan Principle and expected behaviors (I) radial scan Contour of correlation LR Flux surface L|| pitch angle scan L^ Pitch angle scan Correlation Generally, L|| >> L^, LR -> pitch angle measurements Refl. #2 (30GHz fixed) d Refl. #3, #4,.. Pitch angle/radial scan Correlation Field line d/r Ref. #1 (26-40GHz Swept & 31GHz fixsd))

  4. Pitch angle/radial scan Squared Correlation d/r Principle and expected behaviors (II) Frequency Sweep radial scan 40GHz 40GHz 26GHz 26GHz Correlation at the same location Note that, noise reduces the peak and increase the floor. Bandpass filtering is very efficient to reduce the noise effect. 1 Correlation at a different location <1

  5. Calculation of correlation • y0:I-component of Ref.#2(30GHz fixed) • y1:Q-component of Ref.#2 • y2:Swept (homodyne) signal of Ref.#1 Normalized squared cross correlation DT is chosen to be 20 ms. Radial Correlation <y3y2>, <y3y2> Parallel Correlation <y0y2>, <y1y2>

  6. Discharges

  7. Power spectral density of the reflectometer with 30GHz Time window [ms] • Typical frequency range is ~100kHz. • Higher frequency components is small, and contaminated by noise • Effective frequency range is >50kHz due to time window for calculation of correlation Frequency [kHz] H-mode

  8. Tracing a field line (I) y and directions of horns projected a poloidal cross section. • Using EFIT02/EFIT01 results y(Ri,Zj ,tk), F(yl ,tk)=RBt for a fixed grid points, y and F are calculated for an arbitrary (R,Z) at tk.(Br,Bt,Bz) is calculated from y and F. • Using electron density profile ne(Rm,tn), R of the critical density for 30GHz is calculated, and y(R= Rm,Z=0,tk) for the critical density (cutoff) is calculated. tn and tk should be as close as possible. • Intersections between the directions of two horns and y of the cutoff layer are calculated. These points are defined as the reflection points. Density profile and the position of the cutoff layer

  9. Tracing a field line (II) XYZ views of the horns, their directions, reflection points(*) and field lines. • Field lines from the two reflection points are traced toward each other using (Br,Bt,Bz) . Step size is 0.2 mm. • Find the closest points on the field line to the other reflection point, and calculate the distance between them. These lines and points are on the same flux surface. • The distance between the reflection points are. 30GHz 26-40GHz/30.2GHz 0.3m

  10. peaks Results (I) • Black curve represents standard O-mode radial correlation measurements. That is the correlation between 26-40GHz swept signal and 30.2GHz fixed homodyne signal, and it shows peaks (with height 1.0) at the timing of frequency matching. • Red curve represents correlation between 26-40GHz swept signal and 30GHz IQ signal, which is about 30cm depart toroidally. • Blue curve represents correlation between 30.2GHz fixed and 30GHz IQ. • The bottom figure shows minimum (i.e. perpendicular) distance of the two field lines.

  11. Results (I) • Clear peaks in (parallel) correlation were found. However, in some cases no clear peak was found where we expected. • Direct effect of noise, and probably indirect effect resulting poor radial correlation seem to exist. Nose poor corr. peaks No Cutoff

  12. Results (II) Ohmic H-mode • During H-phase, high correlation (red) was found. This suggest long parallel and/or perpendicular correlation length. Note that, high correlation between 30 and 30.2GHz. • Before and after H-phase, no clear peak was found in parallel and/or perpendicular correlation (red), even though radial correlation is good and field line distance is short. ? H-mode

  13. Conclusions • The NSTX reflectometer was operated for pitch angle measurements successfully. • Correlation at different toroidal location (0.3m apart) was found when the distance between the fields is within a few cm. • However, usually, the correlation was low, partly due to the noise. • Fluctuations during H-mode seems to have long perpendicular and parallel correlation length. • More (controlled) experiments, sophisticated analysis, and noise reduction are required to determine the feasibility of pitch angle measurements by correlation reflectometry. A Lot of Thanks to PPPL and UCLA

  14. Results (I) Low Ip case

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