Correlation reflectometry for pitch angle measurements on NSTX

2005/7/20

A. Ejiri

Univ. Tokyo

Outline

Correlation reflectometry and pitch angle measurement

Operation and analysis

Results

Conclusion

Various configurations of correlation reflectometryRadial 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 Antenna B/|B|

(Preliminary analysis was tried in 2003)

Those two have been tried during this visit.(LR was also measured simultaneously.)

Principle and expected behaviors (I)B

Toroidal

Poloidal

Ref. #1

Ref. #2

d

Correlation

d

Ref. #1

(26-40GHz Swept & 31GHz fixsd))

Refl. #2 (30GHz fixed)

Field line

L

L

LR

Contour of correlation

Correlation

d/r

Pitch angle scan

Pitch angle/radial scanRefl. #3, #4,..

Flux surface

Generally, LL, LR

-> pitch angle measurements

radial scan

pitch angle scan

pitch anglescan

Principle and expected behaviors (II)

Squared Correlation

d/r

Pitch angle/radial scan

Frequency Sweep

radial scan

26GHz

40GHz

26GHz

40GHz

Correlationat the samelocation

Correlationat a differentlocation

1

<1

Note that, noise reduces the peak and increase the floor. Bandpass filtering is very efficient to reduce the noise effect.

Calculation of correlation • y0:I-component of Ref.#2(30GHz fixed)• y1:Q-component of Ref.#2• y2:Swept (homodyne) signal of Ref.#1Normalized squared cross correlation

T is chosen to be 20s.

TT

T

TT

T

yyyy

yy

yyyy

yy

2211

2

21

2200

2

20

~~~~

~~

~~~~

~~

ParallelCorrelation <y0y2>, <y1y2>

RadialCorrelation <y3y2>, <y3y2>

Discharges

Power spectral density of the reflectometer with 30GHzTime window [ms]

Frequency [kHz]

H-mode

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

Tracing a field line (I)

Using EFIT02/EFIT01 results (Ri,Zj , tk), F(l ,tk)=RBt for a fixed grid points, and F are calculated for an arbitrary (R,Z) at tk.(Br,Bt,Bz) is calculated from and F.

Using electron density profile ne

(Rm,tn), R of the critical density for 30GHz is calculated, and (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 of the cutoff layer are calculated. These points are defined as the reflection points.

and directions of horns projected a poloidal cross section.

Density profile and the position of the cutoff layer

Tracing a field line (II)

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.

XYZ views of the horns, their directions, reflection points(*) and field lines.

30GHz

26-40GHz/30.2GHz

0.3m

Results (I)

peaks

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.

Results (I)

peaks

poor corr.

Nose

No Cutoff

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.

Results (II) Ohmic H-mode

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.

?

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

Results (I) Low Ip case