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Fast charge exchange spectroscopy using a Fabry-Perot spectrometer in the JIPP TII-U tokamak
Fusion Engineering and Design 34-35 (1997) 219-223 ELSEVIER
Fusion Engineering and Design
Fast charge exchange spectroscopy using a Fabry-Perot spectrometer in the JIPP TII-U tokamak K. I d a *, J. Xu, K.N. Sato, H. S a k a k i t a 1 National Institute for Fusion Science, Nagoya 464-01, Japan
Abstract A new charge exchange spectroscopic technique using a Fabry-Port spectrometer has been developed to increase the photon flux at the detector, and to improve the time resolution of ion temperature and plasma rotation velocity measurements. The spectral resolution is obtained by arranging two-dimensional fiber-optics and a two-dimensional detector at the focal plane of a coupled lens located on both sides of a Fabry-Perot spectrometer. The effective finesse (the ratio of the spectral resolution to the free spectral range) of the Fabry-Perot interferometer in this system is 14. The time evolution of the ion temperature is obtained with a time resolution of 125 gs and with a spatial resolution of 3 cm (eight channels). © 1997 Elsevier Science S.A.
1. Introduction
2. Experimental details
Charge exchange spectroscopy has been routinely used to measure ion temperatures, toroidal and poloidal rotation velocities and radial electric field profiles in tokamaks and stellarators, including heliotrons [1,2]. Recently, fast changes of the poloidal rotation, which indicate a fast change of the radial electric field, have been observed, a n d the improvement of the time resolution of charge exchange spectroscopy has become a more important issue [3]. However, charge exchange spectroscopy using a monochromator and a CCD detector has relatively poor time resolution, although it has excellent spatial resolution [4].
A new charge exchange spectroscopy using a F a b r y - P e r o t spectrometer [5] has been developed and installed in the JIPP TII-U tokamak (where the major radius R is 0.93 m and the minor radius a is 0.23 m) to increase the photon flux at the detector, and to improve the time resolution of the ion temperature and the plasma rotation velocity measurements up to 8 kHz. The spectral resolution is obtained by arranging two-dimensional fiber-optics and a twodimensional detector at the focal plane of a coupled lens located on the both sides of the F a b r y - P e r o t spectrometer. Fig. 1 shows a schematic diagram of the spectrometer system, including an interference filter to limit the spectral bandwidth that illuminates the F a b r y - P e r o t spectrometer. The spacing of the spectrometer is 100 lain. The two-dimensional fiber bundle
* Corresponding author. The JIPP TII-U group also contributed to the authorship of this paper.
0920-3796/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0920-3796(96)00666-7
220
K. Ida et al./Fusion Engineering and Design 34-35 (1997) 219-223
(20 mm x 20 mm) consists of 400 optical fibers each 1 mm in diameter and is divided into eight sections to obtain the spatial resolution. The image of this fiber bundle is focused on to the photocathode of an image intensifier with a tandem microchannel plate (MCP) coupled to a 16 x 16 channel two-dimensional photodiode array (PDA). The PDA (17.6 x 17.6 mm) consists of 256 photodiodes. Each photodiode has dimensions 0.95 x 0.95 mm and a center-to-center separation of 1.1 ram. The wavelength 2 of the light that transmits through the Fabry-Perot spectrometer is given by 2 ~ 20(1 - -
02/2)
where 20 is the wavelength transmitted for the normal incident light, and 0 is the angle between the line of sight of each photodiode and the optical axis of the Fabry-Perot spectrometer. 0 is determined from the distance of each photodiode from the optical axis d and the focal length f of coupled lens as 0 ~ d / f There are eight photodiodes from the center to the edge of the PDA detector, effectively giving eight spectral channels. The spectral resolution and spectral range simultaneously measured with the PDA can be adjusted by changing the focal l e n g t h f of the coupled lens.
3. Discussion
Fig. 2 shows the spectral resolution (full width at half-maximum (FWHM)) of the fast charge exchange system for the pixels located at d = 7 mm (0=0.04 when f - - 1 8 0 mm) and for the incident light with the wavelength of 2 = 632.8 nm (2o = 633.3 nm) for various focal lengths of the coupled lens. We note here that the FWHM differs for different pixels and it increases as the tilted angle 0 is increased, because the wavelength dispersion increases almost linearly as Id2/d0l 200. As the focal length of the coupled lens is increased, the spectral resolution is improved up to 0.14 nm, which is determined by the spectral resolution of the Fabry-Perot spectrometers, but the spectral range also decreases. However, when the focal length of the coupled lens is sufficiently short ( f < 180 mm), the spectral range increases,
approaching the free spectral range (FSR) of the Fabry-Perot spectrometer, and the spectral resolution is determined by the channel resolution of the PDA, as shown in Fig. 2. The FSR of the Fabry-Perot interferometer is 2.0 nm at 632.8 n m - - t h e effective finesse of the Fabry-Perot interferometer in this system is 14. The corresponding finesse is much lower than the original finesse of the Fabry-Perot interferometer of 40, which is obtained when the incident light is parallel to the optical axis (0 = 0.0). This is because of the finite pixel size, especially for the outermost channels, where the incident light is at an angle to the optical axis of the Fabry-Perot spectrometer. Because the off-axis finesse is worse than the on-axis finesse by a factor of three, the original finesse of the Fabry Perot interferometer in this system should be much greater than that required from channel resolution. Fig. 3 shows the intensity distribution of the charge exchange line of the carbon impurity C VI (n = 8 ---,7 transition, 529.05 nm) at the PDA, for the neutral beam injection (NBI)-heated discharge
lens f=lS0mm F=2.8
Fiber 20x20mm ~ \223 A ~//111 /
~
\
~i
I//~
__..t-.-+-t.-rv ~ TI---#-.----.~ 8 v
Filter d)~ =l.5nm 9~0=529nm 1
2 3 4
6 7
\ !8
detector 16 x 16 PDA 17.6xl7.6mm H/-~,,--~,-W-tH+I
Fabry-Perot Interferometer
~
center ,plasma
edge
F=1.25
Fig. 1. Schematic diagram of fast charge exchange spectroscopy systemusing Fabry-Perot interferometer.
221
K. Ida et al./Fusion Engineering and Design 34-35 (1997) 219-223
used in the analysis. However, more than one photodiode corresponds to one wavelength, and the effective wavelength resolution is only eight. Fig. 4 shows the measured spectra of a single chord at two time slices, at R = 1.06 m (p = 0.55), 0.05 ms before ( t = 2 0 8 ms) and 0.45 ms after (t = 208.5 ms) the ice pellet injection. The deviation of the data from the Gaussian fitted curve (for instance, the data at 2 = 529.12 nm) is considered to result from the variation of the fiber transmission, the convolving effects of the spectral instrument functions and the individual sensitivities of the photodiodes; it is not believed to result from the photon statistics. However, these varia-
4
FSR Fa Perot 3
~2
1
0 0
5
10 15 range (~.)
20
25
Fig. 2. Spectral performance (full width at half-maximum) of Fabry-Perot system for the pixels located at 0 = 0.04 and 2 = 632.8 nm (2o = 633.3 nm) for various focal lengths of the coupled lenses. with pellet injection in the JIPP T I I - U tokamak. The C VI background emission, measured just before the NBI, is subtracted from the C VI charge exchange emission. The intensity distribution at the left-hand top section of Fig. 3 corresponds to the C VI emission from the plasma edge, and the intensity distribution at the lefthand b o t t o m section corresponds to the emission from the plasma center. As shown in Fig. 3(b), the two-dimensional image of the C VI emission is divided into eight sections each separated by 45 ° . These eight channels correspond to the viewing chords in the plasma, from the plasma center to the plasma edge. Unfortunately, the two edge chords (chords 1 and 8) experience reflection problems at the viewing port, so they are not used for the analysis. Therefore, the outermost viewing chord near the plasma edge is chord 2 and the innermost viewing chord near the plasma center is chord 7. The photodiodes at and near the boundary between chords are masked (not used) to avoid overlap and 'crosstalk', and these pixels are illustrated with the shaded area in Fig. 3(b). Then, each chord contains 21 spectral pixels (photodiodes)
u
10
0,
0
5
10
15
Fig. 3. (a) Three-dimensional plot and (b) contour plot of intensity distribution of charge exchange line C VI (529.05 nm) detected with 16 x 16 PDA at t = 208.5 ms (0.45 ms after the ice pellet injection).
222
K. Ida et al. /Fusion Engineering and Design 34-35 (1997) 219-223
'
J
~
I
I
o 208ms © 208.5ms
'
'
The ion temperature is derived from the Doppler width of the C VI intensity. Fig. 5 shows the time evolution of the ion temperatures measured at various positions from p =0.13 to 0.82, with a time resolution of 125 gs, which is determined by the sampling speed of data acquisition, for the discharge with NBI and ice pellet injection as an example. The ion temperature at a normalized minor radius (p = 0.13) drops from 0.56 to 0.23 keV, while the edge ion temperature at p = 0.82 drops from 0.1 to 0.03 keV after the pellet injection, and recovers within 5 - 2 0 ms. The fast drop in the ion temperature observed indicates a direct cooling of the ion by pellet injection.
I
~_ ?
=3 cD
©
2
0 5286
Q ~ o
i
5288
5290 5292 wavelength (]~)
i
i
5294
4. Concluding remarks
Fig. 4. Spectra of charge exchange line C VI (529.05 nm) at R = 1.06 m (p = 0.55), 0.05 ms before (t = 208 ms) and 0.45 ms after (t = 208.5 ms) the ice pellet injection. tions will be corrected by calibration in future. The spectra are fitted with Gaussian profiles plus a linear background, and include the convolution of the instrument functions.
0.6
A new charge exchange spectroscopic technique using a F a b r y - P e r o t spectrometer has been developed for the JIPP T-IIU tokamak. The time resolution of the ion temperature measurement is 125 gs, with a spatial resolution of eight channels. The spectral resolution is now limited by the eight channels of the PDA in the spectral direction. The spectral resolution will be improved up to the effective finess of 14 of the F a b r y - P e r o t spectrometer, by increasing the number of photodiodes of the detector from 1 6 x 16 to 3 2 x 3 2 .
Acknowledgements
> 0.4
The authors acknowledge D r B J . Peterson for checking the manuscript.
[_~'"
0.2
References 0
200
220
240
260
280
300
time (ms) Fig. 5. Time evolution of ion temperatures measured by the Fabry-Perot spectrometer at various positions from p = 0.13 to 0.82, for the discharge with ice pellet injection at t = 208.05 ms.
[1] R.J. Fonck, D.S. Darrow and K.P. Jaehnig, Determination of plasma ion velocity distribution via charge-exchange recombination spectroscopy, Phys. Rev. A 29 (1984) 3288. [2] K. Ida et al., Electricfield profile of CHS Heliotron/Torsatron plasma with tangential neutral beam injection, Phys. Fluids B 3 (1991) 515.
K. Ida et a l . / Fusion Engineering and Design 34 35 (1997) 219-223
[3] R.J. Groebner, K.H. Burrell and R.P. Seraydarian, Role of edge electric field and poloidal rotation in the L-H transition, Phys. Rev. Lett. 64 (1990) 3015. [4] K. Ida and S. Hidekuma, Space- and time-resolved measurements of ion temperature with the C VI 5292
223
~, charge-exchange recombination line after subtracting background radiation, Rev. Sci. Instrum. 60 (1989) 867. [5] J.G. Hirschberg and P. Platz, A multichannel FabryPerot interferometer, Appl. Opt. 4 (1965) 1375.
Fusion Engineering and Design
Fast charge exchange spectroscopy using a Fabry-Perot spectrometer in the JIPP TII-U tokamak K. I d a *, J. Xu, K.N. Sato, H. S a k a k i t a 1 National Institute for Fusion Science, Nagoya 464-01, Japan
Abstract A new charge exchange spectroscopic technique using a Fabry-Port spectrometer has been developed to increase the photon flux at the detector, and to improve the time resolution of ion temperature and plasma rotation velocity measurements. The spectral resolution is obtained by arranging two-dimensional fiber-optics and a two-dimensional detector at the focal plane of a coupled lens located on both sides of a Fabry-Perot spectrometer. The effective finesse (the ratio of the spectral resolution to the free spectral range) of the Fabry-Perot interferometer in this system is 14. The time evolution of the ion temperature is obtained with a time resolution of 125 gs and with a spatial resolution of 3 cm (eight channels). © 1997 Elsevier Science S.A.
1. Introduction
2. Experimental details
Charge exchange spectroscopy has been routinely used to measure ion temperatures, toroidal and poloidal rotation velocities and radial electric field profiles in tokamaks and stellarators, including heliotrons [1,2]. Recently, fast changes of the poloidal rotation, which indicate a fast change of the radial electric field, have been observed, a n d the improvement of the time resolution of charge exchange spectroscopy has become a more important issue [3]. However, charge exchange spectroscopy using a monochromator and a CCD detector has relatively poor time resolution, although it has excellent spatial resolution [4].
A new charge exchange spectroscopy using a F a b r y - P e r o t spectrometer [5] has been developed and installed in the JIPP TII-U tokamak (where the major radius R is 0.93 m and the minor radius a is 0.23 m) to increase the photon flux at the detector, and to improve the time resolution of the ion temperature and the plasma rotation velocity measurements up to 8 kHz. The spectral resolution is obtained by arranging two-dimensional fiber-optics and a twodimensional detector at the focal plane of a coupled lens located on the both sides of the F a b r y - P e r o t spectrometer. Fig. 1 shows a schematic diagram of the spectrometer system, including an interference filter to limit the spectral bandwidth that illuminates the F a b r y - P e r o t spectrometer. The spacing of the spectrometer is 100 lain. The two-dimensional fiber bundle
* Corresponding author. The JIPP TII-U group also contributed to the authorship of this paper.
0920-3796/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0920-3796(96)00666-7
220
K. Ida et al./Fusion Engineering and Design 34-35 (1997) 219-223
(20 mm x 20 mm) consists of 400 optical fibers each 1 mm in diameter and is divided into eight sections to obtain the spatial resolution. The image of this fiber bundle is focused on to the photocathode of an image intensifier with a tandem microchannel plate (MCP) coupled to a 16 x 16 channel two-dimensional photodiode array (PDA). The PDA (17.6 x 17.6 mm) consists of 256 photodiodes. Each photodiode has dimensions 0.95 x 0.95 mm and a center-to-center separation of 1.1 ram. The wavelength 2 of the light that transmits through the Fabry-Perot spectrometer is given by 2 ~ 20(1 - -
02/2)
where 20 is the wavelength transmitted for the normal incident light, and 0 is the angle between the line of sight of each photodiode and the optical axis of the Fabry-Perot spectrometer. 0 is determined from the distance of each photodiode from the optical axis d and the focal length f of coupled lens as 0 ~ d / f There are eight photodiodes from the center to the edge of the PDA detector, effectively giving eight spectral channels. The spectral resolution and spectral range simultaneously measured with the PDA can be adjusted by changing the focal l e n g t h f of the coupled lens.
3. Discussion
Fig. 2 shows the spectral resolution (full width at half-maximum (FWHM)) of the fast charge exchange system for the pixels located at d = 7 mm (0=0.04 when f - - 1 8 0 mm) and for the incident light with the wavelength of 2 = 632.8 nm (2o = 633.3 nm) for various focal lengths of the coupled lens. We note here that the FWHM differs for different pixels and it increases as the tilted angle 0 is increased, because the wavelength dispersion increases almost linearly as Id2/d0l 200. As the focal length of the coupled lens is increased, the spectral resolution is improved up to 0.14 nm, which is determined by the spectral resolution of the Fabry-Perot spectrometers, but the spectral range also decreases. However, when the focal length of the coupled lens is sufficiently short ( f < 180 mm), the spectral range increases,
approaching the free spectral range (FSR) of the Fabry-Perot spectrometer, and the spectral resolution is determined by the channel resolution of the PDA, as shown in Fig. 2. The FSR of the Fabry-Perot interferometer is 2.0 nm at 632.8 n m - - t h e effective finesse of the Fabry-Perot interferometer in this system is 14. The corresponding finesse is much lower than the original finesse of the Fabry-Perot interferometer of 40, which is obtained when the incident light is parallel to the optical axis (0 = 0.0). This is because of the finite pixel size, especially for the outermost channels, where the incident light is at an angle to the optical axis of the Fabry-Perot spectrometer. Because the off-axis finesse is worse than the on-axis finesse by a factor of three, the original finesse of the Fabry Perot interferometer in this system should be much greater than that required from channel resolution. Fig. 3 shows the intensity distribution of the charge exchange line of the carbon impurity C VI (n = 8 ---,7 transition, 529.05 nm) at the PDA, for the neutral beam injection (NBI)-heated discharge
lens f=lS0mm F=2.8
Fiber 20x20mm ~ \223 A ~//111 /
~
\
~i
I//~
__..t-.-+-t.-rv ~ TI---#-.----.~ 8 v
Filter d)~ =l.5nm 9~0=529nm 1
2 3 4
6 7
\ !8
detector 16 x 16 PDA 17.6xl7.6mm H/-~,,--~,-W-tH+I
Fabry-Perot Interferometer
~
center ,plasma
edge
F=1.25
Fig. 1. Schematic diagram of fast charge exchange spectroscopy systemusing Fabry-Perot interferometer.
221
K. Ida et al./Fusion Engineering and Design 34-35 (1997) 219-223
used in the analysis. However, more than one photodiode corresponds to one wavelength, and the effective wavelength resolution is only eight. Fig. 4 shows the measured spectra of a single chord at two time slices, at R = 1.06 m (p = 0.55), 0.05 ms before ( t = 2 0 8 ms) and 0.45 ms after (t = 208.5 ms) the ice pellet injection. The deviation of the data from the Gaussian fitted curve (for instance, the data at 2 = 529.12 nm) is considered to result from the variation of the fiber transmission, the convolving effects of the spectral instrument functions and the individual sensitivities of the photodiodes; it is not believed to result from the photon statistics. However, these varia-
4
FSR Fa Perot 3
~2
1
0 0
5
10 15 range (~.)
20
25
Fig. 2. Spectral performance (full width at half-maximum) of Fabry-Perot system for the pixels located at 0 = 0.04 and 2 = 632.8 nm (2o = 633.3 nm) for various focal lengths of the coupled lenses. with pellet injection in the JIPP T I I - U tokamak. The C VI background emission, measured just before the NBI, is subtracted from the C VI charge exchange emission. The intensity distribution at the left-hand top section of Fig. 3 corresponds to the C VI emission from the plasma edge, and the intensity distribution at the lefthand b o t t o m section corresponds to the emission from the plasma center. As shown in Fig. 3(b), the two-dimensional image of the C VI emission is divided into eight sections each separated by 45 ° . These eight channels correspond to the viewing chords in the plasma, from the plasma center to the plasma edge. Unfortunately, the two edge chords (chords 1 and 8) experience reflection problems at the viewing port, so they are not used for the analysis. Therefore, the outermost viewing chord near the plasma edge is chord 2 and the innermost viewing chord near the plasma center is chord 7. The photodiodes at and near the boundary between chords are masked (not used) to avoid overlap and 'crosstalk', and these pixels are illustrated with the shaded area in Fig. 3(b). Then, each chord contains 21 spectral pixels (photodiodes)
u
10
0,
0
5
10
15
Fig. 3. (a) Three-dimensional plot and (b) contour plot of intensity distribution of charge exchange line C VI (529.05 nm) detected with 16 x 16 PDA at t = 208.5 ms (0.45 ms after the ice pellet injection).
222
K. Ida et al. /Fusion Engineering and Design 34-35 (1997) 219-223
'
J
~
I
I
o 208ms © 208.5ms
'
'
The ion temperature is derived from the Doppler width of the C VI intensity. Fig. 5 shows the time evolution of the ion temperatures measured at various positions from p =0.13 to 0.82, with a time resolution of 125 gs, which is determined by the sampling speed of data acquisition, for the discharge with NBI and ice pellet injection as an example. The ion temperature at a normalized minor radius (p = 0.13) drops from 0.56 to 0.23 keV, while the edge ion temperature at p = 0.82 drops from 0.1 to 0.03 keV after the pellet injection, and recovers within 5 - 2 0 ms. The fast drop in the ion temperature observed indicates a direct cooling of the ion by pellet injection.
I
~_ ?
=3 cD
©
2
0 5286
Q ~ o
i
5288
5290 5292 wavelength (]~)
i
i
5294
4. Concluding remarks
Fig. 4. Spectra of charge exchange line C VI (529.05 nm) at R = 1.06 m (p = 0.55), 0.05 ms before (t = 208 ms) and 0.45 ms after (t = 208.5 ms) the ice pellet injection. tions will be corrected by calibration in future. The spectra are fitted with Gaussian profiles plus a linear background, and include the convolution of the instrument functions.
0.6
A new charge exchange spectroscopic technique using a F a b r y - P e r o t spectrometer has been developed for the JIPP T-IIU tokamak. The time resolution of the ion temperature measurement is 125 gs, with a spatial resolution of eight channels. The spectral resolution is now limited by the eight channels of the PDA in the spectral direction. The spectral resolution will be improved up to the effective finess of 14 of the F a b r y - P e r o t spectrometer, by increasing the number of photodiodes of the detector from 1 6 x 16 to 3 2 x 3 2 .
Acknowledgements
> 0.4
The authors acknowledge D r B J . Peterson for checking the manuscript.
[_~'"
0.2
References 0
200
220
240
260
280
300
time (ms) Fig. 5. Time evolution of ion temperatures measured by the Fabry-Perot spectrometer at various positions from p = 0.13 to 0.82, for the discharge with ice pellet injection at t = 208.05 ms.
[1] R.J. Fonck, D.S. Darrow and K.P. Jaehnig, Determination of plasma ion velocity distribution via charge-exchange recombination spectroscopy, Phys. Rev. A 29 (1984) 3288. [2] K. Ida et al., Electricfield profile of CHS Heliotron/Torsatron plasma with tangential neutral beam injection, Phys. Fluids B 3 (1991) 515.
K. Ida et a l . / Fusion Engineering and Design 34 35 (1997) 219-223
[3] R.J. Groebner, K.H. Burrell and R.P. Seraydarian, Role of edge electric field and poloidal rotation in the L-H transition, Phys. Rev. Lett. 64 (1990) 3015. [4] K. Ida and S. Hidekuma, Space- and time-resolved measurements of ion temperature with the C VI 5292
223
~, charge-exchange recombination line after subtracting background radiation, Rev. Sci. Instrum. 60 (1989) 867. [5] J.G. Hirschberg and P. Platz, A multichannel FabryPerot interferometer, Appl. Opt. 4 (1965) 1375.