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A broadband light collection system for ECE diagnostics on the FTU tokamak
infrared Phys. Vol. 34, No. 5, pp. 533-541, 1993 Ail rights reserved
Printed in Great Britain.
Copyright
0
002W891,‘93 56.00 + 0.00 1993 Pergamon Press Ltd
A BROADBAND LIGHT COLLECTION SYSTEM FOR ECE DIAGNOSTICS ON THE FTU TOKAMAK P. WRATTI, 0. Tu~rsco and M. ZERBINI Associazione EURATOM-ENEA sulla Fusione, Centro Richerche Energia Frascati, C.P. 65-~ Frascati, Rome, Italy (Received 8 June 1993) A~~a~-E~~tron temperature profiles are currently measured on the FTU tokamak by spectrai analysis of electron cyciotron emission. Light pipes are used for collecting the light and transporting it to a Fourier transform spectrometer and to a grating polychromator. Use of a light pipe facing the plasma allows one to obtain a large &endue in spite of the restricted diagnostic access, while a narrow field of view is defined by a quasi optical system placed outside the tokamak vacuum. The reliability of this combination of light pipes and quasi optical elements has been checked in the laboratory and has been confirmed by more than 3 years of routine operation on the FTU tokamak.
I.
INTRODUCTION
Measurements of electron cyclotron emission (ECE) spectra from tokamak plasmas have shown that the spatial variation of the cyclotron resonance frequency allow derivation of electron temperature profiles from spectral analysis of optically thick harmonics.“**) The ECE diagnostics for the FTU tokamak (major radius & = 0.935 m; limiter radius r,_= 0.3 m; central toroidal magnetic field B, < 8 T) have been designed to obtain absolutely calibrated electron temperature profiles with a spatial resolution of 3 cm. The radiation temperature for the extraordinary polarization is measured by means of a Fourier transform spectrometer (FTS) over a spectral range including the lirst four harmonics at B, = 8 T, with a 5 ms time resolution. Temperature profiles are derived from the optically thick second harmonic {wavelength in mm: 1 = S.36/~), while the remainder of the spectrum, where the emission is optically thin, is used to monitor either electron density or suprathermal electrons, The electron temperature is also measured at 12 radial positions with microsecond time resolution by means of a 12 channel grating polychromator; this instrument is cross calibrated against the FTS, The performance of the ECE diagnostics critically depends on the characteristics of the light collection and transport system that connects the spectrometers to the tokamak plasma. In fact the width of the field of view must be compatible with the required spatial resolution, while the signal-to-noise ratio (which determines the sensitivity of the polychromator and the quality of the FTS absolute calibration) depends on the &endue of the light collection system and on the losses in the light transport system. The design principles and practical solutions employed in order to obtain a narrow field of view and a large &endue in spite of the restricted diagnostic access are presented in section II; the light transport system is described in section III. Some of the adopted design principles were innovative and the experimental tests performed to check the critical items are described in section IV. II.
THE
LIGHT
COLLECTION
SYSTEM
Isothermal surfaces in a tokamak are nested tori, while the cyclotron resonance frequency varies inversely with major radius R and is constant along vertical chords. Spatially resolved information can then be obtained from spectral measurements, provided that the field of view is narrow enough in the z-direction, while a larger extent can be tolerated in the toroidal direction. The width Wz 533
P. BURATTI et al.
534
resonances
Fig. 1. Effect of field of view width Wi on spatial resolution Ar in the radial direction for central and non-central resonance position. The optic axis lies on the equatorial plane (z = 0); concentric circles represent intersections of the isothermal surfaces with the R, z plane.
from calibration source
to spectrometers
from ilasma t Fig. 2. Optical arrangement of the light collection system allowing control of the field of view width and the spectrometers facing either the plasma or calibration sources. The front-end of the light transport system is also shown.
ECE diagnostics on the FTU tokamak
535
of the field of view is critical near the plasma centre, while for increasing minor radius the z-direction becomes tangent to isothermal surfaces (Fig. 1) and the influence of W, on spatial resolution becomes weaker; hence W, has to be optimized for a fixed distance L from the antenna aperture, corresponding to the plasma centre. An image-forming optical system like the one described in Ref. (3) was considered at first, but it has been ruled out because in FTU the vacuum windows are quite far from the plasma, so that either a large vacuum window or focusing optics in a non-accessible location should have been used to obtain the required Ctendue. In these circumstances the best solution is to use light pipes, provided that an appropriate scheme is devised to control the field of view. A square section straight light pipe has been used for light collection; since the inclination of collected rays increases with the distance from the optic axis, and the light pipe preserves ray inclination, a narrow field of view can be defined outside the torus vacuum by a lens-stop combination that selects rays emerging from the light pipe with a small inclination (Fig. 2). The stop is placed at the focal plane of the lens; its dimensions in the z and toroidal directions are d, and dT respectively; since the field of view is only required to be narrow in the z-direction, dT may be larger than d,. The field of view of this light collection system has tapered edges; ray tracing calculations give a half width at half maximum for the z-direction: ‘3
(1)
where a is the antenna size and f is the focal length. The extent of the grey region is given by min{a, Ld,/f >. For d, 2fa/L, the geometric width of the field of view behaves like that of an image forming system, i.e. both W, and the &endue are reduced for decreasing 4, while, for d, < fa/L, W, remains constant and decreasing dz only reduces the &endue. The maximum Ctendue at fixed W, is obtained for dz = af IL. Broadening of the field of view due to diffraction effects can be accounted for by introducing a ray spreading proportional to the wavelength I in ray tracing calculations; in this way we obtain:
where c is a numerical factor. Matching with far field calculationsC4)would be obtained for c = 0.44, but this value should be increased in the near field (see section IV), so that we assumed c = 1. The ttendue of the light collection system is given by:
the attainable values are limited by the conditions a < 2 W, and dz/f < 2 W,/L, following from equations (1) and (2). This limitation may be overcome by placing a focusing mirror with focal length L before the antenna aperture; in this case the geometric width becomes:
and the restriction on a is removed. A light collection system including the focusing mirror has so far only been tested in the laboratory, and will be employed in an upgraded version of the FTU light collection system; the mirror will not be accessible, but it will be solidly joined with the light pipe (Fig. 3) and will not require any alignment in situ. The FTU light collection system was designed to have a narrow field of view, i.e. Wz < 2.5 cm, and to match the itendue of an InSb detector, i.e. E x 0.4 cm* sr. These requirements become
P. BURATTIet al
536
Plasma edge
Fig. 3. Front-end of the upgraded light collection system for FTU (poloidal section). Only one antenna is shown, the other being symmetric with respect to the equatorial plane. Dashed lines represent rays issued from the plasma centre.
incompatible above a critical wavelength because, for increasing 2 and fixed IV;, both u and dz = af/L have to be decreased. The distance between the antenna front-end and plasma centre is L = 40 cm in FTU; taking a = 3.8 cm, f = 20 cm, dz = 1.9 cm, and dT = 3.8 cm, a reasonable Ctendue E = 0.26 cm’ sr results, while the requirement on Wz is only fulfilled for A < 1.5 mm, i.e. for magnetic fields B > 3.5 T. Increasing the stop width or the light pipe size in the toroidal direction would not be compatible with detector matching. A good spatial resolution at lower magnetic fields will be insured by the upgraded light collection system. The antenna has been realised in copper; its length is 1.6 m; the vacuum window, placed near the antenna back aperture (Fig. 2) is an 80 mm diameter z-cut crystal quartz disk with a thickness of 10 mm; the disk profile is planoconvex, with a focal length of 200 mm, so that the first lens is integrated with the window, and both etalon effects and addition of two air-dielectric interfaces are avoided. The optical axis is folded by 90” using a plane mirror (Fig. 2). The mounting of this mirror can be turned by 180” to interchange the antenna with a replica to be used for calibration (Fig. 2); this solution has been adopted because the size of the ports does not allow insertion of the calibration sources in the torus vacuum chamber. A wire grid polarizer selects the polarization with the electric field in the z-direction, polarization. (‘I The rejection ratio of the polarizer is corresponding to the plasma extraordinary better than l%, while an error of 3% at worst arises from changes in the polarization direction due to time-varying plasma magnetic fields.
III.
THE
LIGHT
TRANSPORT
SYSTEM
A 16 m long, square section aluminium light pipe links the light collection system with the spectrometers. The size b of the light pipe has been determined under the requirement of keeping the resistive losses of the fundamental waveguide mode below 3 dB at i = 0.5 mm. Assuming that real losses double ideal ones, the required size is b = 7.6 cm. The waveguide behaviour is verified for this pipe size, since the diffraction length bz/l is smaller than the pipe length.
ECE diagnostics
on the FTU
531
tokamak
TEol electric field profile
A-
-
To polychromator
Rectangular pipes
t To FTS Fig. 4. Longitudinal
section
of the wave front beam splitter used to connect system to two spectrometers at the same time.
a single light transport
Before entering the aluminium light pipe, radiation emerging from the stop of the light collection system is collimated along the pipe axis by a TPX lens with a focal length of 40 cm. In order to avoid walk-off losses, the light pipe entrance is imaged by a field lens onto the side of the light collecting pipe facing the vacuum window (Fig. 2). A= 0.7 mm
A=0.3
mm
Hz0 vapour absorption lines
0
200
400
600
800
1000
Frequency (GHz) Fig. 5. Spectral
distribution
of radiation detected by an InSb detector arc lamp is used as source.
when a high pressure
mercury
P. BURATTI et al.
538
The light transport
system includes
three E-plane
and two H-plane
mitre bends; a final H-plane
wave front divider splits the square section pipe into two rectangular ones connected to the Fourier transform spectrometer and to the grating polychromator respectively (Fig. 4). The whole system, excepting the calibration antenna, is enclosed in airtight boxes and continuously flushed by dry nitrogen, so that no water vapour absorption lines are observed in the ECE spectra, while two narrow lines at ,l = 0.54 and 0.40 mm (557 and 752 GHz respectively) are present in the calibration spectra (Fig. 5), and have to be eliminated by interpolation. The calibration ventilation;
system could not be enclosed because the hot calibration source requires forced this problem will be solved by stongly flushing the calibration antenna during next
calibration
runs. IV.
The choice of employing
EXPERIMENTAL
a combination
TESTS
of optical
elements
and light pipes allows a compact
design and an easy and reliable alignment, but requires careful experimental tests, because the calculated characteristics could be altered by effects, such as mode scrambling in the light pipes, that are quite difficult to predict. Field of view scans were carried out by moving a high pressure mercury arc lamp transverse to the optic axis (Fig. 6); some cross checks have also been made using a progressively hidden liquid nitrogen bath. The measurements were averaged over a spectrum determined by the cut-off of the InSb detector, the minimum wavelength being ,l z 0.3 mm and the peak 1. z 0.7 mm (Fig. 5). Various values of a and dz have been used in order to check equation (2); the focal length was f = 16 cm. Also a light collection system including a lens with a focal length of 57 cm at the front end has been tested (data with L = 57 cm in Table 1). In order to get rid of the effect of detector optics, the raw data have been divided by those obtained excluding the stop. The widths W, of the resulting profiles have been compared with the predictions of equation (2) for c = 0.5 and 2 = 0.7 mm, while the peak values Q have been compared to (x, = Ld,/Zf Wz, i.e. to the ratio between power passing through the stop and power collected at the antenna front end. As shown in Table 1, a good agreement turns out; the discrepancies found at small d,, i.e. when diffraction effects are dominant, can be reconciled by increasing the numerical factor in the diffraction term
1.2,
I
-2
-1
0
I
1
2
3
4
5
Lamp displacement (cm) Fig. 6. Raw data from field of view scans, with peak values normalized to unity. (a) L = 48 cm, a = 2.3 cm, stop excluded. (b) L = 48 cm, a = 2.3 cm, dz = 1.6 cm. (c) L = 57 cm, a = 4.6 cm, d_ = 0.2 cm; lens with f = 57 cm placed before the antenna aperture.
ECE diagnostics on the FTU tokamak
539
Table 1. Results from field of view scans
L (cm)
a (cm)
48 48 48 48 48 48 48 48 48 48 51 57
1.0 1.0 1.0 1.0 1.0 1.0 2.3 2.3 2.3 2.3 4.6 4.6
4 0-W WM(cm) W:M-4 2 1.6 1.2 0.8 0.6 0.4 1.6 1.2 0.8 0.4 0.2 0.1
3.1 3.1 2.1 2.5 2.2 1.9 2.4 2.0 1.6 1.2 0.50 0.35
3.4 2.9 2.5 2.1 1.9 1.8 2.5 2.0 1.4 1.3 0.56 0.47
aM
a,
0.87 0.78 0.68 0.46 0.30 0.22 0.92 0.80 0.61 0.31 0.67 0.32
0.88 0.83 0.72 0.51 0.47 0.33 0.90 0.90 0.85 0.45 0.64 0.38
to values of c w 0.9. For this reason we assumed c = 1 in the determination of the parameters of the light collection system. The overall attenuation of the system installed on FTU has been measured in order to check the effect of mode scrambling. The signal obtained with the mercury lamp placed at the calibration input has been compared to the one measured for direct illuminations of the FTS; the measured loss was 11.3 dB. The total predicted loss at I = 0.7 mm, i.e. for the peak of the effective spectrum, amounts to 8.8 dB; this value has been calculated including: 0.3 dB for resistive losses in the antenna, 3 dB for reflection and absorption at the lenses, 2.5 dB for resistive losses in the 16 m long aluminium light pipe and 3 dB for area reduction at the final beam splitter. Resistive losses have been calculated by doubling the nominal attenuation for TE,, mode. We attribute the discrepancy of 2.5 dB between measured and predicted attenuation to mode scrambling at the final beam splitter; this conclusion is supported by preinstallation tests, in which 2.0 1
I
115
125
Major radius (cm) Fig. 7. Absolutely calibrated electron temperature profiles measured by the FTS. Solid line: peaked temperature profile as usually observed at low plasma current and low electron density (FTU shot 3646, t = 0.62 s). Dashed line: hollow profile due to cooling of the plasma centre by strong line radiation from impurity ions (FTU shot 4319, t = 0.79 s). Profiles are not available for R < 78.5 cm due to overlap between second and third harmonic resonances.
P. BURATTIel al.
540
Partial reconnection
Full reconnection c
+-
1.90 1.70 2.00 5
1.70 2.10
g
1.70
R = 82.7 cm I
r
I
1
I
R = 86.7 cm
?j 2.10 g &
1.70 2.20
E
1.80
f
1.90
2
1.70
g w
1.80 4I.,” -?n -
670
R= 100.8 cm
675
680
685
690
695
700
Time (ms) Fig. 8. Electron temperature evolution at 9 radial positions for FTU shot 3928. Fast signal oscillations are due to toroidal or poloidal rotation of magnetic islands having a radial extent of about 3 cm. Sudden drops or rises are due to fast radial energy redistribution given by magnetic reconnection processes.
it was verified that an 8 m long light pipe with two mitre bends introduces an attenuation consistent with calculated resistive losses and preserves the polarization. The final beam splitter was configured to fit the requirements of the grating polychromator; as a consequence, if the TE,, mode is propagating in the square section input pipe, the tangential electric field in the rectangular output pipes has a maximum at a wall (Fig. 3) so that extra losses due to mode scrambling are likely to be introduced. The properties of the light collection system have been extensively confirmed in plasma measurements for B > 4 T: very narrow temperature peaks have been measured by the FTS (Fig. 7), with an accuracy of 5%, as verified by comparison with laser scattering results. The good spatial resolution and signal-to-noise ratio also allowed the spatial structure of small temperature oscillations to be resolved; the amplitude of these on the polychromator channels changes dramatically in less than 3 cm (Fig. 8). V. CONCLUSIONS
AND
FUTURE
WORK
A light collection and transport system has been developed, which allows a narrow field of view and a large ttendue to be obtained, in spite of the difficult access to the FTU plasma. The width of the field of view was 2.5 cm for B > 3.5 T, and the signal-to-noise ratio was high enough to allow absolute calibration of the ECE diagnostics to a 5% accuracy and to measure temperature fluctuations of 10 eV on a bandwidth of 20 kHz at B = 6 T. The reliability of the installed system has been checked by periodic calibration runs; no adjustment has been required during 3 years of routine operation.
ECE diagnostics
on the FTU tokamak
541
In order to obtain a good performance of the ECE diagnostics also at magnetic fields down to
B = 2 T, some upgrades will be necessary: in order to avoid the losses at the final beam splitter each spectrometer will be fed by a dedicated light collection system; focusing mirrors will be employed at the front end of the antennas to improve the definition of the field view, and in place of the TPX lenses to reduce losses. As a result of these modifications, the overall losses are expected to be reduced to 4.3 dB, thereby considerably enhancing the signal-to-noise ratio. REFERENCES 1. A. E. Costley, Proc. Course and Workshop ‘Diagnosticsfor Fusion Reactor Conditions’, EUR 8351-l EN, 1, 167. Varenna (1982). 2. M. Bornatici, R. Cano, 0. De Barbieri and F. Engelmann, Nucl. Fusion 23, 1153 (1983). 3. F. J. Stauffer, D. A. Boyd, R. C. Cutler, M. Diesso, M. P. McCarthy, J. Montague and R. Rocco, Rev. Sri. Instrum. 59, 2139 (1988). 4. M. Born and E. Wolf, Principles Optics,p. 393. Pergamon Press, Oxford (1964).
of
Printed in Great Britain.
Copyright
0
002W891,‘93 56.00 + 0.00 1993 Pergamon Press Ltd
A BROADBAND LIGHT COLLECTION SYSTEM FOR ECE DIAGNOSTICS ON THE FTU TOKAMAK P. WRATTI, 0. Tu~rsco and M. ZERBINI Associazione EURATOM-ENEA sulla Fusione, Centro Richerche Energia Frascati, C.P. 65-~ Frascati, Rome, Italy (Received 8 June 1993) A~~a~-E~~tron temperature profiles are currently measured on the FTU tokamak by spectrai analysis of electron cyciotron emission. Light pipes are used for collecting the light and transporting it to a Fourier transform spectrometer and to a grating polychromator. Use of a light pipe facing the plasma allows one to obtain a large &endue in spite of the restricted diagnostic access, while a narrow field of view is defined by a quasi optical system placed outside the tokamak vacuum. The reliability of this combination of light pipes and quasi optical elements has been checked in the laboratory and has been confirmed by more than 3 years of routine operation on the FTU tokamak.
I.
INTRODUCTION
Measurements of electron cyclotron emission (ECE) spectra from tokamak plasmas have shown that the spatial variation of the cyclotron resonance frequency allow derivation of electron temperature profiles from spectral analysis of optically thick harmonics.“**) The ECE diagnostics for the FTU tokamak (major radius & = 0.935 m; limiter radius r,_= 0.3 m; central toroidal magnetic field B, < 8 T) have been designed to obtain absolutely calibrated electron temperature profiles with a spatial resolution of 3 cm. The radiation temperature for the extraordinary polarization is measured by means of a Fourier transform spectrometer (FTS) over a spectral range including the lirst four harmonics at B, = 8 T, with a 5 ms time resolution. Temperature profiles are derived from the optically thick second harmonic {wavelength in mm: 1 = S.36/~), while the remainder of the spectrum, where the emission is optically thin, is used to monitor either electron density or suprathermal electrons, The electron temperature is also measured at 12 radial positions with microsecond time resolution by means of a 12 channel grating polychromator; this instrument is cross calibrated against the FTS, The performance of the ECE diagnostics critically depends on the characteristics of the light collection and transport system that connects the spectrometers to the tokamak plasma. In fact the width of the field of view must be compatible with the required spatial resolution, while the signal-to-noise ratio (which determines the sensitivity of the polychromator and the quality of the FTS absolute calibration) depends on the &endue of the light collection system and on the losses in the light transport system. The design principles and practical solutions employed in order to obtain a narrow field of view and a large &endue in spite of the restricted diagnostic access are presented in section II; the light transport system is described in section III. Some of the adopted design principles were innovative and the experimental tests performed to check the critical items are described in section IV. II.
THE
LIGHT
COLLECTION
SYSTEM
Isothermal surfaces in a tokamak are nested tori, while the cyclotron resonance frequency varies inversely with major radius R and is constant along vertical chords. Spatially resolved information can then be obtained from spectral measurements, provided that the field of view is narrow enough in the z-direction, while a larger extent can be tolerated in the toroidal direction. The width Wz 533
P. BURATTI et al.
534
resonances
Fig. 1. Effect of field of view width Wi on spatial resolution Ar in the radial direction for central and non-central resonance position. The optic axis lies on the equatorial plane (z = 0); concentric circles represent intersections of the isothermal surfaces with the R, z plane.
from calibration source
to spectrometers
from ilasma t Fig. 2. Optical arrangement of the light collection system allowing control of the field of view width and the spectrometers facing either the plasma or calibration sources. The front-end of the light transport system is also shown.
ECE diagnostics on the FTU tokamak
535
of the field of view is critical near the plasma centre, while for increasing minor radius the z-direction becomes tangent to isothermal surfaces (Fig. 1) and the influence of W, on spatial resolution becomes weaker; hence W, has to be optimized for a fixed distance L from the antenna aperture, corresponding to the plasma centre. An image-forming optical system like the one described in Ref. (3) was considered at first, but it has been ruled out because in FTU the vacuum windows are quite far from the plasma, so that either a large vacuum window or focusing optics in a non-accessible location should have been used to obtain the required Ctendue. In these circumstances the best solution is to use light pipes, provided that an appropriate scheme is devised to control the field of view. A square section straight light pipe has been used for light collection; since the inclination of collected rays increases with the distance from the optic axis, and the light pipe preserves ray inclination, a narrow field of view can be defined outside the torus vacuum by a lens-stop combination that selects rays emerging from the light pipe with a small inclination (Fig. 2). The stop is placed at the focal plane of the lens; its dimensions in the z and toroidal directions are d, and dT respectively; since the field of view is only required to be narrow in the z-direction, dT may be larger than d,. The field of view of this light collection system has tapered edges; ray tracing calculations give a half width at half maximum for the z-direction: ‘3
(1)
where a is the antenna size and f is the focal length. The extent of the grey region is given by min{a, Ld,/f >. For d, 2fa/L, the geometric width of the field of view behaves like that of an image forming system, i.e. both W, and the &endue are reduced for decreasing 4, while, for d, < fa/L, W, remains constant and decreasing dz only reduces the &endue. The maximum Ctendue at fixed W, is obtained for dz = af IL. Broadening of the field of view due to diffraction effects can be accounted for by introducing a ray spreading proportional to the wavelength I in ray tracing calculations; in this way we obtain:
where c is a numerical factor. Matching with far field calculationsC4)would be obtained for c = 0.44, but this value should be increased in the near field (see section IV), so that we assumed c = 1. The ttendue of the light collection system is given by:
the attainable values are limited by the conditions a < 2 W, and dz/f < 2 W,/L, following from equations (1) and (2). This limitation may be overcome by placing a focusing mirror with focal length L before the antenna aperture; in this case the geometric width becomes:
and the restriction on a is removed. A light collection system including the focusing mirror has so far only been tested in the laboratory, and will be employed in an upgraded version of the FTU light collection system; the mirror will not be accessible, but it will be solidly joined with the light pipe (Fig. 3) and will not require any alignment in situ. The FTU light collection system was designed to have a narrow field of view, i.e. Wz < 2.5 cm, and to match the itendue of an InSb detector, i.e. E x 0.4 cm* sr. These requirements become
P. BURATTIet al
536
Plasma edge
Fig. 3. Front-end of the upgraded light collection system for FTU (poloidal section). Only one antenna is shown, the other being symmetric with respect to the equatorial plane. Dashed lines represent rays issued from the plasma centre.
incompatible above a critical wavelength because, for increasing 2 and fixed IV;, both u and dz = af/L have to be decreased. The distance between the antenna front-end and plasma centre is L = 40 cm in FTU; taking a = 3.8 cm, f = 20 cm, dz = 1.9 cm, and dT = 3.8 cm, a reasonable Ctendue E = 0.26 cm’ sr results, while the requirement on Wz is only fulfilled for A < 1.5 mm, i.e. for magnetic fields B > 3.5 T. Increasing the stop width or the light pipe size in the toroidal direction would not be compatible with detector matching. A good spatial resolution at lower magnetic fields will be insured by the upgraded light collection system. The antenna has been realised in copper; its length is 1.6 m; the vacuum window, placed near the antenna back aperture (Fig. 2) is an 80 mm diameter z-cut crystal quartz disk with a thickness of 10 mm; the disk profile is planoconvex, with a focal length of 200 mm, so that the first lens is integrated with the window, and both etalon effects and addition of two air-dielectric interfaces are avoided. The optical axis is folded by 90” using a plane mirror (Fig. 2). The mounting of this mirror can be turned by 180” to interchange the antenna with a replica to be used for calibration (Fig. 2); this solution has been adopted because the size of the ports does not allow insertion of the calibration sources in the torus vacuum chamber. A wire grid polarizer selects the polarization with the electric field in the z-direction, polarization. (‘I The rejection ratio of the polarizer is corresponding to the plasma extraordinary better than l%, while an error of 3% at worst arises from changes in the polarization direction due to time-varying plasma magnetic fields.
III.
THE
LIGHT
TRANSPORT
SYSTEM
A 16 m long, square section aluminium light pipe links the light collection system with the spectrometers. The size b of the light pipe has been determined under the requirement of keeping the resistive losses of the fundamental waveguide mode below 3 dB at i = 0.5 mm. Assuming that real losses double ideal ones, the required size is b = 7.6 cm. The waveguide behaviour is verified for this pipe size, since the diffraction length bz/l is smaller than the pipe length.
ECE diagnostics
on the FTU
531
tokamak
TEol electric field profile
A-
-
To polychromator
Rectangular pipes
t To FTS Fig. 4. Longitudinal
section
of the wave front beam splitter used to connect system to two spectrometers at the same time.
a single light transport
Before entering the aluminium light pipe, radiation emerging from the stop of the light collection system is collimated along the pipe axis by a TPX lens with a focal length of 40 cm. In order to avoid walk-off losses, the light pipe entrance is imaged by a field lens onto the side of the light collecting pipe facing the vacuum window (Fig. 2). A= 0.7 mm
A=0.3
mm
Hz0 vapour absorption lines
0
200
400
600
800
1000
Frequency (GHz) Fig. 5. Spectral
distribution
of radiation detected by an InSb detector arc lamp is used as source.
when a high pressure
mercury
P. BURATTI et al.
538
The light transport
system includes
three E-plane
and two H-plane
mitre bends; a final H-plane
wave front divider splits the square section pipe into two rectangular ones connected to the Fourier transform spectrometer and to the grating polychromator respectively (Fig. 4). The whole system, excepting the calibration antenna, is enclosed in airtight boxes and continuously flushed by dry nitrogen, so that no water vapour absorption lines are observed in the ECE spectra, while two narrow lines at ,l = 0.54 and 0.40 mm (557 and 752 GHz respectively) are present in the calibration spectra (Fig. 5), and have to be eliminated by interpolation. The calibration ventilation;
system could not be enclosed because the hot calibration source requires forced this problem will be solved by stongly flushing the calibration antenna during next
calibration
runs. IV.
The choice of employing
EXPERIMENTAL
a combination
TESTS
of optical
elements
and light pipes allows a compact
design and an easy and reliable alignment, but requires careful experimental tests, because the calculated characteristics could be altered by effects, such as mode scrambling in the light pipes, that are quite difficult to predict. Field of view scans were carried out by moving a high pressure mercury arc lamp transverse to the optic axis (Fig. 6); some cross checks have also been made using a progressively hidden liquid nitrogen bath. The measurements were averaged over a spectrum determined by the cut-off of the InSb detector, the minimum wavelength being ,l z 0.3 mm and the peak 1. z 0.7 mm (Fig. 5). Various values of a and dz have been used in order to check equation (2); the focal length was f = 16 cm. Also a light collection system including a lens with a focal length of 57 cm at the front end has been tested (data with L = 57 cm in Table 1). In order to get rid of the effect of detector optics, the raw data have been divided by those obtained excluding the stop. The widths W, of the resulting profiles have been compared with the predictions of equation (2) for c = 0.5 and 2 = 0.7 mm, while the peak values Q have been compared to (x, = Ld,/Zf Wz, i.e. to the ratio between power passing through the stop and power collected at the antenna front end. As shown in Table 1, a good agreement turns out; the discrepancies found at small d,, i.e. when diffraction effects are dominant, can be reconciled by increasing the numerical factor in the diffraction term
1.2,
I
-2
-1
0
I
1
2
3
4
5
Lamp displacement (cm) Fig. 6. Raw data from field of view scans, with peak values normalized to unity. (a) L = 48 cm, a = 2.3 cm, stop excluded. (b) L = 48 cm, a = 2.3 cm, dz = 1.6 cm. (c) L = 57 cm, a = 4.6 cm, d_ = 0.2 cm; lens with f = 57 cm placed before the antenna aperture.
ECE diagnostics on the FTU tokamak
539
Table 1. Results from field of view scans
L (cm)
a (cm)
48 48 48 48 48 48 48 48 48 48 51 57
1.0 1.0 1.0 1.0 1.0 1.0 2.3 2.3 2.3 2.3 4.6 4.6
4 0-W WM(cm) W:M-4 2 1.6 1.2 0.8 0.6 0.4 1.6 1.2 0.8 0.4 0.2 0.1
3.1 3.1 2.1 2.5 2.2 1.9 2.4 2.0 1.6 1.2 0.50 0.35
3.4 2.9 2.5 2.1 1.9 1.8 2.5 2.0 1.4 1.3 0.56 0.47
aM
a,
0.87 0.78 0.68 0.46 0.30 0.22 0.92 0.80 0.61 0.31 0.67 0.32
0.88 0.83 0.72 0.51 0.47 0.33 0.90 0.90 0.85 0.45 0.64 0.38
to values of c w 0.9. For this reason we assumed c = 1 in the determination of the parameters of the light collection system. The overall attenuation of the system installed on FTU has been measured in order to check the effect of mode scrambling. The signal obtained with the mercury lamp placed at the calibration input has been compared to the one measured for direct illuminations of the FTS; the measured loss was 11.3 dB. The total predicted loss at I = 0.7 mm, i.e. for the peak of the effective spectrum, amounts to 8.8 dB; this value has been calculated including: 0.3 dB for resistive losses in the antenna, 3 dB for reflection and absorption at the lenses, 2.5 dB for resistive losses in the 16 m long aluminium light pipe and 3 dB for area reduction at the final beam splitter. Resistive losses have been calculated by doubling the nominal attenuation for TE,, mode. We attribute the discrepancy of 2.5 dB between measured and predicted attenuation to mode scrambling at the final beam splitter; this conclusion is supported by preinstallation tests, in which 2.0 1
I
115
125
Major radius (cm) Fig. 7. Absolutely calibrated electron temperature profiles measured by the FTS. Solid line: peaked temperature profile as usually observed at low plasma current and low electron density (FTU shot 3646, t = 0.62 s). Dashed line: hollow profile due to cooling of the plasma centre by strong line radiation from impurity ions (FTU shot 4319, t = 0.79 s). Profiles are not available for R < 78.5 cm due to overlap between second and third harmonic resonances.
P. BURATTIel al.
540
Partial reconnection
Full reconnection c
+-
1.90 1.70 2.00 5
1.70 2.10
g
1.70
R = 82.7 cm I
r
I
1
I
R = 86.7 cm
?j 2.10 g &
1.70 2.20
E
1.80
f
1.90
2
1.70
g w
1.80 4I.,” -?n -
670
R= 100.8 cm
675
680
685
690
695
700
Time (ms) Fig. 8. Electron temperature evolution at 9 radial positions for FTU shot 3928. Fast signal oscillations are due to toroidal or poloidal rotation of magnetic islands having a radial extent of about 3 cm. Sudden drops or rises are due to fast radial energy redistribution given by magnetic reconnection processes.
it was verified that an 8 m long light pipe with two mitre bends introduces an attenuation consistent with calculated resistive losses and preserves the polarization. The final beam splitter was configured to fit the requirements of the grating polychromator; as a consequence, if the TE,, mode is propagating in the square section input pipe, the tangential electric field in the rectangular output pipes has a maximum at a wall (Fig. 3) so that extra losses due to mode scrambling are likely to be introduced. The properties of the light collection system have been extensively confirmed in plasma measurements for B > 4 T: very narrow temperature peaks have been measured by the FTS (Fig. 7), with an accuracy of 5%, as verified by comparison with laser scattering results. The good spatial resolution and signal-to-noise ratio also allowed the spatial structure of small temperature oscillations to be resolved; the amplitude of these on the polychromator channels changes dramatically in less than 3 cm (Fig. 8). V. CONCLUSIONS
AND
FUTURE
WORK
A light collection and transport system has been developed, which allows a narrow field of view and a large ttendue to be obtained, in spite of the difficult access to the FTU plasma. The width of the field of view was 2.5 cm for B > 3.5 T, and the signal-to-noise ratio was high enough to allow absolute calibration of the ECE diagnostics to a 5% accuracy and to measure temperature fluctuations of 10 eV on a bandwidth of 20 kHz at B = 6 T. The reliability of the installed system has been checked by periodic calibration runs; no adjustment has been required during 3 years of routine operation.
ECE diagnostics
on the FTU tokamak
541
In order to obtain a good performance of the ECE diagnostics also at magnetic fields down to
B = 2 T, some upgrades will be necessary: in order to avoid the losses at the final beam splitter each spectrometer will be fed by a dedicated light collection system; focusing mirrors will be employed at the front end of the antennas to improve the definition of the field view, and in place of the TPX lenses to reduce losses. As a result of these modifications, the overall losses are expected to be reduced to 4.3 dB, thereby considerably enhancing the signal-to-noise ratio. REFERENCES 1. A. E. Costley, Proc. Course and Workshop ‘Diagnosticsfor Fusion Reactor Conditions’, EUR 8351-l EN, 1, 167. Varenna (1982). 2. M. Bornatici, R. Cano, 0. De Barbieri and F. Engelmann, Nucl. Fusion 23, 1153 (1983). 3. F. J. Stauffer, D. A. Boyd, R. C. Cutler, M. Diesso, M. P. McCarthy, J. Montague and R. Rocco, Rev. Sri. Instrum. 59, 2139 (1988). 4. M. Born and E. Wolf, Principles Optics,p. 393. Pergamon Press, Oxford (1964).
of