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An infrared thermometer for tokamak limiter surface temperature measurements
91
Journal of Nuclear Materials 111& 112 (1982) 91-94 North-Holland Publishing Company
AN INFRARED THERMOMETER MEASUREMENTS M. ULRICKSON Princeton
FOR TOKAMAK
LIMITER SURFACE
TEMPERATURE
and G.G. PEARSON
University, Plasma Physics Laboratory
Princeton,
UJ 08544, USA
1. Introduction Assessment of the performance of a tokamak limiter depends strongly on the thermal loads the plasma deposits on the limiter during the discharge. The best method for determining the thermal load is to measure the limiter surface temperature variation during a pulse. The environment surrounding a limiter together with the large electrical potectial at which the limiter operates, require the use of non-contact methods for measuring the surface temperature. Determination of the infrared emission from the limiter surface is a non-contact method which can determine the limiter surface temperature. In fact, infrared emission has been used to measure limiter temperatures on Doublet III [I], ISX [2], and TFR [3]. In all these measurements the limitations on the temporal and/or spatial resolution are such that a two dimensional profile is limited to a time respone of about 30 ms whereas a point or one dimensional profile can have a time response of about 10 ps. Since the two dimensional profile of temperature on the surface of a limiter provides very useful information about the edge properties of the plasma (e.g., scrape-off thickness, and edge transport coefficients), it is desirable to obtain this information with as great a temporal resolution as possible. This paper describes the design and calibration of an infrared device which measures a two dimensional temperature profile with a time response of about 5 ms.
2. Design of the instniment The instrument being described is intended for use primarily on the TFTR tokamak. This severely restricts the choice of optical materials because of radiation damage induced by the neutron flux from TFTR operation. The most stable optical material has been found to be fused quartz [4] with crown and flint glas also being acceptable. The choice of these materials limits the longest usable wavelength to Q 3 pm. The temperature
0022-3 115/82/0000-0000/$02.75
0 1982 North-Holland
range of interest is 100°C to 36OO’C. The lower limit of this range dictates the use of a detector having a large responsivity at wavelengths close to the long wavelength limit of the optics. This leads to the choice of PbS detectors which have maximum responsivity between 2.0 and 2.5 pm. The temperature range of interest also implies that the input signal covers a dynamic range of a little over 6 orders of magnitude. This results in the use of three temperature ranges which are realized through a combination of amplifier gain, optical attenuation, and analogue-to-digital converter dynamic range. Another aspect which determines the basic structure of the device is the desire to do two color pyrometry. If the infrared emissivity of an object is independent of wavelength in a region of interest then the true temperature of the object can be determined solely from the ratio of the radiance at two different wavelengths. Based on the response curves for PbS detectors, available filters, and the position of atmospheric absorpton bands, the wavelength bands chosen were h, = 1.288 pm (band width 0.234 pm) and X,=2.284 pm (band width 0.47 pm). The limiter surface will be either Tic or C in TFTR. The emissivity is nearly constant with wavelength for these two materials over the range of X, and X,. In actual tokamak experiments, however, the emissivity may be modified by surface deposition or erosion and make this assumption invalid. Whatever the results of tokamak experiments show, the two color system has an inherent advantage over a one color system. The final part of the general design is the need to be able to aim the instrument using visible light optics since the application of such devices to an actual tokamak is always complicated by the presence of other diagnostic devices. This is accomplished by using a cold mirror to deflect the visible light out of the infrared optical path. The above choices fully determine the parameters of the remainder of the device. A schematic of the instrument is shown in fig. 1. The imaging and sighting optics consist of standard commercial optical components based on crown and flint
M. Ulrickson, G. G. Pearson / An vfrared
92
thermometer # 82POO87
IR Light \
m SOUI
Imaging Opt its
ce
Optical Chopper
.
-
Ftertrirnl _ __ __.
Mirror co:
\I5
’ Visible Vik*
T--G&Y ;
7
Sighting and Alignment Optics
I I
./
I Phase Sensitive Detectors
of the thermometer.
h
J
Bandpass Filter and Opt icol Attenuator
Reference
Fig. 1. A schematic
r-
S,li.,u
Array
ADC
PlPnmns .__...
Signal
I
Wave length
Compensation Modules
See text for details.
glass. The cold mirror transmits (transmission >0.8) from about 0.9 pm to about 3 pm. The optical chopper is required to increase the signal-to-noise ratio in 100°C to 3OO’C temperature range. The optical attenuator (an infrared neutral density filter having lop3 transmission) prevents saturation of the PbS detector at source temperatures in excess of about 1500°C. Both the wavelength filters and the attenuator are remotely selectable. Neither can be changed rapidly enough to permit either two color or full temperature range (100” - 3000°C) measurements during a tokamak pulse. The PbS detectors are arranged in a 5 X 5 array with four additional detectors on the horizontal and vertical axes for a total of 29 detectors. The active area of the detectors is 1 mm X 1 mm square. The light pipes are necessary to expand the optical image because of the physical size of the detector housings. The light pipes consist of 1.5 mm diameter quartz rods which are bent to the desired shape by heating to their softening point
on a graphite mold. The center-to-center spacing of the light pipes in the image plane is 2.54 mm both horizontally and vertically. The detectors are attached to a thermoelectrically cooled plate which is maintained at 15 f O.l”C. No significant reduction in noise is observed on cooling the detectors to -5°C. The choice of 15°C avoids problems with water condensation in the tokamak environment. The preamplifiers consist of JFET operational amplifiers which have remotely selectable gain (gain = 30 and 2.3). The preamps are in the optical detector unit. The preamps are followed by a balanced line driver which sends the detector output over a 30 meter cable to the remainder of the electronics. The compensation modules correct for emissivity of the object (range 0.1 to 1.0) and the different gain characteristics of the PbS detectors at the two wavelengths. The phase sensitive detectors are Evans Associates Model 4110 units which have been modified to increase the width of the output
93
hf. Ulrickson, G.G. Pearson / An infrared thermometer
bandpass filter to achieve the desired response time of - 5 ms. The processed signals are digitized in a LeCroy model 8212 Data Logger. This device has 12 bits and hence 2.44 mV resolution in its O-10 V range. The control and interface unit allows remote selection of wavelength, attenuator, and preamp gain as well as generating the emissivity signal. The temperature ranges which may be selected are 3OO’C (covers 100°C to 4OO”Q 2000°C (covers 3OO’C to 2OOO”C), and 3OOO’C (covers 1700°C to about 3600°C).
channel sees a spot of about 12 mm diameter. There is no measurable cross-talk between channels. The instrument has been calibrated on both wavelength ranges using two blackbody sources. One blackbody covers the range 1OO’C to 1000°C and the other 800°C to 2500°C. The calibration is not yet complete above 2000°C. A typical calibration curve for one of the channels ranges is shown in fig. 2. The variation of the signal level agrees well with the variation predicted from the integration of the Planck function M( h)dX = rC,h-5(ec~/“‘-
3. Operation and calibration
(E = emissivity, The time respone of the instrument was measured using a step function input having a rise time of < 10 ps. With this input the output was observed to have a rise time (10% to 90%) of 7 ms with a corresponding e-folding time of 3.5 ms. The difference between this time constant and the desired value of 5 ms is due solely to the use of standard values of resistors and capacitors in the modification of the phase sensitive detector boards. The output noise level is 7 mV (RMS) with this output band with on the highest gain range. At a working distance of 1.7m the channel-to-channel separation on the object is about 15 mm, and each
l),
C, = 3.7403 X lOI PW pm4/m2,
C, = 14384 pm K) over the bandpass of the wavelength filters. In the range from 100°C to 1OOO’C the calibration blackbody has a stability of the greater of kO.l% of temperature or -‘0.5”C over one hour and a setability of 20.1 ‘C. With this blackbody the instrument has exhibited a repeatability of *2’C and a short term drift consistent with the stability of the blackbody. In the temperature range above 1000°C the second blackbody is used. That source can only be set to an accuracy of *2% and exhibits a stability of -5°C per hour. This poor performance is due to the controller used for the blackbody. A new controller is being fabricated which should remedy this situation. At this time the accuracy of the thermometer above 1000°C is *2% due strictly to the blackbody source used. This level of accuracy is already adequate for tokamak experiments.
4. Conclusions A device has been constructed which achieves about 7 ms time response for taking two dimensional limiter surface temperatures in the range IOO’C to 3000°C. The instrument is capable of two color measurements which permits in situ determination of the infrared emissivity under the restrictions discussed above. The accuracy and temporal response are adequate for tokamak experiments.
lo-’
0
500
1500
1000 TEMPERATURE
2000
(Cl
Fig. 2. Typical calibration curve for one of the detectors on all three of the temperature ranges up to 2000°C. The curves are identified as follows: (a) A,, 3OO’C range; (b) A,, 300°C range; (c) A,, 2000°C range; (d) A,, 2000°C range; (e) A,, 3000°C range; and (f) A,, 3000°C range.
Acknowledgments This work was supported by US DOE Contract No. DE-AC02 76 CH03073. The authors wish to thank F. Egan and A.J. Newton for their assistance in the construction of this device.
94
M. Wickson.
G. G. Pearson
References [I] T. Taylor, N.H. Brooks, T.R. McMahon, J.F. Pipkins. K. loki, BAPS 26 (1981) 876. [2] C.E. Bush, O.L. Foster, H.E. Ketterer, D.R. Overbey. BAPS 25 (1980) 975.
/ An infrared [3] TFR
thermomerer
Group Report EUR-CEA-FC-1114, Association EURATOM-CEA Sur la Fusion, Fontenay-aux-Roses (1981) J. Nucl. Mater, submitted. [4] K.M. Young, PPPL Report No. 1859.
Journal of Nuclear Materials 111& 112 (1982) 91-94 North-Holland Publishing Company
AN INFRARED THERMOMETER MEASUREMENTS M. ULRICKSON Princeton
FOR TOKAMAK
LIMITER SURFACE
TEMPERATURE
and G.G. PEARSON
University, Plasma Physics Laboratory
Princeton,
UJ 08544, USA
1. Introduction Assessment of the performance of a tokamak limiter depends strongly on the thermal loads the plasma deposits on the limiter during the discharge. The best method for determining the thermal load is to measure the limiter surface temperature variation during a pulse. The environment surrounding a limiter together with the large electrical potectial at which the limiter operates, require the use of non-contact methods for measuring the surface temperature. Determination of the infrared emission from the limiter surface is a non-contact method which can determine the limiter surface temperature. In fact, infrared emission has been used to measure limiter temperatures on Doublet III [I], ISX [2], and TFR [3]. In all these measurements the limitations on the temporal and/or spatial resolution are such that a two dimensional profile is limited to a time respone of about 30 ms whereas a point or one dimensional profile can have a time response of about 10 ps. Since the two dimensional profile of temperature on the surface of a limiter provides very useful information about the edge properties of the plasma (e.g., scrape-off thickness, and edge transport coefficients), it is desirable to obtain this information with as great a temporal resolution as possible. This paper describes the design and calibration of an infrared device which measures a two dimensional temperature profile with a time response of about 5 ms.
2. Design of the instniment The instrument being described is intended for use primarily on the TFTR tokamak. This severely restricts the choice of optical materials because of radiation damage induced by the neutron flux from TFTR operation. The most stable optical material has been found to be fused quartz [4] with crown and flint glas also being acceptable. The choice of these materials limits the longest usable wavelength to Q 3 pm. The temperature
0022-3 115/82/0000-0000/$02.75
0 1982 North-Holland
range of interest is 100°C to 36OO’C. The lower limit of this range dictates the use of a detector having a large responsivity at wavelengths close to the long wavelength limit of the optics. This leads to the choice of PbS detectors which have maximum responsivity between 2.0 and 2.5 pm. The temperature range of interest also implies that the input signal covers a dynamic range of a little over 6 orders of magnitude. This results in the use of three temperature ranges which are realized through a combination of amplifier gain, optical attenuation, and analogue-to-digital converter dynamic range. Another aspect which determines the basic structure of the device is the desire to do two color pyrometry. If the infrared emissivity of an object is independent of wavelength in a region of interest then the true temperature of the object can be determined solely from the ratio of the radiance at two different wavelengths. Based on the response curves for PbS detectors, available filters, and the position of atmospheric absorpton bands, the wavelength bands chosen were h, = 1.288 pm (band width 0.234 pm) and X,=2.284 pm (band width 0.47 pm). The limiter surface will be either Tic or C in TFTR. The emissivity is nearly constant with wavelength for these two materials over the range of X, and X,. In actual tokamak experiments, however, the emissivity may be modified by surface deposition or erosion and make this assumption invalid. Whatever the results of tokamak experiments show, the two color system has an inherent advantage over a one color system. The final part of the general design is the need to be able to aim the instrument using visible light optics since the application of such devices to an actual tokamak is always complicated by the presence of other diagnostic devices. This is accomplished by using a cold mirror to deflect the visible light out of the infrared optical path. The above choices fully determine the parameters of the remainder of the device. A schematic of the instrument is shown in fig. 1. The imaging and sighting optics consist of standard commercial optical components based on crown and flint
M. Ulrickson, G. G. Pearson / An vfrared
92
thermometer # 82POO87
IR Light \
m SOUI
Imaging Opt its
ce
Optical Chopper
.
-
Ftertrirnl _ __ __.
Mirror co:
\I5
’ Visible Vik*
T--G&Y ;
7
Sighting and Alignment Optics
I I
./
I Phase Sensitive Detectors
of the thermometer.
h
J
Bandpass Filter and Opt icol Attenuator
Reference
Fig. 1. A schematic
r-
S,li.,u
Array
ADC
PlPnmns .__...
Signal
I
Wave length
Compensation Modules
See text for details.
glass. The cold mirror transmits (transmission >0.8) from about 0.9 pm to about 3 pm. The optical chopper is required to increase the signal-to-noise ratio in 100°C to 3OO’C temperature range. The optical attenuator (an infrared neutral density filter having lop3 transmission) prevents saturation of the PbS detector at source temperatures in excess of about 1500°C. Both the wavelength filters and the attenuator are remotely selectable. Neither can be changed rapidly enough to permit either two color or full temperature range (100” - 3000°C) measurements during a tokamak pulse. The PbS detectors are arranged in a 5 X 5 array with four additional detectors on the horizontal and vertical axes for a total of 29 detectors. The active area of the detectors is 1 mm X 1 mm square. The light pipes are necessary to expand the optical image because of the physical size of the detector housings. The light pipes consist of 1.5 mm diameter quartz rods which are bent to the desired shape by heating to their softening point
on a graphite mold. The center-to-center spacing of the light pipes in the image plane is 2.54 mm both horizontally and vertically. The detectors are attached to a thermoelectrically cooled plate which is maintained at 15 f O.l”C. No significant reduction in noise is observed on cooling the detectors to -5°C. The choice of 15°C avoids problems with water condensation in the tokamak environment. The preamplifiers consist of JFET operational amplifiers which have remotely selectable gain (gain = 30 and 2.3). The preamps are in the optical detector unit. The preamps are followed by a balanced line driver which sends the detector output over a 30 meter cable to the remainder of the electronics. The compensation modules correct for emissivity of the object (range 0.1 to 1.0) and the different gain characteristics of the PbS detectors at the two wavelengths. The phase sensitive detectors are Evans Associates Model 4110 units which have been modified to increase the width of the output
93
hf. Ulrickson, G.G. Pearson / An infrared thermometer
bandpass filter to achieve the desired response time of - 5 ms. The processed signals are digitized in a LeCroy model 8212 Data Logger. This device has 12 bits and hence 2.44 mV resolution in its O-10 V range. The control and interface unit allows remote selection of wavelength, attenuator, and preamp gain as well as generating the emissivity signal. The temperature ranges which may be selected are 3OO’C (covers 100°C to 4OO”Q 2000°C (covers 3OO’C to 2OOO”C), and 3OOO’C (covers 1700°C to about 3600°C).
channel sees a spot of about 12 mm diameter. There is no measurable cross-talk between channels. The instrument has been calibrated on both wavelength ranges using two blackbody sources. One blackbody covers the range 1OO’C to 1000°C and the other 800°C to 2500°C. The calibration is not yet complete above 2000°C. A typical calibration curve for one of the channels ranges is shown in fig. 2. The variation of the signal level agrees well with the variation predicted from the integration of the Planck function M( h)dX = rC,h-5(ec~/“‘-
3. Operation and calibration
(E = emissivity, The time respone of the instrument was measured using a step function input having a rise time of < 10 ps. With this input the output was observed to have a rise time (10% to 90%) of 7 ms with a corresponding e-folding time of 3.5 ms. The difference between this time constant and the desired value of 5 ms is due solely to the use of standard values of resistors and capacitors in the modification of the phase sensitive detector boards. The output noise level is 7 mV (RMS) with this output band with on the highest gain range. At a working distance of 1.7m the channel-to-channel separation on the object is about 15 mm, and each
l),
C, = 3.7403 X lOI PW pm4/m2,
C, = 14384 pm K) over the bandpass of the wavelength filters. In the range from 100°C to 1OOO’C the calibration blackbody has a stability of the greater of kO.l% of temperature or -‘0.5”C over one hour and a setability of 20.1 ‘C. With this blackbody the instrument has exhibited a repeatability of *2’C and a short term drift consistent with the stability of the blackbody. In the temperature range above 1000°C the second blackbody is used. That source can only be set to an accuracy of *2% and exhibits a stability of -5°C per hour. This poor performance is due to the controller used for the blackbody. A new controller is being fabricated which should remedy this situation. At this time the accuracy of the thermometer above 1000°C is *2% due strictly to the blackbody source used. This level of accuracy is already adequate for tokamak experiments.
4. Conclusions A device has been constructed which achieves about 7 ms time response for taking two dimensional limiter surface temperatures in the range IOO’C to 3000°C. The instrument is capable of two color measurements which permits in situ determination of the infrared emissivity under the restrictions discussed above. The accuracy and temporal response are adequate for tokamak experiments.
lo-’
0
500
1500
1000 TEMPERATURE
2000
(Cl
Fig. 2. Typical calibration curve for one of the detectors on all three of the temperature ranges up to 2000°C. The curves are identified as follows: (a) A,, 3OO’C range; (b) A,, 300°C range; (c) A,, 2000°C range; (d) A,, 2000°C range; (e) A,, 3000°C range; and (f) A,, 3000°C range.
Acknowledgments This work was supported by US DOE Contract No. DE-AC02 76 CH03073. The authors wish to thank F. Egan and A.J. Newton for their assistance in the construction of this device.
94
M. Wickson.
G. G. Pearson
References [I] T. Taylor, N.H. Brooks, T.R. McMahon, J.F. Pipkins. K. loki, BAPS 26 (1981) 876. [2] C.E. Bush, O.L. Foster, H.E. Ketterer, D.R. Overbey. BAPS 25 (1980) 975.
/ An infrared [3] TFR
thermomerer
Group Report EUR-CEA-FC-1114, Association EURATOM-CEA Sur la Fusion, Fontenay-aux-Roses (1981) J. Nucl. Mater, submitted. [4] K.M. Young, PPPL Report No. 1859.