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Application of avalanche photodiode for soft X-ray pulse-height analyses in the Ht-7 tokamak
Nuclear Instruments and Methods in Physics Research A 488 (2002) 566–571
Application of avalanche photodiode for soft X-ray pulse-height analyses in the Ht-7 tokamak Yuejiang Shi*, Baonian Wan, Liqun Hu, Yanjun Sun, Sheng Liu, Bili Ling Institute of Plasma Physics, Academia Sinica, Hefei, Divide 6, P.O. Box 1126, 230031 Hefei, Anhui, China Received 7 June 2001; received in revised form 4 February 2002; accepted 4 February 2002
Abstract An avalanche photodiode (APD) has been used as soft X-ray energy pulse-height analysis system for the measurement of the electron temperature on the HT-7 tokamak. The experimental results obtained with the APD with its inferior energy resolution show a little difference compared to the conventional high energy-resolution Si (Li) detector. Both numerical analysis and experimental results prove that the APD is good enough for application of the electron temperature measurement in tokamaks. r 2002 Elsevier Science B.V. All rights reserved. PACS: 52.70.La; 02.60.Cb Keywords: APD; PHA; Tokamak
1. Introduction As a useful and conventional diagnostic to measure the electron temperature, the soft X-ray pulse-height analysis (PHA) system [1] has been installed on many tokamaks [2–5]. The lithiumdrifted silicon Si (Li) detector is generally used for the PHA system due to its high energy-resolution and absorption efficiency. However, the large volume and frequent maintenance of the LN cooled Si (Li) detector limits the application in spatial multi-channel configurations. Compact and cheap room-temperature detectors are wanted for this purpose due to the limited accessibility to the tokamak machines. Nowadays, several kinds of *Corresponding author. Tel.: +86-551-559-1603; fax: +86551-559-1310. E-mail address: [email protected] (Y. Shi).
room-temperature solid state detectors such as APD [6,7] and cadmium telluride semiconductor (CdTe) [8] have been developed for soft X-ray PHA measurement. The main disadvantage of such room-temperature detectors is their low energy resolution, which is about 10–15% at 5.9 keV, while Si (Li) detector can reach 3% at 5.9 keV. However, the soft X-ray PHA system can be built in the convenient and flexible multichannel configuration in tokamak diagnostics. To evaluate this possibility, a soft X-ray PHA system using APD is built and applied in the HT-7 tokamak to measure the soft X-ray spectrum. To study the uncertainty caused by the finite energy resolution of the detector, the spectra as a function of energy resolution of the detector are derived by numerical analysis in Section 2. The APD PHA system is described in Section 3. The experimental results are given and compared with the
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 0 5 1 3 - 2
Y. Shi et al. / Nuclear Instruments and Methods in Physics Research A 488 (2002) 566–571
measurement of Si (Li) detector and the numerical analysis.
2. Numerical analysis The intensity distribution of soft X-ray photons at the detector from plasma bremsstrahlung and recombination radiation may be simply parameterized as ( 0 EoE0 ; IðEÞ ¼ ð1Þ C expðE=Te Þ E > E0 ; where Te is the electron temperature, E is the Xray photon energy, C is a constant. The electron and ion density, and the ion charge is assumed to be constant. The cutoff energy E0 depends on the thickness of the beryllium entrance windows of the detector, typically E0 ¼ 124 keV. We use the normalized distribution function of photon number, f ðEÞ; that is Z N f ðEÞ dE ¼ 1 ð2Þ E0
and IðEÞ ¼ N0 Ef ðEÞ
ð3Þ
where N0 is the total number of photons in the spectrum. Thus, for bremsstrahlung, ( ðC=EÞ expðE=Te Þ EXE0 ; f ðEÞ ¼ ð4Þ 0 EoE0 ; NðEÞ ¼
Z
567
where n0 is the total number of photons, E is the energy of photons, nðE 0 Þ is the number of photons measured in the energy interval ðE 0 ; E 0 þ DEÞ; sðEÞ represents the energy resolution, i.e., full-width at half-maximum (FWHM), pffiffiffiffiffiffiffiffiffiffiffi FWHM ¼ 2 2 ln 2s: ð8Þ For a given detector, s is a function of photon energy E; spE 1=2 :
ð9Þ
The measured spectrum is the convolution given by Eq. (7). The spectrum of the soft X-ray is measured by a multi-channel analyzer (MCA). In our numerical modeling, the channel width of the PHA system is a tenth of Te ; the cutoff, energy E0 equals the electron temperature Te : Ted is the derived electron temperature from the spectrum including energy resolution effects. Ted is calculated in the energy interval from 2Te to 4Te in our model. The FWHM is the energy resolution at 5.9 keV. Fig. 1 shows the spectra for Te ¼ 1 keV. The spectrum is smeared and particularly distorted in the low energy range. To obtain a correct Te calculation, we can select higher energy region to avoid the serious distortion in low energy range. The relative errors of derived temperature are
EþDE
ðC=EÞ expðE=Te Þ dE
ð5Þ
E
and dP=dE ¼
Z
EþDE
EðC=EÞ expðE=Te Þ dE
ð6Þ
E
where NðEÞ is the number of photons in the energy interval ðE; E þ DEÞ; dP=dE is the power radiated into the photon energy interval (E; E þ DE). However, NðEÞ will be smeared in the measured spectrum due to the finite energy resolution of the detector, which is assumed to be Gaussian Z E 0 þDE n0 ðE 0 EÞ2 0 nðE Þ ¼ pffiffiffiffiffiffi exp dE ð7Þ 2s2 2ps E 0
Fig. 1. Soft X-ray spectra for Te=1 keV from plasma by numerical analysis for different energy resolution of the detection system.
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Y. Shi et al. / Nuclear Instruments and Methods in Physics Research A 488 (2002) 566–571
Fig. 2. Soft X-ray spectra for Te ¼ 5 keV from plasma by numerical analysis for different energy resolution of the detection system.
o10% even when the energy resolution is varied from 3% to 40% at 5.9 keV. Fig. 2 gives the spectra for Te ¼ 5 keV. It can be seen that both the smearing of the spectra and the relative error of derived temperature due to the energy resolution are decreased for the higher electron temperature. These features are very favorable for large tokamak plasmas having higher electron temperature.
3. Expermental instruments and results APDs have been used for the detection of nuclear radiation. Originally, APD were used as optical photodetector coupled to scintillator crystals for gamma ray detection [9,10]. It was found later that APD can also be employed for low energy X-ray spectrometer [6,7]. A number of APDs have been applied on satellite to observe the cosmic X-ray [11]. The APD used in our system is model C30703E made by EG & G Company. This APD with a depletion region of about 100 mm thick is operated with about 400 V biasing. It is sensitive to X-rays of 2–20 keV. Fig. 3 shows the energy spectrum measured by the APD PHA system. The FWHM is about 16% at 5.9 keV with 0.5 ms shaping time.
Fig. 3. The energy spectrum of 55Fe (5.9 keV) measured by the APD PHA system indicates an energy resolution of about 15%.
The Hefei Tokamak 7 (HT-7) machine is a superconducting tokamak [12]. A schematic of the soft X-ray PHA diagnostic installed on the HT-7 tokamak is shown in Fig. 4. The system consists of three liquid-nitrogen-cooled Si (Li) detectors and three 5.0 m long stainless-steel collimating lines. For this experiment one Si (Li) detector is replaced by the APD. Each line includes a valve, bellow units, two apertures and one foil box. The detectors are isolated from the tokamak vacuum vessel by teflon. Each line of sight can be vertically scanned by the bellows unit to view the plasma poloidal cross-section from +0.2a to 0.7a. In our experiment, the sight of the collimating line is aligned in the mid-equatorial plane. Both Si (Li) detector and APD are covered with a 25 mm thick Be window. The absorbing foil is 50 mm thick Be. The total thickness of the Be foil in front of the APD is 75 mm. Fig. 5 shows the detection efficiency for the APD with 75 mm thick Be foil. The correction of the detection efficiency should be taken into account for the calculation of the electron temperature. The cutoff energy of the system due to detector and Be foil is about 2 keV. The electron temperature of HT-7 plasma is in the range of 0.5–1.5 keV. The thermal continuous X-ray spectrum is mainly in the range from 2 to 6 keV. Fig. 6 shows a diagram of the electronics associated with each detector. The linear amplifier is normally operated with a shaping time of 1 ms
Y. Shi et al. / Nuclear Instruments and Methods in Physics Research A 488 (2002) 566–571
569
Fig. 4. Schematic of the soft X-ray PHA systems on the HT-7 tokamak.
Fig. 5. The detection efficiency as a function of photon energy for the APD PHA system with 75 mm thick Be foil. Fig. 7. Time evolution of the electron temperature in an IBW heated plasma discharge.
Fig. 6. Schematic of the electronics associated with the detector.
for the Si (Li) detector and 0.5 ms for the APD. The histogramming memory of the PHA system can store 16 spectra for each detector by operating the ADC with a MCA of 512 channels. The integra-
tion time of each spectrum can be varied according to the pulse length of the plasma discharge. Fig. 7 shows the time evolution of the electron temperature measured by APD and Si (Li) detector in an ion Bernstein wave (IBW) heating plasma. Fig. 8 gives the semilogarithmic spectra of the APD system at two different periods of time for this IBW plasma discharge. The spectra are obtained with an integration time of 70 ms. The electron temperatures are derived from the slope of the continuum for energies between 2 and 3.5 keV. The photons with energy >3.5 keV were not included in the calculation because of limited statistics. The electron temperatures measured by the APD are 10–20% higher than that measured
570
Y. Shi et al. / Nuclear Instruments and Methods in Physics Research A 488 (2002) 566–571
Fig. 8. Measured spectra by the APD PHA system before and during IBW pulse.
by the Si (Li) detector. According to the numerical modeling, the relative error caused by the energy resolution is o10% at Te B1 keV. The discrepancy may be caused by two reasons. One is due to the collimating system. The line of sight of the APD system has a longer path across the plasma in the poloidal cross-section than that of the Si (Li) detection system. The other reason is due to the difference of electronics associated with detectors. The electronics of Si (Li) detector have the capability of pileup rejection, while the electronics of APD cannot reject pileup pulse. The pileup effect will smear the spectrum and cause overestimation of the electron temperature. Fig. 9 gives the basic characteristics of this plasma discharge. Radio frequency power of 250 kW in IBW heating mode was injected into the plasma between 300 and 600 ms. The IBW caused an increase of electron temperature measured both by APD and by Si (Li) detector in Fig. 7. The electron density and the visible bremsstrahlung radiation increased little during the IBW pulse. Therefore, the remarkable increase of the soft X-ray emission intensity (Fig. 9d) is due to an increase of electron temperature during the IBW pulse. The measurement by Thomson scattering also shows an increase of the electron temperature during the IBW pulse. The electron temperatures measured by Thomson scattering are
Fig. 9. The waveform of plasma discharge. (a) Plasma current, (b) loop voltage, (c) power of IBW, (d) intensity of soft X-ray emission, (e) central line averaged electron density, (f) intensity of visible bremsstrahlung emission.
about 0.5 keV in the OH target plasma and 0.8– 1.2 keV during the IBW heating, in good agreement with the measurement of APD PHA system.
4. Conclusions An avalanche photodiode (APD) with a low energy resolution compared to the Si (Li) detector has been successfully applied to soft X-ray PHA system for the measurement of the electron temperature on the HT-7 tokamak. The experimental results show only little difference between the APD and the conventional high energyresolution Si (Li) detector. Both numerical analysis and experimental results prove that APD is suitable in tokamak plasma diagnostics to measure the electron temperature. The features of its small size and room-temperature operation are of great advantage for highly spatially resolved multichannel arrangement, which is important and useful for study of tokamak plasma.
References [1] I.H. Hutchinson, in: Principles of plasma diagnostics, Cambridge University Press, Cambridge, 1987, p. 175.
Y. Shi et al. / Nuclear Instruments and Methods in Physics Research A 488 (2002) 566–571 [2] Y.F. Liang, X.J. Yin, Z.S. Fang, R.Q. Liang, Fusion Eng Des 34 (1997) 201. [3] S. Von Goeler, W. Stodiek, H. Eubank, H. Fishman, S. Grebenshchikov, E. Hinnov, Nucl. Fusion 15 (1975) 301. [4] E.H. Silver, M. Bitter, K. Brau, D. Eames, A. Greenberger, K.W. Hill, D.M. Meade, W. Roney, N.R. Sauthoff, S. Von Goeler, Rev. Sci. Instrum. 53 (1982) 1198. [5] D. Pasini, R.D. Gill, J. Holm, E. van der Goot, Rev. Sci. Instrum. 59 (1988) 693. [6] M.R. Squillante, R. Farrell, J.C. Lund, F. Sinclair, G. Entine, K.R. Keller, IEEE Trans. Nucl. Sci. 33 (1986) 336. [7] R. Farrell, K. Vanderpuye, G. Entine, M.R. Squillante, IEEE Trans. Nucl. Sci. 38 (1991) 144.
571
[8] A.J. Dabrowski, J. Iwanczyk, W.M. Szymczyk, Nucl. Instr. and Meth. 150 (1978) 25. [9] M.R. Squillante, G. Reiff, G. Entine, IEEE Trans. Nucl. Sci. NS-32 (1985) 563. [10] R. Farrell, F. Olschner, E. Frederick, L. McConchie, K. Vanderpuyr, M.R. Squillante, G. Entine, Nucl. Instr. and Meth. A 288 (1990) 137. [11] S.P. Lynch, D.E. Geary, D.J. Forrest, IEEE Trans. Nucl. Sci. 44 (1997) 581. [12] J.K. Xie, HT-7 group, in: Plasma Physics and Controlled Nuclear Fusion Research, 1996 (Proceedings of the 16th International Conference, Montreal, 1996), Vol. 1, IAEA, Vienna, 1997, p. 685.
Application of avalanche photodiode for soft X-ray pulse-height analyses in the Ht-7 tokamak Yuejiang Shi*, Baonian Wan, Liqun Hu, Yanjun Sun, Sheng Liu, Bili Ling Institute of Plasma Physics, Academia Sinica, Hefei, Divide 6, P.O. Box 1126, 230031 Hefei, Anhui, China Received 7 June 2001; received in revised form 4 February 2002; accepted 4 February 2002
Abstract An avalanche photodiode (APD) has been used as soft X-ray energy pulse-height analysis system for the measurement of the electron temperature on the HT-7 tokamak. The experimental results obtained with the APD with its inferior energy resolution show a little difference compared to the conventional high energy-resolution Si (Li) detector. Both numerical analysis and experimental results prove that the APD is good enough for application of the electron temperature measurement in tokamaks. r 2002 Elsevier Science B.V. All rights reserved. PACS: 52.70.La; 02.60.Cb Keywords: APD; PHA; Tokamak
1. Introduction As a useful and conventional diagnostic to measure the electron temperature, the soft X-ray pulse-height analysis (PHA) system [1] has been installed on many tokamaks [2–5]. The lithiumdrifted silicon Si (Li) detector is generally used for the PHA system due to its high energy-resolution and absorption efficiency. However, the large volume and frequent maintenance of the LN cooled Si (Li) detector limits the application in spatial multi-channel configurations. Compact and cheap room-temperature detectors are wanted for this purpose due to the limited accessibility to the tokamak machines. Nowadays, several kinds of *Corresponding author. Tel.: +86-551-559-1603; fax: +86551-559-1310. E-mail address: [email protected] (Y. Shi).
room-temperature solid state detectors such as APD [6,7] and cadmium telluride semiconductor (CdTe) [8] have been developed for soft X-ray PHA measurement. The main disadvantage of such room-temperature detectors is their low energy resolution, which is about 10–15% at 5.9 keV, while Si (Li) detector can reach 3% at 5.9 keV. However, the soft X-ray PHA system can be built in the convenient and flexible multichannel configuration in tokamak diagnostics. To evaluate this possibility, a soft X-ray PHA system using APD is built and applied in the HT-7 tokamak to measure the soft X-ray spectrum. To study the uncertainty caused by the finite energy resolution of the detector, the spectra as a function of energy resolution of the detector are derived by numerical analysis in Section 2. The APD PHA system is described in Section 3. The experimental results are given and compared with the
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 0 5 1 3 - 2
Y. Shi et al. / Nuclear Instruments and Methods in Physics Research A 488 (2002) 566–571
measurement of Si (Li) detector and the numerical analysis.
2. Numerical analysis The intensity distribution of soft X-ray photons at the detector from plasma bremsstrahlung and recombination radiation may be simply parameterized as ( 0 EoE0 ; IðEÞ ¼ ð1Þ C expðE=Te Þ E > E0 ; where Te is the electron temperature, E is the Xray photon energy, C is a constant. The electron and ion density, and the ion charge is assumed to be constant. The cutoff energy E0 depends on the thickness of the beryllium entrance windows of the detector, typically E0 ¼ 124 keV. We use the normalized distribution function of photon number, f ðEÞ; that is Z N f ðEÞ dE ¼ 1 ð2Þ E0
and IðEÞ ¼ N0 Ef ðEÞ
ð3Þ
where N0 is the total number of photons in the spectrum. Thus, for bremsstrahlung, ( ðC=EÞ expðE=Te Þ EXE0 ; f ðEÞ ¼ ð4Þ 0 EoE0 ; NðEÞ ¼
Z
567
where n0 is the total number of photons, E is the energy of photons, nðE 0 Þ is the number of photons measured in the energy interval ðE 0 ; E 0 þ DEÞ; sðEÞ represents the energy resolution, i.e., full-width at half-maximum (FWHM), pffiffiffiffiffiffiffiffiffiffiffi FWHM ¼ 2 2 ln 2s: ð8Þ For a given detector, s is a function of photon energy E; spE 1=2 :
ð9Þ
The measured spectrum is the convolution given by Eq. (7). The spectrum of the soft X-ray is measured by a multi-channel analyzer (MCA). In our numerical modeling, the channel width of the PHA system is a tenth of Te ; the cutoff, energy E0 equals the electron temperature Te : Ted is the derived electron temperature from the spectrum including energy resolution effects. Ted is calculated in the energy interval from 2Te to 4Te in our model. The FWHM is the energy resolution at 5.9 keV. Fig. 1 shows the spectra for Te ¼ 1 keV. The spectrum is smeared and particularly distorted in the low energy range. To obtain a correct Te calculation, we can select higher energy region to avoid the serious distortion in low energy range. The relative errors of derived temperature are
EþDE
ðC=EÞ expðE=Te Þ dE
ð5Þ
E
and dP=dE ¼
Z
EþDE
EðC=EÞ expðE=Te Þ dE
ð6Þ
E
where NðEÞ is the number of photons in the energy interval ðE; E þ DEÞ; dP=dE is the power radiated into the photon energy interval (E; E þ DE). However, NðEÞ will be smeared in the measured spectrum due to the finite energy resolution of the detector, which is assumed to be Gaussian Z E 0 þDE n0 ðE 0 EÞ2 0 nðE Þ ¼ pffiffiffiffiffiffi exp dE ð7Þ 2s2 2ps E 0
Fig. 1. Soft X-ray spectra for Te=1 keV from plasma by numerical analysis for different energy resolution of the detection system.
568
Y. Shi et al. / Nuclear Instruments and Methods in Physics Research A 488 (2002) 566–571
Fig. 2. Soft X-ray spectra for Te ¼ 5 keV from plasma by numerical analysis for different energy resolution of the detection system.
o10% even when the energy resolution is varied from 3% to 40% at 5.9 keV. Fig. 2 gives the spectra for Te ¼ 5 keV. It can be seen that both the smearing of the spectra and the relative error of derived temperature due to the energy resolution are decreased for the higher electron temperature. These features are very favorable for large tokamak plasmas having higher electron temperature.
3. Expermental instruments and results APDs have been used for the detection of nuclear radiation. Originally, APD were used as optical photodetector coupled to scintillator crystals for gamma ray detection [9,10]. It was found later that APD can also be employed for low energy X-ray spectrometer [6,7]. A number of APDs have been applied on satellite to observe the cosmic X-ray [11]. The APD used in our system is model C30703E made by EG & G Company. This APD with a depletion region of about 100 mm thick is operated with about 400 V biasing. It is sensitive to X-rays of 2–20 keV. Fig. 3 shows the energy spectrum measured by the APD PHA system. The FWHM is about 16% at 5.9 keV with 0.5 ms shaping time.
Fig. 3. The energy spectrum of 55Fe (5.9 keV) measured by the APD PHA system indicates an energy resolution of about 15%.
The Hefei Tokamak 7 (HT-7) machine is a superconducting tokamak [12]. A schematic of the soft X-ray PHA diagnostic installed on the HT-7 tokamak is shown in Fig. 4. The system consists of three liquid-nitrogen-cooled Si (Li) detectors and three 5.0 m long stainless-steel collimating lines. For this experiment one Si (Li) detector is replaced by the APD. Each line includes a valve, bellow units, two apertures and one foil box. The detectors are isolated from the tokamak vacuum vessel by teflon. Each line of sight can be vertically scanned by the bellows unit to view the plasma poloidal cross-section from +0.2a to 0.7a. In our experiment, the sight of the collimating line is aligned in the mid-equatorial plane. Both Si (Li) detector and APD are covered with a 25 mm thick Be window. The absorbing foil is 50 mm thick Be. The total thickness of the Be foil in front of the APD is 75 mm. Fig. 5 shows the detection efficiency for the APD with 75 mm thick Be foil. The correction of the detection efficiency should be taken into account for the calculation of the electron temperature. The cutoff energy of the system due to detector and Be foil is about 2 keV. The electron temperature of HT-7 plasma is in the range of 0.5–1.5 keV. The thermal continuous X-ray spectrum is mainly in the range from 2 to 6 keV. Fig. 6 shows a diagram of the electronics associated with each detector. The linear amplifier is normally operated with a shaping time of 1 ms
Y. Shi et al. / Nuclear Instruments and Methods in Physics Research A 488 (2002) 566–571
569
Fig. 4. Schematic of the soft X-ray PHA systems on the HT-7 tokamak.
Fig. 5. The detection efficiency as a function of photon energy for the APD PHA system with 75 mm thick Be foil. Fig. 7. Time evolution of the electron temperature in an IBW heated plasma discharge.
Fig. 6. Schematic of the electronics associated with the detector.
for the Si (Li) detector and 0.5 ms for the APD. The histogramming memory of the PHA system can store 16 spectra for each detector by operating the ADC with a MCA of 512 channels. The integra-
tion time of each spectrum can be varied according to the pulse length of the plasma discharge. Fig. 7 shows the time evolution of the electron temperature measured by APD and Si (Li) detector in an ion Bernstein wave (IBW) heating plasma. Fig. 8 gives the semilogarithmic spectra of the APD system at two different periods of time for this IBW plasma discharge. The spectra are obtained with an integration time of 70 ms. The electron temperatures are derived from the slope of the continuum for energies between 2 and 3.5 keV. The photons with energy >3.5 keV were not included in the calculation because of limited statistics. The electron temperatures measured by the APD are 10–20% higher than that measured
570
Y. Shi et al. / Nuclear Instruments and Methods in Physics Research A 488 (2002) 566–571
Fig. 8. Measured spectra by the APD PHA system before and during IBW pulse.
by the Si (Li) detector. According to the numerical modeling, the relative error caused by the energy resolution is o10% at Te B1 keV. The discrepancy may be caused by two reasons. One is due to the collimating system. The line of sight of the APD system has a longer path across the plasma in the poloidal cross-section than that of the Si (Li) detection system. The other reason is due to the difference of electronics associated with detectors. The electronics of Si (Li) detector have the capability of pileup rejection, while the electronics of APD cannot reject pileup pulse. The pileup effect will smear the spectrum and cause overestimation of the electron temperature. Fig. 9 gives the basic characteristics of this plasma discharge. Radio frequency power of 250 kW in IBW heating mode was injected into the plasma between 300 and 600 ms. The IBW caused an increase of electron temperature measured both by APD and by Si (Li) detector in Fig. 7. The electron density and the visible bremsstrahlung radiation increased little during the IBW pulse. Therefore, the remarkable increase of the soft X-ray emission intensity (Fig. 9d) is due to an increase of electron temperature during the IBW pulse. The measurement by Thomson scattering also shows an increase of the electron temperature during the IBW pulse. The electron temperatures measured by Thomson scattering are
Fig. 9. The waveform of plasma discharge. (a) Plasma current, (b) loop voltage, (c) power of IBW, (d) intensity of soft X-ray emission, (e) central line averaged electron density, (f) intensity of visible bremsstrahlung emission.
about 0.5 keV in the OH target plasma and 0.8– 1.2 keV during the IBW heating, in good agreement with the measurement of APD PHA system.
4. Conclusions An avalanche photodiode (APD) with a low energy resolution compared to the Si (Li) detector has been successfully applied to soft X-ray PHA system for the measurement of the electron temperature on the HT-7 tokamak. The experimental results show only little difference between the APD and the conventional high energyresolution Si (Li) detector. Both numerical analysis and experimental results prove that APD is suitable in tokamak plasma diagnostics to measure the electron temperature. The features of its small size and room-temperature operation are of great advantage for highly spatially resolved multichannel arrangement, which is important and useful for study of tokamak plasma.
References [1] I.H. Hutchinson, in: Principles of plasma diagnostics, Cambridge University Press, Cambridge, 1987, p. 175.
Y. Shi et al. / Nuclear Instruments and Methods in Physics Research A 488 (2002) 566–571 [2] Y.F. Liang, X.J. Yin, Z.S. Fang, R.Q. Liang, Fusion Eng Des 34 (1997) 201. [3] S. Von Goeler, W. Stodiek, H. Eubank, H. Fishman, S. Grebenshchikov, E. Hinnov, Nucl. Fusion 15 (1975) 301. [4] E.H. Silver, M. Bitter, K. Brau, D. Eames, A. Greenberger, K.W. Hill, D.M. Meade, W. Roney, N.R. Sauthoff, S. Von Goeler, Rev. Sci. Instrum. 53 (1982) 1198. [5] D. Pasini, R.D. Gill, J. Holm, E. van der Goot, Rev. Sci. Instrum. 59 (1988) 693. [6] M.R. Squillante, R. Farrell, J.C. Lund, F. Sinclair, G. Entine, K.R. Keller, IEEE Trans. Nucl. Sci. 33 (1986) 336. [7] R. Farrell, K. Vanderpuye, G. Entine, M.R. Squillante, IEEE Trans. Nucl. Sci. 38 (1991) 144.
571
[8] A.J. Dabrowski, J. Iwanczyk, W.M. Szymczyk, Nucl. Instr. and Meth. 150 (1978) 25. [9] M.R. Squillante, G. Reiff, G. Entine, IEEE Trans. Nucl. Sci. NS-32 (1985) 563. [10] R. Farrell, F. Olschner, E. Frederick, L. McConchie, K. Vanderpuyr, M.R. Squillante, G. Entine, Nucl. Instr. and Meth. A 288 (1990) 137. [11] S.P. Lynch, D.E. Geary, D.J. Forrest, IEEE Trans. Nucl. Sci. 44 (1997) 581. [12] J.K. Xie, HT-7 group, in: Plasma Physics and Controlled Nuclear Fusion Research, 1996 (Proceedings of the 16th International Conference, Montreal, 1996), Vol. 1, IAEA, Vienna, 1997, p. 685.