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Pulse height analysis on the balance voltage and acoustic emission signals for the LHD superconducting coils
Fusion Engineering and Design 81 (2006) 2561–2565
Pulse height analysis on the balance voltage and acoustic emission signals for the LHD superconducting coils N. Yanagi a,∗ , K. Seo a , S. Imagawa a , H. Sekiguchi a , K. Takahata a , S. Yamada a , T. Mito a , T. Ishigohka b , A. Ninomiya b a
National Institute for Fusion Science, Toki, Gifu 509-5292, Japan b Seikei University, Musashino, Tokyo 180-8633, Japan Available online 28 August 2006
Abstract It has been observed in the superconducting helical coils of the Large Helical Device (LHD) that the balance voltage signals measured between the corresponding pairs of the coil blocks contain a number of spike signals during ramp-up and ramp-down processes of excitation. The spike signals might be generated by rapid changes of the self-inductances of the coil windings due to mechanical disturbances caused by large electromagnetic forces. Pulse height analysis (PHA) has been applied to analyze these signals in order to investigate the changes of mechanical properties of the coil windings as the excitation and cooling cycles proceed. In addition, acoustic emission (AE) sensors attached to the helical coil cans are also used to detect mechanical disturbances. © 2006 Elsevier B.V. All rights reserved. Keywords: LHD; Helical coils; Balance voltage; Pulse height analysis; Acoustic emission
1. Introduction During the nine experimental cycles of the Large Helical Device (LHD) [1], 17 times of transient normaltransitions have been observed in the bath-cooled helical coils (major radius 3.9 m, minor radius ∼1 m and toroidal pitch number 10) at current and magnetic field values slightly lower than the specified operation point (current 13 kA, magnetic field 6.9 T at temperature ∗ Corresponding author. Tel.: +81 572 58 2126; fax: +81 572 58 2616. E-mail address: [email protected] (N. Yanagi).
0920-3796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2006.07.010
4.4 K) [2]. It has been confirmed both by conducting R&D coil tests and by observing pick-up coil signals along the helical coils of LHD, that the generated normal-zone propagates with a different propagation speed depending on the longitudinal direction along the conductor [3]. Moreover, in some current ranges, the normal-zone propagates only in one direction and it forms a “traveling normal-zone”. This phenomenon seems to be related with the asymmetrical configuration of the conductor structure, i.e., an NbTi/Cu Rutherfordtype superconducting cable and a pure aluminum stabilizer are situated side by side in a copper jacket. It should also be emphasized that the cryogenic stability
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of this type of superconductor is deteriorated transiently as the magnetic (current) diffusion process takes relatively long time due to the low resistivity of pure aluminum. Thus, the (transient) minimum propagation current becomes lower than the (steady-state) cold-end recovery current. The detailed mechanism for precisely explaining the asymmetrical normal-zone propagation is still under investigation [3]. On the other hand, one also needs to pay attention to the mechanism that initiates a normal-transition itself along with the discussion about the cryogenic stability of the winding conductor. We consider that mechanical disturbances, such as conductor motions, occurring in the coil windings play the key role, as the windings are not impregnated due to the structures of the bathcooled helical coils. During excitation tests, a number of spike signals are generally observed on the balance voltage of the helical coils. By analyzing these signals, it seems possible to extract useful information in terms of the changes of mechanical properties of the coil windings as the excitation and cooling cycles proceed. Acoustic emission (AE) signals are also useful for this purpose.
2. Pulse height analysis of the balance voltage spike signals Fig. 1 shows typical waveforms of the balance voltage signals observed in the innermost blocks (where the magnetic field is the highest) of the helical coils during an excitation test. Here, the balance voltage is obtained by providing the terminal voltage signals of the corresponding pair of the helical coil blocks to a “balanced” circuit so that the inductive voltage can be cancelled in order to observe only the resistive component and to detect a normal-transition. It is clearly seen in Fig. 1 that a number of spike signals are observed both in the rampup and ramp-down phases. They might be generated by rapid changes of the self-inductances of the coil windings due to mechanical disturbances, such as conductor motions, caused by large electromagnetic forces [4]. In order to extract quantitative information with respect to the mechanical properties of the coil windings, the observed spike signals are analyzed by pulse height analysis (PHA). In this method, the peak voltage of each spike signal is evaluated and the number of spikes in each step size of voltage is counted [5].
Fig. 1. Typical waveforms of one of the acoustic emission (AE) signals; the balance voltage signals observed for the innermost blocks of the LHD helical coils (HC-I) (with two different frequency resolutions) and for the outer vertical poloidal coils (PC-OV); the magnetic field during an excitation.
It should be noted that the balance voltage signals are measured with different frequency ranges in LHD. In this paper, the signals with the lower frequency measurement (using low-pass filters of 10 Hz) are discussed hereafter, and the voltage step is typically 1 mV in this case. The analysis on the high frequency measurements (1 or 10 kHz) will be discussed elsewhere. Fig. 2(a) shows the obtained distribution function of the balance voltage, which shows that the signals obey exponential distribution functions with two components (high and low energy components) during the ramp-up phase, whereas there is only one component with the lower energy during the ramp-down phase.
3. Measurement of acoustic emissions Before starting the seventh cooling cycle (conducted in 2003), four acoustic emission (AE) sensors (NF AE901DL-B) were attached to the stainless-steel cans of the helical coils to directly monitor the mechanical disturbances. Fig. 3 shows a schematic illustration of the toroidal distribution of the AE sensors along the helical coils. The signal cables of the AE sensors are lead from the LHD cryostat through feed-through connectors, and fed into preamplifiers located near the cryostat. The
N. Yanagi et al. / Fusion Engineering and Design 81 (2006) 2561–2565
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Fig. 3. Schematic illustration of the toroidal distribution of four AE sensors along the helical coils.
Fig. 2. Distribution functions of (a) the spike signals observed on the balance voltage of the innermost blocks of the helical coils and (b) one of the AE signals. The closed and open circles correspond to the ramp-up and ramp-down phases, respectively.
seen in Fig. 1. Pulse height analysis is also applicable to the AE signals, as has been done for some other superconducting magnets [7,8], and a typical result is shown in Fig. 2(b). The obtained distribution functions seem almost the same for the ramp-up and ramp-down phases, which is different from the distribution functions observed for the balance voltage spike signals. It should be noted that many of the AE pulses are correlated with the spike signals of the balance voltage of not only the helical coils but also the poloidal coils. The mechanisms to give these distribution functions should be clarified in our future work. Moreover, detailed analysis with regard to the correlations between the AE signals and the balance voltage spike signals of both the helical coils and poloidal coils is under way.
4. Discussions effect of the stray magnetic field on the preamplifiers is within the acceptable level [6]. Then, the signals are amplified by AE analyzers, which give envelope signals from the raw AE signals. The output signals are digitized with a sampling frequency of 10 kHz and the data are observed and stored by computers in the control room via network with optical fibers. After the installation of AE sensors, the stainless-steel walls of the supporting structures were hit by a hammer and AE signals were confirmed to propagate with the sound velocity along the supporting structures. During the ramp-up and ramp-down processes of excitation, a number of AE pulses are observed as is
The total intensity of spike signals observed in a single excitation is an effective measure to investigate the changes of mechanical properties of the coil windings. It has been found that the total intensity drastically reduces from the second excitation with the same operation condition as is shown in Fig. 4. Moreover, as is seen in Fig. 5, it is confirmed that the total intensity obtained in the first excitation of each cooling cycle continuously decreases, though the intensity becomes once higher than that observed in the successive excitations in the former cooling cycle. Similarly, as shown in Fig. 6, the total intensity of AE signals
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Fig. 4. Variation of the total spike signals of the balance voltage of the helical coils during the ramp-up phase (up to 2.7 T of the central toroidal field) as a function of the excitation number for the cooling cycles of third to ninth.
also decreases from the second excitation with the same operation condition. These observations indicate a favorable trend that the mechanical disturbances in the coil windings decrease with excitations, as has been observed in many other superconducting coils [9–11], which are often discussed also in connection with the training effect. On the other hand, the high frequency measurement of the balance voltage is useful for detecting mechanical disturbances in the windings that may cause normal-
Fig. 6. Variation of the total AE intensity as a function of the excitation number observed in the eighth cooling cycle.
transitions. A clear coincidence between the balance voltage with a high frequency measurement and AE signals was observed when the 17th normal-transition occurred. This indicates that a mechanical disturbance actually initiates a normal-transition, which was formerly only speculated with the low frequency measurement. The details of this observation will be discussed elsewhere.
5. Conclusions Pulse height analysis (PHA) has been applied to analyze the spike signals observed on the balance voltage of the helical coils and the acoustic emission (AE) signals in LHD in order to investigate the changes of mechanical properties of the coil windings as the excitations and cooling cycles proceed. It has been found that the total intensities of the spike signals decrease from the second excitation in each cooling cycle, as has been observed in many other superconducting coils. The intensities observed in the initial excitation in each cooling cycle decreases continuously.
References Fig. 5. Variation of the total spike signals of the balance voltage of the helical coils during the first ramp-up process as a function of the cooling cycle.
[1] O. Motojima, H. Yamada, A. Komori, N. Ohyabu, K. Kawahata, O. Kaneko, et al., Initial physics achievements of Large Helical Device experiments, Phys. Plasmas 6 (1999) 1843–1850.
N. Yanagi et al. / Fusion Engineering and Design 81 (2006) 2561–2565 [2] S. Imagawa, N. Yanagi, H. Sekiguchi, T. Mito, O. Motojima, Performance of the helical coils for the Large Helical Device in six years operation, IEEE Trans. Appl. Supercond. 14 (2004) 1388–1393. [3] N. Yanagi, S. Imagawa, Y. Hishinuma, K. Seo, K. Takahata, S. Hamaguchi, et al., Asymmetrical normal-zone propagation observed in the aluminum-stabilized superconductor for the LHD helical coils, IEEE Trans. Appl. Supercond. 14 (2004) 1507–1510. [4] N. Yanagi, S. Imagawa, T. Mito, A.V. Gavrilin, S. Hamaguchi, H. Sekiguchi, et al., Analysis on the cryogenic stability and mechanical properties of the LHD helical coils, IEEE Trans. Appl. Supercond. 12 (2002) 662–665. [5] N. Yanagi, S. Imagawa, S. Hamaguchi, H. Chikaraishi, K. Takahata, T. Mito, et al., Pulse height analysis of the spike signals on the balance voltage observed in the LHD helical coils, in: Proceedings of ICEC18, Mumbai, 2000, pp. 179–182.
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[6] T. Ishigohka, T. Tsuchiya, Y. Adachi, A. Ninomiya, N. Yanagi, K. Seo, et al., AE measurement of the LHD helical coils, IEEE Trans. Appl. Supercond. 15 (2005) 1423–1426. [7] H. Nomura, K. Takahisa, K. Koyama, T. Sakai, Acoustic emission from superconducting magnets, Cryogenics 17 (1977) 471–481. [8] K. Ikizawa, N. Takasu, Y. Murayama, K. Seo, S. Nishijima, K. Katagiri, T. Okada, Instability of superconducting racetrack magnets, IEEE Trans. Magn. 27 (1991) 2128–2131. [9] Y. Iwasa, Mechanical disturbances in superconducting magnets—a review, IEEE Trans. Magn. 28 (1992) 113–120. [10] O. Tsukamoto, Y. Iwasa, Sources of acoustic emission in superconducting magnets, J. Appl. Phys. 54 (1983) 997–1007. [11] K. Arai, A. Ninomiya, T. Ishigohka, K. Takano, H. Nakajima, P. Michael, et al., Acoustic emission and disturbances in central solenoid model coil for international thermonuclear experimental reactor, Cryogenics 44 (2004) 15–27.
Pulse height analysis on the balance voltage and acoustic emission signals for the LHD superconducting coils N. Yanagi a,∗ , K. Seo a , S. Imagawa a , H. Sekiguchi a , K. Takahata a , S. Yamada a , T. Mito a , T. Ishigohka b , A. Ninomiya b a
National Institute for Fusion Science, Toki, Gifu 509-5292, Japan b Seikei University, Musashino, Tokyo 180-8633, Japan Available online 28 August 2006
Abstract It has been observed in the superconducting helical coils of the Large Helical Device (LHD) that the balance voltage signals measured between the corresponding pairs of the coil blocks contain a number of spike signals during ramp-up and ramp-down processes of excitation. The spike signals might be generated by rapid changes of the self-inductances of the coil windings due to mechanical disturbances caused by large electromagnetic forces. Pulse height analysis (PHA) has been applied to analyze these signals in order to investigate the changes of mechanical properties of the coil windings as the excitation and cooling cycles proceed. In addition, acoustic emission (AE) sensors attached to the helical coil cans are also used to detect mechanical disturbances. © 2006 Elsevier B.V. All rights reserved. Keywords: LHD; Helical coils; Balance voltage; Pulse height analysis; Acoustic emission
1. Introduction During the nine experimental cycles of the Large Helical Device (LHD) [1], 17 times of transient normaltransitions have been observed in the bath-cooled helical coils (major radius 3.9 m, minor radius ∼1 m and toroidal pitch number 10) at current and magnetic field values slightly lower than the specified operation point (current 13 kA, magnetic field 6.9 T at temperature ∗ Corresponding author. Tel.: +81 572 58 2126; fax: +81 572 58 2616. E-mail address: [email protected] (N. Yanagi).
0920-3796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2006.07.010
4.4 K) [2]. It has been confirmed both by conducting R&D coil tests and by observing pick-up coil signals along the helical coils of LHD, that the generated normal-zone propagates with a different propagation speed depending on the longitudinal direction along the conductor [3]. Moreover, in some current ranges, the normal-zone propagates only in one direction and it forms a “traveling normal-zone”. This phenomenon seems to be related with the asymmetrical configuration of the conductor structure, i.e., an NbTi/Cu Rutherfordtype superconducting cable and a pure aluminum stabilizer are situated side by side in a copper jacket. It should also be emphasized that the cryogenic stability
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of this type of superconductor is deteriorated transiently as the magnetic (current) diffusion process takes relatively long time due to the low resistivity of pure aluminum. Thus, the (transient) minimum propagation current becomes lower than the (steady-state) cold-end recovery current. The detailed mechanism for precisely explaining the asymmetrical normal-zone propagation is still under investigation [3]. On the other hand, one also needs to pay attention to the mechanism that initiates a normal-transition itself along with the discussion about the cryogenic stability of the winding conductor. We consider that mechanical disturbances, such as conductor motions, occurring in the coil windings play the key role, as the windings are not impregnated due to the structures of the bathcooled helical coils. During excitation tests, a number of spike signals are generally observed on the balance voltage of the helical coils. By analyzing these signals, it seems possible to extract useful information in terms of the changes of mechanical properties of the coil windings as the excitation and cooling cycles proceed. Acoustic emission (AE) signals are also useful for this purpose.
2. Pulse height analysis of the balance voltage spike signals Fig. 1 shows typical waveforms of the balance voltage signals observed in the innermost blocks (where the magnetic field is the highest) of the helical coils during an excitation test. Here, the balance voltage is obtained by providing the terminal voltage signals of the corresponding pair of the helical coil blocks to a “balanced” circuit so that the inductive voltage can be cancelled in order to observe only the resistive component and to detect a normal-transition. It is clearly seen in Fig. 1 that a number of spike signals are observed both in the rampup and ramp-down phases. They might be generated by rapid changes of the self-inductances of the coil windings due to mechanical disturbances, such as conductor motions, caused by large electromagnetic forces [4]. In order to extract quantitative information with respect to the mechanical properties of the coil windings, the observed spike signals are analyzed by pulse height analysis (PHA). In this method, the peak voltage of each spike signal is evaluated and the number of spikes in each step size of voltage is counted [5].
Fig. 1. Typical waveforms of one of the acoustic emission (AE) signals; the balance voltage signals observed for the innermost blocks of the LHD helical coils (HC-I) (with two different frequency resolutions) and for the outer vertical poloidal coils (PC-OV); the magnetic field during an excitation.
It should be noted that the balance voltage signals are measured with different frequency ranges in LHD. In this paper, the signals with the lower frequency measurement (using low-pass filters of 10 Hz) are discussed hereafter, and the voltage step is typically 1 mV in this case. The analysis on the high frequency measurements (1 or 10 kHz) will be discussed elsewhere. Fig. 2(a) shows the obtained distribution function of the balance voltage, which shows that the signals obey exponential distribution functions with two components (high and low energy components) during the ramp-up phase, whereas there is only one component with the lower energy during the ramp-down phase.
3. Measurement of acoustic emissions Before starting the seventh cooling cycle (conducted in 2003), four acoustic emission (AE) sensors (NF AE901DL-B) were attached to the stainless-steel cans of the helical coils to directly monitor the mechanical disturbances. Fig. 3 shows a schematic illustration of the toroidal distribution of the AE sensors along the helical coils. The signal cables of the AE sensors are lead from the LHD cryostat through feed-through connectors, and fed into preamplifiers located near the cryostat. The
N. Yanagi et al. / Fusion Engineering and Design 81 (2006) 2561–2565
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Fig. 3. Schematic illustration of the toroidal distribution of four AE sensors along the helical coils.
Fig. 2. Distribution functions of (a) the spike signals observed on the balance voltage of the innermost blocks of the helical coils and (b) one of the AE signals. The closed and open circles correspond to the ramp-up and ramp-down phases, respectively.
seen in Fig. 1. Pulse height analysis is also applicable to the AE signals, as has been done for some other superconducting magnets [7,8], and a typical result is shown in Fig. 2(b). The obtained distribution functions seem almost the same for the ramp-up and ramp-down phases, which is different from the distribution functions observed for the balance voltage spike signals. It should be noted that many of the AE pulses are correlated with the spike signals of the balance voltage of not only the helical coils but also the poloidal coils. The mechanisms to give these distribution functions should be clarified in our future work. Moreover, detailed analysis with regard to the correlations between the AE signals and the balance voltage spike signals of both the helical coils and poloidal coils is under way.
4. Discussions effect of the stray magnetic field on the preamplifiers is within the acceptable level [6]. Then, the signals are amplified by AE analyzers, which give envelope signals from the raw AE signals. The output signals are digitized with a sampling frequency of 10 kHz and the data are observed and stored by computers in the control room via network with optical fibers. After the installation of AE sensors, the stainless-steel walls of the supporting structures were hit by a hammer and AE signals were confirmed to propagate with the sound velocity along the supporting structures. During the ramp-up and ramp-down processes of excitation, a number of AE pulses are observed as is
The total intensity of spike signals observed in a single excitation is an effective measure to investigate the changes of mechanical properties of the coil windings. It has been found that the total intensity drastically reduces from the second excitation with the same operation condition as is shown in Fig. 4. Moreover, as is seen in Fig. 5, it is confirmed that the total intensity obtained in the first excitation of each cooling cycle continuously decreases, though the intensity becomes once higher than that observed in the successive excitations in the former cooling cycle. Similarly, as shown in Fig. 6, the total intensity of AE signals
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Fig. 4. Variation of the total spike signals of the balance voltage of the helical coils during the ramp-up phase (up to 2.7 T of the central toroidal field) as a function of the excitation number for the cooling cycles of third to ninth.
also decreases from the second excitation with the same operation condition. These observations indicate a favorable trend that the mechanical disturbances in the coil windings decrease with excitations, as has been observed in many other superconducting coils [9–11], which are often discussed also in connection with the training effect. On the other hand, the high frequency measurement of the balance voltage is useful for detecting mechanical disturbances in the windings that may cause normal-
Fig. 6. Variation of the total AE intensity as a function of the excitation number observed in the eighth cooling cycle.
transitions. A clear coincidence between the balance voltage with a high frequency measurement and AE signals was observed when the 17th normal-transition occurred. This indicates that a mechanical disturbance actually initiates a normal-transition, which was formerly only speculated with the low frequency measurement. The details of this observation will be discussed elsewhere.
5. Conclusions Pulse height analysis (PHA) has been applied to analyze the spike signals observed on the balance voltage of the helical coils and the acoustic emission (AE) signals in LHD in order to investigate the changes of mechanical properties of the coil windings as the excitations and cooling cycles proceed. It has been found that the total intensities of the spike signals decrease from the second excitation in each cooling cycle, as has been observed in many other superconducting coils. The intensities observed in the initial excitation in each cooling cycle decreases continuously.
References Fig. 5. Variation of the total spike signals of the balance voltage of the helical coils during the first ramp-up process as a function of the cooling cycle.
[1] O. Motojima, H. Yamada, A. Komori, N. Ohyabu, K. Kawahata, O. Kaneko, et al., Initial physics achievements of Large Helical Device experiments, Phys. Plasmas 6 (1999) 1843–1850.
N. Yanagi et al. / Fusion Engineering and Design 81 (2006) 2561–2565 [2] S. Imagawa, N. Yanagi, H. Sekiguchi, T. Mito, O. Motojima, Performance of the helical coils for the Large Helical Device in six years operation, IEEE Trans. Appl. Supercond. 14 (2004) 1388–1393. [3] N. Yanagi, S. Imagawa, Y. Hishinuma, K. Seo, K. Takahata, S. Hamaguchi, et al., Asymmetrical normal-zone propagation observed in the aluminum-stabilized superconductor for the LHD helical coils, IEEE Trans. Appl. Supercond. 14 (2004) 1507–1510. [4] N. Yanagi, S. Imagawa, T. Mito, A.V. Gavrilin, S. Hamaguchi, H. Sekiguchi, et al., Analysis on the cryogenic stability and mechanical properties of the LHD helical coils, IEEE Trans. Appl. Supercond. 12 (2002) 662–665. [5] N. Yanagi, S. Imagawa, S. Hamaguchi, H. Chikaraishi, K. Takahata, T. Mito, et al., Pulse height analysis of the spike signals on the balance voltage observed in the LHD helical coils, in: Proceedings of ICEC18, Mumbai, 2000, pp. 179–182.
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[6] T. Ishigohka, T. Tsuchiya, Y. Adachi, A. Ninomiya, N. Yanagi, K. Seo, et al., AE measurement of the LHD helical coils, IEEE Trans. Appl. Supercond. 15 (2005) 1423–1426. [7] H. Nomura, K. Takahisa, K. Koyama, T. Sakai, Acoustic emission from superconducting magnets, Cryogenics 17 (1977) 471–481. [8] K. Ikizawa, N. Takasu, Y. Murayama, K. Seo, S. Nishijima, K. Katagiri, T. Okada, Instability of superconducting racetrack magnets, IEEE Trans. Magn. 27 (1991) 2128–2131. [9] Y. Iwasa, Mechanical disturbances in superconducting magnets—a review, IEEE Trans. Magn. 28 (1992) 113–120. [10] O. Tsukamoto, Y. Iwasa, Sources of acoustic emission in superconducting magnets, J. Appl. Phys. 54 (1983) 997–1007. [11] K. Arai, A. Ninomiya, T. Ishigohka, K. Takano, H. Nakajima, P. Michael, et al., Acoustic emission and disturbances in central solenoid model coil for international thermonuclear experimental reactor, Cryogenics 44 (2004) 15–27.