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Characterization of arcs in ICRF transmission lines
Fusion Engineering and Design 84 (2009) 685–688
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Characterization of arcs in ICRF transmission lines R. D’Inca a,∗ , A. Onyshchenko a , F. Braun a , G. Siegl a , V. Bobkov a , H. Faugel a , J.-M. Noterdaeme a,b a b
Max Planck Institut für Plasmaphysik, EURATOM Association, Garching, Germany EESA Department, Gent University, Belgium
a r t i c l e
i n f o
Keywords: ICRH Arc detection
a b s t r a c t Too high voltages can cause arcs to develop within coaxial transmission lines of ICRF antennas. To avoid damage, arcing must be detected and the RF power switched off. To ensure a high level of safety, the specifications of the detectors have to be based on a fine characterization of arcs. A test bench comprising a coaxial resonator and a fast acquisition system makes it possible to analyze the effects of breakdowns on power, voltage, pressure and DC current in the electrodes. For different types of breakdowns (multipactor, vacuum arc and gas discharge) and different configurations (presence or not of magnetic field, level of injected power), we investigate the processes involved in the specific development of an arc inside a RF coaxial line. The resulting classification of phenomena and the impact on the operational system requirements are discussed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The development of efficient and reliable arc detection systems for ICRF is a major issue for the safety of the components. A promising solution is the SHAD (Sub-Harmonic Arc Detector), a device based on a bandpass filter (cf. [1,2]) that eliminates electronic noise (typically under 5 MHz) and the generator frequencies (over 25 MHz). When an arc appears, it develops itself as a very fast transient phenomenon and therefore excites a broad spectrum of frequencies, especially in the bandpass window. The purpose of this study is to characterize this frequency signature through experiments on a dedicated test bench, the MXP, and on ASDEXUpgrade, in real operational conditions. The results obtained will make it possible to optimize the specifications of the SHAD system for ASDEX-upgrade and for ITER. 2. Manipulator eXPeriment (MXP) The MXP is used to produce and analyze different types of arcs (vacuum arcs, multipactor) under controlled conditions (geometry, pressure and magnetic field).
open probe head confined in a vacuum vessel and the other end is a stub tuner (short-circuit) that minimizes the reflected power to the generator, creating a standing wave inside the resonator with a maximum voltage at the probe head location (cf. [4]). The probe head is made of stainless steel. The power provided ranges between 1 kW and 700 kW and the pressure at the probe head between 10−7 mbar and 10−2 mbar. We limited our experiments to 400 kW. Two magnetic coils are set up in a Helmholtz configuration to create a DC field up to 50 Gauss perpendicular to the coaxial line at the probe head. The diagnostic system includes voltage and current probes inside the resonator, power coupler for forward and reflected power, a DC current probe on the inner conductor and a pressure gauge in the vacuum vessel. All probes are connected to a standard CAMAC acquisition system with up to 2 MS/s sampling rate to save the whole shot. In addition for the voltage, current, reflected and forward power, a fast acquisition system makes it possible to save the data for 10 ms when an arc occurs at a high sampling rate (up to 500 MS/s) to detect high frequencies. A set of tools has also been developed to increase the quality of the signal, among them, a reject filter to eliminate the generator frequency and increase the resolution of side-bands and a low-noise preamplifier (+29 dB) to increase the signal level.
2.1. Experimental setup 2.2. Results The experiment is illustrated in Fig. 1. The MXP is based on a 25 coaxial resonator supplied in power by one of the ASDEXupgrade ICRF generator at 31 MHz. One end of the resonator is the
∗ Corresponding author. E-mail address: [email protected] (R. D’Inca). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.12.045
The features of the spectrograms obtained made it possible to classify them in six types of arcs which are presented in Fig. 2. (a) Vacuum arcs at the probe head: they appear for a voltage above 30 kV at the probe head with a pressure of 10−7 mbar and are located at the probe head (mark A in Fig. 1), i.e. on the
686
R. D’Inca et al. / Fusion Engineering and Design 84 (2009) 685–688
Fig. 1. Description of manipulator eXPeriment.
voltage maximum. They excite discrete frequencies which evolve in time over 200 s. It means that this excitation is not due to the fast apparition of the arc in the RF line but probably to a longer interaction between the arc plasma and the line. To confirm this fact, the time signal reveals that this excitation occurs a few microseconds after the increase of reflected power due to the apparition of an arc. The intensity of the excitation increases slightly with power but no change in the spectrum is observed. (b) Low pressure arcs at the probe head: they appear for a voltage above 30 kV at 10−3 mbar to 10−4 mbar and are located at the probe head. This is still a vacuum discharge and not a glow discharge (the high-Q resonator prevents the apparition of such
discharges). Yet, the presence of gas has an effect on the structure of the spectrogram: the discrete frequencies are less visible. This would confirm the hypothesis of the plasma–RF line interaction as an origin of the excitation. (c) Arcs inside the RF line: these arcs occur for a voltage above 30 kV at the probe head but take place not at the probe head but inside the resonator (the exact position is not known but is evaluated to be near mark B in Fig. 1), i.e. not at a maximum voltage. The spectrum excited has a very low intensity and a short lifetime with a unique discrete frequency. It would mean that the frequency observed is only due to the fast transient inside the line and not to plasma interaction with the RF line. The absence of intense excitations could be linked with the position of the arc
Fig. 2. Classification of arcs spectrograms – the superimposed curve is the voltage at the probe head. (a) Vacuum arc at probe head; (b) pressure arc at probe head; (c) vacuum arc in resonator; (d) series arc on DC break; (e) multipactor induced arc; (f) multipactor induced arc with magnetic field.
R. D’Inca et al. / Fusion Engineering and Design 84 (2009) 685–688
in the line, i.e. with the current available for the development of the arc. This hypothesis has to be checked by a better control on the position of the arc. (d) Series arcs: during the experimental campaign, we got some problems with the DC break (located at mark C) of the resonator: arcs developed on its surface leading to its failure. However, this technical problem made it possible to record the frequency signature of such arcs. The excitation has a very low intensity and can be only detected by adding a low-noise preamplifier after the voltage probe. The excitation lifetime is also very short of about 2 s. (e) Multipactor induced arcs: we studied the case of relatively low voltage (under 10 kV) with gas-filled vessel (10−3 mbar to 10−2 mbar). These are the conditions of the startup phase of the generator (cf. [3]) when power is built up. In this case, the voltage is too low to produce a vacuum discharge and the gas pressure too low to lead to a gas breakdown. Yet a discharge is observed at the probe head: it corresponds to a multipactor induced discharge, where electrons emitted by the walls, collide with the gas and lead to an electronic avalanche and to the breakdown (cf. [5]). In this case, we have also recorded the spectrum of the excitation with the help of a preamplifier: the signal is indeed very low and wideband: no specific frequency can be distinguished. The intensity of the excitation increases slightly with the gas pressure but we do not observe any changes in the value of the frequencies: the spectrum remains wide. (f) Multipactor induced arcs in a magnetic field: we applied a magnetic field on this type of discharge to observe the effect on the spectrum. Our experiments were limited to a voltage of 10 kV (at the probe head). Above a certain value, the magnetic field prevents the apparition of a discharge and this threshold depends on the pressure (6 G for 10−2 mbar and 2 G for 10−3 mbar). Below this threshold, the intensity of the excitation slightly increases when the magnetic field is lowered. Further experiments will be carried out to establish the exact relationship and to analyze the effect of magnetic field with higher power. 3. ASDEX-upgrade ICRF operations
687
Fig. 3. Description of ASDEX-upgrade RF diagnostics.
stake inside the arc plasma and would depend on the nature of the arc. They however correspond to some of the frequencies observed during conditioning (case a). The last observation concerned frequencies generated in absence of arcs by the plasma that could lead to spurious detections. To record such phenomena, we trigger the acquisition system every 20 ms with a time digital controller saving 5 ms of data, so that an overview of the frequency domain of the whole shot could be obtained. The result is that frequencies indeed appear below the generator frequency when the edge density in plasma was low (between 2 × 1019 m−3 and 3 × 1019 m−3 ). They were detected both by the RF probe inside the vacuum vessel and by the voltage probe inside the RF line. An example is given in Fig. 5 for a shot with 4.31 MW ICRF power without NBI at 2.6 × 1019 m−3 . The value of the frequency depends on the generator frequency: 22.5 MHz for f = 30 MHz and 28.5 MHz for f = 36.5 MHz. The source of the frequency is still under investigation.
In parallel with the analysis of arcs signature under controlled conditions, we carried out observations on the AUG ICRF system in real conditions. 3.1. Diagnostics As shown in Fig. 3, the diagnostic system relies on a voltage probe in the ICRH feeding line before the antenna 4 and a RF probe in the vacuum vessel, on the high field side opposite to the antenna. Both are connected to the same fast acquisition system as described before. 3.2. Results We observed arcs during ICRH conditioning operations, without plasma and magnetic field. In Fig. 4a, we see a series of discrete frequencies, comparable with type b observed on the test bench. Other frequencies developed after several microseconds because, on this case, the generator did not stop and the arc carried on burning. Arcs signals were also recorded during plasma operations as shown in Fig. 4b and c. We have discrete frequencies whose value change from one case to the other. These discrete frequencies are either due to resonances inside the vacuum line and the change in frequency from one arc to the other is due to the position of the arc inside the line, or they are consequences of the physical process at
Fig. 4. Arc spectrograms in ASDEX-upgrade ICRF transmission line – arc detected on increased VSWR at 40 s (vertical line).
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R. D’Inca et al. / Fusion Engineering and Design 84 (2009) 685–688
time. A low-noise preamplifier is required to detect them and the attenuation factor of the bandpass filter has to be improved to compensate the effect of the amplifier. The first recommendation for the SHAD design is then to find the best compromise between the preamplifier amplification and the bandpass filter attenuation. The second recommendation concerns the spurious frequencies coming from the plasma. Those observed can perturbate the detection: its mechanism has to be understood to predict their values and tailor the bandpass filter. These two points will be the object of the next steps of this study. Fig. 5. RF probe spectrogram during shot 23438.
4. Conclusion In this study we have collected useful data for the design of arc detectors. The nature of the arc and its environment has an impact on its frequency signature. In the MXP, arcs located at the head feature an important excitation due to interactions with the RF line but for other types of arcs, the frequencies are generated only by the fast transient inside the line and have a low intensity and a short life-
References [1] F. Braun, Th. Sperger, An ARC detection system for ICRF heating, in: Proceedings of the 19th Symposium on Fusion Technology, Lisbon, 1996, pp. 601–603. [2] R. D’Inca, S. Assas, V. Bobkov, F. Braun, B. Eckert, J.-M. Noterdaeme, Comparison of different arc detection methods during plasma operations with ICRF heating on ASDEX Upgrade, AIP Conf. Proc. 933 (2007) 203. [3] G.E. Becerra, Studies of Coaxial Multipactor in the Presence of a Magnetic Field, S.B., Physics Massachusetts Institute of Technology, 2006. [4] V. Bobkov, Studies of high voltage breakdown phenomena on ICRF antennas, Ph.D. Thesis, Max-Planck IPP-EURATOM Association, Garching, Germany. [5] F. Hoehn, W. Jacob, R. Beckmann, R. Wilhelm, The transition of a multipactor to a low-pressure gas discharge, Phys. Plasma 4 (April (4)) (1997).
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Characterization of arcs in ICRF transmission lines R. D’Inca a,∗ , A. Onyshchenko a , F. Braun a , G. Siegl a , V. Bobkov a , H. Faugel a , J.-M. Noterdaeme a,b a b
Max Planck Institut für Plasmaphysik, EURATOM Association, Garching, Germany EESA Department, Gent University, Belgium
a r t i c l e
i n f o
Keywords: ICRH Arc detection
a b s t r a c t Too high voltages can cause arcs to develop within coaxial transmission lines of ICRF antennas. To avoid damage, arcing must be detected and the RF power switched off. To ensure a high level of safety, the specifications of the detectors have to be based on a fine characterization of arcs. A test bench comprising a coaxial resonator and a fast acquisition system makes it possible to analyze the effects of breakdowns on power, voltage, pressure and DC current in the electrodes. For different types of breakdowns (multipactor, vacuum arc and gas discharge) and different configurations (presence or not of magnetic field, level of injected power), we investigate the processes involved in the specific development of an arc inside a RF coaxial line. The resulting classification of phenomena and the impact on the operational system requirements are discussed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The development of efficient and reliable arc detection systems for ICRF is a major issue for the safety of the components. A promising solution is the SHAD (Sub-Harmonic Arc Detector), a device based on a bandpass filter (cf. [1,2]) that eliminates electronic noise (typically under 5 MHz) and the generator frequencies (over 25 MHz). When an arc appears, it develops itself as a very fast transient phenomenon and therefore excites a broad spectrum of frequencies, especially in the bandpass window. The purpose of this study is to characterize this frequency signature through experiments on a dedicated test bench, the MXP, and on ASDEXUpgrade, in real operational conditions. The results obtained will make it possible to optimize the specifications of the SHAD system for ASDEX-upgrade and for ITER. 2. Manipulator eXPeriment (MXP) The MXP is used to produce and analyze different types of arcs (vacuum arcs, multipactor) under controlled conditions (geometry, pressure and magnetic field).
open probe head confined in a vacuum vessel and the other end is a stub tuner (short-circuit) that minimizes the reflected power to the generator, creating a standing wave inside the resonator with a maximum voltage at the probe head location (cf. [4]). The probe head is made of stainless steel. The power provided ranges between 1 kW and 700 kW and the pressure at the probe head between 10−7 mbar and 10−2 mbar. We limited our experiments to 400 kW. Two magnetic coils are set up in a Helmholtz configuration to create a DC field up to 50 Gauss perpendicular to the coaxial line at the probe head. The diagnostic system includes voltage and current probes inside the resonator, power coupler for forward and reflected power, a DC current probe on the inner conductor and a pressure gauge in the vacuum vessel. All probes are connected to a standard CAMAC acquisition system with up to 2 MS/s sampling rate to save the whole shot. In addition for the voltage, current, reflected and forward power, a fast acquisition system makes it possible to save the data for 10 ms when an arc occurs at a high sampling rate (up to 500 MS/s) to detect high frequencies. A set of tools has also been developed to increase the quality of the signal, among them, a reject filter to eliminate the generator frequency and increase the resolution of side-bands and a low-noise preamplifier (+29 dB) to increase the signal level.
2.1. Experimental setup 2.2. Results The experiment is illustrated in Fig. 1. The MXP is based on a 25 coaxial resonator supplied in power by one of the ASDEXupgrade ICRF generator at 31 MHz. One end of the resonator is the
∗ Corresponding author. E-mail address: [email protected] (R. D’Inca). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.12.045
The features of the spectrograms obtained made it possible to classify them in six types of arcs which are presented in Fig. 2. (a) Vacuum arcs at the probe head: they appear for a voltage above 30 kV at the probe head with a pressure of 10−7 mbar and are located at the probe head (mark A in Fig. 1), i.e. on the
686
R. D’Inca et al. / Fusion Engineering and Design 84 (2009) 685–688
Fig. 1. Description of manipulator eXPeriment.
voltage maximum. They excite discrete frequencies which evolve in time over 200 s. It means that this excitation is not due to the fast apparition of the arc in the RF line but probably to a longer interaction between the arc plasma and the line. To confirm this fact, the time signal reveals that this excitation occurs a few microseconds after the increase of reflected power due to the apparition of an arc. The intensity of the excitation increases slightly with power but no change in the spectrum is observed. (b) Low pressure arcs at the probe head: they appear for a voltage above 30 kV at 10−3 mbar to 10−4 mbar and are located at the probe head. This is still a vacuum discharge and not a glow discharge (the high-Q resonator prevents the apparition of such
discharges). Yet, the presence of gas has an effect on the structure of the spectrogram: the discrete frequencies are less visible. This would confirm the hypothesis of the plasma–RF line interaction as an origin of the excitation. (c) Arcs inside the RF line: these arcs occur for a voltage above 30 kV at the probe head but take place not at the probe head but inside the resonator (the exact position is not known but is evaluated to be near mark B in Fig. 1), i.e. not at a maximum voltage. The spectrum excited has a very low intensity and a short lifetime with a unique discrete frequency. It would mean that the frequency observed is only due to the fast transient inside the line and not to plasma interaction with the RF line. The absence of intense excitations could be linked with the position of the arc
Fig. 2. Classification of arcs spectrograms – the superimposed curve is the voltage at the probe head. (a) Vacuum arc at probe head; (b) pressure arc at probe head; (c) vacuum arc in resonator; (d) series arc on DC break; (e) multipactor induced arc; (f) multipactor induced arc with magnetic field.
R. D’Inca et al. / Fusion Engineering and Design 84 (2009) 685–688
in the line, i.e. with the current available for the development of the arc. This hypothesis has to be checked by a better control on the position of the arc. (d) Series arcs: during the experimental campaign, we got some problems with the DC break (located at mark C) of the resonator: arcs developed on its surface leading to its failure. However, this technical problem made it possible to record the frequency signature of such arcs. The excitation has a very low intensity and can be only detected by adding a low-noise preamplifier after the voltage probe. The excitation lifetime is also very short of about 2 s. (e) Multipactor induced arcs: we studied the case of relatively low voltage (under 10 kV) with gas-filled vessel (10−3 mbar to 10−2 mbar). These are the conditions of the startup phase of the generator (cf. [3]) when power is built up. In this case, the voltage is too low to produce a vacuum discharge and the gas pressure too low to lead to a gas breakdown. Yet a discharge is observed at the probe head: it corresponds to a multipactor induced discharge, where electrons emitted by the walls, collide with the gas and lead to an electronic avalanche and to the breakdown (cf. [5]). In this case, we have also recorded the spectrum of the excitation with the help of a preamplifier: the signal is indeed very low and wideband: no specific frequency can be distinguished. The intensity of the excitation increases slightly with the gas pressure but we do not observe any changes in the value of the frequencies: the spectrum remains wide. (f) Multipactor induced arcs in a magnetic field: we applied a magnetic field on this type of discharge to observe the effect on the spectrum. Our experiments were limited to a voltage of 10 kV (at the probe head). Above a certain value, the magnetic field prevents the apparition of a discharge and this threshold depends on the pressure (6 G for 10−2 mbar and 2 G for 10−3 mbar). Below this threshold, the intensity of the excitation slightly increases when the magnetic field is lowered. Further experiments will be carried out to establish the exact relationship and to analyze the effect of magnetic field with higher power. 3. ASDEX-upgrade ICRF operations
687
Fig. 3. Description of ASDEX-upgrade RF diagnostics.
stake inside the arc plasma and would depend on the nature of the arc. They however correspond to some of the frequencies observed during conditioning (case a). The last observation concerned frequencies generated in absence of arcs by the plasma that could lead to spurious detections. To record such phenomena, we trigger the acquisition system every 20 ms with a time digital controller saving 5 ms of data, so that an overview of the frequency domain of the whole shot could be obtained. The result is that frequencies indeed appear below the generator frequency when the edge density in plasma was low (between 2 × 1019 m−3 and 3 × 1019 m−3 ). They were detected both by the RF probe inside the vacuum vessel and by the voltage probe inside the RF line. An example is given in Fig. 5 for a shot with 4.31 MW ICRF power without NBI at 2.6 × 1019 m−3 . The value of the frequency depends on the generator frequency: 22.5 MHz for f = 30 MHz and 28.5 MHz for f = 36.5 MHz. The source of the frequency is still under investigation.
In parallel with the analysis of arcs signature under controlled conditions, we carried out observations on the AUG ICRF system in real conditions. 3.1. Diagnostics As shown in Fig. 3, the diagnostic system relies on a voltage probe in the ICRH feeding line before the antenna 4 and a RF probe in the vacuum vessel, on the high field side opposite to the antenna. Both are connected to the same fast acquisition system as described before. 3.2. Results We observed arcs during ICRH conditioning operations, without plasma and magnetic field. In Fig. 4a, we see a series of discrete frequencies, comparable with type b observed on the test bench. Other frequencies developed after several microseconds because, on this case, the generator did not stop and the arc carried on burning. Arcs signals were also recorded during plasma operations as shown in Fig. 4b and c. We have discrete frequencies whose value change from one case to the other. These discrete frequencies are either due to resonances inside the vacuum line and the change in frequency from one arc to the other is due to the position of the arc inside the line, or they are consequences of the physical process at
Fig. 4. Arc spectrograms in ASDEX-upgrade ICRF transmission line – arc detected on increased VSWR at 40 s (vertical line).
688
R. D’Inca et al. / Fusion Engineering and Design 84 (2009) 685–688
time. A low-noise preamplifier is required to detect them and the attenuation factor of the bandpass filter has to be improved to compensate the effect of the amplifier. The first recommendation for the SHAD design is then to find the best compromise between the preamplifier amplification and the bandpass filter attenuation. The second recommendation concerns the spurious frequencies coming from the plasma. Those observed can perturbate the detection: its mechanism has to be understood to predict their values and tailor the bandpass filter. These two points will be the object of the next steps of this study. Fig. 5. RF probe spectrogram during shot 23438.
4. Conclusion In this study we have collected useful data for the design of arc detectors. The nature of the arc and its environment has an impact on its frequency signature. In the MXP, arcs located at the head feature an important excitation due to interactions with the RF line but for other types of arcs, the frequencies are generated only by the fast transient inside the line and have a low intensity and a short life-
References [1] F. Braun, Th. Sperger, An ARC detection system for ICRF heating, in: Proceedings of the 19th Symposium on Fusion Technology, Lisbon, 1996, pp. 601–603. [2] R. D’Inca, S. Assas, V. Bobkov, F. Braun, B. Eckert, J.-M. Noterdaeme, Comparison of different arc detection methods during plasma operations with ICRF heating on ASDEX Upgrade, AIP Conf. Proc. 933 (2007) 203. [3] G.E. Becerra, Studies of Coaxial Multipactor in the Presence of a Magnetic Field, S.B., Physics Massachusetts Institute of Technology, 2006. [4] V. Bobkov, Studies of high voltage breakdown phenomena on ICRF antennas, Ph.D. Thesis, Max-Planck IPP-EURATOM Association, Garching, Germany. [5] F. Hoehn, W. Jacob, R. Beckmann, R. Wilhelm, The transition of a multipactor to a low-pressure gas discharge, Phys. Plasma 4 (April (4)) (1997).