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Measurement of ion cyclotron emissions by use of ICRF heating antennas in LHD
Fusion Engineering and Design 84 (2009) 1676–1679
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Measurement of ion cyclotron emissions by use of ICRF heating antennas in LHD K. Saito a,∗ , H. Kasahara a , T. Seki a , R. Kumazawa a , T. Mutoh a , T. Watanabe a , F. Shimpo a , G. Nomura a , M. Osakabe a , M. Ichimura b , H. Higaki c , A. Komori a , the LHD experimental group a
National Institute for Fusion Science, Toki, Gifu 509-5292, Japan Tsukuba University, Plasma Research Center, Tsukuba, Ibaraki 305-8577, Japan c Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8530, Japan b
a r t i c l e
i n f o
Article history: Available online 29 January 2009 Keywords: Ion cyclotron emission ICRF heating antenna LHD
a b s t r a c t Ion cyclotron emissions (ICEs) were clearly detected in the Large Helical Device (LHD) during perpendicular neutral beam (NB) injection. Antennas for the ion cyclotron range of frequencies (ICRF) heating were used as probes. The frequencies of ICEs were proportional to magnetic field strength. The location of ICE excitation was a peripheral region of plasma and the source particles were lost-ions injected by the perpendicular NB. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental equipment
The frequencies of ion cyclotron emissions (ICEs) depend on the species of particles, and ICEs may therefore be used to investigate fusion products. ICEs are generated by a positive slope of the perpendicular distribution function, and the exited frequencies are close to multiples of an ion cyclotron frequency, fci at the excited point. They are detected in tokamaks [1–4], where core high-energy particles with some pitch angle excurse to the edge region in the lower magnetic field side of the plasma. ICEs are excited since such particles make the positive slope of the perpendicular distribution function in the edge region. In the Large Helical Device (LHD) [5] a perpendicular neutral beam (NB) injector was installed, and it may produce the positive slope of the perpendicular distribution function. The magnetic fluctuation, which was thought to be one of ICEs, was found in LHD by using a magnetic probe (1 turn, 2.5 mm × 20 mm) installed between a wall armor plate and the chamber wall of LHD [6]. The signal was detected when the perpendicular NB was injected. However, the signal was small, and only a signal with the frequency of 71 MHz was detected. In order to detect the weak magnetic fluctuations excited in plasma, ion cyclotron range of frequencies (ICRF) heating antennas were used as detectors in LHD. In this paper, the detection of ICEs will be described. In Section 2, the devices used for this measurement will be described. Section 3 presents experimental result. Calculation of the particle orbits will be shown in Section 4 to investigate the origin of the ICEs in LHD. A summary is given in Section 5.
In LHD there are three pairs of ICRF heating antennas [7]. As detectors, they have the advantage of high sensitivity owing to the large loop areas of approximately 600 cm2 . In order to measure the magnetic fluctuation such as ICEs with the ICRF heating antennas, two antennas were disconnected from impedance matching devices [8], since the ICE signals reflect back to the antennas and decrease with the impedance matching section. Coaxial cables (>100 m) were connected via DC-cuts to an oscilloscope (LeCroy WR6051A). The maximum sampling rate of the oscilloscope is 5 GHz and the maximum memory length is 12 MW/channel. The acquired signals were analyzed afterwards by using the fast Fourier transform (FFT) method. There is a positive-ion-based NB injector installed perpendicularly to the plasma column at an outer port in LHD. The beam particles are hydrogen and the energy is normally 40 keV.
∗ Corresponding author. Tel.: +81 572 58 2202; fax: +81 572 58 2622. E-mail address: [email protected] (K. Saito). 0920-3796/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.12.053
3. Experimental results From 1.3 s to 2.3 s, the perpendicular NB was injected in the discharge shown in Fig. 1. At the timing of perpendicular NB, multipeaked ICEs were clearly detected in LHD by using ICRF heating antennas. Fig. 2(a) shows a power spectrum measured with an ICRF heating antenna during injection of perpendicular NB. The time window of the FFT was 100 s, where the rectangular window was used. The magnetic field strength on axis (Rax = 3.6 m) was 2.75 T. The lowest 4 peaks of frequency were 24.1, 48.2, 73.3, and 98.0 MHz. They were approximately multiples of the fundamental frequency of 24.1 MHz. The signal detected with the magnetic probe mentioned in the introduction section might be the third harmonic ICE. Fig. 3 shows the cross section of LHD plasma at the horizontally
K. Saito et al. / Fusion Engineering and Design 84 (2009) 1676–1679
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Fig. 1. Plasma parameters and the timing of perpendicular NB. Fig. 4. Relation between fundamental ICE frequencies, f0 and magnetic field strength at the major radius of 3.6 m, B0 .
Fig. 2. Power spectra of ICEs detected with an ICRF heating antenna. Magnetic field strength on axis (Rax = 3.6 m) are (a) 2.75 T and (b) 1.5 T. Sampling frequency was 500 MHz and the time window for the FFT was 100 s. The power levels are arbitrary.
Fig. 3. Cross section of LHD plasma at the horizontally elongated toroidal section. Cyclotron frequency is the same with the detected fundamental ICE frequency (f0 = 24 MHz) on two lines.
Fig. 5. (a) Power spectra of ICE signals before and after turn-off of perpendicular NB injection. (b) Time dependence of ICE signals around turn-off of perpendicular NB injection. The time window for the FFT was 5 s and the power level is arbitrary.
Fig. 6. Orbits of hydrogen ions with the energy of 40 keV starting from the outer edge region.
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K. Saito et al. / Fusion Engineering and Design 84 (2009) 1676–1679
Fig. 7. Relation between toroidal angle and flight time of particles (a) from outer peripheral region and (b) from inner region. Circles are plotted at the ends of orbits.
elongated toroidal section where perpendicular NB was injected from the outer port. The two lines are the location where the cyclotron frequency of beam particles (hydrogen) fcH corresponds to the measured fundamental frequency f0 of 24 MHz. It was found that ICEs are excited at the peripheral region of plasma since ICEs are excited around the layer of f0 = fci according to the theory of ICE excitation [2]. The ICE frequencies changed when the magnetic field strength on the axis (Rax = 3.6 m) decreased to 1.5 T, as shown in Fig. 2(b). The frequencies were changed from 24.1, 48.2, 73.3, 98.0 MHz to 13.0, 26.2, 39.0, 52.4 MHz, which were approximately multiples of the fundamental frequency of 13.0 MHz. As shown in Fig. 4, the fundamental frequencies are proportional to the magnetic field strength. This means that the location where ICEs are excited is fixed on the lines of Fig. 3. The ICE signal was detected even in the high-density plasma (e.g. ne0 > 5 × 1020 m−3 ), where neutral beam penetration from the outer side to inner side is impossible. Therefore, the excitation seems to occur at least in the outer side of the plasma. To investigate where the particles exciting ICEs come from, we investigated the decay of ICE intensity. Before turning off the injection of perpendicular NB (−0.09 ms), ICEs were clearly detected as shown in Fig. 5(a), but only after 0.1 ms they were vanished. Fig. 5(b) shows the time dependence of ICE intensity at the turn-off of the perpendicular NB injection. The timing of the turn-off is not clear, but by the low frequency noise due to the turn-off of NB, it was deduced to be 0.08 ms before the timing of the trigger signal (0 ms). The ICEs seems to be synchronized with the perpendicular NB and the ICEs decay time was less than 0.1 ms; therefore, the particles which excite ICEs do not come from the plasma core to the exciting point in peripheral region. Injected hydrogen ions seem to be lost quickly before slowing down at the peripheral region and the positive slope of the perpendicular distribution function may be formed there.
2 cm. The cyclotron frequency range at these starting points was 22.2–25.9 MHz. The pitch angle of the particles was 87◦ , 90◦ , and 93◦ . The chased particle number was 33. Fig. 6 shows the orbits of the particles. All particles collided with the vacuum vessel wall and were not confined. The relation between the toroidal angle from the start point and the flight time was plotted in Fig. 7(a). There were two branches; one of particles moving toroidally, and the other of particles moving along the ‘loss canal’ [9], which is characteristic of helical devices. The relation between the toroidal angle and the time from the start points of the inner side was also calculated, as shown in Fig. 7(b). The range of the major radius of the start points was from 2.55 m to 2.75 m, corresponding to the position where the range of the fundamental ICE frequency is 22.7–25.7 MHz. In any case, most particles were lost in several ten microseconds and this time is consistent with the decay time of ICEs. Therefore, the short decay time of ICEs are attributed to the short lifetime of particles. 5. Summary Clear ICEs were detected by using ICRF heating antennas in LHD. The frequencies of ICEs were found to be proportional to the magnetic field strength. ICEs were synchronized with perpendicular NB, and the decay time at the turn-off of the injection of perpendicular NB was less than 0.1 ms. This small decay time is consistent with the lifetime of particles at the peripheral region injected by the perpendicular NB. Acknowledgements The authors would like to thank the technical staff of the LHD group at the National Institute for Fusion Science for their helpful support during this work. This work was supported by the NIFS budget NIFS06ULRR504 and NIFS07ULRR504.
4. Calculation of particle orbit References In order to understand the short decay time of ICEs at the turnoff of the perpendicular NB injection, particle orbits were calculated by solving the equation of motion using the calculated vacuum magnetic field [9]. In this calculation, the energy of the hydrogen ion was 40 keV. The major radius of the magnetic axis and magnetic field strength on the axis are 3.6 m and 2.75 T, respectively. The toroidal direction of the magnetic field was counter clockwise. The starting points were set at the outer side of the plasma in the horizontally elongated toroidal plasma section around R = 4.66 m, where f0 corresponds to fcH with the width of 20 cm in every
[1] G.A. Cottrell, V.P. Bhatnagar, O. Da Costa, R.O. Dendy, J. Jacquinot, K.G. McClements, et al., Ion cyclotron emission measurements during JET deuteriumtritium experiments, Nucl. Fusion 33 (1993) 1365–1387. [2] R.O. Dendy, C.N. Lashmore-Davies, K.G. McClements, G.A. Cottrell, The excitation of obliquely propagating fast Alfvén waves at fusion ion cyclotron harmonics, Phys. Plasmas 1 (1994) 1918–1928. [3] S. Cauffman, R. Majeski, K.G. McClements, R.O. Dendy, Alfvénic behaviour of alpha particle driven ion cyclotron emission in TFTR, Nucl. Fusion 35 (1995) 1597–1602. [4] M. Ichimura, H. Higaki, S. Kakimoto, Y. Yamaguchi, K. Nemoto, M. Katano, et al., Observation of spontaneously excited waves in the ion cyclotron frequency range on JT-60U, Nucl. Fusion 48 (2008) 035012.
K. Saito et al. / Fusion Engineering and Design 84 (2009) 1676–1679 [5] O. Motojima, H. Yamada, A. Komori, N. Ohyabu, T. Mutoh, O. Kaneko, et al., Extended steady-state and high-beta regimes of net-current free heliotron plasmas in the Large Helical Device, Nucl. Fusion 47 (2007) S668–S676. [6] H. Higaki, S. Kakimoto, Y. Yamaguchi, M. Ichimura, R. Kumazawa, T. Watari, et al., Annual report of NIFS 2005–2006, p. 75.
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[7] T. Mutoh, R. Kumazawa, T. Seki, K. Saito, F. Shimpo, G. Nomura, et al., Plasma Phys. Control. Fusion 42 (2000) 265–274. [8] R. Kumazawa, T. Mutoh, T. Seki, F. Shimpo, G. Nomura, T. Ido, et al., Liquid stub tuner for ion cyclotron heating, Rev. Sci. Instrum. 70 (1999) 2665–2673. [9] Y. Matsumoto, S. Oikawa, T. Watanabe, Field line and particle orbit analysis in the periphery of the Large Helical Device, J. Phys. Soc. Jpn. 71 (2002) 1684–1693.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Measurement of ion cyclotron emissions by use of ICRF heating antennas in LHD K. Saito a,∗ , H. Kasahara a , T. Seki a , R. Kumazawa a , T. Mutoh a , T. Watanabe a , F. Shimpo a , G. Nomura a , M. Osakabe a , M. Ichimura b , H. Higaki c , A. Komori a , the LHD experimental group a
National Institute for Fusion Science, Toki, Gifu 509-5292, Japan Tsukuba University, Plasma Research Center, Tsukuba, Ibaraki 305-8577, Japan c Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8530, Japan b
a r t i c l e
i n f o
Article history: Available online 29 January 2009 Keywords: Ion cyclotron emission ICRF heating antenna LHD
a b s t r a c t Ion cyclotron emissions (ICEs) were clearly detected in the Large Helical Device (LHD) during perpendicular neutral beam (NB) injection. Antennas for the ion cyclotron range of frequencies (ICRF) heating were used as probes. The frequencies of ICEs were proportional to magnetic field strength. The location of ICE excitation was a peripheral region of plasma and the source particles were lost-ions injected by the perpendicular NB. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental equipment
The frequencies of ion cyclotron emissions (ICEs) depend on the species of particles, and ICEs may therefore be used to investigate fusion products. ICEs are generated by a positive slope of the perpendicular distribution function, and the exited frequencies are close to multiples of an ion cyclotron frequency, fci at the excited point. They are detected in tokamaks [1–4], where core high-energy particles with some pitch angle excurse to the edge region in the lower magnetic field side of the plasma. ICEs are excited since such particles make the positive slope of the perpendicular distribution function in the edge region. In the Large Helical Device (LHD) [5] a perpendicular neutral beam (NB) injector was installed, and it may produce the positive slope of the perpendicular distribution function. The magnetic fluctuation, which was thought to be one of ICEs, was found in LHD by using a magnetic probe (1 turn, 2.5 mm × 20 mm) installed between a wall armor plate and the chamber wall of LHD [6]. The signal was detected when the perpendicular NB was injected. However, the signal was small, and only a signal with the frequency of 71 MHz was detected. In order to detect the weak magnetic fluctuations excited in plasma, ion cyclotron range of frequencies (ICRF) heating antennas were used as detectors in LHD. In this paper, the detection of ICEs will be described. In Section 2, the devices used for this measurement will be described. Section 3 presents experimental result. Calculation of the particle orbits will be shown in Section 4 to investigate the origin of the ICEs in LHD. A summary is given in Section 5.
In LHD there are three pairs of ICRF heating antennas [7]. As detectors, they have the advantage of high sensitivity owing to the large loop areas of approximately 600 cm2 . In order to measure the magnetic fluctuation such as ICEs with the ICRF heating antennas, two antennas were disconnected from impedance matching devices [8], since the ICE signals reflect back to the antennas and decrease with the impedance matching section. Coaxial cables (>100 m) were connected via DC-cuts to an oscilloscope (LeCroy WR6051A). The maximum sampling rate of the oscilloscope is 5 GHz and the maximum memory length is 12 MW/channel. The acquired signals were analyzed afterwards by using the fast Fourier transform (FFT) method. There is a positive-ion-based NB injector installed perpendicularly to the plasma column at an outer port in LHD. The beam particles are hydrogen and the energy is normally 40 keV.
∗ Corresponding author. Tel.: +81 572 58 2202; fax: +81 572 58 2622. E-mail address: [email protected] (K. Saito). 0920-3796/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.12.053
3. Experimental results From 1.3 s to 2.3 s, the perpendicular NB was injected in the discharge shown in Fig. 1. At the timing of perpendicular NB, multipeaked ICEs were clearly detected in LHD by using ICRF heating antennas. Fig. 2(a) shows a power spectrum measured with an ICRF heating antenna during injection of perpendicular NB. The time window of the FFT was 100 s, where the rectangular window was used. The magnetic field strength on axis (Rax = 3.6 m) was 2.75 T. The lowest 4 peaks of frequency were 24.1, 48.2, 73.3, and 98.0 MHz. They were approximately multiples of the fundamental frequency of 24.1 MHz. The signal detected with the magnetic probe mentioned in the introduction section might be the third harmonic ICE. Fig. 3 shows the cross section of LHD plasma at the horizontally
K. Saito et al. / Fusion Engineering and Design 84 (2009) 1676–1679
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Fig. 1. Plasma parameters and the timing of perpendicular NB. Fig. 4. Relation between fundamental ICE frequencies, f0 and magnetic field strength at the major radius of 3.6 m, B0 .
Fig. 2. Power spectra of ICEs detected with an ICRF heating antenna. Magnetic field strength on axis (Rax = 3.6 m) are (a) 2.75 T and (b) 1.5 T. Sampling frequency was 500 MHz and the time window for the FFT was 100 s. The power levels are arbitrary.
Fig. 3. Cross section of LHD plasma at the horizontally elongated toroidal section. Cyclotron frequency is the same with the detected fundamental ICE frequency (f0 = 24 MHz) on two lines.
Fig. 5. (a) Power spectra of ICE signals before and after turn-off of perpendicular NB injection. (b) Time dependence of ICE signals around turn-off of perpendicular NB injection. The time window for the FFT was 5 s and the power level is arbitrary.
Fig. 6. Orbits of hydrogen ions with the energy of 40 keV starting from the outer edge region.
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K. Saito et al. / Fusion Engineering and Design 84 (2009) 1676–1679
Fig. 7. Relation between toroidal angle and flight time of particles (a) from outer peripheral region and (b) from inner region. Circles are plotted at the ends of orbits.
elongated toroidal section where perpendicular NB was injected from the outer port. The two lines are the location where the cyclotron frequency of beam particles (hydrogen) fcH corresponds to the measured fundamental frequency f0 of 24 MHz. It was found that ICEs are excited at the peripheral region of plasma since ICEs are excited around the layer of f0 = fci according to the theory of ICE excitation [2]. The ICE frequencies changed when the magnetic field strength on the axis (Rax = 3.6 m) decreased to 1.5 T, as shown in Fig. 2(b). The frequencies were changed from 24.1, 48.2, 73.3, 98.0 MHz to 13.0, 26.2, 39.0, 52.4 MHz, which were approximately multiples of the fundamental frequency of 13.0 MHz. As shown in Fig. 4, the fundamental frequencies are proportional to the magnetic field strength. This means that the location where ICEs are excited is fixed on the lines of Fig. 3. The ICE signal was detected even in the high-density plasma (e.g. ne0 > 5 × 1020 m−3 ), where neutral beam penetration from the outer side to inner side is impossible. Therefore, the excitation seems to occur at least in the outer side of the plasma. To investigate where the particles exciting ICEs come from, we investigated the decay of ICE intensity. Before turning off the injection of perpendicular NB (−0.09 ms), ICEs were clearly detected as shown in Fig. 5(a), but only after 0.1 ms they were vanished. Fig. 5(b) shows the time dependence of ICE intensity at the turn-off of the perpendicular NB injection. The timing of the turn-off is not clear, but by the low frequency noise due to the turn-off of NB, it was deduced to be 0.08 ms before the timing of the trigger signal (0 ms). The ICEs seems to be synchronized with the perpendicular NB and the ICEs decay time was less than 0.1 ms; therefore, the particles which excite ICEs do not come from the plasma core to the exciting point in peripheral region. Injected hydrogen ions seem to be lost quickly before slowing down at the peripheral region and the positive slope of the perpendicular distribution function may be formed there.
2 cm. The cyclotron frequency range at these starting points was 22.2–25.9 MHz. The pitch angle of the particles was 87◦ , 90◦ , and 93◦ . The chased particle number was 33. Fig. 6 shows the orbits of the particles. All particles collided with the vacuum vessel wall and were not confined. The relation between the toroidal angle from the start point and the flight time was plotted in Fig. 7(a). There were two branches; one of particles moving toroidally, and the other of particles moving along the ‘loss canal’ [9], which is characteristic of helical devices. The relation between the toroidal angle and the time from the start points of the inner side was also calculated, as shown in Fig. 7(b). The range of the major radius of the start points was from 2.55 m to 2.75 m, corresponding to the position where the range of the fundamental ICE frequency is 22.7–25.7 MHz. In any case, most particles were lost in several ten microseconds and this time is consistent with the decay time of ICEs. Therefore, the short decay time of ICEs are attributed to the short lifetime of particles. 5. Summary Clear ICEs were detected by using ICRF heating antennas in LHD. The frequencies of ICEs were found to be proportional to the magnetic field strength. ICEs were synchronized with perpendicular NB, and the decay time at the turn-off of the injection of perpendicular NB was less than 0.1 ms. This small decay time is consistent with the lifetime of particles at the peripheral region injected by the perpendicular NB. Acknowledgements The authors would like to thank the technical staff of the LHD group at the National Institute for Fusion Science for their helpful support during this work. This work was supported by the NIFS budget NIFS06ULRR504 and NIFS07ULRR504.
4. Calculation of particle orbit References In order to understand the short decay time of ICEs at the turnoff of the perpendicular NB injection, particle orbits were calculated by solving the equation of motion using the calculated vacuum magnetic field [9]. In this calculation, the energy of the hydrogen ion was 40 keV. The major radius of the magnetic axis and magnetic field strength on the axis are 3.6 m and 2.75 T, respectively. The toroidal direction of the magnetic field was counter clockwise. The starting points were set at the outer side of the plasma in the horizontally elongated toroidal plasma section around R = 4.66 m, where f0 corresponds to fcH with the width of 20 cm in every
[1] G.A. Cottrell, V.P. Bhatnagar, O. Da Costa, R.O. Dendy, J. Jacquinot, K.G. McClements, et al., Ion cyclotron emission measurements during JET deuteriumtritium experiments, Nucl. Fusion 33 (1993) 1365–1387. [2] R.O. Dendy, C.N. Lashmore-Davies, K.G. McClements, G.A. Cottrell, The excitation of obliquely propagating fast Alfvén waves at fusion ion cyclotron harmonics, Phys. Plasmas 1 (1994) 1918–1928. [3] S. Cauffman, R. Majeski, K.G. McClements, R.O. Dendy, Alfvénic behaviour of alpha particle driven ion cyclotron emission in TFTR, Nucl. Fusion 35 (1995) 1597–1602. [4] M. Ichimura, H. Higaki, S. Kakimoto, Y. Yamaguchi, K. Nemoto, M. Katano, et al., Observation of spontaneously excited waves in the ion cyclotron frequency range on JT-60U, Nucl. Fusion 48 (2008) 035012.
K. Saito et al. / Fusion Engineering and Design 84 (2009) 1676–1679 [5] O. Motojima, H. Yamada, A. Komori, N. Ohyabu, T. Mutoh, O. Kaneko, et al., Extended steady-state and high-beta regimes of net-current free heliotron plasmas in the Large Helical Device, Nucl. Fusion 47 (2007) S668–S676. [6] H. Higaki, S. Kakimoto, Y. Yamaguchi, M. Ichimura, R. Kumazawa, T. Watari, et al., Annual report of NIFS 2005–2006, p. 75.
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[7] T. Mutoh, R. Kumazawa, T. Seki, K. Saito, F. Shimpo, G. Nomura, et al., Plasma Phys. Control. Fusion 42 (2000) 265–274. [8] R. Kumazawa, T. Mutoh, T. Seki, F. Shimpo, G. Nomura, T. Ido, et al., Liquid stub tuner for ion cyclotron heating, Rev. Sci. Instrum. 70 (1999) 2665–2673. [9] Y. Matsumoto, S. Oikawa, T. Watanabe, Field line and particle orbit analysis in the periphery of the Large Helical Device, J. Phys. Soc. Jpn. 71 (2002) 1684–1693.