REVIEW OF SCIENTIFIC INSTRUMENTS 85, 043301 (2014)
Open-loop correction for an eddy current dominated beam-switching magnet K. Koseki,a) H. Nakayama, and M. Tawada High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
(Received 28 February 2014; accepted 16 March 2014; published online 1 April 2014) A beam-switching magnet and the pulsed power supply it requires have been developed for the Japan Proton Accelerator Research Complex. To switch bunched proton beams, the dipole magnetic field must reach its maximum value within 40 ms. In addition, the field flatness should be less than 5 × 10−4 to guide each bunched beam to the designed orbit. From a magnetic field measurement by using a long search coil, it was found that an eddy current in the thick endplates and laminated core disturbs the rise of the magnetic field. The eddy current also deteriorates the field flatness over the required flat-top period. The measured field flatness was 5 × 10−3 . By using a double-exponential equation to approximate the measured magnetic field, a compensation pattern for the eddy current was calculated. The integrated magnetic field was measured while using the newly developed open-loop compensation system. A field flatness of less than 5 × 10−4 , which is an acceptable value, was achieved. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4869779]
The Japan Proton Accelerator Research Complex (J-PARC1 ) was constructed with the collaboration of the Japan Atomic Energy Agency (JAEA) and the High Energy Accelerator Research Organization (KEK). The J-PARC consists of a 181-MeV linac, a 3-GeV rapid-cycle synchrotron (RCS), and a main ring (MR). The layout of the accelerator complex is shown in Fig. 1. Extracted bunch trains from the RCS are delivered both to the Material and Life Science Facility by a beam transport line (3-NBT) and to the MR by the 3-50BT. A beam-switching magnet, which is operated at a repetition rate of about 0.3 Hz, is excited to deliver a portion of the beam bunches to the MR. Because the bunch spacing of the RCS beam is 40 ms, rise and fall times of less than 40 ms are required of the magnet. Another requirement is that the deviation of the magnetic field from the reference field during the flat-top period should be less than 5 × 10−4 to guide every bunched beam into the reference orbit. To meet the operational requirements, a beam-switching magnet (see Fig. 2) and the necessary pulsed power supply were developed. To excite the magnet inductance of 29 mH, the power supply must generate an output pulse voltage of about 4 kV. The pulsed power supply consists of three power modules, namely, two identical full-bridge circuits that use Gate Turn-Off thyristors (GTOs), GTO switch module 1 and 2, and a chopper circuit that uses Insulated Gate Bipolar Transistors (IGBTs), IGBT chopper module. In the chopper circuit, six switching arms that use IGBTs are connected in parallel to increase the equivalent switching frequency for lower output ripple. The rectification circuit for the chopper is a three-phase pulse width modulation (PWM) converter. A DC voltage of 600 V is generated by the converter. The energy required to excite the magnet to a peak current of 2.18 kA is about 70 kJ. To store the excitation energy, each of
the GTO switch modules is equipped with 54-mF aluminum electrolytic capacitors. The initial charging voltage for each capacitor bank is about 2 kV. The circuit schematic of the pulsed power supply is shown in Fig. 3. During the rise of the excitation current, GTO switch modules are turned on synchronously. The stored energy in the capacitor banks is delivered to the magnet while the IGBT chopper module goes into the fly-wheel mode by using diodes. When the excitation current reaches the design value of 2.18 kA, the GTO switch modules are turned off. At this point, the IGBT chopper module starts to control the flat-top current precisely. In the measured integrated magnetic field, serious field deterioration during the flat-top period was observed that was caused by an eddy current.2, 3 The eddy current was induced in the following components of the magnet: the laminated iron cores, endplates, and coil conductors. To reduce the effect of the eddy current, structural optimizations of the magnet such as insertion of slits into the endplates and utilization of stranded wires as the coil material are often adopted.4 For more-precise control of the magnetic field, a correction circuit which is composed by two damping resistors and two capacitors are proposed and adopted.5 Although the required field 30-GeV MR N
I. INTRODUCTION
3rd P.S. Bldg. 3-NBT
3-50BT
2nd P.S. Bldg.
3-GeV RCS
Hadron Experimental Hall
1st P.S. Bldg. 0
a) Electronic mail: [email protected]
0034-6748/2014/85(4)/043301/4/$30.00
Neutrino beam line
50 100 (m)
FIG. 1. Layout of J-PARC facility. 85, 043301-1
© 2014 AIP Publishing LLC
043301-2
Koseki, Nakayama, and Tawada
Rev. Sci. Instrum. 85, 043301 (2014) TABLE I. Parameters extrapolated by using approximation of the measured magnetic field during the flat-top period. α
5.84 × 10−3 A
τ1 β τ2
103.1 ms 7.62 × 10−3 A 18.9 ms
flatness was achieved with this method, a higher-frequency ripple component is excited by a resonant coupling between the magnet inductance and the capacitors. Therefore, in order to solve the problem, an active correction system6, 7 based on the learning control method was evaluated. The purpose of this paper is to illustrate the newly developed method for eddy current compensation and to review the operational results from the magnetic field measurements. FIG. 2. Photograph of the beam-switching magnet of J-PARC.
II. CHARACTERISTICS OF THE EDDY CURRENT
GTO Switch Module 1
IGBT Converter
Magnet Inductance
IGBT Chopper
Coil Resistance
GTO Switch Module 2
Integrated field (T m)
FIG. 3. Circuit schematic of the developed pulsed power supply.
2.0 1.5 1.0 0.5 0.0 -0.2
0.0
0.2
0.4
0.6
The integrated magnetic field of the beam-switching magnet was measured by using a long search coil (length: 2.5 m, width: 20 mm). To increase the signal-to-noise ratio, the number of turns of the search coil was chosen to be 20. The result is shown in Fig. 4. The induced voltage was digitized directly by a precision analog-to-digital converter (ADC) module (16 bit, 1 MS/s) and numerically integrated over time. An enlarged view around the flat-top period of the measured integrated magnetic field is depicted in Fig. 5. Although the excitation current had reached its maximum value of 2.18 kA, the magnetic field strength continued to increase exponentially. The field flatness, BL ≡ (BLmax − BLmin )/BLmax , was measured to be 5 × 10−3 . It is likely that the measured flatness would cause a serious emittance blow-up of the beam of about 100%. To avoid hard radiation damage to the downstream components, the measured deterioration of the magnetic field is not acceptable at the J-PARC. It was also estimated that the driving force of the field deterioration was the eddy current in the thick endplates of the magnet. The measured magnetic field during the flat-top period was approximated by t t − β exp − . (1) Ieffective (t) = I0 1 − α exp − τ1 τ2
0.8 GTO switch 1
Time (s)
IGBT Chopper
FIG. 4. Measured magnetic field of the beam-switching magnet.
600 V
Integrated field (T m)
Gate Signals
1.930 1.925 1.920
Feedback Control Board
1.915 1.910
DCCT
PWM
1.905 0.1
0.2
0.3
0.4
0.5
0.6
Iref 2500 A /7FFF
DAC
+ -
GTO switch 2
K(1+1/sT) Iout
Time (s) FIG. 5. Enlarged view of the measured magnetic field during flat-top period.
FIG. 6. Conceptual schematic of the feedback control unit.
043301-3
Koseki, Nakayama, and Tawada
Rev. Sci. Instrum. 85, 043301 (2014) GTO switch 1
Integrated field (T m)
IGBT Chopper 600V Gate Signals
Feedback Control Board DCCT
PWM Iref 2500A/7FFF
DAC
+ +
-
GTO switch 2
K(1+1/sT)
Time (s)
Iout
FIG. 9. Measured magnetic field with the correction system. Sync. trigger DAC
Correction Pattern (Flash memory) Open-Loop Correction Board
FIG. 7. Conceptual schematic of the open-loop correction system.
Here, α and β are amplitudes of the eddy current, respectively. The time constants for two exponentials are indicated by τ 1 and τ 2 , respectively. The parameters from the approximation are summarized in Table I. III. OPEN-LOOP CORRECTION
The excitation current during the flat-top period is precisely generated by the IGBT chopper module, which is composed of IGBTs (see Fig. 3). The IGBTs are triggered by a feedback control system, in which deviation of the output current from the reference is calculated and pulse-widthmodulation is performed (see Fig. 6). The reference signal, indicated by Iref in Fig. 6, is a constant value. In order to solve the problem by the eddy current in the beam-switching magnet, two exponentials in Eq. (1) should be compensated. Thus, a time varying signal, Icomp in Eq. (2), was produced as a compensation signal. Moreover, an openloop correction system which generates the correction signal, Icomp , was newly developed. The conceptual schematic of the developed open-loop correction system is depicted in Fig. 7. By a trigger from the upper control system, the correction sig-
nal is read by an ADC from a flash memory in a synchronous manner with the GTO switch modules and added to the reference signal, Iref . A photograph of the developed open-loop correction system for the beam-switching magnet is shown in Fig. 8. In order to avoid the temperature drift of the analog correction signal, the open-loop correction board is installed in an aluminum shield box with a temperature stabilization system by a Peltier element: t t + β exp − . (2) Icomp (t) = I0 α exp − τ1 τ2 The integrated magnetic field was measured while using the open-loop correction system. An enlarged view of the measured integrated magnetic field during the flat-top period is shown in Fig. 9. The measurement shows that a field flatness of less than 5 × 10−4 , which is an acceptable value, was achieved. Moreover, it was confirmed that the higherfrequency component measured in the previously applied system was reduced sufficiently with the newly developed system. IV. CONCLUSION
A beam-switching magnet and the pulsed power supply have been developed. A serious deterioration of the integrated magnetic field was observed by using a long search coil. An eddy current in the endplates of the magnet disturbed the rapid increase of the magnetic field. The measured field can be well represented by a double-exponential equation. From parameters determined by the approximation, a correction pattern for the eddy current was produced. By adding the time varying compensation pattern to the reference signal, an open-loop correction system for the eddy current was developed. The integrated magnetic field with the newly developed open-loop correction system was measured. A field flatness of less than 5 × 10−4 , which is an acceptable value, was accomplished. ACKNOWLEDGMENTS
The present study is supported by members of KEKB, KEK-AR, and J-PARC. The authors are grateful for the continued support of Professor H. Kobayashi and Professor M. Yoshioka. 1 Y.
FIG. 8. Photograph of the developed open-loop correction system.
Yamazaki, KEK Report No. 2002-13, 2003. Hildred Blewett, Rev. Sci. Instrum. 24, 856 (1953).
2 M.
043301-4 3 S.
Koseki, Nakayama, and Tawada
Igarashi, T. Adachi, H. Someya, N. Tani, and Y. Watanabe, IEEE Trans. Appl. Supercond. 18(2), 289 (2008). 4 N. Tani, T. Adachi, H. Someya, Y. Watanabe, H. Sato, and J. Kishiro, IEEE Trans. Appl. Supercond. 14(2), 409 (2004). 5 K. Koseki, M. Tawada, H. Nakayama, H. Kobayasi, M. Shirakata, and K. Okamura, in Proceedings of the 10th European Particle Acceler-
Rev. Sci. Instrum. 85, 043301 (2014) ator Conference (EPAC’06), Edinburgh, Scotland, 26–30 June 2006, http://www.jacow.org, pp. 1747–1749. 6 M. A. Morich, D. A. Lampman, W. R. Dannels, and F. Goldie, IEEE Trans. Med. Imaging 7(3), 247 (1988). 7 Y. Chung, D. Barr, G. Decker, J. Galayda, F. Lenkszus, A. Lumpkin, and A. J. Votaw, Rev. Sci. Instrum. 67, 3371 (1996).
Review of Scientific Instruments is copyrighted by the American Institute of Physics (AIP). Redistribution of journal material is subject to the AIP online journal license and/or AIP copyright. For more information, see http://ojps.aip.org/rsio/rsicr.jsp
Open-loop correction for an eddy current dominated beam-switching magnet K. Koseki,a) H. Nakayama, and M. Tawada High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
(Received 28 February 2014; accepted 16 March 2014; published online 1 April 2014) A beam-switching magnet and the pulsed power supply it requires have been developed for the Japan Proton Accelerator Research Complex. To switch bunched proton beams, the dipole magnetic field must reach its maximum value within 40 ms. In addition, the field flatness should be less than 5 × 10−4 to guide each bunched beam to the designed orbit. From a magnetic field measurement by using a long search coil, it was found that an eddy current in the thick endplates and laminated core disturbs the rise of the magnetic field. The eddy current also deteriorates the field flatness over the required flat-top period. The measured field flatness was 5 × 10−3 . By using a double-exponential equation to approximate the measured magnetic field, a compensation pattern for the eddy current was calculated. The integrated magnetic field was measured while using the newly developed open-loop compensation system. A field flatness of less than 5 × 10−4 , which is an acceptable value, was achieved. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4869779]
The Japan Proton Accelerator Research Complex (J-PARC1 ) was constructed with the collaboration of the Japan Atomic Energy Agency (JAEA) and the High Energy Accelerator Research Organization (KEK). The J-PARC consists of a 181-MeV linac, a 3-GeV rapid-cycle synchrotron (RCS), and a main ring (MR). The layout of the accelerator complex is shown in Fig. 1. Extracted bunch trains from the RCS are delivered both to the Material and Life Science Facility by a beam transport line (3-NBT) and to the MR by the 3-50BT. A beam-switching magnet, which is operated at a repetition rate of about 0.3 Hz, is excited to deliver a portion of the beam bunches to the MR. Because the bunch spacing of the RCS beam is 40 ms, rise and fall times of less than 40 ms are required of the magnet. Another requirement is that the deviation of the magnetic field from the reference field during the flat-top period should be less than 5 × 10−4 to guide every bunched beam into the reference orbit. To meet the operational requirements, a beam-switching magnet (see Fig. 2) and the necessary pulsed power supply were developed. To excite the magnet inductance of 29 mH, the power supply must generate an output pulse voltage of about 4 kV. The pulsed power supply consists of three power modules, namely, two identical full-bridge circuits that use Gate Turn-Off thyristors (GTOs), GTO switch module 1 and 2, and a chopper circuit that uses Insulated Gate Bipolar Transistors (IGBTs), IGBT chopper module. In the chopper circuit, six switching arms that use IGBTs are connected in parallel to increase the equivalent switching frequency for lower output ripple. The rectification circuit for the chopper is a three-phase pulse width modulation (PWM) converter. A DC voltage of 600 V is generated by the converter. The energy required to excite the magnet to a peak current of 2.18 kA is about 70 kJ. To store the excitation energy, each of
the GTO switch modules is equipped with 54-mF aluminum electrolytic capacitors. The initial charging voltage for each capacitor bank is about 2 kV. The circuit schematic of the pulsed power supply is shown in Fig. 3. During the rise of the excitation current, GTO switch modules are turned on synchronously. The stored energy in the capacitor banks is delivered to the magnet while the IGBT chopper module goes into the fly-wheel mode by using diodes. When the excitation current reaches the design value of 2.18 kA, the GTO switch modules are turned off. At this point, the IGBT chopper module starts to control the flat-top current precisely. In the measured integrated magnetic field, serious field deterioration during the flat-top period was observed that was caused by an eddy current.2, 3 The eddy current was induced in the following components of the magnet: the laminated iron cores, endplates, and coil conductors. To reduce the effect of the eddy current, structural optimizations of the magnet such as insertion of slits into the endplates and utilization of stranded wires as the coil material are often adopted.4 For more-precise control of the magnetic field, a correction circuit which is composed by two damping resistors and two capacitors are proposed and adopted.5 Although the required field 30-GeV MR N
I. INTRODUCTION
3rd P.S. Bldg. 3-NBT
3-50BT
2nd P.S. Bldg.
3-GeV RCS
Hadron Experimental Hall
1st P.S. Bldg. 0
a) Electronic mail: [email protected]
0034-6748/2014/85(4)/043301/4/$30.00
Neutrino beam line
50 100 (m)
FIG. 1. Layout of J-PARC facility. 85, 043301-1
© 2014 AIP Publishing LLC
043301-2
Koseki, Nakayama, and Tawada
Rev. Sci. Instrum. 85, 043301 (2014) TABLE I. Parameters extrapolated by using approximation of the measured magnetic field during the flat-top period. α
5.84 × 10−3 A
τ1 β τ2
103.1 ms 7.62 × 10−3 A 18.9 ms
flatness was achieved with this method, a higher-frequency ripple component is excited by a resonant coupling between the magnet inductance and the capacitors. Therefore, in order to solve the problem, an active correction system6, 7 based on the learning control method was evaluated. The purpose of this paper is to illustrate the newly developed method for eddy current compensation and to review the operational results from the magnetic field measurements. FIG. 2. Photograph of the beam-switching magnet of J-PARC.
II. CHARACTERISTICS OF THE EDDY CURRENT
GTO Switch Module 1
IGBT Converter
Magnet Inductance
IGBT Chopper
Coil Resistance
GTO Switch Module 2
Integrated field (T m)
FIG. 3. Circuit schematic of the developed pulsed power supply.
2.0 1.5 1.0 0.5 0.0 -0.2
0.0
0.2
0.4
0.6
The integrated magnetic field of the beam-switching magnet was measured by using a long search coil (length: 2.5 m, width: 20 mm). To increase the signal-to-noise ratio, the number of turns of the search coil was chosen to be 20. The result is shown in Fig. 4. The induced voltage was digitized directly by a precision analog-to-digital converter (ADC) module (16 bit, 1 MS/s) and numerically integrated over time. An enlarged view around the flat-top period of the measured integrated magnetic field is depicted in Fig. 5. Although the excitation current had reached its maximum value of 2.18 kA, the magnetic field strength continued to increase exponentially. The field flatness, BL ≡ (BLmax − BLmin )/BLmax , was measured to be 5 × 10−3 . It is likely that the measured flatness would cause a serious emittance blow-up of the beam of about 100%. To avoid hard radiation damage to the downstream components, the measured deterioration of the magnetic field is not acceptable at the J-PARC. It was also estimated that the driving force of the field deterioration was the eddy current in the thick endplates of the magnet. The measured magnetic field during the flat-top period was approximated by t t − β exp − . (1) Ieffective (t) = I0 1 − α exp − τ1 τ2
0.8 GTO switch 1
Time (s)
IGBT Chopper
FIG. 4. Measured magnetic field of the beam-switching magnet.
600 V
Integrated field (T m)
Gate Signals
1.930 1.925 1.920
Feedback Control Board
1.915 1.910
DCCT
PWM
1.905 0.1
0.2
0.3
0.4
0.5
0.6
Iref 2500 A /7FFF
DAC
+ -
GTO switch 2
K(1+1/sT) Iout
Time (s) FIG. 5. Enlarged view of the measured magnetic field during flat-top period.
FIG. 6. Conceptual schematic of the feedback control unit.
043301-3
Koseki, Nakayama, and Tawada
Rev. Sci. Instrum. 85, 043301 (2014) GTO switch 1
Integrated field (T m)
IGBT Chopper 600V Gate Signals
Feedback Control Board DCCT
PWM Iref 2500A/7FFF
DAC
+ +
-
GTO switch 2
K(1+1/sT)
Time (s)
Iout
FIG. 9. Measured magnetic field with the correction system. Sync. trigger DAC
Correction Pattern (Flash memory) Open-Loop Correction Board
FIG. 7. Conceptual schematic of the open-loop correction system.
Here, α and β are amplitudes of the eddy current, respectively. The time constants for two exponentials are indicated by τ 1 and τ 2 , respectively. The parameters from the approximation are summarized in Table I. III. OPEN-LOOP CORRECTION
The excitation current during the flat-top period is precisely generated by the IGBT chopper module, which is composed of IGBTs (see Fig. 3). The IGBTs are triggered by a feedback control system, in which deviation of the output current from the reference is calculated and pulse-widthmodulation is performed (see Fig. 6). The reference signal, indicated by Iref in Fig. 6, is a constant value. In order to solve the problem by the eddy current in the beam-switching magnet, two exponentials in Eq. (1) should be compensated. Thus, a time varying signal, Icomp in Eq. (2), was produced as a compensation signal. Moreover, an openloop correction system which generates the correction signal, Icomp , was newly developed. The conceptual schematic of the developed open-loop correction system is depicted in Fig. 7. By a trigger from the upper control system, the correction sig-
nal is read by an ADC from a flash memory in a synchronous manner with the GTO switch modules and added to the reference signal, Iref . A photograph of the developed open-loop correction system for the beam-switching magnet is shown in Fig. 8. In order to avoid the temperature drift of the analog correction signal, the open-loop correction board is installed in an aluminum shield box with a temperature stabilization system by a Peltier element: t t + β exp − . (2) Icomp (t) = I0 α exp − τ1 τ2 The integrated magnetic field was measured while using the open-loop correction system. An enlarged view of the measured integrated magnetic field during the flat-top period is shown in Fig. 9. The measurement shows that a field flatness of less than 5 × 10−4 , which is an acceptable value, was achieved. Moreover, it was confirmed that the higherfrequency component measured in the previously applied system was reduced sufficiently with the newly developed system. IV. CONCLUSION
A beam-switching magnet and the pulsed power supply have been developed. A serious deterioration of the integrated magnetic field was observed by using a long search coil. An eddy current in the endplates of the magnet disturbed the rapid increase of the magnetic field. The measured field can be well represented by a double-exponential equation. From parameters determined by the approximation, a correction pattern for the eddy current was produced. By adding the time varying compensation pattern to the reference signal, an open-loop correction system for the eddy current was developed. The integrated magnetic field with the newly developed open-loop correction system was measured. A field flatness of less than 5 × 10−4 , which is an acceptable value, was accomplished. ACKNOWLEDGMENTS
The present study is supported by members of KEKB, KEK-AR, and J-PARC. The authors are grateful for the continued support of Professor H. Kobayashi and Professor M. Yoshioka. 1 Y.
FIG. 8. Photograph of the developed open-loop correction system.
Yamazaki, KEK Report No. 2002-13, 2003. Hildred Blewett, Rev. Sci. Instrum. 24, 856 (1953).
2 M.
043301-4 3 S.
Koseki, Nakayama, and Tawada
Igarashi, T. Adachi, H. Someya, N. Tani, and Y. Watanabe, IEEE Trans. Appl. Supercond. 18(2), 289 (2008). 4 N. Tani, T. Adachi, H. Someya, Y. Watanabe, H. Sato, and J. Kishiro, IEEE Trans. Appl. Supercond. 14(2), 409 (2004). 5 K. Koseki, M. Tawada, H. Nakayama, H. Kobayasi, M. Shirakata, and K. Okamura, in Proceedings of the 10th European Particle Acceler-
Rev. Sci. Instrum. 85, 043301 (2014) ator Conference (EPAC’06), Edinburgh, Scotland, 26–30 June 2006, http://www.jacow.org, pp. 1747–1749. 6 M. A. Morich, D. A. Lampman, W. R. Dannels, and F. Goldie, IEEE Trans. Med. Imaging 7(3), 247 (1988). 7 Y. Chung, D. Barr, G. Decker, J. Galayda, F. Lenkszus, A. Lumpkin, and A. J. Votaw, Rev. Sci. Instrum. 67, 3371 (1996).
Review of Scientific Instruments is copyrighted by the American Institute of Physics (AIP). Redistribution of journal material is subject to the AIP online journal license and/or AIP copyright. For more information, see http://ojps.aip.org/rsio/rsicr.jsp
