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Active control system upgrade design for lower hybrid current drive system on Alcator C-Mod
Fusion Engineering and Design 87 (2012) 1981–1984
Contents lists available at SciVerse ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Active control system upgrade design for lower hybrid current drive system on Alcator C-Mod A.D. Kanojia ∗ , G.M. Wallace, D.R. Terry, J.A. Stillerman, W.M. Burke, P.A. MacGibbon, D.K. Johnson Massachusetts Institute of Technology Plasma Science and Fusion Center, Cambridge, MA, United States
h i g h l i g h t s Initial tests of the Hittite microwave components show good or better control of phase and amplitude when compared to the vector modulators used in the current system. With an analog based control component system the system complexity is dramatically reduced. Historically, D-tAcq hardware/software has performed more reliably on DPCS and FFT controllers than the current lower hybrid control system Cost and lead time of the Hittite microwave components is significantly small compared to vector modulators.
a r t i c l e
i n f o
Article history: Available online 8 June 2012 Keywords: Lower hybrid current drive Alcator C-Mod Microwave phase control Amplitude control
a b s t r a c t As a part of the scheduled expansion of the Alcator C-Mod lower hybrid current drive (LHCD) system from 12 to 16 klystrons to accommodate installation of a second LH antenna, the active control system (ACS) is being redesigned to accommodate the additional klystrons. Digitizers and output modules will be cPCI modules provided by D-tAcq Solutions. The real-time application will run on a standard PC server running Linux. Initially, the new ACS system will be designed to control 8 klystrons on the second LH antenna and the existing ACS will control the remaining 8 klystrons on the existing LH antenna. Experience gained operating the existing LHCD system has given us insight into the design of a more robust, compact, efficient and simple system for the new ACS. The design upgrade will be patterned on the digital plasma control system (DPCS [1]) in use on C-Mod. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The current LHCD system was designed for 12 klystrons, rated for 250 kW operating at 50 kV beam voltage, to provide a total source power of 3 MW at 4.6 GHz [2]. The current launcher, denoted LH-2, is driven by 10 klystrons. The existing active control system (ACS) controls the output amplitude and relative phasing of each klystron using I-Q vector modulators (VM). For monitoring and feedback it employs I-Q detectors. A single master oscillator provides drive for each klystron and a reference for the I-Q detectors. The ACS was designed as a pulsed system to run in two real-time modes: during C-Mod pulse (5 s or less) at a control rate of 9.09 kHz and between C-Mod pulses (10 min) at 1 kHz. In the between C-Mod pulse mode it uploads and downloads data and control parameters from the MDSplus system. In order to provide noise isolation and
∗ Corresponding author at: Massachusetts Institute of Technology Plasma Science and Fusion Center, 190 Albany Street, NW-21, Cambridge, MA 02139, United States. Tel.: +1 617 253 6686. E-mail address: [email protected] (A.D. Kanojia). 0920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fusengdes.2012.05.010
better debugging capabilities, the control hardware was located outside the tokamak containment area, called the cell. Custom, in house developed hardware was used to communicate with the microwave components via optical fibers. The ACS uses National Instruments controllers and data acquisition hardware. The controllers are PXI modules and run a QNX real-time operating system. The control software logic was developed in Matlab Simulink and code generation is done using Matlab real-time workshop toolbox. Using Opal-RT’s RT-Lab the generated code is split into real-time and non real-time components and compiled for the QNX platform on the host Windows PC. Hooks in the real-time code along with daemon threads on the targets provide communication and data exchange between the host and PXI. We plan to add an additional antenna (LH-3) and increase the number of klystrons from 12 to 16 with 8 klystrons powering each antenna. Operating the existing ACS over past run campaigns has highlighted the need for a simple, more robust and flexible system. Since maintenance and modification of the current ACS have become difficult and there is no obvious upgrade path, we are proposing a new ACS system which will incorporate the requirements of current ACS performance in addition to design
1982
A.D. Kanojia et al. / Fusion Engineering and Design 87 (2012) 1981–1984
Section 2) we hope to achieve a greater degree of robustness and the flexibility to modify and add to the control system. 2. Control hardware and methodology
Fig. 1. Constant amplitude and phase contours for the attenuator/phase shifter pair.
simplification, robustness and room for expansion. We intend to reduce complexity by minimizing custom hardware solutions, IO and cabling and by simplifying the hardware interface. We plan to provide the same level of equipment and personnel protection but in a more compact form. Using time tested controller hardware (see
One of the major design changes in the recommended upgrade is the replacement of VMs with individual voltage variable attenuators and phase shifters for controlling the klystron amplitude and phase (see Fig. 3). The models selected for the upgrade are manufactured by Hittite microwave, part numbers HMC973LP3E (attenuator) and HMC929LP4E (phase shifter). The typical insertion loss of the attenuator is 5.5 dB and can be controlled over 0–5 V giving an attenuation range of 28 dB. The phase shifter has a insertion loss of 4 dB and can be controlled over 0–10 V giving a phase shift range of 430◦ . For the VMs used in the original control system, the error at minimum attenuation is ±0.2 dB with a phase error of ±2◦ . With 15 dB of additional attenuation, the amplitude error is ±2 dB with a phase error of ±20◦ . The design change was not only prompted by the poor performance of the VM at higher attenuations, but also the lead time (6 months) to procure them and their cost. The cost of attenuator/phase shifter combo is 1/4 that of the VM and they can be easily obtained in two weeks or less. Another major advantage of going with the attenuator/phase shifter based system is the simplicity of design as the Hittite parts are analog controlled while the VM are digitally controlled, thus reducing the amount of cabling and number of outputs. Ideally phase shifters and attenuators are completely orthogonal (i.e., phase shifter has constant attenuation, attenuator has constant phase), however there is a ∼0.5 dB of variation in attenuation with the phase shifter and ∼30◦ variation in phase with the attenuator across their respective ranges. A look up table (LUT) is necessary to map between commanded voltages (VA , V˚ ) and measured amplitude (Aout ) and phase (˚out ). In order to generate a LUT a raster scan
Fig. 2. Schematic of interface board with controller monitor.
A.D. Kanojia et al. / Fusion Engineering and Design 87 (2012) 1981–1984
Fig. 3. System block diagram of lower hybrid current drive active control system. 1983
1984
A.D. Kanojia et al. / Fusion Engineering and Design 87 (2012) 1981–1984
in VA and V˚ is performed and the resulting phase and amplitude are measured using a portable network analyzer (PNA). The phase and amplitude data are inserted into two matrices, the contours of which are shown in Fig. 1. In order to determine the necessary control voltages a contour in each of the matrices is determined and then intersection of the two contours is found, thus identifying the required control voltages for a given Aout and ˚out . These values are inserted in a LUT to get a mapping between (Aout , ˚out ) and (VA , V˚ ). Another approach of solving the mapping problem is fitting surfaces to the calibration data. The two fitted surfaces are VA and V˚ and each of them is a function of Aout and ˚out . Initial tests of the Hittite components, at low as well as high power, show a total amplitude variation of ±0.1 dB at all attenuations values with a total phase variation of ±1◦ . We hope to improve the calibration methodology by using the PNA for phase measurements only and use a fast detector diode for amplitude calibrations for higher accuracy. Due to the nature of the different microwave components, primarily the two different sets of control parameters (Aout , ˚out vs. Iout , Qout ), the original calibration methodology could not be applied to the Hittite components and vice versa, thus the observed performance improvement cannot be qualitatively attributed to the Hittite components or the improved calibration methodology alone. To smoothly control LHCD at 1 kHz we require ACS to control at 10 kHz. The ACS requires the LHCD system to be synchronized to the global C-Mod clock. It also requires inter-channel and inter-board synchronicity. This upgrade will incorporate new data acquisition and control hardware which will meet the requirements. We plan to use a single 32 analog, 64 digital channel output board to drive the system and a single 32 analog and 32 digital channel input board from D-tAcq Solutions. The proposed ACS will incorporate the DtAcq hardware on a standard PC host server running Linux using an extended bus. By masking most of the interrupts on Linux, we can have the single thread control loop run in deterministic fashion. Operating system tools are used to set CPU affinity and vendor supplied drivers allow for interrupt masking similar to the controllers in use on Alcator C-Mod for digital plasma control system (DPCS [1]) and the fast ferrite tuner (FFT [3]). Initial testing shows all IO and computation can be accomplished in less than 80 s thus providing enough head room for future upgrades. 3. Protection It is imperative that we provide adequate protection, not only to protect the personnel but also to protect the expensive equipment which constitutes the LHCD system. A CPLD logic based interface board (between the controller and the low power microwave
system) will monitor the heart beat of the host computer (see Fig. 2). It will also prevent the control signals from going out to the low power microwave system in the event of a fault in the form of fizzle, RF leak or watchdog timer timeout, thus prevent the ACS from driving the klystrons. It will provide handshaking with the upgraded transmitter protection system (TPS [4]) to ensure safe operation. The coupler protection system (CPS [5]) and TPS will communicate with ACS via RF enable signals, and if either of the systems encounters faults the RF enable signals will be disabled to inhibit the ACS from driving the klystrons. In the event that control voltages for the attenuators are lost, the protection system will remove power supply voltage to the attenuators. This is necessary due to the fact that the failure state of the attenuator (control voltage floating or zero) results in minimum attenuation. Depowering the attenuator provides a minimum of 15 dB additional attenuation. 4. Summary Initial tests of the Hittite microwave components show that a combination of attenuator and phase shifter control provides as good or better control of phase and amplitude across a wide range of powers as compared to the VMs used in the current system. We intend to use analog IQ detectors similar to LH-2 system for amplitude and phase feedback using a simple proportional integral (PI) controller implemented in real-time. With an analog based control component system the system complexity is dramatically reduced. Our experience with DPCS and FFT systems has given us insight into a more robust and flexible control hardware and software. Based on the performance of LH-3 control system we intend to upgrade LH-2 system. References [1] J.A. Stillerman, M. Ferrara, T.W. Fredian, S.M. Wolfe, Digital real-time plasma control system for Alcator C-Mod, Fusion Engineering and Design 81 (July (15–17)) (2006). [2] D.R. Terry, et al., Lower hybrid current drive on Alcator C-Mod: system design, implementation, protection, calibration and performance, in: Proc. of the 22nd IEEE/NPSS Symposium on Fusion Engineering—SOFE07, P 3-3, IEEE Catalog Number: 07CH37901C, June 17–21, 2007 (CD-ROM). [3] Y. Lin, et al., Digital real-time control for an ICRF fast ferrite tuning system on Alcator C-Mod, Fusion Engineering and Design 83 (April (2–3)) (2008) 241–244. [4] D.R. Terry, et al., Transmitter protection system upgrade for lower hybrid current drive system on Alcator C-Mod, Fusion Science and Technology 56 (1) (2009) 119–124. [5] W. Burke, et al., The coupler protection system upgrade for lower hybrid current drive on Alcator C-Mod, in: Proc. of the 22nd IEEE/NPSS Symposium on Fusion Engineering—SOFE07, P 3-5, IEEE Catalog Number: 07CH37901C, June 17–21, 2007 (CD-ROM).
Contents lists available at SciVerse ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Active control system upgrade design for lower hybrid current drive system on Alcator C-Mod A.D. Kanojia ∗ , G.M. Wallace, D.R. Terry, J.A. Stillerman, W.M. Burke, P.A. MacGibbon, D.K. Johnson Massachusetts Institute of Technology Plasma Science and Fusion Center, Cambridge, MA, United States
h i g h l i g h t s Initial tests of the Hittite microwave components show good or better control of phase and amplitude when compared to the vector modulators used in the current system. With an analog based control component system the system complexity is dramatically reduced. Historically, D-tAcq hardware/software has performed more reliably on DPCS and FFT controllers than the current lower hybrid control system Cost and lead time of the Hittite microwave components is significantly small compared to vector modulators.
a r t i c l e
i n f o
Article history: Available online 8 June 2012 Keywords: Lower hybrid current drive Alcator C-Mod Microwave phase control Amplitude control
a b s t r a c t As a part of the scheduled expansion of the Alcator C-Mod lower hybrid current drive (LHCD) system from 12 to 16 klystrons to accommodate installation of a second LH antenna, the active control system (ACS) is being redesigned to accommodate the additional klystrons. Digitizers and output modules will be cPCI modules provided by D-tAcq Solutions. The real-time application will run on a standard PC server running Linux. Initially, the new ACS system will be designed to control 8 klystrons on the second LH antenna and the existing ACS will control the remaining 8 klystrons on the existing LH antenna. Experience gained operating the existing LHCD system has given us insight into the design of a more robust, compact, efficient and simple system for the new ACS. The design upgrade will be patterned on the digital plasma control system (DPCS [1]) in use on C-Mod. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The current LHCD system was designed for 12 klystrons, rated for 250 kW operating at 50 kV beam voltage, to provide a total source power of 3 MW at 4.6 GHz [2]. The current launcher, denoted LH-2, is driven by 10 klystrons. The existing active control system (ACS) controls the output amplitude and relative phasing of each klystron using I-Q vector modulators (VM). For monitoring and feedback it employs I-Q detectors. A single master oscillator provides drive for each klystron and a reference for the I-Q detectors. The ACS was designed as a pulsed system to run in two real-time modes: during C-Mod pulse (5 s or less) at a control rate of 9.09 kHz and between C-Mod pulses (10 min) at 1 kHz. In the between C-Mod pulse mode it uploads and downloads data and control parameters from the MDSplus system. In order to provide noise isolation and
∗ Corresponding author at: Massachusetts Institute of Technology Plasma Science and Fusion Center, 190 Albany Street, NW-21, Cambridge, MA 02139, United States. Tel.: +1 617 253 6686. E-mail address: [email protected] (A.D. Kanojia). 0920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fusengdes.2012.05.010
better debugging capabilities, the control hardware was located outside the tokamak containment area, called the cell. Custom, in house developed hardware was used to communicate with the microwave components via optical fibers. The ACS uses National Instruments controllers and data acquisition hardware. The controllers are PXI modules and run a QNX real-time operating system. The control software logic was developed in Matlab Simulink and code generation is done using Matlab real-time workshop toolbox. Using Opal-RT’s RT-Lab the generated code is split into real-time and non real-time components and compiled for the QNX platform on the host Windows PC. Hooks in the real-time code along with daemon threads on the targets provide communication and data exchange between the host and PXI. We plan to add an additional antenna (LH-3) and increase the number of klystrons from 12 to 16 with 8 klystrons powering each antenna. Operating the existing ACS over past run campaigns has highlighted the need for a simple, more robust and flexible system. Since maintenance and modification of the current ACS have become difficult and there is no obvious upgrade path, we are proposing a new ACS system which will incorporate the requirements of current ACS performance in addition to design
1982
A.D. Kanojia et al. / Fusion Engineering and Design 87 (2012) 1981–1984
Section 2) we hope to achieve a greater degree of robustness and the flexibility to modify and add to the control system. 2. Control hardware and methodology
Fig. 1. Constant amplitude and phase contours for the attenuator/phase shifter pair.
simplification, robustness and room for expansion. We intend to reduce complexity by minimizing custom hardware solutions, IO and cabling and by simplifying the hardware interface. We plan to provide the same level of equipment and personnel protection but in a more compact form. Using time tested controller hardware (see
One of the major design changes in the recommended upgrade is the replacement of VMs with individual voltage variable attenuators and phase shifters for controlling the klystron amplitude and phase (see Fig. 3). The models selected for the upgrade are manufactured by Hittite microwave, part numbers HMC973LP3E (attenuator) and HMC929LP4E (phase shifter). The typical insertion loss of the attenuator is 5.5 dB and can be controlled over 0–5 V giving an attenuation range of 28 dB. The phase shifter has a insertion loss of 4 dB and can be controlled over 0–10 V giving a phase shift range of 430◦ . For the VMs used in the original control system, the error at minimum attenuation is ±0.2 dB with a phase error of ±2◦ . With 15 dB of additional attenuation, the amplitude error is ±2 dB with a phase error of ±20◦ . The design change was not only prompted by the poor performance of the VM at higher attenuations, but also the lead time (6 months) to procure them and their cost. The cost of attenuator/phase shifter combo is 1/4 that of the VM and they can be easily obtained in two weeks or less. Another major advantage of going with the attenuator/phase shifter based system is the simplicity of design as the Hittite parts are analog controlled while the VM are digitally controlled, thus reducing the amount of cabling and number of outputs. Ideally phase shifters and attenuators are completely orthogonal (i.e., phase shifter has constant attenuation, attenuator has constant phase), however there is a ∼0.5 dB of variation in attenuation with the phase shifter and ∼30◦ variation in phase with the attenuator across their respective ranges. A look up table (LUT) is necessary to map between commanded voltages (VA , V˚ ) and measured amplitude (Aout ) and phase (˚out ). In order to generate a LUT a raster scan
Fig. 2. Schematic of interface board with controller monitor.
A.D. Kanojia et al. / Fusion Engineering and Design 87 (2012) 1981–1984
Fig. 3. System block diagram of lower hybrid current drive active control system. 1983
1984
A.D. Kanojia et al. / Fusion Engineering and Design 87 (2012) 1981–1984
in VA and V˚ is performed and the resulting phase and amplitude are measured using a portable network analyzer (PNA). The phase and amplitude data are inserted into two matrices, the contours of which are shown in Fig. 1. In order to determine the necessary control voltages a contour in each of the matrices is determined and then intersection of the two contours is found, thus identifying the required control voltages for a given Aout and ˚out . These values are inserted in a LUT to get a mapping between (Aout , ˚out ) and (VA , V˚ ). Another approach of solving the mapping problem is fitting surfaces to the calibration data. The two fitted surfaces are VA and V˚ and each of them is a function of Aout and ˚out . Initial tests of the Hittite components, at low as well as high power, show a total amplitude variation of ±0.1 dB at all attenuations values with a total phase variation of ±1◦ . We hope to improve the calibration methodology by using the PNA for phase measurements only and use a fast detector diode for amplitude calibrations for higher accuracy. Due to the nature of the different microwave components, primarily the two different sets of control parameters (Aout , ˚out vs. Iout , Qout ), the original calibration methodology could not be applied to the Hittite components and vice versa, thus the observed performance improvement cannot be qualitatively attributed to the Hittite components or the improved calibration methodology alone. To smoothly control LHCD at 1 kHz we require ACS to control at 10 kHz. The ACS requires the LHCD system to be synchronized to the global C-Mod clock. It also requires inter-channel and inter-board synchronicity. This upgrade will incorporate new data acquisition and control hardware which will meet the requirements. We plan to use a single 32 analog, 64 digital channel output board to drive the system and a single 32 analog and 32 digital channel input board from D-tAcq Solutions. The proposed ACS will incorporate the DtAcq hardware on a standard PC host server running Linux using an extended bus. By masking most of the interrupts on Linux, we can have the single thread control loop run in deterministic fashion. Operating system tools are used to set CPU affinity and vendor supplied drivers allow for interrupt masking similar to the controllers in use on Alcator C-Mod for digital plasma control system (DPCS [1]) and the fast ferrite tuner (FFT [3]). Initial testing shows all IO and computation can be accomplished in less than 80 s thus providing enough head room for future upgrades. 3. Protection It is imperative that we provide adequate protection, not only to protect the personnel but also to protect the expensive equipment which constitutes the LHCD system. A CPLD logic based interface board (between the controller and the low power microwave
system) will monitor the heart beat of the host computer (see Fig. 2). It will also prevent the control signals from going out to the low power microwave system in the event of a fault in the form of fizzle, RF leak or watchdog timer timeout, thus prevent the ACS from driving the klystrons. It will provide handshaking with the upgraded transmitter protection system (TPS [4]) to ensure safe operation. The coupler protection system (CPS [5]) and TPS will communicate with ACS via RF enable signals, and if either of the systems encounters faults the RF enable signals will be disabled to inhibit the ACS from driving the klystrons. In the event that control voltages for the attenuators are lost, the protection system will remove power supply voltage to the attenuators. This is necessary due to the fact that the failure state of the attenuator (control voltage floating or zero) results in minimum attenuation. Depowering the attenuator provides a minimum of 15 dB additional attenuation. 4. Summary Initial tests of the Hittite microwave components show that a combination of attenuator and phase shifter control provides as good or better control of phase and amplitude across a wide range of powers as compared to the VMs used in the current system. We intend to use analog IQ detectors similar to LH-2 system for amplitude and phase feedback using a simple proportional integral (PI) controller implemented in real-time. With an analog based control component system the system complexity is dramatically reduced. Our experience with DPCS and FFT systems has given us insight into a more robust and flexible control hardware and software. Based on the performance of LH-3 control system we intend to upgrade LH-2 system. References [1] J.A. Stillerman, M. Ferrara, T.W. Fredian, S.M. Wolfe, Digital real-time plasma control system for Alcator C-Mod, Fusion Engineering and Design 81 (July (15–17)) (2006). [2] D.R. Terry, et al., Lower hybrid current drive on Alcator C-Mod: system design, implementation, protection, calibration and performance, in: Proc. of the 22nd IEEE/NPSS Symposium on Fusion Engineering—SOFE07, P 3-3, IEEE Catalog Number: 07CH37901C, June 17–21, 2007 (CD-ROM). [3] Y. Lin, et al., Digital real-time control for an ICRF fast ferrite tuning system on Alcator C-Mod, Fusion Engineering and Design 83 (April (2–3)) (2008) 241–244. [4] D.R. Terry, et al., Transmitter protection system upgrade for lower hybrid current drive system on Alcator C-Mod, Fusion Science and Technology 56 (1) (2009) 119–124. [5] W. Burke, et al., The coupler protection system upgrade for lower hybrid current drive on Alcator C-Mod, in: Proc. of the 22nd IEEE/NPSS Symposium on Fusion Engineering—SOFE07, P 3-5, IEEE Catalog Number: 07CH37901C, June 17–21, 2007 (CD-ROM).