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New achievements in the EAST plasma control system
Fusion Engineering and Design 85 (2010) 474–477
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
New achievements in the EAST plasma control system夽 Q.P. Yuan a,∗ , B.J. Xiao a , B.G. Penaflor b , D.A. Piglowski b , L.Z. Liu a , R.D. Johnson b , M.L. Walker b , D.A. Humphreys b a b
Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China General Atomics, DIII-D National Fusion Facility, San Diego, CA, USA
a r t i c l e
i n f o
Keywords: Tokamak EAST PCS Reflective memory board Digital output structure Density control
a b s t r a c t In order to realize the low latency and distortion-free signal transmission between the plasma control system (PCS) and servo systems, the digital output structure configured with reflective memory board (RFM) was adopted in EAST PCS. And the enhanced performances are reported. Another achievement made in the latest EAST PCS was the implementation of density control algorithm, which controlled the line average density in either voltage or width modulation mode. The new integrated algorithm improved the precision of density calculation and control performance greatly. The details and experiment results are presented in this paper. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The plasma control system (PCS) on EAST, adapted from DIIID plasma control system [1], keeps being developed to satisfy the EAST experimental needs. In original EAST PCS, DAC (digital to analog convertor) was applied to output control commands to the servo systems, such as poloidal field power supply (PFPS), inner coil power supply (ICPS), and gas puffing system. Since these systems are located in different electromagnetic environment and separated distantly away from each other, the v–f/f–v (voltage to frequency or frequency to voltage) convertors were applied to transmit data in digital mode in order to avoid signal mutual interference and attenuation. However, this v–f/f–v photoelectric conversion technique always brought about the problems of system noise and bandwidth limit [2], which affected the control performance and system response especially for the plasma vertical instability control. In order to realize the low latency and distortion-free signal transmission, the PCS hardware structure was updated to realize the direct digital transfer among those systems without any peripheral facilities. The implementation details are described in Section 2.
夽 Supported by National Nature Science Foundation of China with contract number 10835009, the Key Project of Knowledge Innovation Program of Chinese Academy of Sciences, project number KJCX3.SYW.N4, the National 973 Project with number 2009GB103000 and the Knowledge Innovation Program of Hefei Institutes of Physical Science, project number 085FCQ0128. ∗ Corresponding author at: Institute of Plasma Physics, Chinese Academy of Sciences, Computer Application, Building 350, Shu Shan Hu Road, Hefei, Anhui 230031, China. Tel.: +86 5515591354; fax: +86 5515591310. E-mail address: [email protected] (Q.P. Yuan). 0920-3796/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2010.05.017
Density control is one of the most important control aspects in tokamak discharge. The original density control for EAST was performed in an independent system, which was inherited from the one on HT-7 tokamak [3]. In order to enhance the consistency of the whole plasma control process, the density control algorithm has been integrated into the latest EAST PCS. The details are presented in Section 3. The new implemented algorithm enhanced the accuracy of plasma density calculation, since it calculated the effective chord length, which was an important parameter correlating to plasma shapes in calculating the line average density. And the control performance is discussed in Section 4.
2. The digital output structure The EAST PCS is a mini cluster composed of one host and three real-time computing nodes. As shown in Fig. 1, the cluster nodes connecting with peripheral systems and hardware play different roles during the plasma control procedure [4]. In order to ensure the quality of transmitted data including control commands and real-time plasma shapes, the EAST PCS hardware layout keeps being updated and optimized. Now, a digital output structure has been established with a reflective memory board (RFM) installed in the third real-time node (pcsrt3) as shown in Fig. 1. RFM is an ultrahigh speed optic fiber network product which provides a very fast and efficient way of sharing data across distributed computer systems [5]. All the systems configured with RFM, such as PFPS, ICPS and real-time scope, construct a RFM network by connecting to the RFM switch with fibers. In such network, digital data can be transferred directly without any other peripheral facilities. The implementation details and improved performance will be reported in this section.
Q.P. Yuan et al. / Fusion Engineering and Design 85 (2010) 474–477
Fig. 1. Control system hardware structure mainly in digital output mode.
Fig. 2. The RFM memory layout in pcsrt3 of EAST PCS.
During the shot, control commands from pcsrt1 and plasma boundary data from pcsrt2 were transmitted to pcsrt3 through Myrinet network [6]. Then pcsrt3 wrote the date in its local RFM memory. Once the data were written, RFM network transmitted them automatically in direct memory access (DMA) way without interference to the system CPU. Due to the cycle difference in
475
Fig. 3. PS1 voltage comparison between PCS output and PFPS acquired in RFM mode for EAST shot 9003.
updating the commands and boundary data, the RFM memory was separated into two regions as shown in Fig. 2. The control commands for ICPS and PFPS were laid beginning at OFFSET 1 and updated each 50 s, while the plasma boundary data were written each 1 ms starting at OFFSET 2. The DMA transfer speed was fast enough to meet the demands of plasma control in such scenario [2]. For the power systems, the commands could be read at the same offsets in their local RFM memories. As shown in Fig. 3, the data received by PFPS, which is denoted in yellow solid line, completely consist with the PCS output command which is denoted in red dashed line. (For interpretation of the references to color in this text, the reader is referred to the web version of this article.) After reading data from OFFSET 2 of the local RFM memory, the real-time scope computer displays the data on a high-resolution graphic screen in the control room. As shown in Fig. 4, this display shows a cross-section of the EAST vessel and the real-time boundary of the plasma shape during a shot. Subtle shading gives an illumination to experimentalists about the important data such as the proximity of coil currents to their maximum value. In this way, the real-time display has become an indispensable tool to assist the experimentalists.
Fig. 4. Selected real-time boundary frames for EAST shot 10745.
476
Q.P. Yuan et al. / Fusion Engineering and Design 85 (2010) 474–477
Fig. 5. The result of effective chord length and line average density calculation.
Fig. 7. The control performance in voltage modulation mode at shot 10101, plasma current = 250 kA, density target = 1.2e19 /m3 .
3. Implementation of the density control algorithm On EAST, a vertical one-channel far-infrared (FIR) hydrogen cyanide (HCN) laser interferometer is used to measure the plasma density [7]. The acquired raw data is the phasic difference between the diagnostic and referenced laser. When such signal is varied around a period, namely 2, signal analysis must consider the possible 2 overturns in order to get back the real phasic difference ϕ. Then, the line average density is calculated using formula (1), ne =
1 L
z2
ne (z) dz = 3.55 × 1014 z1
ϕ L
(1)
where is the wavelength, and L is the effective chord length. During a real plasma shot, the effective chord length depends on the plasma shape which is variational in different scenarios and immeasurable but reconstructed with magnetic diagnostic data. In original density control system, L was taken as a unity since the plasma shape was unknown. This brought huge errors for the density calculation, especially when the plasma was elongated. In last EAST operation campaign, with the plasma shapes obtained from RTEFIT [8] and the FIR channel position which is in the center with R = 1.82 m, the effective chord length can be calculated in real time as shown in the last plot of Fig. 5. The HCN raw data are sampled at rate 10 kHz as shown in the first plot. With the digital signal processing, the line average density is calculated and denoted in black solid line in the second plot, while the density with L as a unity is represented by blue cross line. Obviously, the density calculation result in the new implemented algorithm is more accurate to illuminate the plasma density. The plasma density is controlled through modulating the voltage or width of valve puffing pulse each 1 ms, since it is a slow
response system. The control process in both control modes is the same as shown in Fig. 6. When gas on/off switch is off for a control cycle, zero command will be transmitted to the gas valve. Otherwise, the feedback loop, a single-input–single-output (SISO) system with a proportionalintegral-derivative (PID) controller is operative. According to the control mode set by operators, the command is interpreted to continuous voltage or pulse width at fixed voltage.
4. Experimental results In voltage modulation mode, the gas puffing quantity depends on the continuous command voltage. For EAST shot 10101, with the calculated chord length (in the third plot) and measured HCN signal (in the second plot), the line average density is calculated and denoted as black triangles in the fourth plot of Fig. 7. Compared with the density target, the error value is generated as shown in the fifth plot. With PID operation, the corresponding control command is shown in the last plot. Before time 0, the first puffing is commanded using the feedforward part. After the plasma is initialized, the feedback control works. But a suitable feedforward setting is always needed as the valve unlocking voltage during a whole shot. The final command is the sum of feedforward setting and feedback calculation. The results indicate that plasma density can be controlled in such voltage modulation mode. But this control has a certain extent delay caused by the non-linear factors of valves. Besides, the accumulative effect of continuous puffing brings the challenges for operators to choose the proper feedforward setting and feedback control parameters.
Fig. 6. The logic of density control algorithm.
Q.P. Yuan et al. / Fusion Engineering and Design 85 (2010) 474–477
477
and flattop phase. The reason is that there is a typical linear relation between the gas puffing quality and the pulse width when the pulse voltage is in a high level [3]. 5. Summary We have reported the recent progresses made in improving EAST PCS performance. By using RFM, digital data are exchanged effectively in low latency among the control system and servo systems. The plasma density control is implemented. Either control mode has been verified in the last EAST campaign. Conclusively, the new implemented algorithm has more accurate density calculation and better control performance.
Fig. 8. The control performance in pulse width modulation mode at shot 12820, plasma current = 300 kA, density target = 2.0e19 /m3 .
Being different with the above voltage control, in pulse width modulation mode, the plasma density is controlled by adjusting the puffing pulse width with fixed pulse amplitude. During a shot, the feedforward setting is only used for the first puffing and a new PID calculation is done only after last puffing pulse is completely finished. Thus, the control command is a series of pulses with different width as shown in the last plot of Fig. 8. Obviously, the density is much better controlled to follow the target during the ramp up
References [1] J.R. Ferron, B. Penaflor, M.L. Walker, et al., Fusion Engineering 2 (1996) 870. [2] Q.P. Yuan, B.J. Xiao, B. Penaflor, et al., Plasma Science and Technology 11 (2009) 734. [3] J.R. Luo, Z.S. Ji, X.M. Wang, et al., Nuclear Fusion and Plasma Physics 21 (2001) 237. [4] B.J. Xiao, D.A. Humphreys, M.L. Walker, et al., Fusion Engineering and Design 83 (2008) 181. [5] M. Jovanovic, V. Milutinovic, IEEE Concurrency 7 (1999) 56. [6] Myricom Inc., Myrinet. http://www.myri.com. [7] X. Gao, The EAST Team, Physics Letters A 372 (2008). [8] J.R. Ferron, M.L. Walker, L.L. Lao, et al., Nuclear Fusion 38 (1998) 1055.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
New achievements in the EAST plasma control system夽 Q.P. Yuan a,∗ , B.J. Xiao a , B.G. Penaflor b , D.A. Piglowski b , L.Z. Liu a , R.D. Johnson b , M.L. Walker b , D.A. Humphreys b a b
Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China General Atomics, DIII-D National Fusion Facility, San Diego, CA, USA
a r t i c l e
i n f o
Keywords: Tokamak EAST PCS Reflective memory board Digital output structure Density control
a b s t r a c t In order to realize the low latency and distortion-free signal transmission between the plasma control system (PCS) and servo systems, the digital output structure configured with reflective memory board (RFM) was adopted in EAST PCS. And the enhanced performances are reported. Another achievement made in the latest EAST PCS was the implementation of density control algorithm, which controlled the line average density in either voltage or width modulation mode. The new integrated algorithm improved the precision of density calculation and control performance greatly. The details and experiment results are presented in this paper. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The plasma control system (PCS) on EAST, adapted from DIIID plasma control system [1], keeps being developed to satisfy the EAST experimental needs. In original EAST PCS, DAC (digital to analog convertor) was applied to output control commands to the servo systems, such as poloidal field power supply (PFPS), inner coil power supply (ICPS), and gas puffing system. Since these systems are located in different electromagnetic environment and separated distantly away from each other, the v–f/f–v (voltage to frequency or frequency to voltage) convertors were applied to transmit data in digital mode in order to avoid signal mutual interference and attenuation. However, this v–f/f–v photoelectric conversion technique always brought about the problems of system noise and bandwidth limit [2], which affected the control performance and system response especially for the plasma vertical instability control. In order to realize the low latency and distortion-free signal transmission, the PCS hardware structure was updated to realize the direct digital transfer among those systems without any peripheral facilities. The implementation details are described in Section 2.
夽 Supported by National Nature Science Foundation of China with contract number 10835009, the Key Project of Knowledge Innovation Program of Chinese Academy of Sciences, project number KJCX3.SYW.N4, the National 973 Project with number 2009GB103000 and the Knowledge Innovation Program of Hefei Institutes of Physical Science, project number 085FCQ0128. ∗ Corresponding author at: Institute of Plasma Physics, Chinese Academy of Sciences, Computer Application, Building 350, Shu Shan Hu Road, Hefei, Anhui 230031, China. Tel.: +86 5515591354; fax: +86 5515591310. E-mail address: [email protected] (Q.P. Yuan). 0920-3796/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2010.05.017
Density control is one of the most important control aspects in tokamak discharge. The original density control for EAST was performed in an independent system, which was inherited from the one on HT-7 tokamak [3]. In order to enhance the consistency of the whole plasma control process, the density control algorithm has been integrated into the latest EAST PCS. The details are presented in Section 3. The new implemented algorithm enhanced the accuracy of plasma density calculation, since it calculated the effective chord length, which was an important parameter correlating to plasma shapes in calculating the line average density. And the control performance is discussed in Section 4.
2. The digital output structure The EAST PCS is a mini cluster composed of one host and three real-time computing nodes. As shown in Fig. 1, the cluster nodes connecting with peripheral systems and hardware play different roles during the plasma control procedure [4]. In order to ensure the quality of transmitted data including control commands and real-time plasma shapes, the EAST PCS hardware layout keeps being updated and optimized. Now, a digital output structure has been established with a reflective memory board (RFM) installed in the third real-time node (pcsrt3) as shown in Fig. 1. RFM is an ultrahigh speed optic fiber network product which provides a very fast and efficient way of sharing data across distributed computer systems [5]. All the systems configured with RFM, such as PFPS, ICPS and real-time scope, construct a RFM network by connecting to the RFM switch with fibers. In such network, digital data can be transferred directly without any other peripheral facilities. The implementation details and improved performance will be reported in this section.
Q.P. Yuan et al. / Fusion Engineering and Design 85 (2010) 474–477
Fig. 1. Control system hardware structure mainly in digital output mode.
Fig. 2. The RFM memory layout in pcsrt3 of EAST PCS.
During the shot, control commands from pcsrt1 and plasma boundary data from pcsrt2 were transmitted to pcsrt3 through Myrinet network [6]. Then pcsrt3 wrote the date in its local RFM memory. Once the data were written, RFM network transmitted them automatically in direct memory access (DMA) way without interference to the system CPU. Due to the cycle difference in
475
Fig. 3. PS1 voltage comparison between PCS output and PFPS acquired in RFM mode for EAST shot 9003.
updating the commands and boundary data, the RFM memory was separated into two regions as shown in Fig. 2. The control commands for ICPS and PFPS were laid beginning at OFFSET 1 and updated each 50 s, while the plasma boundary data were written each 1 ms starting at OFFSET 2. The DMA transfer speed was fast enough to meet the demands of plasma control in such scenario [2]. For the power systems, the commands could be read at the same offsets in their local RFM memories. As shown in Fig. 3, the data received by PFPS, which is denoted in yellow solid line, completely consist with the PCS output command which is denoted in red dashed line. (For interpretation of the references to color in this text, the reader is referred to the web version of this article.) After reading data from OFFSET 2 of the local RFM memory, the real-time scope computer displays the data on a high-resolution graphic screen in the control room. As shown in Fig. 4, this display shows a cross-section of the EAST vessel and the real-time boundary of the plasma shape during a shot. Subtle shading gives an illumination to experimentalists about the important data such as the proximity of coil currents to their maximum value. In this way, the real-time display has become an indispensable tool to assist the experimentalists.
Fig. 4. Selected real-time boundary frames for EAST shot 10745.
476
Q.P. Yuan et al. / Fusion Engineering and Design 85 (2010) 474–477
Fig. 5. The result of effective chord length and line average density calculation.
Fig. 7. The control performance in voltage modulation mode at shot 10101, plasma current = 250 kA, density target = 1.2e19 /m3 .
3. Implementation of the density control algorithm On EAST, a vertical one-channel far-infrared (FIR) hydrogen cyanide (HCN) laser interferometer is used to measure the plasma density [7]. The acquired raw data is the phasic difference between the diagnostic and referenced laser. When such signal is varied around a period, namely 2, signal analysis must consider the possible 2 overturns in order to get back the real phasic difference ϕ. Then, the line average density is calculated using formula (1), ne =
1 L
z2
ne (z) dz = 3.55 × 1014 z1
ϕ L
(1)
where is the wavelength, and L is the effective chord length. During a real plasma shot, the effective chord length depends on the plasma shape which is variational in different scenarios and immeasurable but reconstructed with magnetic diagnostic data. In original density control system, L was taken as a unity since the plasma shape was unknown. This brought huge errors for the density calculation, especially when the plasma was elongated. In last EAST operation campaign, with the plasma shapes obtained from RTEFIT [8] and the FIR channel position which is in the center with R = 1.82 m, the effective chord length can be calculated in real time as shown in the last plot of Fig. 5. The HCN raw data are sampled at rate 10 kHz as shown in the first plot. With the digital signal processing, the line average density is calculated and denoted in black solid line in the second plot, while the density with L as a unity is represented by blue cross line. Obviously, the density calculation result in the new implemented algorithm is more accurate to illuminate the plasma density. The plasma density is controlled through modulating the voltage or width of valve puffing pulse each 1 ms, since it is a slow
response system. The control process in both control modes is the same as shown in Fig. 6. When gas on/off switch is off for a control cycle, zero command will be transmitted to the gas valve. Otherwise, the feedback loop, a single-input–single-output (SISO) system with a proportionalintegral-derivative (PID) controller is operative. According to the control mode set by operators, the command is interpreted to continuous voltage or pulse width at fixed voltage.
4. Experimental results In voltage modulation mode, the gas puffing quantity depends on the continuous command voltage. For EAST shot 10101, with the calculated chord length (in the third plot) and measured HCN signal (in the second plot), the line average density is calculated and denoted as black triangles in the fourth plot of Fig. 7. Compared with the density target, the error value is generated as shown in the fifth plot. With PID operation, the corresponding control command is shown in the last plot. Before time 0, the first puffing is commanded using the feedforward part. After the plasma is initialized, the feedback control works. But a suitable feedforward setting is always needed as the valve unlocking voltage during a whole shot. The final command is the sum of feedforward setting and feedback calculation. The results indicate that plasma density can be controlled in such voltage modulation mode. But this control has a certain extent delay caused by the non-linear factors of valves. Besides, the accumulative effect of continuous puffing brings the challenges for operators to choose the proper feedforward setting and feedback control parameters.
Fig. 6. The logic of density control algorithm.
Q.P. Yuan et al. / Fusion Engineering and Design 85 (2010) 474–477
477
and flattop phase. The reason is that there is a typical linear relation between the gas puffing quality and the pulse width when the pulse voltage is in a high level [3]. 5. Summary We have reported the recent progresses made in improving EAST PCS performance. By using RFM, digital data are exchanged effectively in low latency among the control system and servo systems. The plasma density control is implemented. Either control mode has been verified in the last EAST campaign. Conclusively, the new implemented algorithm has more accurate density calculation and better control performance.
Fig. 8. The control performance in pulse width modulation mode at shot 12820, plasma current = 300 kA, density target = 2.0e19 /m3 .
Being different with the above voltage control, in pulse width modulation mode, the plasma density is controlled by adjusting the puffing pulse width with fixed pulse amplitude. During a shot, the feedforward setting is only used for the first puffing and a new PID calculation is done only after last puffing pulse is completely finished. Thus, the control command is a series of pulses with different width as shown in the last plot of Fig. 8. Obviously, the density is much better controlled to follow the target during the ramp up
References [1] J.R. Ferron, B. Penaflor, M.L. Walker, et al., Fusion Engineering 2 (1996) 870. [2] Q.P. Yuan, B.J. Xiao, B. Penaflor, et al., Plasma Science and Technology 11 (2009) 734. [3] J.R. Luo, Z.S. Ji, X.M. Wang, et al., Nuclear Fusion and Plasma Physics 21 (2001) 237. [4] B.J. Xiao, D.A. Humphreys, M.L. Walker, et al., Fusion Engineering and Design 83 (2008) 181. [5] M. Jovanovic, V. Milutinovic, IEEE Concurrency 7 (1999) 56. [6] Myricom Inc., Myrinet. http://www.myri.com. [7] X. Gao, The EAST Team, Physics Letters A 372 (2008). [8] J.R. Ferron, M.L. Walker, L.L. Lao, et al., Nuclear Fusion 38 (1998) 1055.