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Design of electronics system for Langmuir probes on ITER
Fusion Engineering and Design 152 (2020) 111429
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Design of electronics system for Langmuir probes on ITER a,
a
a
a
a
T b
Yali Wang *, Wei Zhao , Li Zhao , Lin Nie , Guangwu Zhong , Christopher Watts , James Paul Gunnc a b c
Southwestern Institute of Physics, P.O. Box 432, Chengdu, Sichuan, 610041, China ITER Organization, Route de Vinon-sur-Verdon, F-13067, St. Paul Lez Durance Cedex, France CEA, IRFM, F-13108, Saint-Paul-Lez-Durance, France
A R T I C LE I N FO
A B S T R A C T
Keywords: Langmuir probes Probe power supply Signal conditioning Operation switching Current-voltage characteristic
An electronics system for Langmuir probes to be used on ITER has been developing. The electronics system should meet the physics measurement requirements, which means that the electronics system should include power supply for driving probes, single and double probes operation mode switching circuit, measurement signal conditioning circuit, control and long cable transmission. It would make sense to use a single power supply to control a probe pair, which could be configured for one of the two operating modes foreseen and reduce costs while minimizing risk. The R&D of power supply is most critical activity. The power supply is characterized by 2.5 A, 175 V rating. At the same time, the power supply has been designed to ensure output waveforms ripple, high frequency, four quadrant and compatibility with the electromagnetic noise close to the components and with the plasma etc., which behaves as a nonlinear rapidly varying load. For reducing risk of R&D and exploring different technical prototype, one of the technical prototypes based on a full-bridge converter concept, using a pulse width modulation (PWM) technique is relatively simple and adopted first. Relays array circuit and adequate voltage and current signal conditioning circuit are also effective guarantee for data acquisitioning and drawing the probe current-voltage characteristic. The main design activities of electronics system are presented as detailed as possible in this paper. Besides, the detailed design activities should be continuously optimized and changed based on the results of the future relevant tests and changed requirements.
1. Introduction Langmuir probes have been successfully implemented in nearly every tokamak in the world, such as HL-2A [1], EAST [2], Tore Supra [3], DIII-D [4], JET tokamak [5], JT60-SA [6]. The primary aim of the Langmuir probes on ITER is to measure the plasma parameters at the divertor target plates. The essential function of the system is to provide information about the state of plasma attachment in the divertor region, with both advanced control and physics functions. For machine control, it is envisioned that the probes provide a signal indicating the attached/ detached state of the divertor plasma, while for physics studies the system will need to supply more detailed measurements of the divertor plasma density and temperature. The Langmuir probe system can be broken down into several parts: the probe sensors themselves, the driver power supplies, signal conditioning, control, data acquisition and data processing. The backend electronics will be located in the diagnostics building, with 24 electronics cubicles. Ideally, the power supplies and associated electronics should be located as close to the sensors as possible to minimize voltage droop and capacitive effects of the ⁎
cable. However, during ITER operation the port cell and gallery area close to the tokamak are affected by gamma-ray radiation and stray magnetic field which are much more profound than in any existing tokamak device. These special conditions have important impact on design of the ITER Langmuir probe electronics. Any electronics located in the port cell require shielding of for both radiation and magnetic field to keep the electronics from malfunctioning. Further, the space in the port cell is limited for placing probe electronics. Another concern with this option: accessibility to ITER port cells is very limited, only available after the shot down of the ITER tokamak. It means that maintainability of electronics in ITER port cells will be poor. These considerations, therefore, recommend deployment of Langmuir electronics in the diagnostic building. This option avoids the costly, difficult magnetic and radiation shielding. But long cable (the longest distance is 300 m) that is associated with connecting the probe sensors in the tokamak with the backend electronics hinder measurement of fast physics process, such as plasma fluctuation, details of ELMs bursts [7]. In tokamak fusion devices, some probes are usually driven by off the shelf products, other probes could be driven by customized power
Corresponding author. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.fusengdes.2019.111429 Received 17 January 2019; Received in revised form 10 October 2019; Accepted 6 December 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 152 (2020) 111429
Y. Wang, et al.
size and high reliability. So it is necessary to consider comprehensively and design the electronics system for Langmuir probes in ITER. In this paper, the main technical specifications is first presented; cubicle layout, power supply, the probes operation mode switching circuit, measurement signal conditioning circuit are then described, together with control of electronics.
2. The main techinical specifications 2.1. Design of langmuir probe Langmuir probes, or electrostatic probes, are the first plasma diagnostic, developed by Irving Langmuir in the 1920s. The principle is relatively simple: a conductor in contact with the plasma is biased with a voltage V relative to the local plasma potential, collects the I-V curve of the plasma, and then processes the I-V curve to obtain the plasma parameters. In ITER, there are 400 probes attached to the sides of inner and outer target distributed on five divertor cassettes, probes need to survive under 10 MW/m2 high heat flux for 400 s and 20 MW/m2 for 10 s. As shown in Fig. 1, the Langmuir probe consists of a 2-mm-diameter tungsten rod as the actual probe, surrounded by a tungsten cylinder acting as a shield for the heat and plasma, between the probe and the shield is an alumina pipe as electrical insulation [8]. It is foreseen that the probes on ITER will be operated in one of two configurations: 1) operation in DC bias ion saturation current mode, meant to be used to
Fig. 1. Structure of Langmuir Probe.
supplies. But the performances of the power supplies have certain limitations that are only for their respective systems. In recent years, there are great development in power electronics and power semiconductor. The new technology should be used in design. Meantime, there are specific rules and guidelines in ITER, for example, all electronics components need lower heat dissipation and to be placed in cubicle, which require electronics system is of high efficiency, small
Fig. 2. Ion saturation current drawn by 2 mm probe along the divertor inner and outer. (a) Ion saturation current drawn by a 2 mm probe along the divertor outer in standard discharge cases. (b) Ion saturation current drawn by a 2 mm probe along the divertor inner in standard discharge cases. (c) Ion saturation current drawn by a 2 mm probe along the divertor inner in ELMy discharge cases. (d) Ion saturation current drawn by a 2 mm probe along the divertor outer in ELMy discharge cases. 2
Fusion Engineering and Design 152 (2020) 111429
Y. Wang, et al.
Table 1 Dissipation power of each cubicle. Description
Dissipation Power (W)
Height (U)
High performance computer (HPC) Fast controller chassis (PXIe chassis) PLC system (Slow controller) Switching & Conditioning 8 probe Power supplies Network switch cooling Fan Total: 1.8 kW
107 191 10 80 1200 35 150
4 4 4 3 16 1
2.2. The main technical specification To assess whether the probes can meet requirements, two sets of data have been used for analysis, the first was generated as part of the “standard” scenarios for use by diagnostic systems for benchmarking the designs. The second set was provided to understand the effects of ELMs for the probes. Firstly, for Ion saturation current, because the maximum total perpendicular ion flux incident on the divertor target is 10A/mm2, thus, a Langmuir probe bias strongly negative into ion saturation regime would draw this amount of current. The angle of incidence of the magnetic field, however, modifies this value by a factor sinθB, reducing the current to a few percent; for the nominal 3° angle, the maximum current drawn by a flush mounted probe is ∼0.5 A/mm2. A value of θB = 2.7° for the outer target and θB = 3.2° for the inner target in the simulations. Based on the corresponding density and temperature data, the corresponding current drawn by a Langmuir probe biased into ion saturation was calculated. For the plots in Fig. 2, we choose a 2 mm diameter circular, flush mounted probe biased to -150 V. Zeff is assumed to be 1, and the ion mass either 2 or 1. For the standard discharge cases the probes measure as expected when bias to ion saturation, shown in Fig. 2(a),(b), except in the case of discharge 2292. The reason for this is the unusually high electron temperature, > 40 eV. In general, a probe should be biased > ∼3 times the electron temperature to reasonably approach ion saturation (e3 = ∼5 %). However, biasing a probe to much above 100 V risks arcing, which can damage both the probe and backend electronics and even may cause increase cost of backend electronics. For the ELMy discharges, the ion saturation current at the inner target is well behaved, while in the outer target is not, shown in Fig. 2(c) and (d). For measurement of the electron temperature and density the probes will be operated in swept double-probe configuration. Swept single-probe mode is not foreseen as there is significant risk of drawing catastrophically large currents in the electron saturation regime. Fig. 3(a) is the expected double probe trace for standard parameters at the OVT. The characteristic hyperbolic tangent shape as the probe is biased deeply into ion saturation. Fig. 3(b) is the trace for one of the high temperature cases, where there is no clear saturation of the probe, making estimation of the plasma parameters impossible. Actually, in ITER the divertor Langmuir probes are widely separated (12–24 mm), and the interpretation of the characteristic of such a probe pair may not be straightforward. In summary, these simulations indicate that with a maximum probe bias voltage of ± 150 V the probes will draw no more than 0.5A during normal operation. Operating the probes beyond this 150 V range is not recommended due to the risk of arcing. This restriction, however, means that for some divertor plasma conditions the probes cannot provide meaningful measurements because the probes cannot be biased to ion saturation due to the high electron temperature. Secondly, considering ELMs, power supply should not be protected when output current is not higher 10 A in 2.5 ms. This is a reasonable compromise based on the R&D, size and safety of power supply.
Fig. 3. Probe characteristic for several typical cases of a swept probe. The probe is located near the strike point. (a) Expected double probe trace for standard parameters at the OVT. (b) The double probe trace for one of the high temperature cases.
Fig. 4. The functional block diagram of Langmuir probe system.
measure the ion flux for advanced control, 2) operation in swept double probe mode using two adjacent probes to measure the plasma electron temperature and density at the target for offline physics studies. Electronics system must be capable of meeting the configuration requirements via driving relay units. For a single power supply, the powered two probes can work in single or double probe mode.
3
Fusion Engineering and Design 152 (2020) 111429
Y. Wang, et al.
Table 2 Technical specification of PS. Main parameter
Range
Input voltage/current Output voltage/current External control Ripple Stability Bandwidth Distortion Regulation Temperature coefficient Output range Over current in short time periods,
230 V/2.5 A 175 V/2.5 A −10 V to +10 V 80 mV ≦0.1 %/Hr typ 0–15 kHz 0.5 % Line: 0.1 %; load: 0.5 % ≦0.02 %/°C; Four quadrant 10 A in 2.5 ms
3. Layout of the cubicle 3.1. Functional block of Langmuir probe system The functional block diagram of Langmuir probe system is shown in Fig. 4. Power supply for probe is actually a power amplifier and capable of amplifying an arbitrary waveform. It is controlled by external signal. PXIe 6368 produces the signal so as to control power supply. PXIe 6368 is NI X Series multifunction data acquisition device and analogue output (AO) maximum range is ± 10 V. A single power supply is used to control a pair probe, the output of power supply would be applied to the probe via probe operation mode switching circuit (POMSC) and long transmission, the POMSC can finish single and double probe mode switching by using of relays. At the same time, voltage and current of the probe are measured and sent to PXIe 6368. At last, data would be acquired and transferred to Control, Data Access and Communication system (CODAC). 3.2. Layout of the cubicle and components dissipation As mentioned above, the backend electronics will be located in the diagnostics building, with 24 electronics cubicles. Every cubicle contains the entire mentioned components below, Table 1. where fast controller is made up of HPC (high performance computer) and PXIe chassis, will realize the alarm handling, event handling, logging, data communication functions, data acquisition and data analysis, and fast controllers shall be used for the signal monitoring and controller of power supply, the slow controller is used for cubicle health monitoring. To ensure compliance with volume allocation, monitoring and environmental constraints, the main components shall be integrated in standard ITER instrument and control cubicles, shown in Fig. 5. The floor standing cubicle (47 U, 2200 mm (H) × 800 mm (W) × 800 mm (D)) with 718 m³/h fan is recommended, and the total heat dissipation from all electrical and electronic components inside the cubicle is between 1–2 kW. According to Table 1, for the off-the-shelf components, height and power dissipation is defined, so it is necessary to meet high efficiency and small size for in-house developed components. At present, the estimated total heat dissipation is 1.8 kW, with a safety margin of 20 % assumed, total power consumption is about 2.2 kW, which is higher than the heat dissipation capacity (2 kW). It is reasonable and feasible to optimize probes distribution and reduce heat dissipation. That is some probes are from the high-current strike point region and some are from the low-current areas in one cubicle.
Fig. 5. Layout of cubicle.
Thirdly, plasma behaves either as a nonlinear resistor or as a current source requiring a four-quadrant power supply, so the power supply has source and sink current characteristics The plasmas are characterized by a high level of fluctuations, wide spread in frequency. It's predictable that there's a lot of noise and fluctuation on probe current, which requires power supply has high stability and very lower voltage ripple.
4. Design of power supply 4.1. Technical specification of the power supply Considering the design margin, the power supply that is based on a full-bridge converter concept, using a pulse width modulation (PWM) technique is characterized by a 2.5 A, 175 V rating. The technical 4
Fusion Engineering and Design 152 (2020) 111429
Y. Wang, et al.
Fig. 6. topology diagram of power supply.
supply providing power to the H bridge circuit (DC link). 2) H bridge circuit: switch mode amplifier, the principle is using a pulse width modulation (PWM) technology, produce variable duty cycle pulse train which then acts as input, the power stage output take the form of the pulse train, rapidly switching between a zero volt and some maximum (positive or negative) voltage. Because the power mosfet permits high switching frequencies and dissipate 3 % of the controlled power (compared with at least 21 % for class B amplifier design), so the power supply can be made compact and lower dissipation. 3) Absorption and consumption energy circuit: current sink time is determined by the power rating of heat dissipation resistor R1, for safety margin, the sink capacity may match with source capacity of the power supply, As heat dissipation capacity of the power supply increases, so do the cooling and size, at the same time, which may be lead to over-temperature and exceed heat dissipation capacity of cubicle, then trigger health monitor system of cubicle. So it is necessary to control net electron collection operation time. When thus by monitoring the voltage across the energy-storage capacitor C0, Q1 would timely turns on. And the output of power supply need to be stable and with small fluctuation [11,12]. 4) Filter circuit: as switching frequency is 300 kHz and output voltage bandwidth is 0–15 kHz, the cut off frequency of the LC filter can be set 75 kHz, so the L2 = 20 μH, C2 = 220 nF. For RC damping, in general, C3 is 5–10 times the value of C2. So the R2 = 14 Ω. The CM inductor L3 = 47 μH/5 A. CY1, CY2, CY3 and CY4 are safety ceramic cap. The values are100 nF.
Fig. 7. Output voltage and corresponding current for resistive load. (a) Purple curve: 50 V /DIV; blue curve: 1 A/DIV; time: 500 μs/DIV. (b) Purple curve: 50 V/DIV; blue curve: 1 A/DIV; time: 500 μs/DIV.
specification of the power supply is shown in Table 2 [9,10]. 4.3. Preliminary test waveform 4.2. Scheme of the power supply At present, the preliminary test has been performed, output voltage and corresponding current for resistive load are reported in Fig. 7. Fig.7(a) is triangular swept waveform, the peak- peak value of output voltage is +/−175 V (blue curve) and the peak- peak value of output current is +/−2.5 A (purple curve), frequency is 1 kHz; Fig. 7(b) is sine
The power supply comprises four main elements, shown in Fig. 6. 1) AC-DC circuit: AC power is transmitted to DC power. DC power
Fig. 8. Applied voltage trace for probe system. 5
Fusion Engineering and Design 152 (2020) 111429
Y. Wang, et al.
Fig. 9. Realy unit for probe operation mode switching.
performed on some fusion devices.
Table 3 The measurement quantities for voltage and current. Parameter
Range
Relative accuracy
Current voltage
+/−1 mA to +/−5 A +/−200 V
≧0.5 % ≧0.5 %
5. Design of probe operation mode switching and signal conditioning The operation procedure of diagnostics plants in ITER usually includes the following steps [13,14]: Health (check) Configuration Calibration Start and stop measurements Troubleshoot 5.1. Probe operation mode switching The operating procedures will be automated as far as possible. For Langmuir probes system configuration, it shall be automatically configurable such that some of the selected probes are driven in single probe mode and some of the selected probes are driven in swept double probe mode simultaneously. As shown in Fig. 8, S1…S4 are relays contact, when S1, S2 and S3 are closed, it works in in single probe mode. As S1 and S4 are closed, it operates in swept double probe mode. For system security, probes need to be connected to the ground if they do not work for long time, power supply can be connected with probe if the self-check of the electronics is ok. Full consideration should be given for picking up relays. Fig. 9 illustrates a redundancy of quantities of the relays, which can further guarantee security when contact failure.
Fig. 10. Diagram of control design for the Power Supply.
swept waveform, amplitude of the output voltage and current are the same as the triangular swept waveform, frequency is 1 kHz. In the laboratory and factory, the plasma behavior could not be simulated very well. So after some complete test for functionality, the test needs to be
5.2. Signal conditioning Current-voltage relationship of each probe would be achieved after signal conditioning and data acquisition. The measured quantities for
Fig. 11. Diagram of control design for the POMSSC. 6
Fusion Engineering and Design 152 (2020) 111429
Y. Wang, et al.
configuration information, dispatch the information to slave controller (ARM Cortex M3), the slave controller would send signal to open or close relays according to the Langmuir probe operation procedure. Meantime, the controller send back an execution status to master controller, master controller should gather all information and transfer to HPC for verification. Shunt range or conditioning circuit gain can be switched by the slave controller. In every POMSSC, four POMSSC units would realize 8 power supplies driving 16 probes. Diagram of control design for the POMSSC is shown in Fig. 11.
voltage and current is shown in Table 3. Because of the steep gradients in typical I–V, several 10 s of points are required over the sweep period in order to properly fit the data. A reasonable rule of thumb is ∼100 points per full cycle. This implies, for the 1 ms/1 kHz sweep rate of the power supply required for measurement of ne and Te, a time resolution for the electronics and data acquisition of 10 μs, that is 100 kHz is required. In the Fig. 8, shunt and voltage divider is responsible for measuring current and voltage of every probe. The signal conditioning unit provides signal conditioning, isolation and connection function for the shunt and voltage divider. The output of the signal conditioning unit is a difference signal, technical specification is as follows: 1) 2) 3) 4) 5) 6) 7)
7. Summary
signal range :+/−5 V output noise : < 5 mV, output drift: < 5 mV, maximum nonlinear error: < 0.1 %, signal distortion: < 0.1 % gain error: < 0.1 % signal bandwidth: 0–50 kHz
The electronics system for Langmuir probes to be used on ITER has been in preliminary design phase. Cubicle layout and hardware framework have been basically determined, power supply, the probes operation mode switching circuit, and measurement signal conditioning circuit have finished the preliminary design and parts of the R&D work achieving some significant results. The power supply can output stable waveforms in laboratory, but it should be tested under complex electromagnetic environment in a tokamak device to verify the performance, as well as long distance transmission characteristic. For signal conditioning, the main parameters of design circuit are well considered, but some of these will be modified, optimized and finally defined in future R&D as it is analogue circuit. Because ITER is a very large, challenging and long project, requirements may be modified to satisfy measurements or other things, so the detailed design activities should be continuously optimized based on the results of future R&D and updated requirements from ITER.
Although the backend electronics may hinder measurement of fast physics process with long cable transmission, but the power supply woks at 1 kHz sweep rate, the raw data via A/D conversion will then be multiplied by calibration factors that yield applied voltage or collected current because of impact of displacement current and voltage drop and so on. 5.3. Grounding and measured probe potential
Disclaimer
In order to interpret the probe signal, the measured probe potential must be referenced to the nominal plasma ground. In order to add flexibility, the reference ground will be sampled at several locations to provide multiple possible reference points for the probe potential. A good reference point would be picked up at the commissioning. According to ITER Common Bonding Network (CBN) guideline, all signal and power conductors are provided with their own, independent, dedicated return conductors. Shield twist pair cable should be adopted and the outer shield needs to be connected to CBN by multiple points, as shown in Fig. 8, which is different from those probe systems in existing tokamak device.
The views and opinions expressed herein do not necessarily reflect those of the ITER Organization. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References
6. Control design of electronics [1] L. Nie, et al., Experimental evaluation of Langmuir probe sheath potential coefficient on the HL-2A tokamak, Nucl. Fusion 58 (2018) 3. [2] G.S. Xu, et al., First evidence of the role of zonal flows for the L–H transition at marginal input power in the EAST tokamak, Phys. Rev. Lett. 107 (2011) 125001. [3] J.P. Gunn, et al., Scrape-off layer power flux measurements in the Tore Supra tokamak, J. Nucl. Mater. 438S (2013) S184. [4] J.G. Watkins, et al., High heat flux Langmuir probe array for the DIII-D divertor plates, Rev. Sci. Instrum. 79 (10) (2008) 876. [5] D. Tskhakaya, et al., Interpretation of divertor Langmuir probe measurements during the ELMs at JET, J. Nucl. Mater. 415 (2011) S860. [6] M. Fukumoto, et al., Development of Langmuir probes on divertor cassettes in JT60SA, Plasma Fusion Res. 8 (2013) 1405153. [7] M. Walsh, et al., ITER diagnostic challenges, Proceedings of the 2011 IEEE/NPSS 24th SOFE (2011). [8] Y.Z. Jin, et al., Design and analysis of a divertor Langmuir probe for ITER, Fusion Sci. Technol. 75 (2019) 120. [9] A.S. Kukuskin, et al., Finalizing the ITER divertor design: the key role of SOLPS modeling, Fusion Eng. Des 86 (2011) 2865. [10] M. Bagatin, D. Desideri, V. Toigo, The power supply system for Langmuir probes on RFX[J], Meas. Sci. Technol. 4 (11) (1993) 1269–1274 (6). [11] Z. Li, C. Xu, et al., Design of device driver of scanning power supply for Langmuir probe diagnostics, Modern Electron. Tech. (2006). [12] B. Duffey, A. Cheetham, J. Rayner, An automated swept power supply for a Langmuir probe, J. Electr. Electron. Eng. Aust. 16 (2) (1996) 91–96. [13] B. Goncalves, et al., ITER prototype fast plant system controller, Fusion Eng. Des 86 (2011) 556. [14] G.K. Hitesh, et al., Present status and future road map for ITER CODAC networks and infrastructure, Fusion Eng. Des 85 (2010) 549.
Every power supply and POMSSC have their own controllers in every cubicle. These controllers communicated with HPC via RS485, as shown Fig. 4. 6.1. Control design of the power supply For the power supply, external control signal is provided by PXIe 6368, the external signal and a suitable carrier (300 kHz) produce PWM pulse via analog comparator. The controller in the power supply includes DSP and CPLD. CPLD is responsible for producing complementary PWM wave with dead-time and performing protective action of the PS some off-normal cases (over current, over voltage, over temperature). DSP is mainly responsible for exchanging datfia (faults and commands) with HPC and controlling four quadrant operation switching (consumption energy circuit would be open and closed) by acquisition of corresponding voltage and current signals, as shown in Fig. 10. 6.2. Control design of the POMSSC For POMSSC, the control system that is master-slave structure, the master controller (ARM Cortex M4) receives probes operation mode
7
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Design of electronics system for Langmuir probes on ITER a,
a
a
a
a
T b
Yali Wang *, Wei Zhao , Li Zhao , Lin Nie , Guangwu Zhong , Christopher Watts , James Paul Gunnc a b c
Southwestern Institute of Physics, P.O. Box 432, Chengdu, Sichuan, 610041, China ITER Organization, Route de Vinon-sur-Verdon, F-13067, St. Paul Lez Durance Cedex, France CEA, IRFM, F-13108, Saint-Paul-Lez-Durance, France
A R T I C LE I N FO
A B S T R A C T
Keywords: Langmuir probes Probe power supply Signal conditioning Operation switching Current-voltage characteristic
An electronics system for Langmuir probes to be used on ITER has been developing. The electronics system should meet the physics measurement requirements, which means that the electronics system should include power supply for driving probes, single and double probes operation mode switching circuit, measurement signal conditioning circuit, control and long cable transmission. It would make sense to use a single power supply to control a probe pair, which could be configured for one of the two operating modes foreseen and reduce costs while minimizing risk. The R&D of power supply is most critical activity. The power supply is characterized by 2.5 A, 175 V rating. At the same time, the power supply has been designed to ensure output waveforms ripple, high frequency, four quadrant and compatibility with the electromagnetic noise close to the components and with the plasma etc., which behaves as a nonlinear rapidly varying load. For reducing risk of R&D and exploring different technical prototype, one of the technical prototypes based on a full-bridge converter concept, using a pulse width modulation (PWM) technique is relatively simple and adopted first. Relays array circuit and adequate voltage and current signal conditioning circuit are also effective guarantee for data acquisitioning and drawing the probe current-voltage characteristic. The main design activities of electronics system are presented as detailed as possible in this paper. Besides, the detailed design activities should be continuously optimized and changed based on the results of the future relevant tests and changed requirements.
1. Introduction Langmuir probes have been successfully implemented in nearly every tokamak in the world, such as HL-2A [1], EAST [2], Tore Supra [3], DIII-D [4], JET tokamak [5], JT60-SA [6]. The primary aim of the Langmuir probes on ITER is to measure the plasma parameters at the divertor target plates. The essential function of the system is to provide information about the state of plasma attachment in the divertor region, with both advanced control and physics functions. For machine control, it is envisioned that the probes provide a signal indicating the attached/ detached state of the divertor plasma, while for physics studies the system will need to supply more detailed measurements of the divertor plasma density and temperature. The Langmuir probe system can be broken down into several parts: the probe sensors themselves, the driver power supplies, signal conditioning, control, data acquisition and data processing. The backend electronics will be located in the diagnostics building, with 24 electronics cubicles. Ideally, the power supplies and associated electronics should be located as close to the sensors as possible to minimize voltage droop and capacitive effects of the ⁎
cable. However, during ITER operation the port cell and gallery area close to the tokamak are affected by gamma-ray radiation and stray magnetic field which are much more profound than in any existing tokamak device. These special conditions have important impact on design of the ITER Langmuir probe electronics. Any electronics located in the port cell require shielding of for both radiation and magnetic field to keep the electronics from malfunctioning. Further, the space in the port cell is limited for placing probe electronics. Another concern with this option: accessibility to ITER port cells is very limited, only available after the shot down of the ITER tokamak. It means that maintainability of electronics in ITER port cells will be poor. These considerations, therefore, recommend deployment of Langmuir electronics in the diagnostic building. This option avoids the costly, difficult magnetic and radiation shielding. But long cable (the longest distance is 300 m) that is associated with connecting the probe sensors in the tokamak with the backend electronics hinder measurement of fast physics process, such as plasma fluctuation, details of ELMs bursts [7]. In tokamak fusion devices, some probes are usually driven by off the shelf products, other probes could be driven by customized power
Corresponding author. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.fusengdes.2019.111429 Received 17 January 2019; Received in revised form 10 October 2019; Accepted 6 December 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 152 (2020) 111429
Y. Wang, et al.
size and high reliability. So it is necessary to consider comprehensively and design the electronics system for Langmuir probes in ITER. In this paper, the main technical specifications is first presented; cubicle layout, power supply, the probes operation mode switching circuit, measurement signal conditioning circuit are then described, together with control of electronics.
2. The main techinical specifications 2.1. Design of langmuir probe Langmuir probes, or electrostatic probes, are the first plasma diagnostic, developed by Irving Langmuir in the 1920s. The principle is relatively simple: a conductor in contact with the plasma is biased with a voltage V relative to the local plasma potential, collects the I-V curve of the plasma, and then processes the I-V curve to obtain the plasma parameters. In ITER, there are 400 probes attached to the sides of inner and outer target distributed on five divertor cassettes, probes need to survive under 10 MW/m2 high heat flux for 400 s and 20 MW/m2 for 10 s. As shown in Fig. 1, the Langmuir probe consists of a 2-mm-diameter tungsten rod as the actual probe, surrounded by a tungsten cylinder acting as a shield for the heat and plasma, between the probe and the shield is an alumina pipe as electrical insulation [8]. It is foreseen that the probes on ITER will be operated in one of two configurations: 1) operation in DC bias ion saturation current mode, meant to be used to
Fig. 1. Structure of Langmuir Probe.
supplies. But the performances of the power supplies have certain limitations that are only for their respective systems. In recent years, there are great development in power electronics and power semiconductor. The new technology should be used in design. Meantime, there are specific rules and guidelines in ITER, for example, all electronics components need lower heat dissipation and to be placed in cubicle, which require electronics system is of high efficiency, small
Fig. 2. Ion saturation current drawn by 2 mm probe along the divertor inner and outer. (a) Ion saturation current drawn by a 2 mm probe along the divertor outer in standard discharge cases. (b) Ion saturation current drawn by a 2 mm probe along the divertor inner in standard discharge cases. (c) Ion saturation current drawn by a 2 mm probe along the divertor inner in ELMy discharge cases. (d) Ion saturation current drawn by a 2 mm probe along the divertor outer in ELMy discharge cases. 2
Fusion Engineering and Design 152 (2020) 111429
Y. Wang, et al.
Table 1 Dissipation power of each cubicle. Description
Dissipation Power (W)
Height (U)
High performance computer (HPC) Fast controller chassis (PXIe chassis) PLC system (Slow controller) Switching & Conditioning 8 probe Power supplies Network switch cooling Fan Total: 1.8 kW
107 191 10 80 1200 35 150
4 4 4 3 16 1
2.2. The main technical specification To assess whether the probes can meet requirements, two sets of data have been used for analysis, the first was generated as part of the “standard” scenarios for use by diagnostic systems for benchmarking the designs. The second set was provided to understand the effects of ELMs for the probes. Firstly, for Ion saturation current, because the maximum total perpendicular ion flux incident on the divertor target is 10A/mm2, thus, a Langmuir probe bias strongly negative into ion saturation regime would draw this amount of current. The angle of incidence of the magnetic field, however, modifies this value by a factor sinθB, reducing the current to a few percent; for the nominal 3° angle, the maximum current drawn by a flush mounted probe is ∼0.5 A/mm2. A value of θB = 2.7° for the outer target and θB = 3.2° for the inner target in the simulations. Based on the corresponding density and temperature data, the corresponding current drawn by a Langmuir probe biased into ion saturation was calculated. For the plots in Fig. 2, we choose a 2 mm diameter circular, flush mounted probe biased to -150 V. Zeff is assumed to be 1, and the ion mass either 2 or 1. For the standard discharge cases the probes measure as expected when bias to ion saturation, shown in Fig. 2(a),(b), except in the case of discharge 2292. The reason for this is the unusually high electron temperature, > 40 eV. In general, a probe should be biased > ∼3 times the electron temperature to reasonably approach ion saturation (e3 = ∼5 %). However, biasing a probe to much above 100 V risks arcing, which can damage both the probe and backend electronics and even may cause increase cost of backend electronics. For the ELMy discharges, the ion saturation current at the inner target is well behaved, while in the outer target is not, shown in Fig. 2(c) and (d). For measurement of the electron temperature and density the probes will be operated in swept double-probe configuration. Swept single-probe mode is not foreseen as there is significant risk of drawing catastrophically large currents in the electron saturation regime. Fig. 3(a) is the expected double probe trace for standard parameters at the OVT. The characteristic hyperbolic tangent shape as the probe is biased deeply into ion saturation. Fig. 3(b) is the trace for one of the high temperature cases, where there is no clear saturation of the probe, making estimation of the plasma parameters impossible. Actually, in ITER the divertor Langmuir probes are widely separated (12–24 mm), and the interpretation of the characteristic of such a probe pair may not be straightforward. In summary, these simulations indicate that with a maximum probe bias voltage of ± 150 V the probes will draw no more than 0.5A during normal operation. Operating the probes beyond this 150 V range is not recommended due to the risk of arcing. This restriction, however, means that for some divertor plasma conditions the probes cannot provide meaningful measurements because the probes cannot be biased to ion saturation due to the high electron temperature. Secondly, considering ELMs, power supply should not be protected when output current is not higher 10 A in 2.5 ms. This is a reasonable compromise based on the R&D, size and safety of power supply.
Fig. 3. Probe characteristic for several typical cases of a swept probe. The probe is located near the strike point. (a) Expected double probe trace for standard parameters at the OVT. (b) The double probe trace for one of the high temperature cases.
Fig. 4. The functional block diagram of Langmuir probe system.
measure the ion flux for advanced control, 2) operation in swept double probe mode using two adjacent probes to measure the plasma electron temperature and density at the target for offline physics studies. Electronics system must be capable of meeting the configuration requirements via driving relay units. For a single power supply, the powered two probes can work in single or double probe mode.
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Table 2 Technical specification of PS. Main parameter
Range
Input voltage/current Output voltage/current External control Ripple Stability Bandwidth Distortion Regulation Temperature coefficient Output range Over current in short time periods,
230 V/2.5 A 175 V/2.5 A −10 V to +10 V 80 mV ≦0.1 %/Hr typ 0–15 kHz 0.5 % Line: 0.1 %; load: 0.5 % ≦0.02 %/°C; Four quadrant 10 A in 2.5 ms
3. Layout of the cubicle 3.1. Functional block of Langmuir probe system The functional block diagram of Langmuir probe system is shown in Fig. 4. Power supply for probe is actually a power amplifier and capable of amplifying an arbitrary waveform. It is controlled by external signal. PXIe 6368 produces the signal so as to control power supply. PXIe 6368 is NI X Series multifunction data acquisition device and analogue output (AO) maximum range is ± 10 V. A single power supply is used to control a pair probe, the output of power supply would be applied to the probe via probe operation mode switching circuit (POMSC) and long transmission, the POMSC can finish single and double probe mode switching by using of relays. At the same time, voltage and current of the probe are measured and sent to PXIe 6368. At last, data would be acquired and transferred to Control, Data Access and Communication system (CODAC). 3.2. Layout of the cubicle and components dissipation As mentioned above, the backend electronics will be located in the diagnostics building, with 24 electronics cubicles. Every cubicle contains the entire mentioned components below, Table 1. where fast controller is made up of HPC (high performance computer) and PXIe chassis, will realize the alarm handling, event handling, logging, data communication functions, data acquisition and data analysis, and fast controllers shall be used for the signal monitoring and controller of power supply, the slow controller is used for cubicle health monitoring. To ensure compliance with volume allocation, monitoring and environmental constraints, the main components shall be integrated in standard ITER instrument and control cubicles, shown in Fig. 5. The floor standing cubicle (47 U, 2200 mm (H) × 800 mm (W) × 800 mm (D)) with 718 m³/h fan is recommended, and the total heat dissipation from all electrical and electronic components inside the cubicle is between 1–2 kW. According to Table 1, for the off-the-shelf components, height and power dissipation is defined, so it is necessary to meet high efficiency and small size for in-house developed components. At present, the estimated total heat dissipation is 1.8 kW, with a safety margin of 20 % assumed, total power consumption is about 2.2 kW, which is higher than the heat dissipation capacity (2 kW). It is reasonable and feasible to optimize probes distribution and reduce heat dissipation. That is some probes are from the high-current strike point region and some are from the low-current areas in one cubicle.
Fig. 5. Layout of cubicle.
Thirdly, plasma behaves either as a nonlinear resistor or as a current source requiring a four-quadrant power supply, so the power supply has source and sink current characteristics The plasmas are characterized by a high level of fluctuations, wide spread in frequency. It's predictable that there's a lot of noise and fluctuation on probe current, which requires power supply has high stability and very lower voltage ripple.
4. Design of power supply 4.1. Technical specification of the power supply Considering the design margin, the power supply that is based on a full-bridge converter concept, using a pulse width modulation (PWM) technique is characterized by a 2.5 A, 175 V rating. The technical 4
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Fig. 6. topology diagram of power supply.
supply providing power to the H bridge circuit (DC link). 2) H bridge circuit: switch mode amplifier, the principle is using a pulse width modulation (PWM) technology, produce variable duty cycle pulse train which then acts as input, the power stage output take the form of the pulse train, rapidly switching between a zero volt and some maximum (positive or negative) voltage. Because the power mosfet permits high switching frequencies and dissipate 3 % of the controlled power (compared with at least 21 % for class B amplifier design), so the power supply can be made compact and lower dissipation. 3) Absorption and consumption energy circuit: current sink time is determined by the power rating of heat dissipation resistor R1, for safety margin, the sink capacity may match with source capacity of the power supply, As heat dissipation capacity of the power supply increases, so do the cooling and size, at the same time, which may be lead to over-temperature and exceed heat dissipation capacity of cubicle, then trigger health monitor system of cubicle. So it is necessary to control net electron collection operation time. When thus by monitoring the voltage across the energy-storage capacitor C0, Q1 would timely turns on. And the output of power supply need to be stable and with small fluctuation [11,12]. 4) Filter circuit: as switching frequency is 300 kHz and output voltage bandwidth is 0–15 kHz, the cut off frequency of the LC filter can be set 75 kHz, so the L2 = 20 μH, C2 = 220 nF. For RC damping, in general, C3 is 5–10 times the value of C2. So the R2 = 14 Ω. The CM inductor L3 = 47 μH/5 A. CY1, CY2, CY3 and CY4 are safety ceramic cap. The values are100 nF.
Fig. 7. Output voltage and corresponding current for resistive load. (a) Purple curve: 50 V /DIV; blue curve: 1 A/DIV; time: 500 μs/DIV. (b) Purple curve: 50 V/DIV; blue curve: 1 A/DIV; time: 500 μs/DIV.
specification of the power supply is shown in Table 2 [9,10]. 4.3. Preliminary test waveform 4.2. Scheme of the power supply At present, the preliminary test has been performed, output voltage and corresponding current for resistive load are reported in Fig. 7. Fig.7(a) is triangular swept waveform, the peak- peak value of output voltage is +/−175 V (blue curve) and the peak- peak value of output current is +/−2.5 A (purple curve), frequency is 1 kHz; Fig. 7(b) is sine
The power supply comprises four main elements, shown in Fig. 6. 1) AC-DC circuit: AC power is transmitted to DC power. DC power
Fig. 8. Applied voltage trace for probe system. 5
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Fig. 9. Realy unit for probe operation mode switching.
performed on some fusion devices.
Table 3 The measurement quantities for voltage and current. Parameter
Range
Relative accuracy
Current voltage
+/−1 mA to +/−5 A +/−200 V
≧0.5 % ≧0.5 %
5. Design of probe operation mode switching and signal conditioning The operation procedure of diagnostics plants in ITER usually includes the following steps [13,14]: Health (check) Configuration Calibration Start and stop measurements Troubleshoot 5.1. Probe operation mode switching The operating procedures will be automated as far as possible. For Langmuir probes system configuration, it shall be automatically configurable such that some of the selected probes are driven in single probe mode and some of the selected probes are driven in swept double probe mode simultaneously. As shown in Fig. 8, S1…S4 are relays contact, when S1, S2 and S3 are closed, it works in in single probe mode. As S1 and S4 are closed, it operates in swept double probe mode. For system security, probes need to be connected to the ground if they do not work for long time, power supply can be connected with probe if the self-check of the electronics is ok. Full consideration should be given for picking up relays. Fig. 9 illustrates a redundancy of quantities of the relays, which can further guarantee security when contact failure.
Fig. 10. Diagram of control design for the Power Supply.
swept waveform, amplitude of the output voltage and current are the same as the triangular swept waveform, frequency is 1 kHz. In the laboratory and factory, the plasma behavior could not be simulated very well. So after some complete test for functionality, the test needs to be
5.2. Signal conditioning Current-voltage relationship of each probe would be achieved after signal conditioning and data acquisition. The measured quantities for
Fig. 11. Diagram of control design for the POMSSC. 6
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configuration information, dispatch the information to slave controller (ARM Cortex M3), the slave controller would send signal to open or close relays according to the Langmuir probe operation procedure. Meantime, the controller send back an execution status to master controller, master controller should gather all information and transfer to HPC for verification. Shunt range or conditioning circuit gain can be switched by the slave controller. In every POMSSC, four POMSSC units would realize 8 power supplies driving 16 probes. Diagram of control design for the POMSSC is shown in Fig. 11.
voltage and current is shown in Table 3. Because of the steep gradients in typical I–V, several 10 s of points are required over the sweep period in order to properly fit the data. A reasonable rule of thumb is ∼100 points per full cycle. This implies, for the 1 ms/1 kHz sweep rate of the power supply required for measurement of ne and Te, a time resolution for the electronics and data acquisition of 10 μs, that is 100 kHz is required. In the Fig. 8, shunt and voltage divider is responsible for measuring current and voltage of every probe. The signal conditioning unit provides signal conditioning, isolation and connection function for the shunt and voltage divider. The output of the signal conditioning unit is a difference signal, technical specification is as follows: 1) 2) 3) 4) 5) 6) 7)
7. Summary
signal range :+/−5 V output noise : < 5 mV, output drift: < 5 mV, maximum nonlinear error: < 0.1 %, signal distortion: < 0.1 % gain error: < 0.1 % signal bandwidth: 0–50 kHz
The electronics system for Langmuir probes to be used on ITER has been in preliminary design phase. Cubicle layout and hardware framework have been basically determined, power supply, the probes operation mode switching circuit, and measurement signal conditioning circuit have finished the preliminary design and parts of the R&D work achieving some significant results. The power supply can output stable waveforms in laboratory, but it should be tested under complex electromagnetic environment in a tokamak device to verify the performance, as well as long distance transmission characteristic. For signal conditioning, the main parameters of design circuit are well considered, but some of these will be modified, optimized and finally defined in future R&D as it is analogue circuit. Because ITER is a very large, challenging and long project, requirements may be modified to satisfy measurements or other things, so the detailed design activities should be continuously optimized based on the results of future R&D and updated requirements from ITER.
Although the backend electronics may hinder measurement of fast physics process with long cable transmission, but the power supply woks at 1 kHz sweep rate, the raw data via A/D conversion will then be multiplied by calibration factors that yield applied voltage or collected current because of impact of displacement current and voltage drop and so on. 5.3. Grounding and measured probe potential
Disclaimer
In order to interpret the probe signal, the measured probe potential must be referenced to the nominal plasma ground. In order to add flexibility, the reference ground will be sampled at several locations to provide multiple possible reference points for the probe potential. A good reference point would be picked up at the commissioning. According to ITER Common Bonding Network (CBN) guideline, all signal and power conductors are provided with their own, independent, dedicated return conductors. Shield twist pair cable should be adopted and the outer shield needs to be connected to CBN by multiple points, as shown in Fig. 8, which is different from those probe systems in existing tokamak device.
The views and opinions expressed herein do not necessarily reflect those of the ITER Organization. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References
6. Control design of electronics [1] L. Nie, et al., Experimental evaluation of Langmuir probe sheath potential coefficient on the HL-2A tokamak, Nucl. Fusion 58 (2018) 3. [2] G.S. Xu, et al., First evidence of the role of zonal flows for the L–H transition at marginal input power in the EAST tokamak, Phys. Rev. Lett. 107 (2011) 125001. [3] J.P. Gunn, et al., Scrape-off layer power flux measurements in the Tore Supra tokamak, J. Nucl. Mater. 438S (2013) S184. [4] J.G. Watkins, et al., High heat flux Langmuir probe array for the DIII-D divertor plates, Rev. Sci. Instrum. 79 (10) (2008) 876. [5] D. Tskhakaya, et al., Interpretation of divertor Langmuir probe measurements during the ELMs at JET, J. Nucl. Mater. 415 (2011) S860. [6] M. Fukumoto, et al., Development of Langmuir probes on divertor cassettes in JT60SA, Plasma Fusion Res. 8 (2013) 1405153. [7] M. Walsh, et al., ITER diagnostic challenges, Proceedings of the 2011 IEEE/NPSS 24th SOFE (2011). [8] Y.Z. Jin, et al., Design and analysis of a divertor Langmuir probe for ITER, Fusion Sci. Technol. 75 (2019) 120. [9] A.S. Kukuskin, et al., Finalizing the ITER divertor design: the key role of SOLPS modeling, Fusion Eng. Des 86 (2011) 2865. [10] M. Bagatin, D. Desideri, V. Toigo, The power supply system for Langmuir probes on RFX[J], Meas. Sci. Technol. 4 (11) (1993) 1269–1274 (6). [11] Z. Li, C. Xu, et al., Design of device driver of scanning power supply for Langmuir probe diagnostics, Modern Electron. Tech. (2006). [12] B. Duffey, A. Cheetham, J. Rayner, An automated swept power supply for a Langmuir probe, J. Electr. Electron. Eng. Aust. 16 (2) (1996) 91–96. [13] B. Goncalves, et al., ITER prototype fast plant system controller, Fusion Eng. Des 86 (2011) 556. [14] G.K. Hitesh, et al., Present status and future road map for ITER CODAC networks and infrastructure, Fusion Eng. Des 85 (2010) 549.
Every power supply and POMSSC have their own controllers in every cubicle. These controllers communicated with HPC via RS485, as shown Fig. 4. 6.1. Control design of the power supply For the power supply, external control signal is provided by PXIe 6368, the external signal and a suitable carrier (300 kHz) produce PWM pulse via analog comparator. The controller in the power supply includes DSP and CPLD. CPLD is responsible for producing complementary PWM wave with dead-time and performing protective action of the PS some off-normal cases (over current, over voltage, over temperature). DSP is mainly responsible for exchanging datfia (faults and commands) with HPC and controlling four quadrant operation switching (consumption energy circuit would be open and closed) by acquisition of corresponding voltage and current signals, as shown in Fig. 10. 6.2. Control design of the POMSSC For POMSSC, the control system that is master-slave structure, the master controller (ARM Cortex M4) receives probes operation mode
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