DESIGN OF REMOTE CONTROL SYSTEM FOR FAR-INFRARED LASER INTERFEROMETER

Proceedings of the 18th International Conference on Nuclear Engineering ICONE18 May 17-21, 2010, Xi'an, China ICONE18-29500 DESIGN OF REMOTE CONTROL SYSTEM FOR FAR-INFRARED LASER INTERFEROMETER Shi Nan Institute of Plasma Physics, Chinese Academy of Sciences. Hefei, Anhui, China. Gao Xiang Institute of Plasma Physics, Chinese Academy of Sciences. Hefei, Anhui, China. Jie Yinxian Institute of Plasma Physics, Chinese Academy of Sciences. Hefei, Anhui, China. Wang Erhui Institute of Plasma Physics, Chinese Academy of Sciences. Hefei, Anhui, China. Yang Yao Institute of Plasma Physics, Chinese Academy of Sciences. Hefei, Anhui, China. ABSTRACT A remote control system is designed to achieve the stable and economical operation of far-infrared laser interferometer on the EAST Machine (Experimental Advanced Superconducting Tokamak). Based on the PLC controller, the system is set up as two parts: gas flow control for adjusting working gas, feedback control for stabilizing output power of the laser. We demonstrate the feasibility that, according to the need of experiment, the working gas ratio mode can be changed in order to save gas (CD 4 ) instead of keeping a single-mode ratio. Meanwhile, the changes of the output power and power supply of the laser have been measured due to the mode conversion. The output power of the DCN laser, as a laser source, is influenced by a number of parameters, especially the gas ratio and a variety of disturbances. Therefore, the feedback control system has been developed to make the laser power stable near the maximum available power. The whole system has been applied to the DCN laser, and has been proved to be effective. 1 INTRODUCTION Far-Infrared (FIR) laser interferometer is widely used to determine plasma electron density, electron density fluctuations, and the local poloidal magnetic field for tokamak plasma research [1]. It is based on the fact that an electromagnetic wave, on its passage through the plasma, experiences a phase change with respect to that in vacuum [2]. Now the diagnostic system on the EAST machine is DCN laser interferometer. It consists of the DCN laser, the light path, and the detecting part. The working gas of the DCN laser, a gas mixture containing N 2, D 2, CD 4 and He [3], should always be kept on an optimization ratio. Because of the axial flow DCN laser, the working gas is injected from one side of the laser and pumped from the other. The working gas ratio maybe changed. So the ratio needs to be adjusted in real time to keep the laser s optimal state. Additionally, the CD 4 is often difficult to acquire and the Tokamak operation is pulse discharge. The gas flow of the CD 4 should be cut off between two discharges for economical reason. Therefore we confirmed two gas working modes: one mode with the CD 4 as the operating mode and the other without CD 4 as the non-operating mode. Both of them should convert each other automatically. In the EAST campaign, the output power of the laser is unstable and easily influenced by the gas working modes conversion, environmental temperature, the variety of disturbances and so on. For maintaining the maximum available power, the resonator cavity length of the laser needs to be adjusted in real time. To avoid entering the EAST hall and keep the researchers safe during the fusion experiment, a remote control system should be developed to solve the problems above. The rest of paper is organized as follows. The system setup and description are presented in Section 2. Section 3 shows the experiment results and demonstrates the effectiveness of our system. Finally, the concluding remarks followed. 2 EXPERIMENTAL SETUP FIR laser interferometer system as a basic diagnostics is a part of diagnostic system in the EAST. So all of the control signals should be synchronization with the EAST s discharge signal, which is given by the super-control room. The flowchart of whole control system shows in Fig.1. Before discharging, a beginning signal is sent by the supercontrol room, then the gas flow control system shift the working gas mode to operating mode. Then the feedback control system starts to find the peak value of the output power of the laser, at last it sends a ready signal to super-control room 1 Copyright 2010 by ASME

to allow discharging. When the discharge is over, super-control room send a finishing signal to gas flow control system to change the gas mode as non-operating mode. Meanwhile we developed a supervising system to monitor all processes. In the following section, both parts of this control system above will be presented, respectively. A beginning signal Gas flow control system (switch the gas mode to the operating mode) Output power control system (find and maintain the output power after the mode-shifting) Super-control room A ready signal A finishing signal Gas flow control system (switch the gas mode to the nonoperating mode) Supervising system (supervise all control signal and running states) Figure 1. Flow chart of the whole control system. 2.1 Gas flow control system As mentioned in the previous section, the working gas of the DCN laser is a gas mixture containing N 2, D 2, CD 4 and He. In order to optimize the gas ratio and control the gas flow in real time during the experiment, four-way mass flow meter has been used. The schematic layout is shown below. In Fig.2, the mass flow meter connects with the PLC controller (the A/D and D/A modules for the Siemens EM231, EM232) which exchanged the instructions and the value of gas flow with the supervisory computer. Mass flow meter Connect Cable Flow signal output In-Set Set Display 0V Out-Set A/D In(EM231) 0 V D/A Out(EM232) 0 V PLC controller Supervisory computer Figure 2. Schematic diagram of the gas flow control system. The linear relation between the gas flow and A/D transformed value can be represented in the following form, supposing the range of flow is A 0 ~A m, the range of setting/output electrical signal is B 0 ~B m, the A/D (D/A) transformed range for PLC is C 0 ~C m,(b as an intermediary process is elimination) X= (A m-a 0) (Z-C 0) / (C m-c 0) +A 0 (1) where X and Z denote the real-time value of gas flow and A/D (D/A) transformed value, respectively. We utilized the linear relation to program the PLC to control the gas flow ratio. Then the adjustment result could be observed on the supervisory computer. 2.2 Mode shift and the effects on the power supply and the output power of the laser For using the CD 4 economically, the gas mode should be shift to non-operating mode (without the CD 4 ) during the discharge interval. Trying several modes of gas mixture, it was found that N 2 has significant impact on the power supply. By comparing the mode of He, D 2 with the mode of He, N 2, D 2, it was observed that the output voltage of the former had been more easily overflowed than the latter and the laser become unstable. So the N 2 must be reserved. In addition, the mode of He, N 2, D 2 and the mode of He, N 2 had the similar impact on the power supply and both of them made the power supply stay within limits. Considering economically, finally the mode of He and N 2 has been chosen as the non-operating gas mode. The best time of the modes conversion is also an important issue. The conversion time from the non-operating mode to the operating mode is about 1.5mins, which is less than the EAST s shortest discharge interlude. The details of how to confirm this time will be introduced in Section 3, Fig.5. We found that the maximum laser s output power declined gradually and continually along with the incremental times of gas mode conversions and could never restore the initial peak value. A typical phenomenon shows in Section 3, Fig.6. For keeping the laser power stable near the maximum available power after each conversion, the feedback control must be established. 2.3 The control system for the output power The important function of this control system is to find and maintain the peak value of the laser s output power after each conversion from the non-operating mode to the operating mode. The resonator cavity of the laser consists of a pyrex tube of 54mm diameter and two plane reflectors against its ends. One is a gilded glass plane mirror, and the other is a metal mesh. The mirror can be moved horizontally by means of a micrometer to adjust the cavity length [3-10]. Some changes have been made on this side of the laser: fixing a pulley on the micrometer and fixing another one on a stepper to adjust the cavity length. A position control module of the PLC drives the stepper by the signal from PLC controller. The laser output power varies with the length of the cavity and it has been shown in Fig. 3. Each peak corresponds to one (or more) modes of propagation in the wave-guide, the pattern repeating every half-wavelength of mirror displacement [4-10]. From the distance between each two peaks, it is concluded that λ 1 is for 195μm and λ 2 is for 190μm. We need to adjust the length of the cavity to acquire the laser of the 195μm mode, which corresponds to the peak value of the output power. Figure 4. shows the framework of this control system. The detected output power signal is converted to voltage, as the 2 Copyright 2010 by ASME

feedback signal. According to a movement of the output power, the PLC controller make the judgment drive the stepper to do the corresponding motion. The EM253 is a position control module which sends the pulse signal to the stepper. The unit change of the cavity length corresponding to every pulse signal is 1μm. The CP243-1 is the Ethernet module to connect this control system with the supervisory computer. gas is a gas mixture containing He and N 2 in a ratio of 2:20. The discharge chamber without an oil jacket (a constant temperature system) is exposed in atmosphere directly. The experiment of the EAST is pulse discharge, so it is divided into the discharge and interval. This mode was simulated with and without the controller for two hours, respectively. It was supposed that there were six discharges during these two hours, and each discharge lasted for 1min. In the interval, the gas mode was non-operating mode, and the output power of the laser was noise power. We also supposed the intervals were 10mins, 1 5mins, 30mins, 10mins, 15mins and 15mins in order. There is a self-restore time of the output power from nonoperating mode to operating mode. During the self-restore time, the output power can restore to a stable value without control. Through several measures, the time is believed as 1.5mins. A typical situation of the output power s self-restore course is shown in Fig.5. It s clear that the time point corresponding to the output power s maximum value appeared at 31 st min to 31.5 th min after the conversion happened. Then the peak value declined gradually. Figure 3. The observation of output modes in the DCN laser (output power in a.u, and the change of cavity length L-L 0 in mm. ) This control system is started up automatically after every conversion which is from the non-operating mode to the operating mode to find the peak value of the output power anew. In addition, it equips a manual operation to cope with an emergency yet. In this operation, the researcher can control the EM253 manually by sending the pulse signal one by one to drive the stepper and also can reboot the whole control system. DCN laser Optical system InSb detector stepper driver EM253 PLC control system CP243-1 Supervisory computer CPU224XP Figure 4. Schematic diagram of the control system of the output power. 3 RESULTS AND DISCUSSION The experiments below are conducted under the same conditions: the discharge current of the laser is 0.7A, the environment humidity is 30%, the operating mode of the working gas is a gas mixture containing N 2, D 2, CD 4 and He in a ratio of 2:2:2:20, and the non-operating mode of the working Figure 5. Schematic diagram of the output power s selfrestore course from non-operating mode to operating mode. In order to illuminate the Fig.6 and Fig.7, some details are presented as follow: Without feedback control, a beginning signal from the super-control room converted the working gas mode to operating mode and then after passing the self-restore time, the EAST discharged. The overall time of the operating mode is 2.5mins (1.5mins +1min). Then, the gas mode shifted to the non-operating mode again. Comparably, with feedback control, after passing the self-restore time, it would enter feedback control course to adjust the output power and it would sustain 2mins. Then the EAST discharged. So in this situation, the overall time of the operating mode is 4.5mins (1.5mins+2mins+1min). By the way, the experiment s sampling rate was set to 30 s. Figure 6. represents the change of output power of the laser under the non-control condition after each gas mode conversion. It is observed that the output power declined gradually after 3 Copyright 2010 by ASME

several gas mode conversions, especially after the long interval, like the third pulse in the Fig.6. Figure 7. shows the situation with the feedback control system. It is observed that the declined tendency got effectively suppressed. The results demonstrate the feedback control system could stabilize the output power near the maximum scope well. couldn t restore again. With the help of the PLC controller, the feedback control system could find the peak value of the output power, as shown in Fig 7. Figure 8 shows clearly the difference of the power stability before and after operating the feedback control. Curve (a) shows the output power under the control condition, and the curve (b) shows the output power without control. The time axis is the time point before each discharge. In sum, the feedback control system could not only stabilize the output power after the gas mode conversions, but have the effective function of anti-disturbance. Figure 6. The observation of the output power of the DCN laser with gas mode conversion under the non-control condition. ((1) ~ (6) represent each course, from interlude to discharge, respectively.) Figure 7. The observation of the output power of the DCN laser with gas mode conversion under the control condition. ((1) ~ (6) represent each course, from interlude to discharge, respectively.) In our experiment, the disturbances caused by the gas ratio changed or electromagnetic field were also simulated. Normally, these disturbances have crucial effects on the output power. In Fig.6 the disturbance signal was generated at the fifth discharge pulse. The output power declined rapidly and Figure 8. The comparison diagram of the two conditions. In the following section, we will discuss the details of the gas mode control and feedback control system. The second pulse and the third pulse in Fig.7 were chosen to decompose and the result was shown in Fig. 9. As mentioned above, when the EAST is ready for next discharge, a beginning signal will trigger the working gas mode to operating mode. The point of the time, when the beginning signal was sent, was the 32 nd min in Fig.9.(a) and the 67 th min in Fig.9.(b). Then, after the self-restore time, the feedback control system started up to adjust the output power automatically. The time point when the feedback control started up was 33.5 th min in Fig.9.(a) and the 68.5 th min in Fig.9.(b). And the output power at 33.5 th min and 68.5 th min corresponded to the maximum value of the self-restore. The control signal lasted 2mins. After 2mins, the output power achieved the peak value of this time. The obvious difference after using the control system was observed. In Fig.9.(a), the maximum value of the self-restore was 1.13, but after controlling, the peak value was 1.17. The output power had been improved by 3.5%. In Fig.9.(b), the maximum value of the self-restore was 0.95, but after controlling, the peak value was 1.10. The output power had been improved by 16%. The length of the non-operating time has strong effect on the self-restore value of the output power. The longer the nonoperating time is, the lower the self-restore value of the laser s output power is. The situation, that the non-operating mode 4 Copyright 2010 by ASME

lasted for 1 hour, was measured too. After shifting to the operating mode, the output power had been improved by more than 50%, after using the feedback control. It is shown that after a long interval, the PLC control system is extremely useful to restore the maximum value of the laser s output power automatically. the clear change of the output power is shown along with the control signal. Under the feedback control, if the movement of the output power more than the preset value, the controller will send the corresponding signal and drive the stepper to change the cavity length. Accordingly, the output power change. In the control procedure, once the judgment comes into existence, the output power will be adjusted per 10s. Because of the sampling rate is 30s, each adjustment cannot be observed clearly. Therefore, we present the output power signal given by the detector in Fig.10.(c). It corresponds to the point of time from 123.5 th min to 124.5 th min. By the Fig.10.(c), the change of the output power according to the judgment signal per 10s has been shown. The details of the detector signal in time point that was randomly chosen has been plotted in Fig.10.(d). Through all of experiments above, it is shown that after several gas mode conversions, the working state of the laser keeps good and the output power is stabilized near the peak value scope. Meanwhile, a large amount of working gas has been saved. Taking the control condition into consideration, the use of the D 2 and CD 4 was decreased by near 78%. (a) (a) (b) Figure 9. The decomposition chart of the output power of the DCN laser with gas mode conversion under the control condition. ((a) The second course. (b) The third course.) More details about the anti-disturbance will be given in this section. In the following experiment, a disturbance was generated by changing the cavity length artificially. This disturbance signal was supposed to appear during the EAST discharge campaign, and the output power declined to 0.22 rapidly. The output power could self-restore to about 0.52 only without control. But after adjusted by the feedback control, the output power could restore to the peak value 1.14, as the Fig.10.(a) showed. The output power had been improved by 119%. It proves that the control system has effective antidisturbance. The details of this graph has been provided by the Fig.10.(b). It is expanded from 122.5 th min to126.5 th min and (b) 5 Copyright 2010 by ASME

utilizing the CD 4 economically. Meanwhile, the feedback control system has been developed to make the laser power stable near the maximum available power. The experiment results perfectly demonstrate the effectiveness of the system. Additionally, all of the control operations above can be accomplished in the control room and the researchers avoid entering the EAST hall during the discharge campaign. (c) (d) Figure 10. (a) The decomposition chart of the output power of the DCN laser with a disturbance and gas mode conversion. (b) The expanding graph of the time from 122.5 th min to 126.5 th min. (c) The output power signal from the detector directly. (d) The signal of the detector. 4 CONCLUSION The remote control system has been designed and established for far-infrared laser interferometer. Two gas modes have been confirmed, which are the operating mode and the non-operating mode, respectively. Two modes can convert automatically for 5 REFERENCES [1]Gao, X., Lu, H.J., Guo, L. Q., et al, 1995, Far-Infrared Laser Diagnostics on the HT-6M Tokamak, Rev.Sci.Instrum., Vol.66 (1), pp.139-142. [2]Xu, Q., Gao, X., Jie, Y. X., et al, 2008, HCN Laser Interferometer on the EAST Superconducting Tokamak, Plasma Science and Technology, Vol.10 (4), pp.519. [3]Jie, Y. X., Gao, X., Liu, H. Q., et al, 2003, Design of CW High-Power Discharge-Pumped DCN Laser, International Journal of Infrared and Millimeter Waves, Vol.24 (12), pp.2079-2081. [4]Gao, L., Gao, X., Hu, X. W., et al, 2004, The Beam Property of DCN Laser, International Journal of Infrared and Millimeter Waves, Vol. 25 (6), pp.891-892. [5]Veron, D., Belland, P., 1978, Contiunous 250 mv Gas Discharge DCN Laser at 195μm,Infrared Physics,Vol.18, pp.465-486. [6]Liu, H. Q., Gao, X., Jie, Y. X., et al, 2004, Optimization and Maximum Output Power of CW DCN Laser, International Journal of Infrared and Millimeter Waves, Vol.25 (4), pp.649-655. [7]Jie, Y. X., Gao, X., Liu, H. Q., et al, 2004, CW Discharge- Pumped DCN Laser with Novel Mixture Gas, International Journal of Infrared and Milimeter Waves, Vol.25 (12), pp.1765-1771. [8]Rebuffi, L., Grenn, J. P., 1989, Radiation Patterns the HE 11 Mode and Gaussian Approximations, International Journal of Infrared and Milimeter Waves, Vol.10 (4), pp.291-311. [9]Bruneau, J.L., Belland, P., and Veron, D., 1978, A CW DCN Waveguide Laser of High Volumetric Efficiency, Optical Communications, Vol.24 (3), pp.259-264. [10]Asif, M., Gao, X., Jie, Y. X., et al, 2004, Experimental Study with LaB 6 Cathode on DCN Laser, International Journal of Infrared and Milimeter Waves, Vol.25 (5), pp.809-813. 6 Copyright 2010 by ASME