REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11D404 (2014)
Design of vibration compensation interferometer for Experimental Advanced Superconducting Tokamaka) Y. Yang,1 G. S. Li,1,2 H. Q. Liu,1 Y. X. Jie,1,b) W. X. Ding,3 D. L. Brower,3 X. Zhu,1 Z. X. Wang,1 L. Zeng,1 Z. Y. Zou,1 X. C. Wei,1 and T. Lan2 1
Institute of Plasma Physics, Chinese Academy of Science, Hefei, Anhui 230031, People’s Republic of China University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China 3 Department of Physics and Astronomy, University of California at Los Angeles, Los Angeles, California 90095, USA 2
(Presented 2 June 2014; received 31 May 2014; accepted 18 June 2014; published online 11 July 2014) A vibration compensation interferometer (wavelength at 0.532 μm) has been designed and tested for Experimental Advanced Superconducting Tokamak (EAST). It is designed as a sub-system for EAST far-infrared (wavelength at 432.5 μm) poloarimeter/interferometer system. Two Acoustic Optical Modulators have been applied to produce the 1 MHz intermediate frequency. The path length drift of the system is lower than 2 wavelengths within 10 min test, showing the system stability. The system sensitivity has been tested by applying a periodic vibration source on one mirror in the system. The vibration is measured and the result matches the source period. The system is expected to be installed on EAST by the end of 2014. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4886455] I. INTRODUCTION
A far-infrared laser polarimeter/interferometer system has been designed and installed for Experimental Advanced Superconducting Tokamak (EAST).1 This diagnostic will measure the density and Faraday angle profile information and contribute to the plasma current profile control. In the polarimeter/interferometer system, the plasma density measurement is carried out by the interferometer, where the phase shift between the reference channel and detecting channel gives the line integrated electron density. The vibration (either from the machine or the system) will introduce an additional change in the path length of the optics system, which is an error for the phase of the density measurement.2–6 The error is higher, especially, during the plasma start-up phase since the density term is low and the vibration term is large or during the change of the toroidal magnetic field. The most severe potential vibration of EAST is expected from the retro reflectors locating at the high field side wall in the chamber. In order to compensate the vibration, we designed a second laser (wavelength at 0.532 μm) interferometer as a sub-system for EAST far-infrared polarimeter/interferometer. The system has been tested in the lab and planned to be installed on EAST by the end of 2014. II. SYSTEM DESCRIPTION
The principle of interferometer for plasma density measurement is known as ϕ = ϕp = c ne dl, (1) a) Contributed paper, published as part of the Proceedings of the 20th
Topical Conference on High-Temperature Plasma Diagnostics, Atlanta, Georgia, USA, June 2014. b) Author to whom correspondence should be addressed. Electronic mail: [email protected]. 0034-6748/2014/85(11)/11D404/3/$30.00
where ϕ p is the phase shift by plasma, c is a constant proportional to the wave length, and ne is the local plasma density. By considering the vibration in the optical path, the phase shift between the reference channel and the detecting channel is 2π ϕ = ϕp + ϕv = c ne dl + L, (2) λ where L is the path length change by vibration, ϕ v is the vibration term and is proportional to L, and λ is the wave length of the working laser. For CO2 pumped HCOOH laser (wavelength at 432.5 μm) and EAST low density (∼1 × 1019 m−3 ) case, the ϕ v term is ∼8% (80%) in the total phase shift for a 0.1 mm (1 mm) vibration. So the vibration compensation is needed. A visible laser (wavelength at 0.532 μm) has been selected as the second laser to compensate the vibration. For this wavelength, the dominant term in Eq. (2) is the vibration term and the plasma term can be neglected. The modulation frequency is set at 1 MHz by two Acoustic Optical Modulators (AOMs). One AOM produces a first order beam which has an 80 MHz frequency shift from the original/zero order beam. The other AOM produces an 81 MHz frequency shifted beam. These two first order beams then interfere and an IF signal at 1 MHz is created carrying the vibration information. The layout of the vibration compensation system is presented in Figure 1. The system will settle on a vertical optical board in front of the “O” window on EAST. The main part of the board is for the HCOOH laser polarimeter/interferometer. The upper part of the board is currently reserved for the vibration compensation system. The vibration compensation system shares part of the optical path with one HCOOH laser interferometer channel (5 in all and possibly upgraded to 11). Such arrangement implies that the assumed vibration is dominant from the retro reflectors and is identical for all channels. The outputs from one AOM are composed of the zero order
85, 11D404-1
© 2014 AIP Publishing LLC
11D404-2
Yang et al.
Rev. Sci. Instrum. 85, 11D404 (2014)
FIG. 1. System layout of vibration compensation interferometer. The black/red/blue dashed box shows the optical path for 1 channel HCOOH laser/visible laser/both lasers.
beam, the first order beam and the higher order beams. An aperture is applied after the AOM, to allow only the first order beam to pass. A heterodyne interferometer scheme is designed to measure the phase shift. Avalanche Photodetectors (APD120 Series, Thorlabs Company) have been selected as the detectors for the 1 MHz IF signal. Other main components in the system are beam splitters (for visible laser and FIR laser, respectively), mirrors, quarter wave plate (for FIR laser), retro reflector, Polymethylpentene (TPX) lens, and TPX window. Bench test of these components has been finished. For each component, we tested the transmission efficiency for the visible laser. The power dependence (decreasing) on the optical path length has been measured and fit. Based on these data, the relative power (to the source power) at the two detectors are 0.0935% and 0.046%, respectively. For a 20 mW laser source, such optical arrangement can produce 18.7 μW and 9.2 μW signal in front of the detector. According to the detector’s bench performance, these signals could be detected. III. BENCH TEST OF THE SYSTEM A. System stability test
As described in Sec. II, the IF of the system is 1 MHz. These IF data are shown in Figure 2 (upper part). To test the system stability, this experiment was done when the system is mechanically stationary and without plasma. There is a phase difference between the two channels signal, which means the initial path length difference between the reference and detecting channels. In an ideal case (without spatial perturbation or any drift), such phase difference value should be constant in time. The phase difference, δϕ, has been measured for 400s (bottom part of Figure 2). Such time scale is currently the longest EAST pulse length. The result shows that during this time, the largest phase drift is within 3π . This means the drift in the system’s path length is within 1.5λ (∼0.8 μm). The
FIG. 2. The intermediate frequency signal and the stability of optical path.
drift could from any spatial perturbation in the lab. Comparing with the possible vibration from the machine which needs to be compensated (∼0.1 mm), this drift is in the order of 1%. On the other hand, if the machine vibration is ∼0.1 mm, there will be ∼180 fringe jumps for such a vibration. This may bring some difficulty in the fast fringe counting during the plasma discharges. B. Measurement of applied vibration
In order to simulate the machine vibration in the experiment, an external vibration has been applied to the system. This artificial vibration is introduced by setting an electric motor on one mirror (by screws). The frequency of the motor can be controlled and modified (
Design of vibration compensation interferometer for Experimental Advanced Superconducting Tokamaka) Y. Yang,1 G. S. Li,1,2 H. Q. Liu,1 Y. X. Jie,1,b) W. X. Ding,3 D. L. Brower,3 X. Zhu,1 Z. X. Wang,1 L. Zeng,1 Z. Y. Zou,1 X. C. Wei,1 and T. Lan2 1
Institute of Plasma Physics, Chinese Academy of Science, Hefei, Anhui 230031, People’s Republic of China University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China 3 Department of Physics and Astronomy, University of California at Los Angeles, Los Angeles, California 90095, USA 2
(Presented 2 June 2014; received 31 May 2014; accepted 18 June 2014; published online 11 July 2014) A vibration compensation interferometer (wavelength at 0.532 μm) has been designed and tested for Experimental Advanced Superconducting Tokamak (EAST). It is designed as a sub-system for EAST far-infrared (wavelength at 432.5 μm) poloarimeter/interferometer system. Two Acoustic Optical Modulators have been applied to produce the 1 MHz intermediate frequency. The path length drift of the system is lower than 2 wavelengths within 10 min test, showing the system stability. The system sensitivity has been tested by applying a periodic vibration source on one mirror in the system. The vibration is measured and the result matches the source period. The system is expected to be installed on EAST by the end of 2014. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4886455] I. INTRODUCTION
A far-infrared laser polarimeter/interferometer system has been designed and installed for Experimental Advanced Superconducting Tokamak (EAST).1 This diagnostic will measure the density and Faraday angle profile information and contribute to the plasma current profile control. In the polarimeter/interferometer system, the plasma density measurement is carried out by the interferometer, where the phase shift between the reference channel and detecting channel gives the line integrated electron density. The vibration (either from the machine or the system) will introduce an additional change in the path length of the optics system, which is an error for the phase of the density measurement.2–6 The error is higher, especially, during the plasma start-up phase since the density term is low and the vibration term is large or during the change of the toroidal magnetic field. The most severe potential vibration of EAST is expected from the retro reflectors locating at the high field side wall in the chamber. In order to compensate the vibration, we designed a second laser (wavelength at 0.532 μm) interferometer as a sub-system for EAST far-infrared polarimeter/interferometer. The system has been tested in the lab and planned to be installed on EAST by the end of 2014. II. SYSTEM DESCRIPTION
The principle of interferometer for plasma density measurement is known as ϕ = ϕp = c ne dl, (1) a) Contributed paper, published as part of the Proceedings of the 20th
Topical Conference on High-Temperature Plasma Diagnostics, Atlanta, Georgia, USA, June 2014. b) Author to whom correspondence should be addressed. Electronic mail: [email protected]. 0034-6748/2014/85(11)/11D404/3/$30.00
where ϕ p is the phase shift by plasma, c is a constant proportional to the wave length, and ne is the local plasma density. By considering the vibration in the optical path, the phase shift between the reference channel and the detecting channel is 2π ϕ = ϕp + ϕv = c ne dl + L, (2) λ where L is the path length change by vibration, ϕ v is the vibration term and is proportional to L, and λ is the wave length of the working laser. For CO2 pumped HCOOH laser (wavelength at 432.5 μm) and EAST low density (∼1 × 1019 m−3 ) case, the ϕ v term is ∼8% (80%) in the total phase shift for a 0.1 mm (1 mm) vibration. So the vibration compensation is needed. A visible laser (wavelength at 0.532 μm) has been selected as the second laser to compensate the vibration. For this wavelength, the dominant term in Eq. (2) is the vibration term and the plasma term can be neglected. The modulation frequency is set at 1 MHz by two Acoustic Optical Modulators (AOMs). One AOM produces a first order beam which has an 80 MHz frequency shift from the original/zero order beam. The other AOM produces an 81 MHz frequency shifted beam. These two first order beams then interfere and an IF signal at 1 MHz is created carrying the vibration information. The layout of the vibration compensation system is presented in Figure 1. The system will settle on a vertical optical board in front of the “O” window on EAST. The main part of the board is for the HCOOH laser polarimeter/interferometer. The upper part of the board is currently reserved for the vibration compensation system. The vibration compensation system shares part of the optical path with one HCOOH laser interferometer channel (5 in all and possibly upgraded to 11). Such arrangement implies that the assumed vibration is dominant from the retro reflectors and is identical for all channels. The outputs from one AOM are composed of the zero order
85, 11D404-1
© 2014 AIP Publishing LLC
11D404-2
Yang et al.
Rev. Sci. Instrum. 85, 11D404 (2014)
FIG. 1. System layout of vibration compensation interferometer. The black/red/blue dashed box shows the optical path for 1 channel HCOOH laser/visible laser/both lasers.
beam, the first order beam and the higher order beams. An aperture is applied after the AOM, to allow only the first order beam to pass. A heterodyne interferometer scheme is designed to measure the phase shift. Avalanche Photodetectors (APD120 Series, Thorlabs Company) have been selected as the detectors for the 1 MHz IF signal. Other main components in the system are beam splitters (for visible laser and FIR laser, respectively), mirrors, quarter wave plate (for FIR laser), retro reflector, Polymethylpentene (TPX) lens, and TPX window. Bench test of these components has been finished. For each component, we tested the transmission efficiency for the visible laser. The power dependence (decreasing) on the optical path length has been measured and fit. Based on these data, the relative power (to the source power) at the two detectors are 0.0935% and 0.046%, respectively. For a 20 mW laser source, such optical arrangement can produce 18.7 μW and 9.2 μW signal in front of the detector. According to the detector’s bench performance, these signals could be detected. III. BENCH TEST OF THE SYSTEM A. System stability test
As described in Sec. II, the IF of the system is 1 MHz. These IF data are shown in Figure 2 (upper part). To test the system stability, this experiment was done when the system is mechanically stationary and without plasma. There is a phase difference between the two channels signal, which means the initial path length difference between the reference and detecting channels. In an ideal case (without spatial perturbation or any drift), such phase difference value should be constant in time. The phase difference, δϕ, has been measured for 400s (bottom part of Figure 2). Such time scale is currently the longest EAST pulse length. The result shows that during this time, the largest phase drift is within 3π . This means the drift in the system’s path length is within 1.5λ (∼0.8 μm). The
FIG. 2. The intermediate frequency signal and the stability of optical path.
drift could from any spatial perturbation in the lab. Comparing with the possible vibration from the machine which needs to be compensated (∼0.1 mm), this drift is in the order of 1%. On the other hand, if the machine vibration is ∼0.1 mm, there will be ∼180 fringe jumps for such a vibration. This may bring some difficulty in the fast fringe counting during the plasma discharges. B. Measurement of applied vibration
In order to simulate the machine vibration in the experiment, an external vibration has been applied to the system. This artificial vibration is introduced by setting an electric motor on one mirror (by screws). The frequency of the motor can be controlled and modified (
