Improvement of the dual C02 laser interferometer

Fusion Engineering and Design 34-35 (1997)375-378 ELSEVIER

Improvement of the dual

CO 2

Fusion Engineering and Design

laser interferometer

Yasunori Kawano, Akira Nagashima, Katsuhiko Tsuchiya, Takaki Hatae, Soichi Gunji Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, Naka-machi, Naka-gun, Ibaraki-ken, 311-01, Japan

Abstract A new phase comparator named VRPC (Very high Resolution Phase Comparator) has been developed to improve the phase and density resolutions of a dual CO2 laser interferometer in JT-60U tokamak. The designed time discrimination of the VRPC (39.0625 ps) can provide a phase resolution of 1/12 800 of a fringe for a 2 MHz interference beat signal, which corresponds to an effective density resolution of 1.32 x 1017 m - 2 . The first operation of the VRPC for plasma discharges has been executed. The phase resolution is improved significantly in comparison with that of a previous comparator which has a resolution of 1/100 of a fringe. The density resolution is also well improved to extract noise components which were comparable to a bit noise of the previous comparator. The VRPC has the capability to enable the dual CO 2 interferometer to measure precise density changes in tokamak fusion plasmas. © 1997 Elsevier Science S.A.

1. Introduction To measure the electron density of plasmas with sufficient reliability is one of the important topics of tokamak fusion research [1-5]. Laser interferometry has been playing an important role in this field and it is further expected to be useful for future large devices such as I T E R (International Thermonuclear Experimental Reactor). For the diagnostics of I T E R a CO2 laser interferometer is proposed for electron density monitoring and for cross calibration for L I D A R [6]. The wavelength of 10.6 gm of the CO2 laser is not too long to suffer from refraction effects and the Faraday rotation effect, and it is not too short as to be in the visible region where interferometry is less sensitive to a density change. A dual CO2 laser interferometer has been devel-

oped for electron density measurement on JT-60U [7]. Two different wavelength interferometers of 10.6 and 9.27 gm are utilized for simultaneous measurement of the density and the optical path length changes. This interferometer provides a lot of advantages for large devices such as a better robustness to the darkening of vacuum windows and mirrors, a better capability for large mechanical vibration and displacement of reflection mirrors, ease of laser beam monitoring and simplified layout of optical components. As a result, the electron density of JT-60U was successfully measured, and density behaviour during fast major disruptions was diagnosed without a fringe loss. According to such previous works, the feasibility of the dual CO 2 laser interferometer as a density monitor for large tokamaks were well demonstrated.

0920-3796/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0920-3796(96)00610-2

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Y. Kawano et al./Fusion Engineering and Design 34-35 (1997) 375-378

The observed density resolution of 2 x 1019 m -2, however, was relatively poor. Due to the closeness of the wavelengths of the CO2 interferometers, it was seen that a bit noise of the phase comparator with 1/100 fringe resolution (its expected density resolution is 1.6 1019 m - 2 ) was one of major sources of the resolution limit [7]. This resolution is enough for the density monitoring with appropriate data smoothing or for a study of large density changes. On the other hand, when it is required to diagnose small density changes, more accurate resolution is necessary for the dual CO2 interferometer. For this purpose, we developed a new phase comparator, VRPC (Very high Resolution Phase Comparator). In this paper, the first operational result of the VRPC by applying an actual density measurement for a large tokamak is described.

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2. Principle and specifications of the VRPC Fig. 1 shows the schematic principle of phase discrimination of the VRPC. A very high phase resolution is achieved using a two-stage time difference measurement in the VRPC [5,7]. For the first stage, a time difference between a local oscillator signal and a probing signal of the interferometer, t D, is measured by a scale clock of 100 MHz. Secondly, a residual time interval between the last scale clock pulse and the probing signal At is measured by the precise analog time discriminator which has a resolution of 1/256 between two clock pulses. Therefore, the expected time discrimination becomes 39.0625 ps ( = (1/100 MHz)/256). In fact, the time discrimination accuracy of ~ 30 ps was achieved in a stand-alone test [7]. In the case of an usual 2 MHz beat signal of the dual CO2 laser interferometer, the phase resolution becomes 1/12 800 x 2re. Measured phase difference data is accumulated in the VRPC and they are sampled by external strobe signals with maximum rates of 1 MHz. An acceptable frequency of input signals is widely ranged from 10 kHz to 50 MHz in contrast with the previous one's signal of 2 MHz only. The phase resolution is directly connected to the input frequency and ranges from 1/512 to 1/2.56 x 106 of a fringe. Thus, a wide

range for the selection of time response and phase resolution is available.

3. Operational results 3.1. Phase resolution

Fig. 2 shows phase signals measured by the VRPC for a 13 M W NB heated tokamak discharge in JT-60U. A phase signal of the 10.6 gm interferometer is shown in Fig. 2a, c, and e and a phase signal of the 9.27 gm interferometer is shown in Fig. 2b, d, and f, respectively. The sampling rate is 100 kHz. Here, the plasma current was started at 3.1 s, its flat top of 1.8 MA was sustained from 5.7 to 8.2 s and it was ended at 11.5 s. A large mechanical vibration is observed after the beginning and the ending of the discharge as shown in Fig. 2a and b. The frequency

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Y. Kawano et al.// Fusion Engineering and Design 34-35 (1997) 375-378

pared with the previous one. Thus, it is concluded that the phase resolution of the interferometer is significantly improved by the VRPC.

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of the vibration is ~ 30 Hz as can be seen in Fig. 2c and d, which mainly originates from a vibration of a mirror mounting structure near the vacuum vessel. As shown in Fig. 2e and f, a 3.3 kHz oscillating fluctuation is observed only in the phase signal of the 10.6 pm interferometer. The origin of the fluctuation is not clear yet but is supposed to be an electrical noise. An amplitude of the fluctuation, about 2/100 of a fringe, is of the same order as the bit noise of the previous comparator. Therefore, the 3.3 kHz noise can not be easily distinguish when the previous comparator is used. A comparison of the phase resolutions between the VRPC and the previous comparator is shown in Fig. 3. It is to be noted that very small changes of the phase signal are well detected in the case of the VRPC (Fig. 3a) corn-

Fig. 4 shows a line electron density calculated from phase data in Fig. 2. Line densities along the CO2 beam path calculated from density profiles measured by Thomson scattering are plotted in Fig. 4a as closed circles, showing good agreement. There, however, is little improvement in the density resolution. The density resolution (the thickness of the density trace) is almost the same as that of the previous case and amounts to 2 x 1019 m -2. Sources of the thickness of the trace are relatively low frequency (200 ~ 300 Hz) fluctuations with amplitudes of 0 . 5 ~ 2 x 1019 m 2, shown in Fig. 4b and a 3.3 kHz fluctuation with an amplitude of 1 x 10 ~9 m -2, shown in Fig. 4c. Here, the 3.3 kHz noise originates from the one discussed in the former section. Since those two fluctuations exist even during a no plasma period, they are considered to be noises.

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A small fluctuation with an amplitude of 2 x 1018 m -2 is seen on the 3.3 kHz trace in Fig. 4c, and this seems to be a critical density resolution in the present status of the interferometer using the VRPC. In order to achieve this critical resolution, the identification of the low-frequency noise and an appropriate noise reduction are required.

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Acknowledgements

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The authors acknowledge Y. Endo, S. Chiba and M. Uramoto for their technical cooperation. The authors thank Drs M. Mori, R. Yoshino, H. Ninomiya, M. Shimada and M. Nagami for their continuous support and encouragement.

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Fig. 4. Line electron density calculated from Fig. 2. Line electron densities along the CO2 beam path calculated from density profiles measured by Thomson scattering are plotted as closed circles.

4. Discussion A spectrum analysis of the density trace shows that there are several different frequency components of noise in the lower frequency region ( < 1 kHz). The lowest component is at ~ 30 Hz, which is close to the characteristic vibration frequency of the mirror mounting structure. This indicates that the lowest frequency noise might be related to an accuracy of the vibration compensation. Origins of other components are not clear yet but are possibly related to higher orders of the vibration.

References [1] T. Fukuda and A. Nagashima, Frequency-stabilized single-mode cw 118.8 gm CH3OH waveguide laser for large tokamak diagnostics, Rev. Sci. Instrum., 60 (1989) 1080. [2] D.K. Mansfield, H.K. Park, L.C. Johnson et al., Multichannel far-infrared laser interferometer for electron density measurements on the tokamak fusion test reactor, App. Optics, 26 (1987) 4469. [3] G. Braithwaite, N. Gottardi, G. Magyar et al., JET polari-interferometer, Rev. Sci. Instrum., 60 (1989) 2825. [4] T.N. Carlstrom, D.R. Ahlgren and J. Crosbie, Real-time, vibration-compensation C Q interferometer operation on the DIII-D tokamak, Rev. Sci. Instrum., 59 (1988) 1063. [5] Y. Kawano, A. Nagashima, S. Ishida et al., CO2 laser interferometer for electron density measurement in JT60U tokamak, Rev. Sci. Instrum., 63 (1992) 4971. [6] V.S. Mukhovatov for ITER Team, ITER operation and diagnostics, Rev. Sci. Instrum., 61 (1990) 3241. [7] Y. Kawano, A. Nagashima, T. Hatae et al., Dual CO2 laser interferometer with a wavelength combination of 10.6 and 9.27 gm for electron density measurement on large tokamaks, Rev. Sci. Instrum., 67 (1996) 1520.