The fiber-optic current sensor (FOCS) will be installed in ITER to measure the plasma current for plasma control and machine protection. FOCS uses the Faraday effect in the fiber installed on the outer surface of the vacuum vessel. During plasma operation in ITER, vibrations may change polarization properties of optical fiber installed in the cryostat bridge, and it may affect the sensor accuracy. In this paper, we analyze the vibration effect on the FOCS measurement by applying the Jones matrix formalism. The effect of vibrations on the Jones matrix was addressed using an experimental set-up. A fiber-inserted helical shape metal tube was prepared according to the ITER cryostat bridge design. Vibrations were applied using a shaker, assuming a worst-case scenario of ITER operation. Using experimental data, we were able to estimate the influence of vibrations on the accuracy of plasma current measurement using FOCS. We have also estimated requirements for the spun fiber which is planned to be used for FOCS. It is concluded that it is not possible to satisfy the ITER requirements when using commercially Hi-Bi spun fibers, while a Lo-Bi fiber with a ratio of the linear beat length to the spun period of ~200 allows to achieve the goal.
The International Thermonuclear Experimental Reactor (ITER) Project is the next step in the transition from experimental studies of plasma physics to full-scale electricity-producing fusion power stations. There is a need for the regular measuring of the erosion and deposition at the wall once the reactor starts operating. An erosion and deposition monitor able to measure the changes in the surface shape is planned. We have shown that long distance shape measurements in challenging environmental conditions (strong vibrations) can be done by two-wavelength digital holography and thus this technique could be used for the erosion monitoring inside the ITER-Tokamak.
In the International Thermonuclear Experimental Reactor (ITER) Project, under construction in southern France, there will be a need for continually measuring the erosion at the wall, after the machine starts operating. A multiple wavelength interferometric technique based on digital holography is proposed for the erosion measurement. This technique has the ability to tackle the challenging environmental conditions within the reactor by a long distance measurement, where a relay optic will be used for imaging the investigated surface on the detector. We will show that shape measurements of objects located at a distance of more than 13 m from the measuring head can be carried out by the multiple wavelength interferometric technique. A depth accuracy of ±10 μm is achieved.
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