Influence of the optical fiber type on the performances of fiber-optics current sensor dedicated to plasma current measurement in ITER

Abstract

In this paper, we compare, by means of simulations using the Jones formalism, the performances of several optical fiber types (low birefringence and spun fibers) for the measurement of plasma current in international thermonuclear experimental reactor (ITER). The main results presented in this paper concern the minimum value of the ratio between the beat length and the spun period, which allows meeting the ITER current measurement specifications. Assuming a high-birefringence spun fiber with a beat length of 3 mm, we demonstrate that the minimum ratio between the beat length and the spun period is 4.4 when considering a 28 m long sensing fiber surrounding the vacuum vessel. This minimum ratio rises to 10.14 when a 100 m long lead fiber connecting the interrogating system to the sensing fiber is taken into account.

Contactless measurement of electric current using magnetic sensors

“…These sensors often use a phase modulation technique that was originally developed for fiber-optic gyroscopes to measure the phase shift between the left and right circular light beams in a fiber coil around the measured conductor. The accuracy depends on the fibre quality [33] and also on the optical connectors [34].…”

Effect of Faraday mirror imperfections in a fiber optic current sensor dedicated to ITER

“…1), a circular birefringence is induced in the fiber resulting in a rotation of the SOP along the fiber loop. Since the light beam passes twice in the loop (after reflecting from the Faraday mirror) and the Faraday mirror adds a 90°extra rotation, the plasma current is given by [16]:…”

“…Recent works published by the authors [16,17] demonstrated that when the FOCS uses a spun fiber with adequate parameters, the ITER specifications regarding the required plasma current measurement accuracy (see Table 1 [1]) can be fulfilled. A spun fiber is characterized by some intrinsic linear birefringence quantified by its beat length L B [26] and by a rotation of its linear birefringence axis along the fiber length.…”

“…Note that the merit of using spun fibers for current measurement [12,18] lies in the fast rotation (small S P ) of the birefringence axis along the fiber without generation of shear stresses, so that the detrimental effect of the intrinsic birefringence is greatly reduced. In [16], it was shown that a L B /S P ratio larger than 10.14 allows to satisfy the ITER requirements. However, that study did not include temperature effects that modify both the fiber Verdet constant and the linear birefringence.…”

Measurement of plasma current in Tokamaks using an optical fibre reflectometry technique

“…This is problematic for high plasma current measurement like in the future ITER (20 MA). Second, the presence of a lead fiber (fiber section situated between the interrogating device and the loop surrounding the vacuum vessel) deteriorates the FOCS performance [6]. To eliminate these issues, a POTDR (Polarization Optical Domain Reflectometer) can be used since a POTDR trace contains information about the SOP spatial evolution along the fibre.…”