Simulation of vibration-induced effect on plasma current measurement using a fiber optic current sensor

Abstract

An accurate measurement of the plasma current is of paramount importance for controlling the plasma magnetic equilibrium in tokamaks. Fiber optic current sensor (FOCS) technology is expected to be implemented to perform this task in ITER. However, during ITER operation, the vessel and the sensing fiber will be subject to vibrations and thus to time-dependent parasitic birefringence, which may significantly compromise the FOCS performance. In this paper we investigate the effects of vibrations on the plasma current measurement accuracy under ITER-relevant conditions. The simulation results show that in the case of a FOCS reflection scheme including a spun fiber and a Faraday mirror, the error induced by the vibrations is acceptable regarding the ITER current diagnostics requirements.

Assessment of the Structural Vibration Effect on Plasma Current Measurement Using a Fiber Optic Current Sensor in ITER

“…The spun fiber length to be installed in the bridge section ( ) was 6 m, and the perimeter of the loop around the VV ( ) was 28 m. The spin rate ( ) in forward propagation was set to . To define the Faraday effect-induced rotation ( f ) for each section, the shape of the VV and bridge could be assumed to be a circle (comparison with a D shape showed only a small difference, as investigated in [ 6 ]) and a straight line, as shown in Figure 2 .…”

“…The sensing fiber of the FOCS will be installed on the external surface of the vacuum vessel and will be connected by optical fibers to the optical devices installed in the cubicle area for sensor operation and polarization state measurement. During plasma operation, sensor accuracy can be degraded due to the temperature changes and vibrations induced on the fibers [ 6 , 7 ]. These effects induce additional birefringence that changes the polarization properties of the fiber.…”

“…A previous study [ 7 ] showed that a ratio of the intrinsic beat length ( ) to the spun period ( ) greater than 10 is required to satisfy the plasma current measurement accuracy required for the ITER when considering the temperature range undergone by the fibers. The effect of vibrations on the FOCS accuracy was also investigated when considering vibrations applied to the ITER’s vacuum vessel (VV) [ 6 ]. However, with the ITER FOCS project’s progress, it became clear that a more realistic vibration analysis should also include vibrational properties of other parts of the reactor.…”

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

“…by ρ = VB where B = μ 0 I P /(2πR), R being the average radius of the vacuum vessel. We can assume a circular shape of the vacuum vessel because it has been shown in [23] that the performance of FOCS is only slightly dependent on the distribution of the magnetic field along the fiber (the D-shape of the ITER configuration is therefore not modeled).…”

Intrinsic Fabry-Perot Sensors for Magnetic Field Detection

“…Working to develop point detectors for magnetic fields, our group has undertaken theoretical and practical work to apply Bragg grating technology in this domain [15], [16]. We have previously demonstrated a detector with good linearity that distinguishes between inherent birefringence in the optical fibre and externally induced birefringence, proving the concept and showing the potential for distributed magnetic field sensing.…”