Measurement of plasma current in tokamak system using Faraday fiber-optic sensor

Fiber-optic sensors are of interest for many industrial applications. Such sensors are passive, have high immunity to electromagnetic influence, and prospective for remote control of electric hostile equipment. Here we present a fiber-optic current sensor based on Faraday effect in an isotropic single-mode fiber for plasma current measurement. This sensor allows measuring both direct and alternative components of the plasma current. The measurement frequency band depended on parameters of used photodetectors, and was obtained from DC to a few MHz. The sensor sensitivity was limited by the noise level of used light source (standard He-Ne laser at the wavelength of 1.15 microns) and by mechanical perturbations. Two variations of the sensor were experimentally studied. The first variation was based on single pass optical scheme, and was characterized by simplicity of a technical realization. However it had relatively low sensitivity (around 30 Ampere/fiber turn) because of a random linear birefringence, which was produced by mechanical influence. For another sensor variation there was used two-pass optical scheme with compensation for the linear birefringence, utilizing Faraday mirror. There was obtained the mechanical perturbation suppression maximum of 20 dB. The sensor was used for tokamak plasma current measurements and demonstrated a good operation reliability under the mechanical perturbations.