Faraday plasma current sensor with compensation for reciprocal birefringence induced by mechanical perturbations

Abstract

A Faraday fiber-optic current sensor was employed to measure the tokamak plasma current. In order to decrease the influence of mechanical perturbations on the sensor sensitivity, a two-pass optical scheme with a variable Faraday mirror at the fiber end is proposed. A decrease, by two orders of magnitude, in the influence of the linear birefringence produced by an external piezoceramic fiber modulator was experimentally observed.

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

“…Thanks to the FM, both and can be neglected. Since the polarization properties are reciprocal in the fiber bundle section, the FM compensates for their effect [ 8 ]. The equation can then be rewritten as where , , and correspond to a spun fiber section whose Jones matrix can be generally expressed as a retarder-rotator pair, yielding [ 12 ] where l is the spun fiber length, the retardance, is the angle of the retarder’s fast eigenmode, and is the rotation angle of the rotator.…”

“…These effects induce additional birefringence that changes the polarization properties of the fiber. It is well known that using a Faraday mirror can compensate for the unwanted reciprocal effect of the fibers [ 8 , 9 ]. However, this compensation is not perfect when the non-reciprocal Faraday effect coexists [ 10 , 11 ].…”

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

“…Plasma current measurement accuracy High range [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Relative accuracy of %1 Low range [0- 1 MA] Absolute accuracy of 10 kA Fig. 1.…”

“…with a classical mirror in between [13]. However, for a real Faraday mirror, the rotation angle can differ from 45deg (rotation angle detuning).…”

“…An important problem is the presence of the linear birefringence, which originates from both fiber intrinsic properties and external mechanical perturbations [10,11]. In order to alleviate these detrimental effects, the use of the FOCS operating in the reflection mode with a Faraday mirror has been proposed [12][13][14]. A Faraday mirror reflects light and rotates the SOP with a total angle of 90 deg in order to exchange the x and y components of the lightwave.…”