Simple detection of a plasma horizontal position by optical fibers

The horizontal position of a small Tokamak plasma can be simply measured by a pair of optical fibers mounted over a torus vacuum vessel without being affected by the electromagnetic noises as in the case of the measurement by magnetic probes. The horizontal position Ho is given by Ho=0.3ln(Fo/Fi) from the optoelectrically transformed outputs of the fibers. Unless the plasma changes violently, the measurement by the fibers is accurate and coincident well with the measurement by magnetic porbes.