We will finally design our magnetic shielding after preliminary tests of the effect of the stray magnetic field of our dipole magnet on the optical apparatus. An effect like this has been reported in (note 33). Indirect effects on the optics may also be generated by a time varying field because of the electromagnetic force proportional to ∂B/∂t acting on the apparatus. As far as we know, no measurement of a Cotton–Mouton ellipticity induced on linearly polarized light when reflected by an interferential mirror in the presence of a magnetic field parallel to the reflective surface of the mirror has been published up to now. These results depend on the nature and number of the mirror reflective layers and cannot be easily generalized. 7 × 10 − 6 rad T − 1 per reflection at normal incidence was found. As the dominant effect, a Faraday rotation of the light polarization of the order of 3. The effect of a magnetic field perpendicular to the mirror coating on an interferential mirror has been measured and reported in. Some magnetic shielding of the optics will be necessary. The stray magnetic field on the mirror location has been evaluated by calculation and computer simulation to be around 0.003 T. The stray magnetic field of the dipole magnet could induce some systematic effect on the optics and, in particular, on the mirrors of the Fabry–Perot cavity that are also the elements nearest to the magnet. In table 3 we list the indicative required partial pressure for some of the most common gases following Cotton–Mouton effect values reported in. ![]() of the components of the residual gas we assume that the gas must not give an effect higher than 10% of the vacuum effect. The conductor has a square cross section of 5. The dipole is wound with a hollow Cu–Nb–Ti composite conductor. The magnet has been operated immersed in a superfluid helium bath at a temperature of 1.8 K and a pressure of 1 bar. This magnet has been designed, realized and tested at CERN within the framework of the tests following the 1979 proposal. In the PVLAS experiment an 8 T superconducting dipole magnet cooled by superfluid helium will be used. Two windows W and W allow the light to enter and exit the vacuum chamber. The lens L T focuses the transmitted light onto this photodiode. The photodiode PD T, on the other hand, collects the light transmitted by the cavity and analysed by the polarizer prism AP. The photodiode PD R collects the reflected light, focused by the lens L R, giving the main signal for the Pound–Drever locking scheme. Appropriate manual feedthroughs for vacuum allow us to precisely align the optical elements from outside the vertical vacuum chamber. The cavity mirrors CM 1 and CM 2, the polarizer prisms PP and AP and the PEM are contained in a vacuum chamber and mounted on stages designed to rotate the optical elements around the z axis, to tilt them around the x and y axes and to translate along the x and y axes (see figure 2). The polarizer prism AP is then used to analyse the polarization state of the light transmitted by the cavity. This device consists of a slab of fused silica on which a piezoelectric device induces a high stress birefringence longitudinally when driven by a sinusoidal wave at the mechanical resonance frequency of the longitudinal mode of the slab itself. ![]() The light transmitted by the cavity passes through a photoelastic ellipticity modulator PEM. equation (8) it is straightforward to see that the resulting ellipticity will be modulated at a frequency = 2 m and that the phase θ = 2 θ m.
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