Measurement of azimuthal magnetic fields in a Z-pinch gas discharge using the faraday rotation of a probe beam

Abstract

The Faraday rotation of the plane of polarisation of a probe beam by azimuthal magnetic fields in the presence of beam deflection caused by refractive index gradients is discussed for a plasma carrying an axial current. A method for calculation of the magnetic field profile from experimental data is described. Bθ can be found by Abel inversion if the electron density is known and if deflected rays can be collected by an optical system and focussed onto a detector. Typical Faraday rotations calculated for the Bennett pinch assuming small beam deflection are found to be proportional to the plasma current and to the angle of deflection. If the probe beam wavelength is chosen to satisfy the approximate relation N 0λ 2 ≈ 3.5 × 10 13 m -1, where N 0 is the electron density on the axis, measurement of Bθ with beam deflections less than 2 × 10 -2 radians should be possible in cases where small rotations can be detected in the mid-to-far infra-red part of the spectrum.