Fourier optics approach to far forward scattering and related refractive index phenomena in laboratory plasmas

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Abstract
The transmission of a gaussian beam of electromagnetic radiation through a refracting medium such as plasma containing within it a localized sinusoidal disturbance, is described entirely in terms of refraction and diffraction. The lowest order terms of an expansion for the intensity profile of a beam in the front focal plane of a lens situated a focal length beyond a beam waist in the medium are identified with well-known optical effects usually treated independently, including shadowgraph, scattering, and schlieren. The latter is shown to be indistinguishable from spontaneous heterodyning of the scattered radiation with the unperturbed beam, and is sensitive to frequency, wavelength, and the location of the disturbance in the medium. An experiment using a HeNe laser with monochromatic ultrasound in atmospheric air has verified the predictions of the model over a wide range of conditions. As a plasma diagnostic, the method may offer a unique means of measuring otherwise inaccessible very long wavelength instabilities such as trapped ion modes in large Tokamaks like JET.