Propagation of a Gaussian Wigner Function Through a Matrix-Aperture Beamline
In the framework of statistical optics, a Wigner function represents partially coherent radiation. A Gaussian Wigner function, which is an equivalent representation of the more commonly used Gaussian Schell-model cross-spectral density, may be defined in terms of its covariance matrix and centroid. Starting from the relationship between Gaussian Wigner functions and the Gaussian Schell model, we derive coherence properties of the Gaussian Wigner function, including coherence length and degree of coherence. We define a simplified beamline called a matrix-aperture beamline composed of linear transport sections separated by physical apertures. This is an idealized form for a transport beamline in a synchrotron light source or X-ray free electron laser. An envelope model provides a basic foundation for understanding the optics of a given beamline, in a manner analogous to how linear optics are treated in particle beam dynamics, with corresponding definitions of emittance and Twiss parameters. One major challenge to such an envelope model lies in the hard-edge apertures which break the Gaussian condition, raising the question as to the adequacy of a Gaussian model. We present a consistent way to construct a Gaussian approximation of the far-field Wigner function following the hard edge aperture. To this end, we introduce the concept of a Gaussian aperture and analyze its effects on the radiation Wigner function. A software implementation of this model is described as well.