Measure of the transverse coherence of a self amplified spontaneous emission of a free electron laser with the heterodyne speckle method

We describe the method of Heterodyne Near Field Speckles (HNFS) for the characterization of spatial and temporal coherence of radiation. The method relies on the statistical properties of the speckle field produced by spherical particles randomly distributed and suspended in a fluid. We report preliminary results obtained with broadband light sources. We discuss the results obtained with the Self-Amplified Spontaneous Emission free electron laser SPARC LAB. This method will enable to calibrate and realize a diagnostics for the X-ray, broadband betatron radiation emitted in laser-plasma accelerators.