Refraction of an electromagnetic probe wave in a laser plasma

A method for reconstructing the refraction angle of electromagnetic probe radiation, based on a field-visualisation interferogram, was proposed and implemented in laser plasma experiments. A reconstruction of the refraction angle involved determination of partial derivatives of an interference phase shift ∂δ/∂x and ∂δ/∂y at each point of the wavefront reconstructed from an interferogram. An example is given of an analysis of a laser-plasma interferogram obtained on irradiation of an aluminium target with a 2 ns pulse from a powerful Nd:YAG laser of 25 J energy and 527 nm wavelength (the intensity on the target surface was ~10¹⁴ W cm^&ndashl2). The refraction angle of the probe radiation reached ≈3° and the maximum electron density in the plasma was ~2.6 × 10²⁰ cm^–3. The proposed method for reconstructing the refraction angle makes it possible to visualise the structure of an inhomogeneity in the investigated object and to answer unambiguously the question of the cause of the appearance of an ‘opaque zone’, observed in experiments on laser plasmas and Z pinches.