Direct measurement of the matched spot size in a slow capillary discharge optical waveguide

This communication presents direct method for experimental determining the matched spot size in a plasma optical waveguide, created in a slow capillary discharge. It can be used for Laser Wakefield Acceleration experiments in addition to interferometry for fast control of optical properties of discharge plasma. The measurements are done by means of the comparison of the laser beam size at the entrance and at the exit of the plasma channel. They are direct in the sense that the interpretation is made in terms of the refractive index without usage of the information about electron density distribution. The method can be used for matched spot size measurement in conditions of the nonlinear effects (transmission of high power laser pulses).