Comparison of 2ω₀ plasma photographs with far field laserphotographs

A plasma was produced by focussing a pulse from a Nd:glass laser onto a solid aluminum target. The plasma was photographed along a direction perpendicular to the laser axis utilizing the light at the second harmonic of the laser frequency which is generated at the critical surface. This photograph was compared with a far field photograph that showed the intensity distribution of the laser focal spot. The comparison showed that structures which appeared in the focal intensity distribution also appeared in the distribution of the emitted second harmonic light. It was concluded that the laser intensity profile did not undergo any major modifications due to propagation through the underdense plasma to the critical surface at intensities up to 10¹⁴ watts/cm². Also, the second harmonic photograph could be used as a diagnostic of the laser focal distribution.