Experimental studies on shock wave propagation in laser produced plasmas using double wavelength holography

Nd-glass light has been focused from two opposite directions simultaneously on the centre of a glass spherical cell filled with He at 3.5 atm pressure. The optical system adopted guarantees an illumination over two wide solid angles with their apexes coinciding on the centre of the cell. This arrangement provides a spherically symmetric shock wave surrounding the laser induced plasma. The measurements have been performed at an absorbed energy E0=0.2 J. A double pulse-double wavelength holographic laser system is used to study the diverging shock wave. The use of two holographic wavelengths (0.6943 and 0.3472 ?m) allows to calculate the plasma electron density radial distribution without making use of any assumption about the heavy particle distribution, with a sensitivity of about one order of magnitude higher compared with earlier measurements. The spatial variation of the plasma electron density is reported. The shock wave parameters have been determined and found to be in close agreement with the predictions of the point strong explosion theory taking into consideration the counter-pressure ahead of the shock wave.