Experimental arrangement for studying a plasma produced by laser breakdown in a gas

An experimental arrangement for spectroscopic investigation of dense plasmas produced by laser breakdown in gases is described. A TEA CO2 laser delivering 3 J in 50 nsec at 10.6 microns was used to initiate breakdown in helium at 1.5 atm. The spectral analysis system consists of a monochromator 1.15 m in focal length equipped with a holographic screen and a wideband photomultiplier. The spectral range covers from 2500 to 7500 A. The photomultiplier signal is visualized on an oscilloscope and is stored in digital form on magnetic tape. During each laser shot, the time evolution of the plasma for given wavelength and position is traced. After a series of shots, the time evolution of a spectral band or of the spatial structure of the plasma can be studied. Examples show the system's suitability for studies of plasma expansion, radiation transfer phenomena, and recombination and excitation transfer among the excited species.