Comparative study on the copper plasma confined with upward and downward conical cavities in laser-induced breakdown spectroscopy

In this study, the confinement effects of upward and downward conical cavities on laser-induced copper plasma were investigated by optical emission spectroscopy and fast imaging technique. It follows from the time-integrated spectra, the time-resolved spectra, and the spatial distributions of spectral intensity that the superior enhancement effects on spectral intensity are obtained in the upward conical cavity. Compared to the plasmas without confinement, plasma temperature and electron number density are increased in the presence of upward conical cavity, while no enhancements are obtained in the downward conical cavity. Reflection model of shockwave in a conical cavity is established to interpret these phenomena. The enhancement of the plasma plume confined in the upward conical cavity results from the compression of the plume towards its central region by the shockwave, as well as the accelerated collisions between the particles therein. No enhancement effect of the plasma in the downward conical cavity is obtained because of the radial expansion of plasma plume and the population reduction of excited species in the plasma primary core due to the pressure of oblique downward shockwave. This shockwave reflection model is further verified by the fast imaging, where the direct observation of the plasma plume agrees well with the enhancement effects of the two cavities on the spectral intensity.