Observation of blackbody-like emission from laser-induced plasma at early times and implications for thermal equilibrium

Abstract

Abstract We present spectroscopic investigation of laser-induced plasma at an early stage of development when the Nd:Yag laser still illuminates the copper target. According to experiments during this phase the plasma nearly meets the three requirement characteristics for a black body: the spectrum is continuous and close in form to Planck’s distribution, the plasma is nontransparent, while reflectivity is close to zero. The surface temperature of the plasma is derived from continuous nearly black-body spectra, emitted by the plasma. The spectra are recorded with temporal resolution of 1 ns, and with spatial resolution of 50 μm in the axial direction. At a later stage, when the plasma becomes transparent, the absorption properties of the plasma are estimated by measuring attenuation of two low-power diagnostic CW lasers (a He–Ne 633 nm and a diode laser 400 nm) applied side-on. By assuming that inverse bremsstrahlung is the dominant absorption mechanism and by comparing absorption coefficients for 633 nm and 400 nm wavelength radiation we derived values for electron density and electron temperature inside the plasma.