Evaporation of material and influence of auxiliary spark gap on the spectral excitation by means of laser emission spectroscopy for local analysis of graphite

For the determination of impurities in graphite, laser-micro-emission spectroscopy allows the analysis of much smaller sample areas than possible by spectroscopic analysis of arc or spark discharges.Because the maximum quantity of material which can be evaporated by a laser beam is only approximately 8μg, it was found necessary to introduce a spark gap above the evaporation pit to provide additional excitation of the plasma, thereby increasing the sensitivity of the analysis. In this way, the radiation intensity of the plasma was increased by two orders of magnitude.The effects of the spark gap parameters, voltage, capacitance and inductance, on the spectral excitation were investigated. Voltage and capacitance determine the energy supplied to the spark gap, whereas capacitance, together with inductance, controls the duration and characteristic of the discharge. To obtain the optimum additional excitation, the duration of the spark discharge had to be matched with the time taken for the material to evaporate.The catalytic effect of some impurity elements on the corrosion of graphite was identified by using the technique to analyse material in the corrosion pits of irradiated graphite fuel elements.