Development of a laser-induced plasma probe to measure gas phase plasma signals at high pressures and temperatures

Abstract

The ability of laser induced breakdown spectroscopy (LIBS) technique for on line simultaneous measurement of elemental concentrations has led to its application in a wide number of processes. The simplicity of the technique allows its application to harsh environments such as present in boilers, furnaces and gasifiers. This paper presents the design of a probe using a custom optic which transforms a round beam into a ring (Donut) beam, which is used for forming a plasma in an atmosphere of nitrogen at high pressure (20bar) and temperature (200°C). The LIBS experiments were performed using a high pressure cell to characterize and test the effectiveness of the donut beam transmitted through the LIBS probe and collect plasma signal in back scatter mode. The first tests used the second harmonic of a Nd:YAG laser, pulse width 7ns, to form a plasma in nitrogen gas at five different pressures (1, 5, 10, 15 and 20bar) and three different gas temperatures (25, 100 and 200°C). The uniqueness of this probe is the custom made optic used for reshaping the round laser beam into a ring (Donut) shaped laser beam, which is fed into the probe and focused to form a plasma at the measurement point. The plasma signal is collected and collimated using the laser focusing lens and is reflected from the laser beam axis onto an achromatic lens by a high reflection mirror mounted in the center section of the donut laser beam. The effect of gas pressure and temperature on N(I) lines in the high pressure cell experiment shows that the line intensity decreases with pressure and increases with temperature. Mean plasma temperature was calculated using the ratios of N(I) line intensities ranging from 7400K to 8900K at 1bar and 2400K to 3200K at 20bar for the three different gas temperatures. The results show that as a proof of principle the donut beam optics in combination with the LIBS probe can be used for performing extensive LIBS measurements in well controlled laboratory environment as well as harsh and demanding environments of practical devices at both high pressures and temperatures.