The accurate determination of plasma electron density is of prime importance for the analysis of plasma dynamic process and quantitative analysis in Laser-Induced Breakdown Spectroscopy(LIBS) and Spark Discharge assisted Laser-induced Breakdown Spectroscopy (SD-LIBS). Among the techniques for experimental determination of the electron density in LIBS and SD-LIBS, the most widely used is the purely spectroscopic method based on the measurement of the Stark-broadened profiles of reference lines with known values of their Stark widths. However, it is difficult to get precise line Stark broadening parameters of complex atoms and ions theoretically, this method is mostly used for spectra of simple atoms and ions. In general, the optical thin H beta line of hydrogen is preferable for the diagnosis of plasma electron density due to its strong enough profile, the negligible effect of temperature on Stark broadening, and reliable parameters. There are several problems in determination of plasma electron density, such as due to the limitation of the resolution of the spectrometer, it is difficult to measure the Stark broadening of the spectral line, the Stark broadening coefficient of the measured element spectral line is unknown, and sometimes there are no hydrogen spectral lines that can be used. In order to solve these problems, measurement of electron density utilizing the Stark broadened profiles of H beta and Ag I lines in SD-LIBS was studied. The laser pulse was focused on the surface of the sample along the direction perpendicular to the sample surface to ablate the sample and generate the plasma. The inorganic salt powder containing hydrogen was selected as a sample. The potassium dihydrogen phosphate (KH2PO4) powder was pressed into a pellet under 21.4 MPa of pressure. The sample was mounted on a two-dimensional motion platform to ensure that each laser beam irradiates on the new target surface. Two pure silver needles were selected as the anode and cathode of the spark discharge. The voltage was added to the spark gap through the charged capacitor which was connected to the two electrodes. Once the laser ablated the sample then the laser plasma was generated, spark discharge would happen automatically, and enhanced optical emissions could be observed immediately. The plasma emission was collected by a compact multichannel fiber spectrometer through a light collection system consisted of two quartz lenses under non-gated signal recording mode. The whole experiment was carried out in the atmosphere, and each data point was the average of 10 laser pulses. Firstly, the emission spectra of the plasma in SP-LIBS and SD-LIBS were discussed. The results show that both H beta line and Ag I at 520.91 nm signal intensities are significantly enhanced in SD-LIBS and interference of close lines of other elements with H beta profile are less. Secondly, Stark broadened profiles of H beta line and Ag I at 520.91 nm were determined. The Stark full width at half maximum was obtained from precise fitting of the measured line shapes to Lorentzian and Voigt shapes. The average electron densities inferred from H beta line was 1.1 x 10(17) cm(-3) under current experimental condition. Thirdly, the effect of discharge capacitance on plasma electron density under different discharge voltages was experimentally studied. The results show that with the increase of discharge capacitance, the energy of injected plasma increase, and more particles are excited from low energy level to high energy level. The average electron density of plasma in SD-LIBS also increases. With the increase of discharge capacitance, the change of discharge voltage will also bring obvious changes in the average electron density of plasma. Fourthly, the relation between the Stark broadening of Ag I line at 520.91 nm and the calculated electron density from H beta line under different discharge parameters was determined. The plasma electron densities obtained from H beta line shows strong linear dependence on the Stark broadening of Ag I line at 520.91 nm which is clear from the least square fit of 0.982. Although there is no precise Stark broadening parameter for Ag I 520.91 nm spectral line, it is impossible to calculate the plasma electron density directly through the Stark broadening of Ag I at 520.91 nm in SD-LIBS, the Stark broadening of Ag I spectral line can be used to indirectly determine the plasma electron density according to the correction curve measured experimentally. In the SD-LIBS technology, the plasma electron density can be measured indirectly by using the pure silver needle as the discharge electrode according to the Stark broadening of the silver atomic spectrum line when detecting the sample without hydrogen. This method will be helpful to accurately determine the plasma parameters in SD-LIBS technology, and has good scientific significance for further expanding its application field.
