Real time measurement of the electron density of a laser generated plasma using a RC circuit

A Nd:YAG laser pulse was focused, in air or on a Cu target, between the plates of a planar charged capacitor. The plasma generates a transient redistribution of the electrical charges on the plates that can be easily measured as a voltage drop across a resistor connected to the ground plate. At the same time, the Stark broadening of the Hα spectral line (656.3 nm) obtained from the optical emission spectrum of the plasma was measured. In this work, we show that the peak of electrical signal measured on the resistor is, in the energy range of our laser (30 mJ to 220 mJ) and at time delays typical of Laser-Induced Breakdown Spectroscopy applications (500–5000 ns), univocally related to the temporal evolution of the Stark broadening of the Hα line. Therefore, after a proper calibration depending on the material and the experimental geometry, the peak of the electrical signal can be used to predict the temporal evolution of the electron density of the generated plasma.