Effect of External Electric Field on Laser-Induced Breakdown Spectroscopy of Vanadium Plasma Under Different Pressures of Environmental Gases

The effect of external electric field employment on laser-induced breakdown spectroscopy (LIBS) of vanadium (V) plasma has been investigated. An external electric field of strength from 3 to 9 kV/cm was applied perpendicular to the direction of propagation of the plasma plume by using two circularly shaped brass electrodes and high-voltage dc power supply. The plasma is generated by employing a Q-switched Nd:YAG laser (1064 nm, 10 ns, and 10 Hz) at a pulse energy of 120 mJ at laser irradiance of 2.2 GWcm⁻² under argon (Ar) and neon (Ne) environments at various pressures ranging from 5 to 40 torr. LIBS analysis reveals that with increasing ambient gas pressure from 5 to 20 torr, the electron temperature “T_e” and electron density “n_e” are increased due to the enhanced collisions and confinement effects. Further increase in pressure from 20 to 25 torr causes a drastic decrease in plasma parameters, which is attributed to the shielding effect. With further increase in pressure up to a maximum of 40 torr, the saturation with insignificant changes in both plasma parameters is observed, which is explainable on the basis of the formation of a self-regulating regime. It is also observed that emission intensity, electron temperature, and electron number density of vanadium plasma are higher in the presence of external electric field as compared to the field-free case under all pressures of both environmental gases, Ar and Ne. The externally applied electric field provides additional energy by accelerating the sheath of the plasma species. The higher electric field strength is responsible for higher T_e and n_e due to more excitation, ionization, collisional frequency, and rate of recombination processes within plasmas. The emission intensity and plasma parameters are also found to be higher in Ar compared to Ne in the absence and presence of an externally applied electric field. For example, in field-free case, T_e of vanadium plasma varies slightly from 9121 to 9402 K, and n_e varies from $1.06\times 10^{18}$ cm⁻³ to $1.14\times 10^{18}$ cm⁻³ as a function of Ar pressure variation, whereas in the case of employment of external electric field with field strength of 9 kV/cm, these values of T_e are enhanced with the variation from 9445 to 9683 K, and n_e varies from $1.29\times 10^{18}$ cm⁻³ to $1.34\times 10^{18}$ cm⁻³. Thus, the electric field strength of 9 kV/cm is responsible for a percentage increase of T_e from 2.9% to 3.6% and a percentage increase in n_e of vanadium plasma from 17.5% to 21.6% in the case of Ar. Similarly, trends have been observed for Ne.