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<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> 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 “<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_e</i>” and electron density “<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n_e</i>” 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 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_e</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n_e</i> 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, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_e</i> of vanadium plasma varies slightly from 9121 to 9402 K, and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n_e</i> varies from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.06\times 10^{18}$ </tex-math></inline-formula> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.14\times 10^{18}$ </tex-math></inline-formula> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> 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 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_e</i> are enhanced with the variation from 9445 to 9683 K, and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n_e</i> varies from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.29\times 10^{18}$ </tex-math></inline-formula> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.34\times 10^{18}$ </tex-math></inline-formula> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup>. Thus, the electric field strength of 9 kV/cm is responsible for a percentage increase of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_e</i> from 2.9% to 3.6% and a percentage increase in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n_e</i> of vanadium plasma from 17.5% to 21.6% in the case of Ar. Similarly, trends have been observed for Ne.