Characterization of High-Pressure Argon Plasma Generated by Femtosecond Laser

Abstract

When high-intensity laser light is focused, the breakdown at focal spot occurs and the plasma develops toward laser irradiation. The study about electron density and temperature of low-pressure femtosecond laser plasma has already been.][1] In this experiment, titanium sapphire laser having, a maximum laser energy of 100mJ, a pulse half width of 100 fs, and a wavelength of 780 nm, is used. Incidentally, the laser energy in the chamber decreases to 50 % of laser energy, because the chamber windows diameter is smaller than laser light diameter. The electron density and electron temperature of laser induced high-pressure Ar plasma are respectively measured. Electron density up to 100 atm is measured by Mach-Zender interferometer. The Ar-ion laser is used as a probe laser source. It is difficult to find out a turning point of interferometric signal at which the electron density reaches a maximum. Therefore, the peak electron density is estimated by extrapolating the observed electron density up to the time at which laser pulse is terminated. Dense plasma with an electron density of the order of 1025 - 1026 m-3 at focal spot is obtained. The theoretical calculation of electron density is done and compares to experimental results. The calculated results are lower than the experimental results, because this theoretical calculation does not include effect of multiphoton ionization. The electron temperature up to 50 atm is measured by using emission spectroscopy of the laser induced Plasma. The measured electron temperatures are respectively obtained by the Boltzmann plot method and the continuous spectrum of the light intensity distribution. The measured electron temperatures are about 10000 K. On the other hands, the calculated electron temperature is obtained from the energy balance equation taking into consideration the energy gain due to inverse bremsstrahlung and the energy lose due to elastic collision and collision ionization. Theoretical calculations and experimental results are qualitatively agreed above 10 atm. However, they are disagreed at gas pressure below 10 atm, because multiphoton ionization is not considered in theoretical calculation.