Deionization of afterglow plasma of pulsed lasers through intensified ambipolar diffusion

摘要

Method of electron concentration (n_e) reducing in an interpulse period is proposed, based on ambipolar diffusion (AD) rate increase to the tube walls. As the AD coefficient D_a = D(1-T_e/T), the heating of electrons by weak electric field to the T_e ≤ 1, yet insufficient for ionization although much higher than the gas temperature T, considerably hastens the process of ion removal to the walls. Effect of the intensified ambipolar diffusion (IAD) was registered experimentally by us in the afterglow of zinc and cadmium metal vapor ion lasers. In this work, computer modeling of the IAD is performed for He-Cu and Ne-Cu discharge afterglows with parameters typical for self-terminated copper vapor laser. As the calculations demonstrate, at helium pressure of about 10 Torr and tube diameter ～ 1 cm, when heating field E of 0.8-0.9 V/cm is imposed on afterglow for a time period of 100 μs, n_e slides down to values of 10~2-10~4 times smaller as compared to those without heating. The effect is stronger in helium. The use of IAD in interpulse period would allow, as calculations reveal, up to 20-40 % increase of pulse repetition rate in the copper vapor laser with small tube diameters and low buffer gas pressures. The excess build up of copper metastables density is prevented by interruption of plasma heating 3-5 μs prior the onset of a main current pulse.