Studies of electron energy distribution in plasma produced by a resonant laser pulse

Abstract

Using the Langmuir probe method we evaluated the electron energy distribution (EED) in plasma produced in a Na+He mixture by a laser pulse tuned to the 3S–3P transition. We proposed a new method of retrieving the EED from the probe characteristics at high gas pressures, when the free path of the electrons is less than the probe radius, i.e. in the range of 2–100 Torr. The method consists of using a combination of the first and second derivatives of the electron current. The systematic error that usually occurs when the EED is treated by means of traditional Langmuir and diffusion probes was reduced by two to three times when treated this way. Two phase temporal evolution of the electron distribution was observed. Initially the shape of the EED was close to the Maxwellian function. Its temperature corresponded to the effective excitation temperature, which can characterize the total population of excited sodium atoms. When the concentration of excited atoms decreased, one observed a transformation of the EED function. At the second phase it became well Maxwellized only in the low energy region. The high energy part of the EED had a non-equilibrium shape. This was caused by relaxation of fast electrons produced by super-elastic collisions with residual excited atoms.