Electron temperature measurement of Ba photoplasma by using an electrostatic probe

Abstract

We study experimentally and theoretically the time evolution of the electron temperature Te of a photoionized barium vapor which expands freely into a vacuum. Using the gas dynamics fundamental equations and assuming the plasma expansion to be adiabatic, a model is built and analytical expressions are derived for the electron and ion temperatures, velocity, and density time evolutions. The experimental apparatus consists essentially of a vacuum chamber, a Joule effect furnace which produces the Ba vapor. A cylindrical plasma created between the two vertical plates is produced by two-step ionization of the vapor. The electron temperature is measured with a cylindrical electrostatic probe biased by a slowly variable voltage ramp. The current-voltage curve is built step by step with a boxcar averager. The results involve different parameter variations like vapor density or sampling time of the probe current for studying the time evolution of electron temperature. Finally, it was found that the adiabatic cooling model agrees well with the experimental electron temperature evolution, but was limited below 0.025 eV by the Langmuir probe accuracy.