An increase in the expansion rate towards vacuum of a plasma column with density of order 108 cm−3 and radius 0·5–2·0 mm produced by photoionization in the presence of a 9·4 GHz microwave field is found. The microwave field imposed by the TEM005 Gaussian mode of a spherical Fabry–Pérot resonator acts on the plasma through its ponderornotive force. First, the use of a potential barrier spectrometer allows us to measure the increase, in the number and energy of escaping electrons compared with the same plasma without microwave field. Secondly, the expansion of the modified photoplasma is checked by applying a small polarization voltage on the Fabry-Pérot mirrors to collect the ions. In the presence of the microwave field the time-resolved ion peak. Which presents a two-lobe profile faster than the single one observed without the field, indicates strong modification of the plasma dynamics. All these observations are interpreted by a simple model including the ponderomotive microwave force and the electrostatic plasma force, which act in opposite directions.

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.