Ponderomotive effects on the expansion of a photoplasma in the microwave range

摘要

An increase in the expansion rate towards vacuum of a plasma column with density of order 10B cm"3 and radius 0-5-2-0 mm produced by photoionization in the presence of a 94 GHz microwave field is found. The microwave field imposed by the TEMl)05 Gaussian mode of a spherical Fabry-Perot resonator acts on the plasma tlirough its ponderomotive 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-Perot 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 Avithout the field, indicates strong modification of the plasma dynamics. All these observations arc interpreted by a simple model including the pon?deromotive microwave force and the electrostatic plasma force, which act in opposite directions.