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Published online by Cambridge University Press: 13 March 2009
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.