Experiments of ion extractions of cesium plasmas produced between two parallel plates by the fourth harmonics of a YAG laser (266 nm) have been carried out. The influences of applied voltage and initial ion density on the ion extraction process have been discussed and a scaling relation has been formulated. But the ion extraction time does not always decrease with the applied voltage. In the experiment, the ion extraction time increases when the applied voltage is above 1200 V. The existence of ion sputtering peak has been observed above a certain applied voltage, which is coincident with the theoretical analysis. A best applied voltage is obtained from the experiments concerning not only the ion extraction time but the collection of target isotope ions.