Measurement of the electrical size of a laser-induced plasma in a uniform field

Measurements of the electric field produced by laser-induced plasma in ambient air under an external (bias) are presented. The transient field induced by the plasma was monitored using a D-dot type probe. The plasma field was found to have a dipolar structure with its moment aligned with the bias field. The measurement of the induced dipolar moment gives the electric size of the plasma. These results were compared with fast photography and shadowgraphy which gave sizes of about 2-3 mm in the time interval from 50 to 100 ns. The electric size in the same interval was approximately three times larger than that obtained by the optical techniques. The difference is attributed to an ionized layer 2.6 mm thick which reflects the spatial extent of the air ionized by UV radiation around the plasma.