RF from two pulse laser plasmas in atmospheric pressure gas

Summary form only given. We have measured the production of RF produced by a mode-locked (M-L) CO/sub 2/ laser (100 mJ macro pulse, 3 ns pulselets @ 25 Mhz) focused onto a target of copper in a background of atmospheric pressure N/sub 2/ and He. Prior to the arrival of the M-L laser pulse, a small (few mm to cm) "vacuum bubble" is prepared at the focus by a pre-pulse laser (brought to the same focus) which is either another CO/sub 2/ laser or a YAG laser. In all cases RF in the 0.1 to 20 GHz region is produced as measured by an encircling, near-field, "band sensor" of /spl sim/5 cm diameter. Use of the pre-pulse laser enhances the RF power by a factor of up to 100. We present data showing RF signals vs pre-pulse laser energy and the time interval between the pre-pulse laser and the M-L laser. We also present open shutter photographs of plasma plumes generated with the pre-pulse lasers as well as the M-L CO/sub 2/ laser. We give calculations of plasma bubble formation mechanisms, as well as bubble volume and time to form.