First experimental measurement of the free streaming plasma mode

Summary form only given, as follows. The "free streaming" mode is magneto-static (w=0) plasma eigenmode sustained by steady currents flowing in the plasma in alternate directions. For this reason it has also been referred to as the "picket fence" mode. The existence of this mode has been theorized for more than 30 years, but has never been observed experimentally. This mode is excited in the DARC source where a static electric field alternating with a spatial wavenumber. k/sub 0/ is transmitted into a plasma through a relativistic ionization front. We present the first experimental measurement of the "free streaming" mode. This was accomplished by placing an array of microstrip coils, (whose impedance is matched to a 50 ohm load from zero to 1 GHz), inside of the proof of principle DC to AC radiation convertor (DARC source). With this arrangement the phasing of the interaction region and the microstrip coil array is held constant, thus allowing measuring each coil in the array independently, instead of simultaneously. The wavenumber of the free streaming mode, variations of the magnetic field amplitude with the incident electric field strength and the plasma density were measured, and are consistent with theoretical predictions. The implications of the existence of the free streaming mode is the validation of the assumption that the electrons born via laser ionization are essentially at rest, or at most, born with an isotropic velocity distribution.