Measurements of a Precursor Electron Front

An electron current front of a few thousand amperes has been produced in the expansion chamber of an electric shock tube. It precedes the shock and follows the (much faster) precursor breakdown wave. The electron front velocity of up to 2 × 107 cm/sec is linear with applied voltage gradient and gas conductivity, as measured by double probes. As the operating parameters are varied over a wide range the electron temperature varies from 600 to 8000°K, but the number density in the front remains about 2 × 1013 cm−3. The front heats the gas and increases the speed of the subsequent shock by a factor of about two. This field-driven wave may exist as a precursor in other shock tubes and must be distinguished from breakdown and hot electron diffusion precursor waves already reported by other authors.