A plasma diode electron-beam for low-density flow visualization

An electron beam is capable of illuminating gas flows at densities that are too low for refractive visualization techniques to operate. The electrons impact upon the gas molecules, causing electronic excitation. As the molecules fluoresce down to their unexcited state, the photons given off indicate their location. Because there is a direct relationship between the luminosity and the density, a photograph of the flowing gas reveals the flow structure. The need to examine short-time events, such as chemical laser mixing flows, led to the choosing of a 6-in. diam plasma diode electron beam, capable of operating between 20 - 120 kV with current densities as high as 100 mA per sq cm for times as long as 200 microns. By clipping the beam to a thin sheet (10 cm x 0.5 mm), it is possible to extract a 2-D cross-sectional image of the flow. The technique is studied by firing the electron beam into the plume of an axisymmetric jet exhausting into a vacuum chamber.