MEASUREMENT OF THE DENSITY DISTRIBUTION IN A RAREFIED GAS FLOW USING THE FLUORESCENCE INDUCED BY A THIN ELECTRON BEAM

The concept of utilizing a confined gaseous fluorescence, induced by a thin but energetic electron beam, for the measurement of local rarefied gas densities, notably in a low-density flow field, was applied in the development of an optical electron beam density probe. The instrument consists of a high-voltage electron gun sealed to the test section of a low-density aerodynamics facility. One or more small-diameter exit tubes allow t e generated electron beam to enter the flow field. The beam-induced fluorescence which is coincident with the beam and whose intensity depends on the local gas density, is detected by a photomultiplier arrangement. The measured photocurrent is calibrated directly against the gas density. Point measurements are made feasible by controlling the location of the gun with the help of a traversing mechanism, and by selecting the radiation coming from a small section of the electron beam. The following topics are discussed relevant properties of the fluorescence mechanism, sealing of an electron gun to a low-density aerodynamics facility, design considerations for an electron gun, the calibration, and limitations of the method. Results are presented from an experimental investigation of the radial density distribution of the molecular flux that emerges from a short cylindrical tube. Comparison with a theoretical solution is made. Author