Propagation of medium energy electrons (10–20 keV) in carbon dioxide

Abstract

Propagation of medium energy electrons (10–20 keV) in carbon dioxide is studied for application to using the Electron Beam Fluorescence (EBF) technique with CO2 as a rarefied gas dynamic diagnostic tool. A collimated (1 mm nominal diameter) electron beam at energies of 10, 15, and 20 keV was injected into a vacuum chamber containing ambient CO2 maintained at pressures of 50, 100, 200, 400, 600, and 800 mTorr. Through deconvolution of recorded two-dimensional images of the emitted fluorescence, the growth of the root-mean-square (rms) radius of the beam was obtained as a function of axial distance. An empirical form of the differential scattering cross section which includes the contributions of both elastic and inelastic collisions between fast electrons and CO2 molecules is used in single scattering, multiple scattering, and scattering envelope models developed for describing the electron distribution along the electron beam. Using these models, the rms radius of the electron beam is calculated and compared with experimental results obtained from imaging of the electron beam fluorescence. It is found that the multiple scattering and envelope models significantly overestimate experimental rms radius data. The single scattering model underestimates radius data at target thickness higher than ∼2 and 4 Torr cm for the 10 and 15 keV beams, respectively. The single scattering model matches well with the 20 keV beam data.