Electron densities and drift velocities in a positive column He-Cd laser discharge

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Abstract
Electron densities and drift velocities have been measured in the positive column of a He-Cd laser discharge under conditions appropriate to laser oscillation at 4416 AA and 3250 AA. In a pure helium discharge the electron drift velocity is proportional to the E/p value of the discharge, and drift velocities measured (3-12 * 106 cm s-1 for E/p values in the range 4-16 volt cm-1 Torr-1) are in agreement with those of earlier investigators for low current densities. The behaviour of drift velocity with E/p can be explained in terms of a momentum transfer cross section for electrons with helium which is inversely proportional to electron velocity. The associated electron collision frequency at unit pressure is 2*109 s-1Torr-1. At constant discharge current and helium frequency at unit pressure is 2*109 s-1Torr-1. At constant discharge current and helium pressure the presence of a small partial pressure of cadmium in the discharge increases the electron density (by about 40% to 2.5*1012cm-3 under conditions appropriate to lasing) and decreases the electron drift velocity and E/p value.