Effect of Variable Ionic Mobility on Ambipolar Diffusion

Recent measurements of the mobility of atomic rare gas ions in their parent gases have shown that the mobility falls with increasing field strength, becoming inversely proportional to the square root of the field at high fields. This paper presents the solution to the ambipolar diffusion problem for cylindrical geometry and with the observed field dependence of ionic mobility. Results for ${\mathrm{He}}^{+}$ ions in He indicate that ambipolar diffusion in a low-pressure positive column may be reduced by a factor of two or more from that predicted by using constant mobility. The theory is also applicable to ${\mathrm{Ne}}^{+}$ ions in Ne and to ${\mathrm{A}}^{+}$ ions in A.