Note on Ionization Coefficients in Methane

Measurements have been made of the spatial growth of ionization and of formative time lags in methane at pressures p in the range 9.7 < p < 52.6 mmHg corresponding to values of the parameter E/p (where E is the electric field) in the range 50 < E/p < 300 v cm^-1 mmHg^-1 and for over-voltages up to 5%. Experimental values of α/p were readily obtained as a function of E/p from measurements only in the early stages of the steady state of spatial growth of ionization, and the value of the generalized ω/α secondary ionization coefficient was very small as found by previous observers. Reliable measurements in the later stages of ionization growth, whether in the spatial steady state or temporal growth in the non-steady state, were difficult to make and reproducible values of the formative time lag t_f could not be obtained. This and other experimental effects were consistent with the view that dissociation of the methane occurred under the action of the ionization current when this was large, as in the later stages of ionization growth, and so changed the electrical properties of the gas. Such dissociation showed that it was not possible to make quantitative calculations relating t_f to α and the individual secondary processes composing ω/α, and so to establish a criterion for setting the value of the static breakdown potential in methane. The physical significance of the criterion αd similar 20 is discussed.