Continuum analysis of the photoionization chamber in the transition from low to high rates of ionization

The photoionization chamber consists of a test gas which is bounded by parallel plates and ionized to a small extent by incident radiation. The charged particles formed by the ionization process diffuse to the walls of the chamber where they recombine. The particle densities and electric field distributions as well as the current-voltage characteristic are studied. At low levels of ionization the electron density is neglected with respect to the ion density. By coupling this simplification with correct scaling, it is possible to obtain analytic solutions. The resulting current-voltage characteristic reflects the rate of ionization as well as the ratio of the ion to the electron temperature. When this ratio is equal to unity, the ion and electron current fluxes saturate at the same rate. If this ratio is less than unity, the electron current flux saturates more slowly than the ion current flux. At higher levels of ionization the electron density can no longer be neglected with respect to the ion density and the problem is solved numerically. The current-voltage characteristic obtained in this case can be approximated analytically at small and large voltages. The expression which applies to small voltages agrees with the results of other double probe studies.