Attachment of initial secondary electrons in an electron-beam sustained discharge

The switch opening time, after e-beam turn-off, is determined by the electron loss processes: recombination and attachment. In order to achieve opening times of less than a microsecond at initial electron densities <1014cm-3 the dominate loss process must be attachment, which means that the switch gas mixture must contain an electro-negative gas. Additives of attacher will, however, also increase the power losses during conduction. Low forward voltage drop and fast opening can only be obtained by choosing gas mixtures which satisfy the following conditions: (1) for low values of EnN (conduction phase) the gas mixture should have a high drift velocity and a low attachment rate coefficient; (2) for high values of E/N (opening phase) the gas mixture should have a lower drift velocity and a high attachment rate coefficient; The required attachment rate can be achieved with attachers with an attachment cross section peaking at a few eV. For an electron beam sustained discharge operated at low values of E/N the energy of the initial secondary electrons may be well above the energy range of the steady state electron energy distribution, and also above the energy range in which the attacher has a significant attachment cross section. These initial secondary electrons will subsequently during their relaxation move through the energy range where attachment may occur even if the steadystate distribution function does not overlap with the attachment cross section