Time-of-flight electron-swarm studies of ionisation and attachment in gases

A technique has been developed to evaluate ionisation and attachment coefficients for electrons travelling in gases across a uniform electric field by measuring the integrated-anode-current/time profile for a pulse of electrons released from the cathode. The main advantage of the method is that the time-resolved technique clearly distinguishes the current due to delayed effects, such as the arrival of negative ions in attaching gases or electrons produced by the metastable collisional ionisation of trace gases present in the inert gases. The method has been adopted to determine ionisation coefficients in hydrogen [85¿E/N¿169 Td (1 Td=10¿21Vm2)], nitrogen (226¿E/N¿546) and helium (14¿E/N¿226) and both ionisation and attachment coefficients in oxygen (14¿E/N¿56) and carbon tetraflouride (42¿E/N¿197).