Effective ionization, electron and ion transport in SF6-He mixtures

This paper reports on the measurement of the electron drift velocity, the density normalized effective ionization coefficients (alpha - eta)/N, and the ratio eta/alpha (eta and alpha are the attachment and ionization coefficients, respectively), over a wide range of the density-reduced field strength, E/N, from 40 to 560 Td (1 Td = 10(-17) V cm(2)). From the measured values of (alpha - eta)/N and eta/alpha, the density normalized ionization and attachment coefficients were obtained. Further analysis of avalanche development past the first electron transit allowed us to derive the average mobility of both positive and negative ions. The mobility of negative ions was found to agree very well with Blanc's law at small fields, thereby indicating that the majority negative ion in the avalanche is SF6-. The critical field strength was found to vary almost linearly with the SF6 content in the mixture, comparing well with previous measurements.