Ionization and breakdown in oxygen

Measurements are reported of uniform-field breakdown voltages and of effective primary and secondary ionization coefficients in oxygen in the range 0.825*10 18 <or=N<or=9.90*10 18 cm -3 , 1.1*10 -15 <or=E/N<or=2.1*10 -15 V cm -2 , where N is the gas number density and E is the electric field strength. Oscillographic observations show that, over the range covered, the secondary electrons are produced only by photoelectric processes. Also reported are theoretical values of the secondary ionization coefficient, calculated using a one-dimensional model. The results show that for N=0.825*10 18 cm -3 the secondary process can be described in terms of a single radiation component with an absorption coefficient of 38 cm -1 (at N=25.1*10 18 cm -3 ). The secondary process appears to be mainly cathode photoemission but photo-ionization of the gas also plays a significant part. For N=3.30*10 18 cm -3 , on the other hand, the calculated and measured secondary coefficients differ considerably and additional radiation components must be assumed to be active.