The electron density at the axis, the electron temperature and the electric field strength of the positive column of low-pressure Hg/Ne discharges have been measured using electrostatic probes. The measurements have been carried out for discharge currents between 0·1 and 0·8 A, noble gas pressures between 0·3 and 10 Torr, tube wall temperatures in the range of 13-70°C, and an inner tube radius of 18 mm. The addition of argon to neon as a filling gas has also been studied.By computing the volume and wall losses from the measured data, the authors have worked out the effect of the discharge conditions on the radiation efficiency of the column. These data have been compared with the results of a theoretical model of the positive column, showing a reasonable agreement. From this comparison it has been found that elastic collisions of electrons and of ions with neutral mercury atoms have a decisive influence on column properties at mercury pressures above similar 10−2 Torr. Furthermore a higher radiation efficiency of the column has been found in Hg/Ne discharges compared to Hg/Ar discharges for 3 Torr noble gas pressure, wall temperature 42°C, and discharge current 0·4 A.

The positive column of 50-Hz ac discharges in a mixture of sodium vapor and noble gas (99 vol% Ne–1 vol% Ar) has been investigated experimentally. The experimental conditions were as follows: discharge tube radius 10 mm, noble-gas pressure 5.5 Torr at 20°C, and sodium density (2.4–5.7)  × 1019 m−3 which corresponds to a tube wall temperature of 519–538 K. The rms values of the current ranged from 0.25 to 1.5 A. The discharge properties, measured with the aid of electrostatic probes, show hysteresis effects especially for high rms current values. This behavior can be explained from the combined effects of depletion of sodium atoms and diffusion. It is found that the experimental data on the time-resolved electron temperature agree with calculated values. Qualitative agreement is obtained between the measured and calculated data on the electron density, electric field strength, and radiated power.

A new type of discharge instability is reported for a low-pressure Na–noble-gas discharge (∼5×10−3 Torr Na; 5.5 Torr Ne-Ar, 1% Ar). The voltage-current (V-I) characteristic is shown to be multivalued in the voltage. The resulting instability is a discontinuity in the positive column which propagates in the direction of electron particle flow with velocity between about 105 and 5×105 cm/s.