He metastable density in a double layer formed by a diameter discontinuity in a positive column

The population density profile of triplet metastable He (2 3S1) is measured in the multiple space-charge (double) layer formed at an abrupt transition of the tube diameter in a positive column discharge. The double layer is formed at the transition from a 2.6 cm diameter to a 0.6 cm diameter tube in a static pressure discharge. The line integrated He 2 3S1 population density profile in the double-layer region is determined by diode-laser absorption. The triplet metastable density profiles are measured with discharge currents of 1, 2, and 5 mA at 1 Torr pressure for both polarity conditions: (a) when the cathode end of the discharge is in the large diameter tube and (b) when the anode end of the discharge is in the large diameter tube. The diode-laser absorption profiles show enhanced production of triplet metastable He 2 3S1 in the double-layer region as compared to the unperturbed positive column (0.6 cm diameter) with the magnitude being strongly dependent on the discharge polarity.