Excitation by fast atoms at very high electric field to gas-density ratios in argon
- Joint Institute for Laboratory Astrophysics, University of Colorado National Institute of Standards and Technology, Boulder, Colorado 80309 (US)
Measurements of transient and steady-state light emission and of current transients from Ar at very high electric field to gas-density ratios {ital E}/{ital n} and low gas densities show that the excitation of the 811-nm lines of Ar is primarily by fast neutral Ar atoms. A drift tube with parallel-plane electrodes produces a spatially uniform electric field. Pulsed laser irradiation at 266 nm of the semitransparent cathode produces current pulses of {approx}10 mA and {approx}10 ns width. The time and spatial dependences of the line emission at 811{plus minus}1 and 750{plus minus}1 nm were measured for {ital E}/{ital n} in the range 280 townsend (Td) to 84 kTd at gas densities from 1.3{times}10{sup 23} to 9{times}10{sup 20} m{sup {minus}3}, where 1 Td=10{sup {minus}21} V m{sup 2}. At {ital E}/{ital n}{lt}4 kTd the time dependence of the 811-nm emission is consistent with direct and cascade excitation due to electron collisions with Ar. At {ital E}/{ital n}{gt}18 kTd the emission transient consists of an early peak caused by the initial electron avalanche followed by a delayed peak that moves toward the cathode with an apparent velocity of about twice that of the ions. Steady-state 811-nm emission is also observed in a region of very low electric field near the cathode where the Ar{sup +} ions have insufficient energy for excitation and the fast atoms are injected from a high-field region. These observations of 811-nm emission, plus previous measurements of steady-state spatial distributions of emission, are interpreted as demonstrating that excitation of the 811-nm lines is primarily by fast Ar atoms formed by charge transfer in Ar{sup +-}Ar collisions. For 750-nm emission the prompt portion is much larger and the delayed portion is much weaker than for 811-nm emission showing the importance of excitation by electrons even at these high {ital E}/{ital n} values.
- OSTI ID:
- 5622155
- Journal Information:
- Physical Review, A; (USA), Vol. 43:6; ISSN 1050-2947
- Country of Publication:
- United States
- Language:
- English
Similar Records
Excitation of Balmer lines in low-current discharges of hydrogen and deuterium
Ionization and current growth in N/sub 2/ at very high electric field to gas density ratios