Spatial profiles of copper atom density in a Cu/Ne hollow cathode discharge

1. 12 Spatial profiles of copper atom density in a Cu/Ne hollow cathode discharge P. Hartmann*, T.M. Adamowicz**, E. Stoffels and W.W. Stoffels, Department of Physics, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven. *Research Institute of Solid State Physics and Optics, PO Box 49, 1525 Budapest, Hungary. **Institute of Micro- and Optoelectronics, Warsaw University of Technology, Koszykowa 75, Warsaw, Poland. The work has been supported by the Nato Science for Peace program, the Royal Dutch Academy of Sciences (KNAW), the Hungarian Academy of Sciences and the Polish Ministry of Science and Education. /n

2. Introduction • Cathode-sputtered Cu–Ne discharge are used to produce metal vapour lasers. • The NATO Science for Peace projects aims at improving laser operation (stability, lifetime, band-width and repetition rate). • To model the discharge accurate data are necessary. • Present work: absorption measurement to obtain spatially resolved Cu density in a Cu–Ne hollow cathode discharge. • These data will be used to check the numerical model.

3. Principle of laser operation • At the operating pressures of several mbars, high cathode voltages are responsible for efficient sputtering of metal atoms from the cathode by noble gas ions. • Sputtered metal atoms undergo charge transfer collisions with noble gas ions, e.g. Cu + Ne+ => (Cu+)* + Ne. • This leads to population inversion and laser oscillations on several Cu II transitions in the UV (240-270 nm). • The same principle is used for other metals e.g. gold, silver, zink etc. resulting in various wavelength in the UV region.

4. The tube Experimental tube • The experimental tube is a 60 cm long pyrex cylinder. • Quartz Brewster windows • Only the 10 cm long cathode is copper to avoid anode effects. • The long electrodes reduce axial gradients • Neon is flushed to allow for stable reproducible results. Picture of the tube: Left Al, right Cu, in the middle the plasma can be seen

5. The optical Set-up • Cu densities are measured by light absorption. • Light source is a broad band halogen lamp • A quartz lens produces a parallel beam. • Diaphragms allow for a space resolution of 1 mm. • An optical chopper can discriminate the plasma emission from the absorption signal • A monochromator with photomultiplier allows for wavelength selective detection.

6. Operation parameters Side view: 0.5 mbar Side view: 2.0 mbar • spatial resolution: 1 mm ( 19 data points) • pressure range: 0.22 – 3 mbar (HC effect ~ 0.5 mbar) • gas flow: ~ 0.5 sccm Ne • currents: 50 and 100 mA d.c. • wavelength: 324.7 nm

7. First results • Radially resolved absorption profiles for various operating pressures and plasma currents. • The the y-axis shows: • The chopper allows both the numerator and the denominator to be measured directly by the photomultiplier as peak to peak values.

9. Conclusions • At low neon pressures we observe the maximum Cu density at the tube axis. This indicates high re-absorption of Cu atoms by the walls and relatively little volume loss processes. • At higher pressures profiles become flat. • The optimal (highest) copper density in the discharge is obtained at an intermediate pressure of 1 mbar. • In future these results will be compared with a model of a hollow cathode discharge.