Speaker
Mrs
Veronica Gonzalez-Fernandez
(University of Valladolid)
Description
Low-pressure plasmas generated in hollow-cathode discharges (HCD) devices have been widely used in industry and in research. Namely, discharges working in glow discharge regime have been used in material processing, etching, thin-film deposition, spectroscopy etc. All these applications require a deep knowledge of plasma behaviour, theoretically and experimentally.
During many years the Laser Spectroscopy Laboratory (University of Valladolid) has developed and improved an experimental arrangement to perform high-quality electric field strength (E-field) measurements in the cathode fall region in a HCD in pure hydrogen working in abnormal glow-discharge regime. This is done by a non-invasive technique with high temporal and spatial resolution based on the Stark shifting and splitting of the 2S level of hydrogen and optogalvanic detection. The E-field is determined by the frequency difference of the 2P1/2 and 2P3/2 components.
Pulsed UV-radiation (243 nm) used in these measurements is generated by an injection seeded Q-switched Nd:YAG (10 Hz) and a second laser based in non-linear crystals. This system provides single-longitudinal mode radiation, 300 MHz bandwidth, 5 mJ and 2.5 ns temporal duration. In the experimental arrangement, the UV radiation is divided in two counter-propagating beams circularly polarized in opposite directions (following the selection rules for two-photon absorption, $\Delta$L=0) and focused on the upper central part of the discharge, in a 120 μm focus. The discharge source is a cylindrical cathode placed between two cylindrical cone peaked anodes. All pieces have an axial perforation that allows end-on spectroscopic measurements at different distances from the cathode surface. The discharge can be handled in pressures from 400 Pa to 1350 Pa, and currents from 50 mA to 300 mA.
This experimental arrangement has provided very good results in stainless steel cathodes with an inner diameter of 10 mm. In this work, a more exhaustive study of the discharge is performed, studying the influence of the diameter and the cathode material. Therefore, four different cathodes have been used: two stainless steel cathodes, with inner diameters of 10 and 15 mm; and two tungsten cathodes of the same diameters.
To obtain the maximum information available from the discharge, several theoretical models are fitted to the experimental E-fields. From these studies important parameters can be obtained as the length of the cathode dark space, the voltage employed to maintain the discharge, etc. The early results analysis show very good agreement between theory and the experimental results.
Primary author
Mrs
Veronica Gonzalez-Fernandez
(University of Valladolid)
Co-authors
Dr
Concha Pérez
(University of Valladolid)
Dr
Klaus Grützmacher
(University of Valladolid)
Dr
Luis Maria Fuentes
(University of Valladolid)
Dr
Maria Inmaculada de la Rosa
(University of Valladolid)
