Spectroscopic study on the Hα line shape in spray discharge

Complete text of publication follows. The spray discharge (Spritzentladung) was discovered by Guentherschulze and Fricke, Z. Phys. 86, 451-63, 821-7 (1933). This discharge can be obtained with cathode of carbon covered by a layer of an insulating powder like e. g. Al2O3. In both cases a discharge without cathode dark space was observed and the negative glow beginning was detected at the cathode surface. The discharge potential can be low and current density much higher than in the glow discharge. The electrons are not accelerated in a cathode sheath layer as in the glow discharge, but in a thin solid insulating layer. This discharge has much resemblance to the vacuum discharge, where the positive charge on an insulating layer is brought about by emission of secondary electrons (Malter effect). In this work Doppler spectroscopy was used to study the Hα line shapes in a spray discharge with porous and solid Al2O3 thin layers on Al cathode. The light source was a standard Grimm type glow discharge operating in dc mode. To operate discharge in dc mode, a voltage stabilized power supply is used. The ballast resistor of 5 kΩ is placed in series with the discharge and power supply. The cathode was either plate of Al (purity 99.999%) or Al covered with ∼1 μm porous or 350 nm Al2O3 layer. The spectroscopic measurements were realized with 2 m focal length Ebert spectrometer (reciprocal dispersion 0.74 nm/mm in first diffraction order). The instrumental profile was very close to Gaussian, with full half-width of 0.018 nm. The end-on discharge image, unity magnification, was projected onto the entrance slit of the spectrometer by an achromatic lens (focal length 75.8 mm). Signals from the CCD line detector (3648 pixels, 8 μm pixel width) are A/D converted, collected and processed by PC. The results of line shape study will be presented at the Conference. The authors kindly acknowledge the financial support from the the Ministry of Science and Technological Development of the Republic of Serbia (Project 141032) and ECC (FP-6, Marie Curie Initiative, GLADNET, and Project 035459).