The hollow cathode discharge tube has proved an excellent source for emission spectroscopy of metals, but in the red and near-infrared regions the limit of sensitivity for the detection of weak lines is set by scattered light or photon noise from the strong lines of the carrier gas, usually neon or argon. This paper describes an 'active filter' consisting of a long, low-current, positive column discharge in which an appreciable population of excited atoms is maintained in the lower levels of the lines in question, resulting in selective absorption of the hollow cathode radiation. Reduction factors of about 8 were achieved for lines starting from the two metastable levels. From subsidiary experiments on the reversal temperatures and equivalent widths of the filter discharge lines and on the widths and shifts of the hollow cathode lines, a simple radiative transfer model was constructed so that the effects of different discharge parameters on the reduction factors could be predicted. Of the various ways of increasing these, the most promising is to broaden the filter discharge lines by, for example, applying a magnetic field to the discharge.
We experimentally formed snow dimples on artificial snow layers via a snow-and rainfall device and observed these in cross-section. Time sequences of density, water content, and thermo-graphic images show local and little water penetrations in the low-density layer under the water-saturated layer during the initial stage. After these penetrations were observed across the cross-section, snow dimples formed on the snow surface. Initial water penetrations at 0ºC affected the growth of snow particles and their aperture. This metamorphism of snow under the water-saturated layer caused both the decrease of the water entry capillary pressure and the increase of the volume of impounded water. Consequently, high penetration in the metamorphosed snow layer rapidly changed to granular snow, forming snow dimples on the snow surface.
The longitudinal confinement of a plasma in a pulsed multi-mirror fusion reactor is investigated. It is shown that the initial plasma energy W p per unit of reactor cross-section and the initial plasma temperature T o serve as similarity parameters. One-dimensional numerical calculations of plasma flow along the reactor axis were performed for various W p and T o and were used to determine the plasma gain function QE(W P , T o ). The region of maximum values of QE lies near T o = 5 keV for all W p . Plasma heating by a-particles becomes substantial for values of Q exceeding unity. At W p = 20 MJcm" 2 , QE calculated with plasma heating by a-particles exceeds QE as calculated without including such heating by a factor of five. If the mirror ratio is increased by enlarging the cross-section towards the reactor ends, Q is further increased by a factor of two. A value of QE = 1 is achieved at an initial plasma energy of W p « 5 MJ-cm" 2 , and QE = 10 is achieved for W p «12MJ-cm" 2 .
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