The density of the neutral gases in Hg-rare gas discharges and their spatial distribution is controlled by the discharge parameters as well as by the externally adjustable partial pressures. Essential quantities in this context are the gas temperature produced by elastic collisions, the ion transport by the discharge current, the processes governing the wall temperature, and ambipolar diffusion. Despite equal partial pressures different densities may occur, which, i n turn, will influence the parameters of the discharge. This has effects on the assessment of the methods of Hgvapour pressure adjustment and on the evaluation of the measured values. These effects are demonstrated, especially by field strength measurements, for a wide parameter range.

After the investigation of the influence of current and tube radius on the power concentration of the Hg-rare gas discharge in the two previous parts, part I11 deals with the corresponding effects of additional recombination surfaces. The plasma parameters (radiant flux of the resonance lines, field strength, electron density, electron energy distribution function) are measured in a discharge tube with an axial glass rod. With increasing rod diameter and increasing distance from the tube axis radial radiation intensity distribution becomes asymmetric. The field strength increases with the radius of the rod. The dependence of the field strength and that of the radiant flux on the parameters of the discharge do not basically change. The measured UV-radiation efficiency of the positive column is higher than that of discharges without axial rod for equal tube radius. This is attributed primarily to the changed radial electron density distribution and its effect on the collision processes.

In a preceding part of the paper, based on experimentid methods and on a corresponding kinetic model, especially the impact of the effective life times of the 63P1 and 61P1 Hg resonance levels on the main properties of the low pressure Hg-Ar positive column has been studied. In the presented second part these investigations, which have been performed under the aspect of their applicnt.ion in fluorescent lamps, will be continued. They are related to the effects of tube radius and discharge current density. New fluorescent lamps have reduced tube radii from R = 18 t o 13 mm and compact lamps down to R = 5 mm. Thus it is of interest to study the electron distribution function and main macroscopic properties of such low pressure discharge plasmas, where the latter properties nre obtained by adequate energy space averaging. At first, results are reported and discussed which are related to the dependence of main plasma parameters on the Hg partizrl pressure when reducing the discharge tube radius for constant discharge current i and argon pressure P,,~. Starting from the electron distribution function thc particle and energy budget will be studied in detail, especially of course the change of the ultraviolet radiation output from both Hg resonmce levels 63P, and 61P1 and, in addition, of visible radiation. Then, to understand the effect of the discharge current density, as a reprcsentntive example two cases with different v i h e s of R, i and pAr but with equal current density have been investigated and discussed.Altogether the investigations made will shed light upon the complex relntions in the mentioned mixture plasma and give hints to select suitable pnrnmetcr values which can be usefill to irnprovc light soiirccs.

By measuring the radiant flux of the resonance lines of mercury, the field strength and the electron energy distribution function the influence of the current on the parameters, especially on the energy balance of the Hg-Ar positive column is investigated, compared with a kinetic model, iind interpreted. The decrease in yield of the 254 nm radiation with increasing current is linked to a corresponding increase of the losses due to elastic collisions with the rare gas atoms and is caused by the changes in the electron energy distribution function. Moreover, the influence of gas temperature, Hg-densities and effective life times is investigated.

The reduction of the tube radius is an effective technique for enhancing power concentration. The increased wall losses are compensated by larger field strengths, which counteract the decrease in radiation efficiency with rising current density. The causal relationships are investigated for tube radii of 18-4 mm a t constant discharge current 1 = 200 mA and constant current density j = 1.2 mA/mm2 and for parameter variation of the field strength only. The measured efficiencies of the Hg-resonance lines 254 and 185 nm and the electron energy distribution functions agree well with the calculated values. Moreover, the results of the model calculations provide information on the elementary processes involved, such as elastic losses, collisions of second kind, stepwise processes and Coulomb interaction. The results show that Hg-rare gas discharge with tube radii R = 5-10 mm are no less suitable for light production than those with radii between 13 and 18 mm.