We investigated the breakdown in low-pressure helium gas both experimentally and by computer simulations. At low breakdown voltages (V BR 1000 V) the experimental and simulation results show a good agreement (differences are within 20%), while at higher voltages the simulations and experiments agree qualitatively. Our simulations indicate that several processes contribute to the particular shape of the Paschen curve in helium at low pressures. These processes are: (1) the dependence of the (ion-induced) secondary electron emission yield on the ion energy, (2) the appearance of ion impact ionization of the gas at high electric fields and (3) the secondary electron emission from the cathode due to fast neutral atoms.

We carried out molecular dynamics experiments to determine the reduced heat diffusion coefficient D ‫ء‬ th and the reduced thermal conductivity l ‫ء‬ of the three-dimensional classical electron one-component plasma, for the 1 # G # 20 range of the plasma coupling parameter G. In our simulations the temperature of the system was spatially modulated, and D ‫ء‬ th and l ‫ء‬ were calculated from the relaxation time of the temperature profile. D ‫ء‬ th was found to decrease with increasing G, while l ‫ء‬ decreased with increasing G for G # 4 (from l ‫ء‬ Х 2.3 at G 1.1 to l ‫ء‬ Х 0.6 at G 4) and was approximately constant (l ‫ء‬ Х 0.4) in the 8 # G # 20 range. [S0031-9007(98)06853-7]

Voltage-current characteristics and the Cu-II 780.8 nm laser performances are described for a novel segmented hollow cathode and for three-and four-slot hollow-anode cathode (HAC) tubes. Each of these operate at a higher voltage and with higher slope resistance than a conventional hollow cathode and produce improved laser performance. The best laser performance is obtained with the segmented tube. The application of a longitudinal magnetic field raises the discharge voltage and enhances the laser performance for the segmented tube and raises the voltage for the four-slot HAC tube. The magnetiqfield lowers the voltage and reduces the laser performance with the three-slot HAC tube. The voltage effects are attributed to the deflection of the fast electrons by the magnetic field and represent experimental evidence for the oscillation of electrons in a hollow-cathode discharge. .

We have investigated the effect of constriction on the characteristics of low-pressure glow discharges in argon. In a series of experiments four discharge tubes with plane-parallel electrodes of different diameters (D = 31.4, 20, 10 and 5 mm) and same electrode separation (L 0 = 45 mm) were studied. The discharges were surrounded by floating-potential metal tubes. We measured voltage-current characteristics of the discharges and recorded the spatial intensity distribution of selected spectral lines (Ar-I 750.3 nm, 811.5 nm and Ar-II 476.5 nm) in the electrode gap at current densities 0.2 mA cm −2 j 1 mA cm −2. We also observed copper lines in the spectrum originating from the sputtering of the copper cathode, even at these relatively low discharge current densities. The 'effective surface' of the cathode-that actively participates in discharge operation-is always smaller than the cathode surface. This results from the radial electric field formed in the cathode dark space due to the accumulation of charges on the metal tube surrounding the discharge. Using a simple model we could calculate the effective cathode diameter (which determines the current density for given current). Taking into account the current density obtained this way, the voltage of the tubes was found to obey the V = f (j/p 2) scaling relation. The increasing loss of charge on the metal wall with decreasing tube diameter also resulted in structural changes in the discharge. While at large diameter the cathode dark space and the negative glow filled the interelectrode gap, at low D a positive column-like part was also formed.