Volume 96, Issue 6, 15 September 2004
Index of content:
- PLASMAS AND ELECTRICAL DISCHARGES (PACS 51-52)
96(2004); http://dx.doi.org/10.1063/1.1782951View Description Hide Description
A two-dimensional modified fluid model for a capacitively coupled rf discharge in methane, used for the deposition of diamond-like carbon layers, is presented. The gas velocity calculated with a computational fluid dynamics code is used as an input for the modified fluid model.Convection is taken into account as an additional transport mechanism as well as diffusion and migration. The calculations show that the gas flow results in a shift of the maximum of the densities of the plasma species toward the grounded electrode. It is shown that this shift has a large effect on the fluxes of the ions and radicals toward the substrate where the deposition occurs. As a result, the deposition rate will increase if the gas velocity has a component directed to the surface and it will decrease in the opposite case. However, the uniformity of the layer can become worse when the convection velocity is too high for the reactor geometry studied here.
Differences in the metallic plasma-neutral gas structure in a vacuum arc operated with nitrogen and argon96(2004); http://dx.doi.org/10.1063/1.1776628View Description Hide Description
Ion current and electron temperature are measured using electrostatic probes in a dc, nonfiltered vacuum arc operated with argon and nitrogen as filling gases in the pressure range . It is found that the measured ion current for argon is between two and six times larger than for nitrogen, for similar operating conditions. Also, the electron temperature is smaller for . These differences can be satisfactorily explained with a simple one-dimensional model, which includes the most relevant elastic and inelastic processes that take place in the interelectrodic plasma:elastic scattering of metallic ions by neutral gas, charge exchange, electron impact ionization of gas, dissociative recombination of gas ions, and conversion of atomic ions into molecular ions. The observed differences between nitrogen and argon are attributed to the low rate of conversion of atomic argon into molecular argon, that makes inoperative the channel of dissociative recombination for this gas, together with the higher ionization rate of argon.
96(2004); http://dx.doi.org/10.1063/1.1784554View Description Hide Description
Optical breakdown generated by laser pulses in helium, argon, and xenon atmospheres has been investigated by fast plasma plume imaging and measurements of energy and temporal shape of the transmitted laser pulse. The role of preionization due to material evaporation has been examined by placing targets with low ablation threshold in the vicinity of the laser beam focus. Ambient gas pressure, laser pulse energy, and distance between target and laser beam focus have been varied in order to identify the physical mechanisms involved into the ignition and propagation of the laser-induced breakdownplasma. The understanding of the generation of absorption waves at low power density is useful for the design of plasma shutters with low threshold in the infrared spectral range.
96(2004); http://dx.doi.org/10.1063/1.1778212View Description Hide Description
Water bath calorimetry was used to demonstrate one more peculiar phenomenon associated with a certain class of mixed gas plasmas, termed resonant transfer (RT) plasmas. Specifically, , , and (500 and ) plasmas generated with an Evenson microwave cavity consistently yielded on the order of 50% more heat than non-RT plasma (controls) such as , , under identical conditions of gas flow, pressure, and microwave operating conditions. The excess power density of RT plasmas was of the order . In earlier studies with these same RT plasmas it was demonstrated that other unusual features were present including dramatic broadening of the hydrogen Balmer series lines, unique vacuum ultraviolet lines, and, in the case of waterplasmas, population inversion of the hydrogen excited states. Both the current results and the earlier results are completely consistent with the existence of a hitherto unknown exothermic chemical reaction, such as that predicted by Mills, occurring in RT Plasmas.