The interaction of 1.07-μm laser radiation with plasma of a continuous optical discharge (COD) in xenon and argon at a pressure of p = 3–25 bar and temperature of T= 15 kK has been studied. The...

Laser-sustained plasma or continuous optical discharge (COD), predicted and first obtained in pioneering works initiated and inspired by Professor Raizer et al (1970 JETP Lett. 11 120–3; 1970 JETP Lett. 11 302–4; 1972 Sov. Phys.—JETP 34 763–9), is still a unique method of sustaining localized dense plasma in the laboratory and some special devices such as high-brightness broadband radiation sources [6–10]. Although the parameters determining the COD state can be stabilized, in practice, COD tends to regular oscillations. As stated in the paper, the oscillations may be attributed to the regular pulsing of thermal gravitational convective plumes around the COD. Application problems arising from instability of this type is pulsing of the emission from promising radiation sources utilizing COD plasma. This paper presents a detailed analysis of the experimental data on COD convective plume radii and oscillation frequencies depending on the plasma-forming gas pressure. The analysis leads to a similarity relation for the oscillation frequency. The similarity relation turns out to be common for optical discharges and laminar flames under conditions of prevailing buoyancy forces. This indicates the hydrodynamic nature of the instability considered, regardless of whether the energy input method is in the form of absorbed laser radiation or a chemical reaction. To support the idea of a hydrodynamic origin of the pulsations, a numerical simulation of a convective plume from a concentrated heat source was carried out. The thermal dissipation power of the heat source was equal to that of the COD. The results of numerical simulations demonstrate good agreement with those of the experiments. The dynamic temperature, density and gas velocity distributions obtained show that the direct cause of the oscillations is the dynamics of the toroidal vortices developed in the convection plume. The pulsing frequency equal to the frequency of the vortex formation may be increased under incoming additional forced gas flow that may also suppress the convective oscillations. The results obtained may be useful for studying optical discharges, and improving the parameters of high-brightness broadband laser-plasma radiation sources based on them.

The features of oxidation of ultra-high-temperature ceramic material HfB₂-30 vol.%SiC modified with 1 vol.% graphene as a result of supersonic flow of dissociated CO₂ (generated with the use of high-frequency induction plasmatron), as well as under the influence of combined heating by high-speed CO₂ jets and ytterbium laser radiation, were studied for the first time. It was found that the addition of laser radiation leads to local heating of the central region from ~1750 to ~2000–2200 °C; the observed temperature difference between the central region and the periphery of ~300–550 °C did not lead to cracking and destruction of the sample. Oxidized surfaces and cross sections of HfB₂-SiC-C_G ceramics with and without laser heating were investigated using X-ray phase analysis, Raman spectroscopy and scanning electron microscopy with local elemental analysis. During oxidation by supersonic flow of dissociated CO₂, a multilayer near-surface region similar to that formed under the influence of high-speed dissociated air flows was formed. An increase in surface temperature with the addition of laser heating from 1750–1790 to 2000–2200 °C (short term, within 2 min) led to a two to threefold increase in the thickness of the degraded near-surface area of ceramics from 165 to 380 microns. The experimental results indicate promising applications of ceramic materials based on HfB₂-SiC as part of high-speed flying vehicles in planetary atmospheres predominantly composed of CO₂ (e.g., Venus and Mars).

Optical discharges find now ever expanding applications in science, engineering and medicine. In present study authors discuss the phenomenon of periodic pulsing of a continuous optical discharge (COD) on the base of original experimental results. In the literature there is no general agreement regarding the origin of the considered instability. Authors have obtained the dependence of oscillation frequency on the gas pressure and proposed semi-empirical relations to estimate pulse frequency pressure dependence observed. Regular self-sustained oscillations of COD plasma were explained by the pulsations of heated gas bubble surrounding COD followed by buoyancy driven vortex formation that was detected on schlieren images of convection zone. This kind of buoyant convection instability was determined to obey the same similarity law as cycling frequency of puffing or flickering in diffusion and premixed flames. The instability discussed can affect the performance of COD in some important applications such as high brightness broadband light sources, for instance.