Laser light scattering (LLS) to observe plasma impact on the adhesion of micrometer-sized particles to a surface

Laser light scattering (LLS) method, combined with a long-distance microscope was utilized to detect micrometer-sized particles on a smooth substrate. LLS was capable to detect individual particle release, shrink, or fragmentation during exposure to a plasma or a gas jet. In-situ monitoring of hundreds of particles was carried out to investigate the effect of hydrogen plasma exposure on particle adhesion, morphology, and composition. LLS was calibrated with monodisperse melamine resin spheres with known sizes of 2.14 µm, 2.94 µm, and 5.26 µm in diameter. The lowest achievable noise level of approximately 3% was demonstrated for counting 5.26 µm spherical melamine particles. The accuracy for melamine particle size measurements ranged from 50% for 2.14 µm particles to 10% for 5.26 µm particles. This scatter was taken as the imprecision of the method. Size distribution for polydisperse particles with known refractive index was obtained by interpolating to an effective scattering cross-section of a sphere using Mie theory. While the Abbe diffraction limit was about 2 µm in our system, the detection limit for Si particles in LLS according to Mie approximation was assessed to about 3 µm, given the limitations of the laser flux, microscope resolution, camera noise, and particle composition. Additionally, the gradual changes in forward scattering cross-sections for Si particles during the exposure to the hydrogen plasma were consistent with Si etching reported in the literature.</p

Probing background ionization: Positive streamers with varying pulse repetition rate and with a radioactive admixture

Positive streamers need a source of free electrons ahead of them to propagate. A streamer can supply these electrons by itself through photo-ionization, or the electrons can be present due to external background ionization. Here we investigate the effects of background ionization on streamer propagation and morphology by changing the gas composition and the repetition rate of the voltage pulses, and by adding a small amount of radioactive Krypton 85. We find that the general morphology of a positive streamer discharge in high purity nitrogen depends on background ionization: at lower background ionization levels the streamers branch more and have a more feather-like appearance. This is observed both when varying the repetition rate and when adding Krypton 85, though side branches are longer with the radioactive admixture. But velocities and minimal diameters of streamers are virtually independent of the background ionization level. In air, the inception cloud breaks up into streamers at a smaller radius when the repetition rate and therefore the background ionization level is higher. When measuring the effects of the pulse repetition rate and of the radioactive admixture on the discharge morphology, we found that our estimates of background ionization levels are consistent with these observations; this gives confidence in the estimates. Streamer channels generally do not follow the paths of previous discharge channels for repetition rates of up to 10 Hz. We estimate the effect of recombination and diffusion of ions and free electrons from the previous discharge and conclude that the old trail has largely disappeared at the moment of the next voltage pulse; therefore the next streamers indeed cannot follow the old trail.Comment: 30 pages, 13 figure

On-board plasma assisted fuel reforming

It is well known that the addition of gaseous fuels to the intake manifold of diesel engines can have significant benefits in terms of both reducing emissions of hazardous gases and soot and improving fuel economy. Particularly, the addition of LPG has been investigated in numerous studies. Drawbacks, however, of such dual fuel strategies can be found in storage complexity and end-user inconvenience. It is for this reason that on-board refining of a single fuel (for example, diesel) could be an interesting alternative. A second-generation fuel reformer has been engineered and successfully tested. The reformer can work with both gaseous and liquid fuels and by means of partial oxidation of a rich fuel-air mix, converts these into syngas: a mixture of H2 and CO. The process occurs as partial oxidation takes place in an adiabatic ceramic reaction chamber. High efficiency is ensured by the high temperature inside the chamber due to heat release. Thus, efficient thermal insulation is crucial to maintain said temperature. Heat recuperation from the reformer exhaust also improves the thermal efficiency. The prototype yields up to 20% of H2 (80% of the theoretical maximum) and 22% of CO with all kinds of fuels tested, including automotive diesel fuel. Efficient thermal insulation allows to keep the dimensions below 40 cm in any direction for a full burning power of 10-30 kW while outer wall of the reformer is exposed to air at normal temperature

The impact of hydrogen plasma on the structure and morphology of tin and lead micrometer sized particles

The stability of micrometer sized particles in hydrogen plasma is essential for extreme ultraviolet lithography, the ITER fusion program and the application of hydrogen plasma etching. We experimentally investigated the morphological evolution of tin (Sn), lead (Pb), and lead (II) oxide (PbO) micrometer sized particles on a surface that is exposed to a low pressure hydrogen plasma. Post exposure particle cross sections obtained by a scanning electron microscope accompanied by a focused ion beam demonstrated a significant influence of hydrogen plasma exposure on both the surface and the bulk material of the particles. Chemical sputtering at the surface and accumulation of pressurized hydrogen bubbles in cavities in the bulk material are the main drivers of the morphological changes. These mechanisms may influence the adhesion of particles to the surface through the introduction of asperities, increase of contact spot area, or fragmentation after the accumulation of mechanical stress

Non-equilibrium plasma ignition for internal combustion engines

High-voltage nanosecond gas discharge has been shown to be an efficient way to ignite ultra-lean fuel air mixtures in a bulk volume, thanks to its ability to produce both high temperature and radical concentration in a large discharge zone. Recently, a feasibility study has been carried out to study plasma-assisted ignition under high-pressure high-temperature conditions similar to those inside an internal combustion engine. Ignition delay times were measured during the tests, and were shown to be decreasing under high-voltage plasma excitation. The discharge allowed instant control of ignition, and specific electrode geometry designs enabled volumetric ignition even at high-pressure conditions

Development of KM-5 Hall effect thruster and its flight testing onboard GEO spacecraft “Express-A4”

The paper presents the outcomes of development of Keldysh Research Center (KeRC) KM-5 Hall effect thruster and experience obtained from the in-orbit exploiting of the thruster onboard Reshetnev's Scientific and Production Association of Applied Mechanics (NPO PM) “Express-A4” GEO communication spacecraft (SC). KM-5 is a multimode thruster with operational power range from 1 to 2.5 kW in contrast to other thrusters applied in space missions. It has high thrust characteristics. It makes possible solution of orbit raising tasks to geostationary orbit (GEO) for medium and heavy weight SC in addition to tasks of station keeping for geostationary SC already being solved by Hall-effect thrusters. Furthermore, a thruster with such performance can be an efficient choice as a main engine for small interplanetary probes