Bimodal ion energy distribution functions in a hydrogen magnetized plasma

The evolution of the IEDFs in a low-pressure HiPIMS discharge

In the present work we systematically study the ion energy distribution functions (IEDFs) in argon discharges produced by a combination of pulsed (1–2 kHz) microwave (MW) and continuous wave (cw) radio frequency (rf) excitations. We show that the IEDFs for the pulsed MW discharges are structured, with individual features originating from different periods of the pulse. In the dual-mode MW/rf discharge, significant modulation of the self-bias voltage, Vb, during the MW pulse cycle is observed, which we attribute to changes in the overall plasma impedance: We demonstrate that in the pulsed-MW/cw-rf mode the impedance is highly resistive when the MW signal is on, while it is predominantly capacitive during the period between individual pulses. Using the measured time evolution of Vb in combination with time-resolved measurements of individual ion species, IEDFs at the rf-powered electrode at each instant of the MW pulse have been obtained. This approach is then used to reconstruct the total IEDF in pulsed-MW/cw-rf plasma in order to determine the total energy delivered by the impacting ions.

A simple model is developed to examine the internal energy distribution for metastable ions and the probability of reaction in the field‐free regions of sector mass spectrometers in systems where the reactant ions in the ion source react by dissociation in competition with reversible isomerization to non‐dissociating species. The internal energy distribution for the reacting ions is strongly dependent on whether dissociation is preceded by isomerization and back‐isomerization, and it is shown that a range of reasonable combinations of rate constants for the forward and reverse isomerization result in a bimodal distribution of the internal energy for the ions that dissociate in the field‐free regions. The isomerization effectively provides a kinetic reservoir of higher energy reactant ions. The reaction probability of the metastable ions varies strongly with internal energy, and the back‐isomerized ions may in some instances be responsible for most of the reactions observed. A bimodal energy distribution will give rise to composite metastable peaks even though neither non‐statistical behavior nor multiple reaction channels are involved.

A plasma diagnostic study of the Ion Energy Distribution Functions (IEDFs) of H+, H2+, and H3+ ions in a 2.45 GHz hydrogen plasma reactor called TIPS is presented. The measurements are conducted by using a Plasma Ion Mass Spectrometer with an energy sector and a quadrupole detector from HIDEN Analytical Limited in order to select an ion species and to measure its energy distribution. The reactor is operated in the pulsed mode at 100 Hz with a duty cycle of 10% (1 ms pulse width). The IEDFs of H+, H2+, and H3+ are obtained each 5 μs with 1 μs time resolution throughout the entire pulse. The temporal evolution of the plasma potential and ion temperature of H+ is derived from the data. It is shown that the plasma potential is within the range of 15–20 V, while the ion temperature reaches values of 0.25–1 eV during the pulse and exhibits a fast transient peak when the microwave radiation is switched off. Finally, the ion temperatures are used to predict the transverse thermal emittance of a proton beam extracted from 2.45 GHz microwave discharges.

O2+ Temperature Profiles Measured in the Martian Ionosphere

Bacterial populations often have complex spatial structures, which can impact their evolution. Here, we study how spatial structure affects the evolution of antibiotic resistance in a bacterial population. We consider a minimal model of spatially structured populations where all demes (i.e., subpopulations) are identical and connected to each other by identical migration rates. We show that spatial structure can facilitate the survival of a bacterial population to antibiotic treatment, starting from a sensitive inoculum. Specifically, the bacterial population can be rescued if antibiotic resistant mutants appear and are present when drug is added, and spatial structure can impact the fate of these mutants and the probability that they are present. Indeed, the probability of fixation of neutral or deleterious mutations providing drug resistance is increased in smaller populations. This promotes local fixation of resistant mutants in the structured population, which facilitates evolutionary rescue by drug resistance in the rare mutation regime. Once the population is rescued by resistance, migrations allow resistant mutants to spread in all demes. Our main result that spatial structure facilitates evolutionary rescue by antibiotic resistance extends to more complex spatial structures, and to the case where there are resistant mutants in the inoculum.

It is known that in very-high-frequency (VHF) capacitively coupled plasmas, the higher harmonics generated by nonlinear sheath motion can enhance the standing wave effect (SWE), which can lead to center-peaked plasma density profiles. In this work, an improved nonlinear electromagnetic model incorporating a transmission line model, an electron momentum balance model, a bulk plasma model, a collisionless nonlinear numerical sheath model, and an ion Monte-Carlo collision (MCC) model is developed to study the effects of low-frequency (LF) voltage VL on the nonlinear standing wave excitation, plasma uniformity, and ion energy and angular distribution functions (IEDFs and IADFs) in dual-frequency (DF) asymmetric capacitive argon discharges at relatively low pressure of 3 Pa. The plasma diffusion in the radial direction and ion dynamics within the LF oscillating sheath are self-consistently considered. The LF voltage VL at 2 MHz varies from 0 to 700 V while the HF voltage VH at 60 MHz is fixed at 100 V. Simulation results indicate that without the addition of an LF source (i.e. VL= 0 V), there are a considerable number of high-order harmonics with short wavelengths, leading to significant SWE and central peak in the radial plasma density profile. Nevertheless, the high-order harmonic excitations tend to be weakened and merely occur around the phase of the full LF sheath collapse due to a shorter characteristic damping time of the surface waves as VL increases. This, combined with increased surface wavelengths of both the driving frequency and the higher harmonics at a higher VL , leads to suppressed standing waves and improved plasma uniformity. Meanwhile, the simulations show that both the low and the high energy peaks of IEDF move towards higher energies, and the energy peak separation width ΔE becomes wider with the increase of VL . The IEDF at the radial center of the powered electrode exhibits a broader ΔE than that at the edge. For the IADF, an increased VL results in more ions incident on the electrode with a smaller deflection angle. Because of a thinner sheath and a higher sheath voltage at the electrode center, the peak value of IADF at the electrode center is greater than that at the edge.

Radio-frequency capacitively coupled plasmas (RF-CCPs) are central to microelectronics processing, yet their behavior is highly sensitive to the working gas. We employ a self-developed two-dimensional particle-in-cell/Monte Carlo collision (PIC/MCC) model to investigate He-Ar mixtures in He-rich, Ar-dilute regimes. Introducing a modest Ar fraction into He increases plasma density and improves spatial uniformity via complementary electron-impact processes on the two species. With increasing Ar fraction and pressure, the Ar ion energy distribution function (IEDF) shifts to slightly lower peak energy and broadens,whereas the overall He ions energy spectrum remains largely unchanged but exhibits broadening with partial extension into the high-energy region. These trends are consistent with a modified sheath potential and altered ion acceleration path lengths in a more collisional sheath. The results demonstrate that adjusting the Ar content provides a simple knob to tune IEDFs and uniformity in RF-CCPs without changing the external driving waveform.

An etching simulator is developed to study the two-dimensional (2D) silicon etch profile evolution under SF6/O2 inductively coupled plasma discharge. The simulator is composed of three modules: plasma kinetic module, sheath module, and etching module. With this approach, the authors can predict the 2D etch profile evolution versus reactor parameters. Simulation results from the sheath model show that the shape of the bimodal ion energy distribution function for each incident angle depends on the ion mass. It is all the larger that the ion mass is low. As shown in the experiment, the simulation results reveal that the atomic oxygen plays an important role in the passivation process along the side-wall. Indeed, the simulation results show the decrease of the undercut when the %O2 increases. This improves the etching anisotropy. However, the decrease in the etch rate is observed for a high %O2. Moreover, for a moderate direct current (DC) bias (some 10 V), a low variation of the silicon etch profile versus DC bias is observed. The moderate ion energy only allows removing of the passivation layer on the surface bottom. The etching process is mainly controlled by the chemical etching under fluorine flux.

The techniques of optical beam shaping have enabled progress in a broad range of interdisciplinary science and engineering, owing to the unique properties and promising applications of their created structured light. However, the conventional methods, which are based on fully coherent optics approaches, introduce several adverse effects such as speckles noise in the generated beams and susceptible to be disturbed in complex environment (e.g. turbulent atmospheres), because of the sensitive coherent light-matter interaction. To overcome those side effects, a new protocol relied on the partially coherent beam shaping has been developed. By elaborately tailoring the complex spatial coherence structure of a partially coherent beam, the desired beam profile and trajectory with high beam quality and robust propagation feature in complex environment can be generated. In this review, we present an overview of such unconventional partially coherent beam shaping with a focus on the important role of the complex spatial coherence structure engineering. Partially coherent beam shaping not only provides an efficient means for resisting the disadvantages in coherent optics methods but also enables new applications in novel optical imaging and tweezers.

Natural microbial populations often have complex spatial structures. This can impact their evolution, in particular the ability of mutants to take over. While mutant fixation probabilities are known to be unaffected by sufficiently symmetric structures, evolutionary graph theory has shown that some graphs can amplify or suppress natural selection, in a way that depends on microscopic update rules. We propose a model of spatially structured populations on graphs directly inspired by batch culture experiments, alternating within-deme growth on nodes and migration-dilution steps, and yielding successive bottlenecks. This setting bridges models from evolutionary graph theory with Wright-Fisher models. Using a branching process approach, we show that spatial structure with frequent migrations can only yield suppression of natural selection. More precisely, in this regime, circulation graphs, where the total incoming migration flow equals the total outgoing one in each deme, do not impact fixation probability, while all other graphs strictly suppress selection. Suppression becomes stronger as the asymmetry between incoming and outgoing migrations grows. Amplification of natural selection can nevertheless exist in a restricted regime of rare migrations and very small fitness advantages, where we recover the predictions of evolutionary graph theory for the star graph.

It has been established that in order to study thermo- and electrophysical phenomena in wet steam turbines, studies of the wet steam flow volume charge effect on its dielectric and thermophysical properties have recently been carried out at A. Pidhornyi Institute of Mechanical Engineering Problems of the National Academy of Sciences of Ukraine. According to their results, it was established that the most representative electrophysical parameter, which allows to evaluate changes in the thermophysical properties of steam, which occur under the action of its own volume charge, is its dielectric constant. It is assumed that the value of the dielectric constant of an electrified steam can be significantly different from the value for a neutral steam, and a mathematical assessment of its possible change is made. It has been confirmed that the influence of electrophysical phenomena caused by the wet steam flow electrification is significant, but is not taken into account in the existing physical and mathematical thermodynamic models of the steam expansion process. It is proved that in order to clarify the main thermodynamic parameters and calculated characteristics of the electrified wet steam flow, it is necessary to determine how its dielectric constant changes. On the basis of the analysis, the relevance of experimental determination of the dielectric constant of a wet steam flow with a volume charge in order to obtain the dependence of its change on the temperature and pressure of the flow, as well as the density of the volume charge, is substantiated. To perform the research, a gas dynamic laboratory plant, which allows to obtain a wet steam flow with a volume charge, was used. The internal space of the flow part of a real wet steam turbine has a significant size and allows the formation of a flow with a volume charge of a complex spatial configuration and structure. It is emphasized that in the flow part of the plant of a small volume, in contrast to the turbine flow part, significant technical difficulties arise when organizing the conditions for the occurrence of a wet steam flow with a volume charge. Taking this into account, at the first stage, it was decided to conduct a study of a steam flow with a volume charge flowing into the atmosphere in a laboratory room with a sufficient volume to form its spatial structure. To estimate the value of its dielectric constant, the inductive method (L-method) of determining dielectric properties, in which the substance under study is introduced into the inductive solenoid cell, was chosen. Experiments were conducted and, according to the obtained data, it is possible to make a preliminary assessment of the change in dielectric constant in the presence of a volume electric charge in the steam flow

In recent years, emerging sequencing technologies and computational tools have driven a tidal wave of research on host-associated microbiomes, particularly the gut microbiome. These studies demonstrate numerous connections between the gut microbiome and vital host functions, primarily in humans, model organisms, and domestic animals. As the adaptive importance of the gut microbiome becomes clearer, interest in studying the gut microbiomes of wild populations has increased, in part due to the potential for discovering conservation applications. The study of wildlife gut microbiomes holds many new challenges and opportunities due to the complex genetic, spatial, and environmental structure of wild host populations, and the potential for these factors to interact with the microbiome. The emerging picture of adaptive coevolution in host-microbiome relationships highlights the importance of understanding microbiome variation in the context of host population genetics and landscape heterogeneity across a wide range of host populations. We propose a conceptual framework for understanding wildlife gut microbiomes in relation to landscape variables and host population genetics, including the potential of approaches derived from landscape genetics. We use this framework to review current research, synthesize important trends, highlight implications for conservation, and recommend future directions for research. Specifically, we focus on how spatial structure and environmental variation interact with host population genetics and microbiome variation in natural populations, and what we can learn from how these patterns of covariation differ depending on host ecological and evolutionary traits.

We report experimental observation of ion heating and subsequent development of a prominent ion temperature gradient in the core of a linear magnetized plasma device, and the controlled shear de-correlation experiment. Simultaneously, we also observe the development of strong sheared flows at the edge of the device. Both the ion temperature and the azimuthal velocity profiles are quite flat at low magnetic fields. As the magnetic field is increased, the core ion temperature increases, producing centrally peaked ion temperature profiles and therefore strong radial gradients in the ion temperature. Similarly, we observe the development of large azimuthal flows at the edge, with increasing magnetic field, leading to strong radially sheared plasma flows. The ion velocities and temperatures are derived from laser induced fluorescence measurements of Doppler resolved velocity distribution functions of argon ions. These features are consistent with the previous observations of simultaneously existing radially separated multiple plasma instabilities that exhibit complex plasma dynamics in a very simple plasma system. The ion temperature gradients in the core and the radially sheared azimuthal velocities at the edge point to mechanisms that can drive the multiple plasma instabilities, that were reported earlier.

A method is developed to estimate harbor seiche at Marina di Carrara, Italy, from the properties of wind-generated incident waves outside the harbor. A linear model of the spatial structure of amplified seiche modes is combined with empirical estimates of the response of each mode to variable incident wave forcing. These empirical coefficients parameterize the complex nonlinear transfer of energy from wind waves to lower frequency seiche. As at other small harbors (<1 km2 surface area) on ocean coasts, and consistent with previous analyses at Carrara, the observed seiche is relatively energetic at several periods between about 1 and 15 min that are highly amplified theoretically, and the spatial structure of modeled and observed seiches agree as well. The longest seiche (≈15 min) mode is almost spatially uniform within the harbor and dominates with low-energy, short-period incident wind waves (measured 1 km offshore of the harbor). Increased wave energy and longer periods excite shorter period (1–3 min) seiche modes with more complex spatial structure, including small areas of high amplification, which have led to operational issues. The energy in each of the six most energetic seiche modes is related in this paper empirically to offshore incident wind wave height and peak period, allowing detailed predictions of harbor seiche from routine wind wave forecasts. The approach appears applicable to relatively small, shallow harbors with reflective quay walls, in which the exterior harbor mouth is exposed, and the interior sheltered from energetic wind-generated waves.