143 research outputs found
The effect of ambipolar electric fields on the electron heating in capacitive RF plasmas
We investigate the electron heating dynamics in electropositive argon and
helium capacitively coupled RF discharges driven at 13.56 MHz by Particle in
Cell simulations and by an analytical model. The model allows to calculate the
electric field outside the electrode sheaths, space and time resolved within
the RF period. Electrons are found to be heated by strong ambipolar electric
fields outside the sheath during the phase of sheath expansion in addition to
classical sheath expansion heating. By tracing individual electrons we also
show that ionization is primarily caused by electrons that collide with the
expanding sheath edge multiple times during one phase of sheath expansion due
to backscattering towards the sheath by collisions. A synergistic combination
of these different heating events during one phase of sheath expansion is
required to accelerate an electron to energies above the threshold for
ionization. The ambipolar electric field outside the sheath is found to be time
modulated due to a time modulation of the electron mean energy caused by the
presence of sheath expansion heating only during one half of the RF period at a
given electrode. This time modulation results in more electron heating than
cooling inside the region of high electric field outside the sheath on time
average. If an electric field reversal is present during sheath collapse, this
time modulation and, thus, the asymmetry between the phases of sheath expansion
and collapse will be enhanced. We propose that the ambipolar electron heating
should be included in models describing electron heating in capacitive RF
plasmas
Customized ion flux-energy distribution functions in capacitively coupled plasmas by voltage waveform tailoring
We propose a method to generate a single peak at a distinct energy in the ion
flux-energy distribution function (IDF) at the electrode surfaces in
capacitively coupled plasmas. The technique is based on the tailoring of the
driving voltage waveform, i.e. adjusting the phases and amplitudes of the
applied harmonics, to optimize the accumulation of ions created by charge
exchange collisions and their subsequent acceleration by the sheath electric
field. The position of the peak (i.e. the ion energy) and the flux of the ions
within the peak of the IDF can be controlled in a wide domain by tuning the
parameters of the applied RF voltage waveform, allowing optimization of various
applications where surface reactions are induced at particular ion energies
Effects of fast atoms and energy-dependent secondary electron emission yields in PIC/MCC simulations of capacitively coupled plasmas
In most PIC/MCC simulations of radio frequency capacitively coupled plasmas
(CCPs) several simplifications are made: (i) fast neutrals are not traced, (ii)
heavy particle induced excitation and ionization are neglected, (iii) secondary
electron emission from boundary surfaces due to neutral particle impact is not
taken into account, and (iv) the secondary electron emission coefficient is
assumed to be constant, i.e. independent of the incident particle energy and
the surface conditions. Here we question the validity of these simplifications
under conditions typical for plasma processing applications. We study the
effects of including fast neutrals and using realistic energy-dependent
secondary electron emission coefficients for ions and fast neutrals in
simulations of CCPs operated in argon at 13.56 MHz and at neutral gas pressures
between 3 Pa and 100 Pa. We find a strong increase of the plasma density and
the ion flux to the electrodes under most conditions, if these processes are
included realistically in the simulation. The sheath widths are found to be
significantly smaller and the simulation is found to diverge at high pressures
for high voltage amplitudes in qualitative agreement with experimental
findings. By switching individual processes on and off in the simulation we
identify their individual effects on the ionization dynamics and plasma
parameters. We conclude that fast neutrals and energy-dependent secondary
electron emission coefficients must be included in simulations of CCPs in order
to yield realistic results
Kinetic Interpretation of Resonance Phenomena in Low Pressure Capacitively Coupled Radio Frequency Plasmas
The kinetic origin of resonance phenomena in capacitively coupled radio
frequency plasmas is discovered based on particle-based numerical simulations.
The analysis of the spatio-temporal distributions of plasma parameters such as
the densities of hot and cold electrons, as well as the conduction and
displacement currents reveals the mechanism of the formation of multiple
electron beams during sheath expansion. The interplay between highly energetic
beam electrons and low energetic bulk electrons is identified as the physical
origin of the excitation of harmonics in the current
Impact of Research About the Early Development of Children With Intellectual Disability: A Science Mapping Analysis
The āWarnock Reportā (Department for Education and Science, 1978) underlined the importance of early intervention for children with a range of special educational needs and the importance of partnership with families. This paper focuses on young children with intellectual disability to describe the longitudinal research on early development that has emerged since the report, and to describe the scholarly literature that has been impacted by this longitudinal work. First, we conducted a systematic literature search for primary reports of longitudinal studies on the early development of children with intellectual disability. Included studies were those that measured dependent (i.e., developmental outcomes) and independent variables (i.e., risk and resilience factors) on at least two measurement occasions (i.e., truly longitudinal), starting before the end of the 7th year of life, with samples including children with intellectual disability (or related terms). The topics of these studies, and of the publications that have cited these longitudinal studies, were extracted from titles and abstracts using machine reading and subjected to multidimensional clustering (VOSviewer; Van Eck and Waltman, 2016). The resulting body of 101 research studies (about 2.5 studies per year) covered a scattering of topics without a dominant focus. The literature that was impacted by these longitudinal studies consisted of 3,491 scientific publications. Three clusters of topics emerged from mapping the terms used in these publications, which were dominated by (1) syndrome and disorder related terms; (2) autism-related terms; and (3) disability and parent related terms. Topics related to autism and, to a lesser extent, parents showed the strongest increase over time. Topics related to intervention and programmes were mostly linked to the topics disability and parents. Taking into account the science mapping as well as features of the context in which research on intellectual disability takes place, we suggest a collaborative research agenda that systematically links topics relevant for intervention with longitudinal research, in co-creation with families
The effect of the driving frequency on the confinement of beam electrons and plasma density in low pressure capacitive discharges
The effect of changing the driving frequency on the plasma density and the
electron dynamics in a capacitive radio-frequency argon plasma operated at low
pressures of a few Pa is investigated by Particle in Cell/Monte Carlo
Collisions simulations and analytical modeling. In contrast to previous
assumptions the plasma density does not follow a quadratic dependence on the
driving frequency in this non-local collisionless regime. Instead, a step-like
increase at a distinct driving frequency is observed. Based on the analytical
power balance model, in combination with a detailed analysis of the electron
kinetics, the density jump is found to be caused by an electron heating mode
transition from the classical -mode into a low density resonant heating
mode characterized by the generation of two energetic electron beams at each
electrode per sheath expansion phase. These electron beams propagate through
the bulk without collisions and interact with the opposing sheath. In the low
density mode, the second beam is found to hit the opposing sheath during its
collapse. Consequently, a high number of energetic electrons is lost at the
electrodes resulting in a poor confinement of beam electrons in contrast to the
classical -mode observed at higher driving frequencies. Based on the
analytical model this modulated confinement quality and the related modulation
of the energy lost per electron lost at the electrodes is demonstrated to cause
the step-like change of the plasma density. The effects of a variation of the
electrode gap, the neutral gas pressure, the electron sticking and secondary
electron emission coefficients of the electrodes on this step-like increase of
the plasma density are analyzed based on the simulation results
The first 20,000 strange situation procedures: A meta-analytic review
The Strange Situation Procedure (SSP) was developed five decades ago to assess infantāparent attachment relationships. Although the procedure itself has remained relatively constant in over 285 studies (20,720 dyads) conducted to date, there have been vast sociological changes during this time, and research foci shifts to studying diverse populations. Since its inception, the SSP has also been adopted in over 20 countries. In this meta-analysis, we collate this large body of work, with the objectives of producing reliable estimates of the distribution of the four SSP attachment classifications, assessing temporal trends and geographical differences, and determining if and when distributions are different across various populations. Results revealed that the global distribution of SSP attachment was 51.6% secure, 14.7% avoidant, 10.2% resistant, and 23.5% disorganized. There were no differences in the distribution among mothers and fathers, and no child age or sex differences. We found a temporal trend in which there was less avoidant attachment over time and there were attachment distribution differences between samples from North America versus other regions of the world, particularly Asia, Middle East/Israel and South America. We found higher rates of avoidant and disorganized attachment in populations with sociodemographic risks and higher rates of disorganized attachment in samples where parents had psychopathology and when the child experienced maltreatment or was adopted from foster or institutional care. The implications of these findings for future research and practice are discussed. (PsycInfo Database Record (c) 2023 APA, all rights reserved
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