2,097 research outputs found
Electron heating mode transitions in dual frequency capacitive discharges
The authors consider electron heating in the sheath regions of capacitive discharges excited by a combination of two frequencies, one much higher than the other. There is a common supposition that in such discharges the higher frequency is the dominant source of electron heating. In this letter, the authors discuss closed analytic expressions quantifying the Ohmic and collisionless electron heating in a dual frequency discharge. In both cases, the authors show that the lower frequency parameters strongly influence the heating effect. Moreover, this influence is parametrically different, so that the dominant heating mechanism may be changed by varying the low frequency current density
Uncertainty and sensitivity analysis in complex plasma chemistry models
The purpose of a plasma chemistry model is prediction of chemical species densities, including understanding the mechanisms by which such species are formed. These aims are compromised by an uncertain knowledge of the rate constants included in the model, which directly causes uncertainty in the model predictions. We recently showed that this predictive uncertainty can be large—a factor of ten or more in some cases. There is probably no context in which a plasma chemistry model might be used where the existence of uncertainty on this scale could not be a matter of concern. A question that at once follows is: which rate constants cause such uncertainty? In the present paper we show how this question can be answered by applying a systematic screening procedure—the so-called Morris method—to identify sensitive rate constants. We investigate the topical example of the helium–oxygen chemistry. Beginning with a model with almost four hundred reactions, and focussing on conditions relevant to biomedical applications, we show that only about fifty rate constants materially affect the model results, and as few as ten cause most of the uncertainty. This means that the model can be improved, and the uncertainty substantially reduced, by focussing attention on this tractably small set of critical rate constants. We discuss strategies that might be used to accomplish this refinement. The present results apply to a particular chemistry, but we suggest that possibly, and perhaps probably, investigations of other plasma chemistry models will arrive at similar results. In that case, an opportunity exists to systematically improve the quality of plasma chemistry modelling
Verification of particle-in-cell simulations with Monte Carlo collisions
Widespread recent interest in techniques for demonstrating that computer simulation programs are correct ('verification') has been motivated by evidence that traditional development and testing procedures are disturbingly ineffective. Reproducing an exact solution of the relevant model equations is generally accepted as the strongest available verification procedure, but this technique depends on the availability of suitable exact solutions. In this paper we consider verification of a particle-in-cell simulation with Monte Carlo collisions. We know of no exact solutions that simultaneously exercise all of the functions of this code. However, we show here that there can be found in the literature a number of non-trivial exact solutions, each of which exercises a substantial subset of these functions, and which in combination exercise all of the functions of the code. That the code is able to reproduce these solutions is correctness evidence of a stronger kind than has hitherto been elucidated
A model for tailored-waveform radiofrequency sheaths
The sheath physics of radiofrequency plasmas excited by a sinusoidal waveform is reasonably well understood, but the existing models are complicated and are not easily extended to the more complex waveforms recently introduced in applications. Turner and Chabert (2014 Appl. Phys. Lett. 104 164102) proposed a model for collisionless sheaths that can easily be solved for arbitrary waveforms. In this paper we extend this model to the case of collisional sheaths in the intermediate pressure regime. Analytical expressions are derived for the electric field, the electric potential and the density profiles in the sheath region. The collisionless and collisional models are compared for a pulsed-voltage waveform
Atomic oxygen patterning from a biomedical needle-plasma source
A ”plasma needle” is a cold plasma source operating at atmospheric pressure. Such sources interact strongly with living cells, but experimental studies on bacterial samples show that this interaction has a surprising pattern resulting in circular or annular killing structures. This paper presents numerical simulations showing that this pattern occurs because biologically active reactive oxygen and nitrogen species are produced dominantly where effluent from the plasma needle interacts with ambient air. A novel solution strategy is utilised coupling plasma produced neutral(uncharged) reactive species to the gas dynamics solving for steady state profiles at the treated biological surface. Numerical results are compared with experimental reports corroborating evidence for atomic oxygen as a key bactericidal species. Surface losses are considered for interaction
of plasma produced reactants with reactive solid and liquid interfaces. Atomic oxygen surface reactions on a reactive solid surface with adsorption probabilities above 0.1 are shown to be limited by the flux of atomic oxygen from the plasma. Interaction of the source with an aqueous surface showed hydrogen peroxide as the dominant species at this interface
Flux and energy asymmetry in a low pressure capacitively coupled plasma discharge excited by sawtooth-like waveform -- a harmonic study
Control over plasma asymmetry in a low-pressure capacitively coupled plasma
(CCP) discharges is vital for many plasma processing applications. In this
article, using the particle-in-cell simulation technique, we investigated the
asymmetry generation by a temporally asymmetric waveform (sawtooth-like) in
collisionless CCP discharge. A study by varying the number of harmonics (N)
contained in the sawtooth waveform is performed. The simulation resultspredict
a non-linear increase in the plasma density and ion flux with N i.e., it first
decreases, reaching a minimum value for a critical value of N, and then
increases almost linearly with afurther rise in N. The ionization asymmetry
increases with N, and higher harmonics on the instantaneous sheath position are
observed for higher values of N. These higher harmonics generate multiple
ionization beams that are generated near the expanding sheath edge and are
responsible for an enhanced plasma density for higher values of N. The ion
energy distribution function (IEDF) depicts a bi-modal shape for different
values of N. A strong DC self-bias is observed on the powered electrode, and
its value with respect to the plasma potential decreases with an increase in N
due to which corresponding ion energy on the powered electrode decreases. The
simulation results conclude that by changing the number of harmonics of a
sawtooth-like in collisionless CCP discharges, the ion flux asymmetry is not
generated, whereas sheath symmetry could be significantly affected and
therefore a systematic variation in the ion energy asymmetry is observed. Due
to an increase in the higher harmonic contents in the sawtooth waveform with N,
a transition from broad bi-modal to narrow-shaped IEDFs is found
Using the resonance hairpin probe and pulsed photodetachment technique as a diagnostic for negative ions in oxygen plasma
In this work the resonance hairpin probe technique has been used for detection of photoelectrons generated during photodetachment experiments performed to determine negative ion density in an inductively coupled oxygen plasma. An investigation of the temporal development of the photoelectron population was recorded with the hairpin probe located inside the laser beam region and at various points outside the beam. Varying the external microwave frequency used to drive the probe resonator allowed the local increase in electron density resulting from photoelectrons to be determined. At a fixed probe frequency, we observed two resonance peaks in the photodetachment signal as the photoelectron density evolved as a function of time. Inside the laser beam the resonance peaks were asymmetric, the first peak rising sharply as compared with the second peak. Outside the laser beam region the peaks were symmetric. As the external frequency was tuned the resonance peaks merge at the maximum electron density. The resonance peak corresponding to maximum density outside the beam occurs at a delay of typically 1–2 µs as compared with the centre of the beam allowing an estimate of the negative ion velocity. Using this method, negative ion densities were measured under a range of operating conditions inside and outside the beam
Plasma asymmetry, electron and ion energy distribution function in capacitive discharges excited by tailored waveforms
Using particle-in-cell simulation technique, we investigate the plasma and
ionization asymmetry, electron and ion energy distribution function in
capacitive discharges excited by tailored waveforms. At a base frequency of
13.56 MHz, three different waveforms namely, sinusoidal, saw-tooth, and square
are applied for a constant current density of 50 A/m2 and 5 mTorr argon gas
pressure. The simulation results show that the square waveform produces the
highest plasma density in the discharge, whereas maximum asymmetry is observed
for plasma excited by sawtooth like waveform. Both square and sawtooth
waveforms generate multiple beams of high-energy electrons from near to the
expanding phase of the sheath edge followed by the high-frequency modulations
up to 100 MHz on the instantaneous sheath position. The electron energy
distribution function depicts 3 electron temperature and highly elevated
tail-end electrons for the square waveform in comparison to the sinusoidal and
sawtooth waveform. The ion energy distribution function is bimodal at both
powered and grounded electrodes with a large asymmetry and narrow type
distribution in the case of sawtooth like waveform. These results suggest that
the choice of the waveform is highly critical for achieving maximum asymmetry
and plasma density simultaneously in the discharge
A systematic investigation of electric field nonlinearity and field reversal in low pressure capacitive discharges driven by sawtooth-like waveforms
Understanding electron and ion heating phenomenon in capacitively coupled
radio-frequency plasma discharges is vital for many plasma processing
applications. In this article, using particle-in-cell simulation technique we
investigate the collisionless argon discharge excited by temporally asymmetric
sawtooth-like waveform. In particular, a systematic study of the electric field
nonlinearity and field reversal phenomenon by varying the number of harmonics
and its effect on electron and ion heating is performed. The simulation results
predict higher harmonics generation and multiple field reversal regions
formation with an increasing number of harmonics along with the local charge
separation and significant displacement current outside sheath region. The
field reversal strength is greater during the expanding phase of the sheath
edge in comparison to its collapsing phase causing significant ion cooling. The
observed behavior is associated with the electron fluid compression/rarefaction
and electron inertia during expanding and collapsing phase respectively
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