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

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