Felipe Iza

Three distinct modes in a cold atmospheric pressure plasma jet

Cold atmospheric pressure helium plasma jets are increasingly used in many processing applications, due to a distinct combination of their inherent plasma stability with excellent reaction chemistry often enhanced downstream. Despite their widespread usage, it remains largely unknown whether cold atmospheric plasma jets maintain similar characteristics from breakdown to arcing or whether they possess different operating modes. In addition to their known ability to produce a fast moving train of discrete luminous clusters along the jet length, commonly known as plasma bullets, this paper reports evidence of two additional modes of operation, namely a chaotic mode and a continuous mode in an atmospheric helium plasma jet. Through detailed electrical and optical characterization, it is shown that immediately following breakdown the plasma jet operates in a deterministic chaotic mode. With increasing input power, the discharge becomes periodic and the jet plasma is found to produce at least one strong plasma bullet every cycle of the applied voltage. Further increase in input power eventually leads to the continuous mode in which excited species are seen to remain within the inter-electrode space throughout the entire cycle of the applied voltage. Transition from the chaotic, through the bullet, to the continuous modes is abrupt and distinct, with each mode having a unique set of operating characteristics. For the bullet mode, direct evidence is presented to demonstrate that the evolution of the plasma jet involves a repeated sequence of generation, collapse and regeneration of the plasma head occurring at locations progressively towards the instantaneous cathode. These offer previously unavailable insight into plasma jet formation mechanisms and the potential of matching plasma jet modes to specific needs of a given processing application.

Global model of low-temperature atmospheric-pressure He + H2O plasmas

A detailed global model of atmospheric-pressure He + H2O plasmas is presented in this paper. The model incorporates 46 species and 577 reactions. Based on simulation results obtained with this comprehensive model, the main species and reactions are identified, and simplified models capable of capturing the main physicochemical processes in He + H2O discharges are suggested. The accuracy of the simplified models is quantified and assessed for changes in water concentration, input power and electrode configuration. Simplified models can reduce the number of reactions by a factor of similar to 10 while providing results that are within a factor of two of the detailed model. The simulation results indicate that Penning processes are the main ionization mechanism in this kind of discharge (1-3000 ppm of water), and water clusters of growing size are found to be the dominant charged species when the water concentration is above similar to 100 ppm. Simulation results also predict a growing electronegative character of the discharge with increasing water concentration. The use of He + H2O discharges for the generation of reactive oxygen species of interest in biomedical applications and the green production of hydrogen peroxide are also discussed. Although it would be unrealistic to draw conclusions regarding the efficacy of these processes from a zero-dimensional global model, the results indicate the potential suitability of He + H2O plasmas for these two applications.

Air plasma coupled with antibody-conjugated nanoparticles: a new weapon against cancer

Ambient air plasmas have been known to kill cancer cells. To enhance selectivity we have used antibody-conjugated nanoparticles. We achieved five times enhancement of melanoma cell death over the case of the plasma alone by using an air plasma with gold nanoparticles bound to anti-FAK antibodies. Our results show that this new interdisciplinary technique has enormous potential for use as a complement to conventional therapies.

Plasma–liquid interactions: a review and roadmap

Plasma–liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas.

Particle and fluid simulations of low-temperature plasma discharges: benchmarks and kinetic effects

Fluid, particle-in-cell and hybrid models are the numerical simulation techniques commonly used for simulating low-temperature plasma discharges. Despite the complexity of plasma systems and the challenges in describing and modelling them, well-organized simulation methods can provide physical information often difficult to obtain from experiments. Simulation results can also be used to identify research guidelines, find optimum operating conditions or propose novel designs for performance improvements. In this paper, we present an overview of the principles, strengths and limitations of the three simulation models, including a brief history and the recent status of their development. The three modelling techniques are benchmarked by comparing simulation results in different plasma systems (plasma display panels, capacitively coupled plasmas and inductively coupled plasmas) with experimentally measured data. In addition, different aspects of the electron and ion kinetics in these systems are discussed based upon simulation results.

Low-power microwave plasma source based on a microstrip split-ring resonator

Microplasma sources can be integrated into portable devices for applications such as bio-microelectromechanical system sterilization, small-scale materials processing, and microchemical analysis systems. Portable operation, however, limits the amount of power and vacuum levels that can be employed in the plasma source. This paper describes the design and initial characterization of a low-power microwave plasma source based on a microstrip split-ring resonator that is capable of operating at pressures from 0.05 torr (6.7 Pa) up to one atmosphere. The plasma sources microstrip resonator operates at 900 MHz and presents a quality factor of Q=335. Argon and air discharges can be self-started with less than 3 W in a relatively wide pressure range. An ion density of 1.3/spl times/10/sup 11/ cm/sup -3/ in argon at 400 mtorr (53.3 Pa) can be created using only 0.5 W. Atmospheric discharges can be sustained with 0.5 W in argon. This low power allows for portable air-cooled operation. Continuous operation at atmospheric pressure for 24 h in argon at 1 W shows no measurable damage to the source.

Split-ring resonator microplasma: microwave model, plasma impedance and power efficiency

The microstrip split-ring resonator (MSRR) microplasma source is analysed and characterized using a microwave model of the device. Throughout the discussion, experimental data for three MSRR designs are also presented. The model identifies the key parameters that control the performance of the device and results in the formulation of closed-form expressions useful for designing, analysing and comparing MSRR designs. Matching the microstrip characteristic impedance to the microplasma impedance is found to be a key factor in the performance of these devices and it can be even more critical than the quality factor of the ring resonator. Based on the model, average rf electric fields of up to 4 MV m−1 at 1 W of input power are estimated to be generated in a 45 µm gap device. Furthermore, the model is used to determine the plasma impedance and thereby obtain information on physical properties of the microdischarge. Electron densities of the order of 1014 cm−3 are estimated in a 1 W argon discharge at atmospheric pressure. Based on the values of the plasma impedance, it is also determined that up to 70% of the power input to the MSRR is coupled to the electrons in the microdischarge.

Contrasting characteristics of sub-microsecond pulsed atmospheric air and atmospheric pressure helium–oxygen glow discharges

Glow discharges in air are often considered to be the ultimate low-temperature atmospheric pressure plasmas for numerous chamber-free applications. This is due to the ubiquitous presence of air and the perceived abundance of reactive oxygen and nitrogen species in air plasmas. In this paper, sub-microsecond pulsed atmospheric air plasmas are shown to produce a low concentration of excited oxygen atoms but an abundance of excited nitrogen species, UV photons and ozone molecules. This contrasts sharply with the efficient production of excited oxygen atoms in comparable helium–oxygen discharges. Relevant reaction chemistry analysed with a global model suggests that collisional excitation of O2 by helium metastables is significantly more efficient than electron dissociative excitation of O2, electron excitation of O and ion–ion recombination. These results suggest different practical uses of the two oxygen-containing atmospheric discharges, with air plasmas being well suited for nitrogen and UV based chemistry and He–O2 plasmas for excited atomic oxygen based chemistry.

Particle-in-cell simulation of gas breakdown in microgaps

Gas breakdown in large scale systems has been widely studied and is reasonably well understood. Deviations from the well-known Paschen law, however, have been reported in microgaps. One possible mechanism responsible for these deviations is the increase of the secondary electron emission yield due to the quantum tunnelling of electrons from the metal electrodes to the gas phase. The high electric fields obtained in small gaps combined with the lowering of the potential barrier seen by the electrons in the cathode as an ion approaches lead to the onset of ion-enhanced field emissions. Particle-in-cell/Monte Carlo simulations including ion-enhanced field emission have been performed to evaluate the importance of these mechanisms in the discharge breakdown. Deviations from the Paschen curve in gaps smaller than 5 µm can be explained based on this mechanism.

Electronic quenching of OH(A) by water in atmospheric pressure plasmas and its influence on the gas temperature determination by OH(A–X) emission

In this paper it is shown that electronic quenching of OH(A) by water prevents thermalization of the rotational population distribution of OH(A). This means that the observed ro-vibrational OH(A?X) emission band is (at least partially) an image of the formation process and is determined not only by the gas temperature. The formation of negative ions and clusters for larger water concentrations can contribute to the non-equilibrium. The above is demonstrated in RF excited atmospheric pressure glow discharges in He?water mixtures in a parallel metal plate reactor by optical emission spectroscopy. For this particular case a significant overpopulation of high rotational states appears around 1000?ppm H2O in He. The smallest temperature parameter of a non-Boltzmann (two-temperature) distribution fitted to the experimental spectrum of OH(A?X) gives a good representation of the gas temperature. Only the rotational states with the smallest rotational numbers (J ? 7) are thermalized and representative for the gas temperature.