Atmospheric Helium Plasma Jet Research Articles

Microwave emissions from the cold atmospheric helium plasma jet

One of the recently observed effects of plasma in medical applications is the physical effect, suggesting that the electromagnetic (EM) emission of cold atmospheric plasmas can lead to cell membrane oscillations and sensitization to the chemical active ingredient of treatments such as cancer drugs. This is a new aspect that must be considered along with the plasma chemical effects for the future dose definition which is the most urgent research topic of plasma medicine. However, unlike the reactive oxygen and nitrogen species generated from plasma chemistry which is well-known as playing a key role in apoptosis cancer cells, the EM emission power spectrum and emission mechanism are still unquantified. This makes the uncertainty of the physical dosage of the therapy and thus impedes the further understanding and optimization of the plasma therapy. In this paper, we compute the 3D spatial distribution of the power density spectrum of EM emission from a cold atmospheric helium plasma jet. The simulations indicate that the plasma oscillations following the plasma streamer propagation are the main source of EM emission, while the emissions of the bulk current caused by net charge movements and the bremsstrahlung due to charge collisions are negligible. The results are also verified by a microwave power measurement using a heterodyne frequency sweep. These findings will thus fill out the last missing piece of the jigsaw before the plasma medicine community can define the dose in the future.

Brewster angle-cavity ringdown spectroscopy for low temperature plasma measurements in multiphases

We report on the development of a Brewster angle-cavity ringdown spectroscopy (BA-CRDS) system for low temperature plasma diagnostics. The system can measure gas species in solutions, with a detection limit (minimum detectable absorbance) of 9.1 × 10−5, which is equivalent to a detection limit of 0.04 parts per billion for measuring OH radicals in water at 308 nm. With higher reflectivity ringdown mirrors and improved design of a Brewster angle cell, the detection limit can potentially be up to 10−6 or lower. In this exploratory study, the absorption cross sections of HgBr2 and H2O2 in the aqueous phase at 256 nm are measured to be (1.8 ± 0.1) × 10−18 cm2 and (5.2 ± 0.5) × 10−20 cm2, respectively. Furthermore, temporal profiles of absorbance from distilled water, HgBr2, and H2O2 solutions when interacting with a helium atmospheric plasma jet are individually characterized at different plasma powers, gas flow rates, and/or solute concentrations. The observed linear temporal profiles of absorbance from the plasma-interacted water suggest formation of H2O2 from plasma-generated OH radicals, while the nonlinear temporal profiles from the plasma-treated HgBr2 solutions reveal possible removal of HgBr2 by OH radicals. Our results demonstrate that the new BA-CRDS system is a powerful tool for quantification of reactive plasma species in multiphases or other complex settings.

Features of electric field distribution along helium atmospheric plasma jet in stepwise propagation mode of guided streamer

Features of electric field distribution along helium atmospheric plasma jet in stepwise propagation mode of guided streamer

Plasma Electrochemistry: Redox Reactions in Gases

Electrochemistry underpins numerous commercially important processes ranging from energy generation to sensors and coating deposition. A common and –so far– essential feature of all these processes involve a redox active species dissolved in a solvent which serves to allow charge exchange between electrodes. Rather than being inert, the solvent is without exception redox-active at extreme potentials and thus restrict the breadth of observable reactions to a narrow potential window of approx. -2.5 to +2.5 V vs. NHE. Gaseous electrolytes have typically been ignored due to their feeble electrical conductivity. However, recently with the advent of new accessible approaches to form stable plasmas, these electrically conducting gases are attracting some significant interest and are now being investigated as exotic electrochemical environments. The scope of this work is to apply the well-healed cornerstones of electrochemistry developed almost exclusively in liquids, to the new context of gaseous plasma.The defining property of plasmas is presence of free electrons; because of this they may be considered as both electrodes or electrolytes1&2. We describe results supporting both modes. As electrodes, we show that metal oxides on surfaces may be reduced to zero valent metals using a helium atmospheric plasma jet.3&4 We show that free electrons do indeed reduce a copper oxide film, which may be carefully controlled by surface bias.5 A gaseous flame doped with electroactive species may be considered as electrolytes. Using a three-electrode system6, we may measure unique voltammograms for a series of small organic molecules and amino acids.7 Except for leucine and isoleucine, all were distinguishable. The reduction signatures originate from specific electron attachment reactions of radicals formed via incomplete combustion and fragmentation of the parent molecules. In this case without a solvent we have an extended potential window and our voltammograms extend between 0 and -10 V, which gives unprecedented access to chemistry not previously accessible in liquids. Moreover, mass transport properties are far better than in liquids, as such the fluxes of electroactive species to the electrode a much greater. References Rumbach, P.; Bartels, D. M.; Sankaran, R. M.; Go, D. B., The solvation of electrons by an atmospheric-pressure plasma. Nature Communications 2016, 6, 7248.Elahi, A.; Fowowe, T.; Caruana, D. J., Dynamic Electrochemistry in Flame Plasma Electrolyte. Angewandte Chemie-International Edition 2012, 51 (26), 6350-6355.Sener, M. E., Quesada Cabrera, R., Parkin, I.P., Caruana, D. J., Facile formation of black titania films using an atmospheric-pressure plasma jet, Green Chemistry 2022, 24, 2499-2505.Sener, M. E., Palgrave, R., Quesada Cabrera, R., Caruana, D. J., Patterning of Metal Oxide thin Films using H2/He Atmospheric Pressure Plasma Jet. Green Chemistry 2020, 22, 1406-1413.Sener, M. E.; Caruana, D. J., Modulation of copper(I) oxide reduction/oxidation in atmospheric pressure plasma jet. Electrochemistry Communications 2018, 95, 38-42.Fowowe, T.; Hadzifejzovic, E.; Hu, J. P.; Foord, J. S.; Caruana, D. J., Plasma Electrochemistry: Development of a Reference Electrode Material for High Temperature Plasma. Advanced Materials 2012, 24 (47), 6305-6309.Calleja, M.; Elahi, A.; Caruana, D. J., Gas phase electrochemical analysis of amino acids and their fragments. Communications Chemistry 2018, 1, 48.

(Invited) Plasma Electrochemistry: Plasmas Are so Much More Fun for Electrochemistry Than Liquids!

As electrochemists, we are interested in electron attachment and detachment processes. Traditionally, we control the availability of electrons via an electrically conducting solid and measure electron transfer across the solid/liquid interface. Of course, there are exceptions to this picture, e.g. liquid/liquid interfaces, but often liquids are involved to provide an electrolyte medium to support the chemical species. Gaseous electrolytes have typically been ignored due to their feeble electrical conductivity. However, recently with the advent of new accessible approaches to form stable plasmas, these electrically conducting gases are attracting some significant interest and are now being investigated as exotic electrochemical environments.The defining property of plasmas is presence of free electrons; because of this they may be considered as both electrodes or electrolytes1, 2. We describe results supporting both modes.As electrodes, we show that metal oxides on surfaces may be reduced to zero valent metals using a helium atmospheric plasma jet.3 We show that free electrons do indeed reduce a copper oxide film, which may be carefully controlled by surface bias.4 A gaseous flame doped with electroactive species may be considered as electrolytes. Using a three-electrode system5, we may measure unique voltammograms for a series of small organic molecules and amino acids.6 Except for leucine and isoleucine, all were distinguishable. The reduction signatures originate from specific electron attachment reactions of radicals formed via incomplete combustion and fragmentation of the parent molecules. In this case without a solvent we have an extended potential window and our voltammograms extend between 0 and -10 V, which gives unprecedented access to chemistry not previously accessible in liquids. Moreover, mass transport properties are far better than in liquids, as such the fluxes of electroactive species to the electrode a much greater. References Rumbach, P., Bartels, D.M., Sankaran, R.M. & Go, D.B. The solvation of electrons by an atmospheric-pressure plasma (vol 6, pg 7248, 2015). Nature Communications 7 (2016).Elahi, A., Fowowe, T. & Caruana, D.J. Dynamic Electrochemistry in Flame Plasma Electrolyte. Angewandte Chemie-International Edition 51, 6350-6355 (2012).Sener, M.E., Palgrave, R., Quesada Cabrera, R., Caruana, D.J.*, Patterning of Metal Oxide thin Films using H2/He Atmospheric Pressure Plasma Jet. Green Chemistry,22, 1406-1413 (2020).Sener, M.E. & Caruana, D.J. Modulation of copper(I) oxide reduction/oxidation in atmospheric pressure plasma jet. Electrochemistry Communications 95, 38-42 (2018).Fowowe, T., Hadzifejzovic, E., Hu, J.P., Foord, J.S. & Caruana, D.J. Plasma Electrochemistry: Development of a Reference Electrode Material for High Temperature Plasma. Advanced Materials 24, 6305-6309 (2012).Calleja, M., Elahi, A. & Caruana, D.J. Gas phase electrochemical analysis of amino acids and their fragments. Communications Chemistry 1, 48 (2018).

(Invited) Electrochemistry in Plasmas: From Flames to RF Plasma Jets

As electrochemists, we are interested in electron attachment and detachment processes. Traditionally, we control the availability of electrons via an electrically conducting solid and measure electron transfer across the solid/liquid interface. Of course, there are exceptions to this picture, e.g. liquid/liquid interfaces, but often liquids are involved to provide an electrolyte medium to support the chemical species. Gaseous electrolytes have typically been ignored due to their feeble electrical conductivity. However, recently with the advent of new accessible approaches to form stable plasmas, these electrically conducting gases are attracting some significant interest and are now being investigated as exotic electrochemical environments.The defining property of plasmas is presence of free electrons; because of this they may be considered as both electrodes or electrolytes1, 2. We describe results supporting both modes.As electrodes, we show that metal oxides on surfaces may be reduced to zero valent metals using a helium atmospheric plasma jet.3 We show that free electrons do indeed reduce a copper oxide film, which may be carefully controlled by surface bias.4 A gaseous flame doped with electroactive species may be considered as electrolytes. Using a three-electrode system5, we may measure unique voltammograms for a series of small organic molecules and amino acids.6 Except for leucine and isoleucine, all were distinguishable. The reduction signatures originate from specific electron attachment reactions of radicals formed via incomplete combustion and fragmentation of the parent molecules. In this case without a solvent we have an extended potential window and our voltammograms extend between 0 and -10 V, which gives unprecedented access to chemistry not previously accessible in liquids. Moreover, mass transport properties are far better than in liquids, as such the fluxes of electroactive species to the electrode a much greater.

Interaction between a helium atmospheric plasma jet and targets and dynamics of the interface

This paper investigates the interaction of a helium atmospheric plasma jet impinging onto liquid and metal targets using experimental and numerical techniques. The primary motivation of this study is to experimentally understand the effect of liquid and metal targets on the propagation of a plasma jet, and to numerically characterize chemical and electrical behavior at the plasma-water interface. This study is relevant in the ongoing research of plasma self-organization for the development of medical devices capable of self-adaptation. Experimental measurements are made to obtain ionization wave (IW) propagation, average electron density, and optical emission spectrum of the plasma near the interface for cases with copper and water targets and a freely expanding jet. The induced acidity on the plasma-treated water is measured by pH balance. Results confirm that both liquid and metal targets alter the IW propagation velocity and electric potential. IW average propagation speed is highest with the copper target and lowest with the water target by a factor of two. Corresponding IW head potentials are highest for the metal target and lowest in the presence of the water target. Spatial average of electron density in the plasma column is of the same order of magnitude for all cases, with the case of the water target being the lowest. For the copper target, single IW contact areas on the plasma-target interface are observed to last well into the micro-second range after initial impact within each discharge period. A reflected IW is observed in the case of the metal but not the liquid target. The plasma jet delivers a small acid dose which changes the pH of the liquid. The numerical model uses parameters from plasma-liquid experiments to simulate reactive oxygen and nitrogen, acidic, and charged species in interacting gaseous and aqueous layers by employing a transient advection-diffusion-reaction-solvation equation. The interacting layers are connected by a two-way coupling governed by solvation through Henry’s law. Accumulation of charges in the aqueous layer results from varying diffusion and mobility time scales of charged species. The electric potential on the aqueous layer is characterized to be in the order of tens of volts, a small fraction of the kilo-Volt measurement of the IW head potential. Electric fields and Maxwell stresses on the aqueous layer are also simulated and are proposed to affect the movement of the plasma contact spot on the gaseous layer. The mechanism for the plasma contact spot movement on the gaseous layer is implemented using a level-set technique which uses charge-induced electric stresses to calculate local advection velocity. Energy equations are solved in both phases for neutrals, ions and electrons by considering the effect of the Joule heating term. Current density through the gaseous-aqueous interface is calculated to be in the order of micro-Amperes, which creates a small enough Joule heating term so that the temperature of heavy species remains unchanged.

Effect of N2 and O2 on OH radical production in an atmospheric helium microwave plasma jet

UV-pulsed laser cavity ringdown spectroscopy of the hydroxyl radical OH(A–X) (0–0) band in the wavelength range of 306–310 nm was employed to determine absolute number densities of OH in the atmospheric helium plasma jets generated by a 2.45 GHz microwave plasma source. The effect of the addition of molecular gases N2 and O2 to He plasma jets on OH generation was studied. Optical emission spectroscopy was simultaneously employed to monitor reactive plasma species. Stark broadening of the hydrogen Balmer emission line (Hβ) was used to estimate the electron density ne in the jets. For both He/N2 and He/O2 jets, ne was estimated to be on the order of 1015 cm−3. The effects of plasma power and gas flow rate were also studied. With increase in N2 and O2 flow rates, ne tended to decrease. Gas temperature in the He/O2 plasma jets was elevated compared to the temperatures in the pure He and He/N2 plasma jets. The highest OH densities in the He/N2 and He/O2 plasma jets were determined to be 1.0 × 1016 molecules/cm3 at x = 4 mm (from the jet orifice) and 1.8 × 1016 molecules/cm3 at x = 3 mm, respectively. Electron impact dissociation of water and water ion dissociative recombination were the dominant reaction pathways, respectively, for OH formation within the jet column and in the downstream and far downstream regions. The presence of strong emissions of the bands in both He/N2 and He/O2 plasma jets, as against the absence of the emissions in the Ar plasma jets, suggests that the Penning ionization process is a key reaction channel leading to the formation of in these He plasma jets.

Investigation of the mechanism of enhanced and directed differentiation of neural stem cells by an atmospheric plasma jet: A gene-level study

Cold atmospheric plasmas (CAPs) have been shown to be capable of enhancing stem cell differentiation, especially directed differentiation of neural stem cells (NSCs). Consequently, one-step CAP treatment shows promise as an aid to tissue transplantation. However, the mechanisms involved in the enhancement of NSCs differentiation by CAP treatment are not yet fully understood. We have previously shown that in atmospheric helium plasma jet treatment, nitric oxide (NO) is the main factor involved in promoting NSC differentiation. This article further investigated the possible signaling pathways stimulated by NO in the neuronal differentiation of C17.2-NSCs after plasma treatment. Extracellular and intracellular NO concentrations were measured at different time points of incubation to monitor NO production. Meanwhile, the expressions of related genes and proteins were detected by quantitative real-time polymerase chain reaction and western blot, respectively. It is found that plasma treatment could both generate extracellular NO and increase extracellular NO concentration by inducing inducible nitric oxide synthase expression. The synergetic effect of extracellular and intracellular NO then downregulated Notch1 and Id2, and upregulated Ngn2 and Ascl1, thereby activating downstream NeuroD expression and finally enhancing and directing differentiation of NSCs into neurons.Cold atmospheric plasmas (CAPs) have been shown to be capable of enhancing stem cell differentiation, especially directed differentiation of neural stem cells (NSCs). Consequently, one-step CAP treatment shows promise as an aid to tissue transplantation. However, the mechanisms involved in the enhancement of NSCs differentiation by CAP treatment are not yet fully understood. We have previously shown that in atmospheric helium plasma jet treatment, nitric oxide (NO) is the main factor involved in promoting NSC differentiation. This article further investigated the possible signaling pathways stimulated by NO in the neuronal differentiation of C17.2-NSCs after plasma treatment. Extracellular and intracellular NO concentrations were measured at different time points of incubation to monitor NO production. Meanwhile, the expressions of related genes and proteins were detected by quantitative real-time polymerase chain reaction and western blot, respectively. It is found that plasma treatment could both genera...

Schlieren flow visualization of helium atmospheric plasma jet and influence of the gas flow rate and applied voltage frequency

We used schlieren photography to visualize the influence of gas flow rates of 1, 2.5, 5, 10 L/min and of the applied voltage frequency on a helium atmospheric plasma jet induced at the nozzle of a capillary tube. The expansion of the gas in the surrounding medium (air) was analyzed in the two different modes – plasma on/plasma off. Changes in the above parameters affect the gas flow regime and the hydrodynamics of the jet.

Modified drug release using atmospheric pressure plasma deposited siloxane coatings

This pilot study evaluates the potential of atmospheric plasma polymerised coatings to modify the rate of drug release from polymeric substrates. The antibiotic rifampicin was deposited in a prototype multi-layer drug delivery system, consisting of a nebulized layer of active drug between a base layer of TEOS deposited on a plastic substrate (polystyrene) and an overlying layer of plasma polymerised PDMS. The polymerised TEOS and PDMS layers were deposited using a helium atmospheric plasma jet system. Elution of rifampicin was measured using UV–VIS spectroscopy, in addition to a antimicrobial well diffusion assay with an established indicator organism. The multi-layered plasma deposited coatings significantly extended the duration of release of the rifampicin from 24 h for the uncoated polymer to 144 h for the coated polymer.

Schlieren imaging investigation of the hydrodynamics of atmospheric helium plasma jets

This work investigates the hydrodynamic characteristics of a coaxial double-ring electrode helium plasma jet by means of a “Z-type” Schlieren imaging system. The Schlieren images and visual optical photographs made show that a transition point from a laminar region to a turbulent region exists for gas flow without plasma when the helium flow rate exceeds a certain value. After plasma ignition, the laminar region shrinks with voltage increases, and the maximum length of the plasma plume is confined to the laminar region. The heat transfer equation and the spectral broadening of the He I 667.8 nm were used to estimate the increased gas temperature in the plasma jet, and the change in gas velocity by ionic momentum transfer was found by application of a double sphere collision model. As a result, gas heating is considered to be the dominant factor for the earlier onset of turbulence after plasma ignition, whereas the role of ion momentum transfer to neutral gas molecules is comparatively weak. The hydrodynamic behaviors of the plasma jet at the impact region for organic glass and silicon substrates are also researched. The ionization front propagates along the organic glass surface and contracts at the impact point on the silicon surface. More visible vortices are observed from Schlieren images with silicon substrates than with organic glass substrates. Possible mechanisms related to the different treatment effects are discussed.

Cold atmospheric plasma jet-generated RONS and their selective effects on normal and carcinoma cells.

Cold atmospheric helium plasma jets were fabricated and utilized for plasma–cell interactions. The effect of operating parameters and jet design on the generation of specific reactive oxygen and nitrogen species (RONS) within cells and cellular response were investigated. It was found that plasma treatment induced the overproduction of RONS in various cancer cell lines selectively. The plasma under a relatively low applied voltage induced the detachment of cells, a reduction in cell viability, and apoptosis, while the plasma under higher applied voltage led to cellular necrosis in our case. To determine whether plasma-induced reactive oxygen species (ROS) generation occurs through interfering with mitochondria-related cellular response, we examined the plasma effects on ROS generation in both parental A549 cells and A549 ρ0 cells. It was observed that cancer cells were more susceptible to plasma-induced RONS (especially nitric oxide (NO) and nitrogen dioxide (NO2−) radicals) than normal cells, and consequently, plasma induced apoptotic cell responses mainly in cancer cells.

Electron properties in an atmospheric helium plasma jet determined by Thomson scattering

In this work we present Thomson scattering measurements on a nanosecond pulsed high voltage dielectric barrier discharge (DBD)-like helium plasma jet, operated in ambient air. With the low detection limit offered by a triple grating spectrograph equipped with a high quantum efficiency intensified charge-coupled device (ICCD) camera, temporally and spatially resolved electron densities and mean energies have been mapped. 7 kV peak with 250 ns width pulses at 20 kHz are applied to the inner cylindrical shaped electrode of a DBD. This results in a peculiar hollow electron density profile in the vicinity of the jet nozzle with maximum values of ne = 5 × 1018 m−3 and mean energies of up to 2.5 eV. Further downstream, the profile collapses radially and contracts. A much higher electron density is found (2 × 1019 m−3) while the mean energy is lower (0.5 eV).

Computational study of the interaction of cold atmospheric helium plasma jets with surfaces

We describe a computational modeling study of a cold atmospheric pressure plasma jet interacting with a dielectric surface placed normal to the jet axis. The plasma jet is generated by the application of a nanosecond pulse voltage applied to a dielectric tube through which the jet issues into ambient air. A base fluid flow field is pre-computed using a Navier–Stokes model for the helium jet impinging on the dielectric target surface with a two-species description for laminar diffusional mixing of the helium and ambient air streams. A self-consistent, multiple species, two-temperature model is used to describe the non-equilibrium plasma discharge dynamics in the presence of the base jet flow field. A single nanosecond pulse discharge event starting from initial breakdown in the dielectric tube, to propagation into the open gap, and finally the interaction with the dielectric surface is simulated. Initially, the plasma forms within the dielectric tube and propagates along the tube surface as a surface discharge driven by large induced electric fields produced by trapped charge on the dielectric surface. When the discharge reaches the end of the dielectric tube, the discharge transitions to a constricted fast ionization wave that propagates along the helium–air interface. The fast ionization wave eventually reaches the dielectric target surface where charged species are deposited as the discharge propagates parallel to the wall as a surface driven discharge. The surface driven discharge ceases to propagate once the quantity of air to helium is sufficient enough to quench the hot electrons and prevent further ionization. Due to the low speed of the flow discharge and the short life times of the radical species such as O, most of the radical species delivered to the surface are a result of the surface discharge that forms after the plasma bullet impinges against the surface. It is found that factors such as the thickness of the target dielectric and the profile of the stagnation helium–air jet significantly impact the net quantity of reactive particles delivered to the surface.

Characterization of an atmospheric helium plasma jet by relative and absolute optical emission spectroscopy

The characteristics of plasma temperatures (gas temperature and electron excitation temperature) and electron density in a pulsed-dc excited atmospheric helium plasma jet are studied by relative and absolute optical emission spectroscopy (OES). High-resolution OES is performed for the helium and hydrogen lines for the determination of electron density through the Stark broadening mechanism. A superposition fitting method composed of two component profiles corresponding to two different electron densities is developed to fit the investigated lines. Electron densities of the orders of magnitude of 1021 and 1020 m−3 are characterized for the center and edge regions in the jet discharge when the applied voltage is higher than 13.0 kV. The atomic state distribution function (ASDF) of helium demonstrates that the discharge deviates from the Boltzmann–Saha equilibrium state, especially for the helium lower levels, which are significantly overpopulated. Local electron excitation temperatures T13 and Tspec corresponding to the lower and upper parts of the helium ASDF are defined and found to range from 1.2 eV to 1.4 eV and 0.2 eV to 0.3 eV, respectively. A comparative analysis shows that the Saha balance is valid in the discharge for helium atoms at high excited states.

Surface treatment comparison using atmospheric helium plasma jets with different frequencies and target objects

This paper details a plasma surface treatment feasibility study conducted for different driving frequencies of low frequency (LF, 50 kHz) and radio-frequency (RF, 13.56 MHz) sources by adding a target plate to a single pin electrode plasma jet system. The plasmas were generated in ambient air with a helium gas supply. The electrical characteristics of the plasmas were studied and compared for cases with and without a plane electrode. The LF plasma was more affected by the presence of the dielectric plate than was the RF plasma, showing a larger increase in its current. In addition, pork samples were treated to assess the electrical and thermal safety of plasma biomaterial surface treatments. The LF plasma conduction current and temperature during the pork treatment were 0.5 mA and 22 °C, and those of the RF plasma were 40 mA and 31 °C at maximum.

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.