Seth A. Norberg

Atmospheric pressure plasma jets interacting with liquid covered tissue: Touching and not-touching the liquid

In the use of atmospheric pressure plasma jets in biological applications, the plasma-produced charged and neutral species in the plume of the jet often interact with a thin layer of liquid covering the tissue being treated. The plasma-produced reactivity must then penetrate through the liquid layer to reach the tissue. In this computational investigation, a plasma jet created by a single discharge pulse at three different voltages was directed onto a 200 µm water layer covering tissue followed by a 10 s afterglow. The magnitude of the voltage and its pulse length determined if the ionization wave producing the plasma plume reached the surface of the liquid. When the ionization wave touches the surface, significantly more charged species were created in the water layer with H3O+aq, O3−aq, and O2−aq being the dominant terminal species. More aqueous OHaq, H2O2aq, and O3aq were also formed when the plasma plume touches the surface. The single pulse examined here corresponds to a low repetition rate plasma jet where reactive species would be blown out of the volume between pulses and there is not recirculation of flow or turbulence. For these conditions, NxOy species do not accumulate in the volume. As a result, aqueous nitrites, nitrates, and peroxynitrite, and the HNO3aq and HOONOaq, which trace their origin to solvated NxOy, have low densities.

Helium atmospheric pressure plasma jets touching dielectric and metal surfaces

Atmospheric pressure plasma jets (APPJs) are being investigated in the context plasma medicine and biotechnology applications, and surface functionalization. The composition of the surface being treated ranges from plastics, liquids, and biological tissue, to metals. The dielectric constant of these materials ranges from as low as 1.5 for plastics to near 80 for liquids, and essentially infinite for metals. The electrical properties of the surface are not independent variables as the permittivity of the material being treated has an effect on the dynamics of the incident APPJ. In this paper, results are discussed from a computational investigation of the interaction of an APPJ incident onto materials of varying permittivity, and their impact on the discharge dynamics of the plasma jet. The computer model used in this investigation solves Poissons equation, transport equations for charged and neutral species, the electron energy equation, and the Navier-Stokes equations for the neutral gas flow. The APPJ ...

Formation of reactive oxygen and nitrogen species by repetitive negatively pulsed helium atmospheric pressure plasma jets propagating into humid air

Atmospheric pressure plasma jets have many beneficial effects in their use in surface treatment and, in particular, plasma medicine. One of these benefits is the controlled production of reactive oxygen and nitrogen species (RONS) in the active discharge through the molecular gases added to the primary noble gas in the input mixture, and through the interaction of reactive species in the plasma effluent with the ambient air. In this computational investigation, a parametric study was performed on the production of RONS in a multiply pulsed atmospheric pressure plasma jet sustained in a He/O2 mixture and flowing into ambient humid air. The consequences of flow rate, O2 fraction, voltage, and repetition rate on reactant densities after a single discharge pulse, after 30 pulses, and after the same total elapsed time were investigated. At the end of the first discharge pulse, voltage has the greatest influence on RONS production. However, the systematic trends for production of RONS depend on repetition rate and flow rate in large part due to the residence time of RONS in the plasma zone. Short residence times result in reactive species produced by the previous pulse still being in the discharge tube or in the path of the ionization wave at the next pulse. The RONS therefore accumulate in the tube and in the near effluent on a pulse-to-pulse basis. This accumulation enables species requiring multiple reactions among the primary RONS species to be produced in greater numbers.

Propagation mechanisms of guided streamers in plasma jets: the influence of electronegativity of the surrounding gas

Atmospheric pressure plasma jets for biomedical applications are often sustained in He with small amounts of, for example, O2 impurities and typically propagate into ambient air. The resulting poorly controlled generation of reactive species has motivated the use of gas shields to control the interaction of the plasma plume with the ambient gas. The use of different gases in the shield yields different behavior in the plasma plume. In this paper, we discuss results from experimental and computational investigations of He plasma jets having attaching and non-attaching gas shields. We found that negative ion formation in the He-air mixing region significantly affects the ionization wave dynamics and promotes the propagation of negative guided streamers through an electrostatic focusing mechanism. Results from standard and phase resolved optical emission spectroscopy ratios of emission from states of N2 and He imply different electric fields in the plasma plume depending on the composition of the shielding gas. These effects are attributed to the conductivity in the transition region between the plasma plume and the shield gas, and the immobile charge represented by negative ions. The lower conductivity in the attaching mixtures enables more extended penetration of the electric field whereas the negative ions aid in focusing the electrons towards the axis.

Helium atmospheric pressure plasma jets interacting with wet cells: delivery of electric fields

The use of atmospheric pressure plasma jets (APPJs) in plasma medicine have produced encouraging results in wound treatment, surface sterilization, deactivation of bacteria, and treatment of cancer cells. It is known that many of the reactive oxygen and nitrogen species produced by the APPJ are critical to these processes. Other key components to treatment include the ion and photon fluxes, and the electric fields produced in cells by the ionization wave of the APPJ striking in the vicinity of the cells. These relationships are often complicated by the cells being covered by a thin liquid layer-wet cells. In this paper, results from a computational investigation of the interaction of APPJs with tissue beneath a liquid layer are discussed. The emphasis of this study is the delivery of electric fields by an APPJ sustained in He/O-2 = 99.8/0.2 flowing into humid air to cells lying beneath water with thickness of 200 mu m. The water layer represents the biological fluid typically covering tissue during treatment. Three voltages were analyzed-two that produce a plasma effluent that touches the surface of the water layer and one that does not touch. The effect of the liquid layer thickness, 50 mu m to 1 mm, was also examined. Comparisons were made of the predicted intracellular electric fields to those thresholds used in the field of bioelectronics.

Dynamics of repetitive plasma bullets in He plasma jets into air

Atmospheric-pressure plasma jets formed by dielectric barrier discharges and injected into ambient air are effective sources for production of chemically active non-thermal plasmas [1]. The jets are repetitively pulsed and so are composed of a sequence of ionization waves with speeds up to 108 cm/s. The luminous plume of the plasma jets can be longer than 10 cm. With typical applied voltages of a few kV, the mean electric field in the plume can be less than 1 kV/cm which is less than the avalanche field in air or in He. As a result, a conventional ionization wave (the bullet) cannot be sustained for such long distances. However, for repetition frequencies of a few to tens of kHz, each new plasma bullet propagates in a gas excited and preionized by the previous plasma bullet. The pulse-periodic plasma jet then must develop from the initially unionized (and unexcited gas) having a short extent to a preionized channel having a longer extent.

Reactive oxygen and nitrogen species (RONS) produced by repetitive pulses in atmospheric pressure plasma jets

Summary form only given. Reactive oxygen and nitrogen species (RONS) are desired in numerous applications from the destruction of harmful proteins and bacteria for sterilization in the medical field to taking advantage of the metastable characteristics of O2(1 Δ) to transfer energy to other species. Advances in atmospheric pressure plasma jets in recent years have shown the possibility of using this technology as a source of RONS. The plasma jets consist of small diameter tubes (a few mm) through which rare gas mixtures (e.g., He seeded with a few percent of O2) are flowed into room air. They are typically operated in a dielectric barrier discharge (DBD) configuration which produces an ionization wave (or plasma bullet) with repetition rates of many kHz to tens or hundreds of MHz. In this paper, we report on results of a computational investigation of the production of RONS from repetitively pulsed plasma jets at frequencies from many kHz to many MHz consisting of He/O2 mixtures discharged into ambient air. The computer model used in this study, nonPDPSIM, solves transport equations for charged and neutral species, Poissons equation for the electric potential, the electron energy conservation equation for the electron temperature, and Navier-Stokes equations for the neutral gas flow. Rate coefficients and transport coefficients for the bulk plasma are obtained from local solutions of Boltzmanns equation for the electron energy distribution. The length of the interpulse period has significant effects on the density and distribution of the RONS in the effluent of the plasma jet. At high repetition rates (producing interpulse periods shorter than the gas clearing time), there is accumulation of RONS in the plume on a pulse-to-pulse basis, enabling further reactions between these species. The ionization wave of the following pulse samples the reactive environment produced by the previous pulse. At lower repetition rates, the interpulse periods are commensurate or longer than the clearing time of the gas through the device. In these cases, the ionization wave enters a more pristine and controllable environment.