Arrays of atmospheric pressure microplasma jets: He/O2 and Ar jets into air

Summary form only given. Atmospheric-pressure microplasma jets are finding increasing use in biomedical applications to controllably deliver radical fluxes to tissue. A typical plasma jet consists of a capillary tube having a diameter of less than a few mm through which noble gases or mixtures with a molecular gas flow. An electrode, excited using sinusoidal or high-voltage pulses, produces a plasma in the flowing gas, resulting in a luminous plume that extends for up to several cm. Plasma plumes are formed by propagation of ionization waves (IWs) through the tubes and then through the gas phase channel formed by the noble gas injected into the surrounding air. The luminous fronts of the IWs have been termed plasma bullets. For plasma jets having a single aperture, the plasma plume covers only a few mm², which may not be optimum for large area applications. One remedy is to group jets together to form an array of jets¹. In this talk, we discuss results from a computational investigation of small arrays of 2-5 microplasma jets whose tubes either touch or are separated by air. The investigation was conducted with the modeling platform nonPDPSIM. An atmospheric pressure He/O₂ = 99.8/0.2 mixture was flowed through the tubes into humid room air (N₂/O₂/H₂O = 79.5/20/0.5). We found that the properties of the plasma plume depend on air diffusing into the jets of He/O₂ (and vice-versa). He jets from tubes that touch quickly merge into a single stream, which gets narrower with flow distance as air diffuses into the jets. The separated tubes maintain more unique He channels for longer flow distances. The plasma bullets are typically confined to regions which have a high He/O2 mole fraction as the ionization rate in the air dominated regions is too low to sustain the bullet. With synchronized negative pulses applied to pin electrodes of all the tubes, simultaneous plasma bullets are initially produced- Due to the negative space charge of the bullets, the heads of the IWs initially repel each other. Once the bullets emerge into the He/O₂ jets, the bullets tend to converge into regions of higher He/O₂ mole fraction, being guided by the “dielectric walls” of the non-ionizing air. For three widely spaced tubes, the bullets interact through photoionization from their neighbor. As a result, the center bullet accelerates. Interaction of jets of He and Ar into humid air will be also discussed.