Simulation study of reactive oxygen species production in the self-pulsing regime of a micro hollow cathode discharge in He/O₂

Abstract A global equivalent-circuit model is used to study the generation of reactive oxygen species (ROS) in the self-pulsing regime of a microhollow cathode discharge (MHCD) operating in He/O 2, and the results are compared with those of Ar/O 2 . The numerical simulations revealed that the most populated ROS are the atomic oxygen O and the metastable singlet delta oxygen O <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> (a 1 Δ g ), followed by O * (1D) and O 3 , whereas <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msubsup> <mml:mo stretchy="false">(</mml:mo> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">b</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:msub> <mml:mrow> <mml:mo mathvariant="normal">Σ</mml:mo> </mml:mrow> <mml:mrow> <mml:mi>g</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:math> is always the least populated species. The ROS number densities are increased with an increase in the pressure up to P = 200 Torr and with O 2 fraction up to 5%, due to enhancement in O 2 concentration as the source of ROS. However, further increase in these parameters results in saturation of the ROS production because of the reduction in the specific energy deposition. O*(1D) concentration always decreases with O 2 fraction as its energy is effectively consumed in dissociation reactions with ground state O 2 molecules. The maximum electron temperature in He/O 2 MHCD is 1.5 eV higher than that of Ar/O 2 . This leads to much higher ROS production rates in He/O 2 , such that the corresponding number densities are almost two times larger than those of Ar/O 2 . Also, He/O 2 demonstrates a higher O 2 dissociation and a higher electronegativity than Ar/O 2, which is mainly attributed to the higher electron temperature and the lower electron number density in He/O 2 plasma. Because of the reduction in the specific energy deposition, O 2 dissociation always decreases with O 2 fraction. The production yield of O <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> (a 1 Δ g ) shows a rapid growth with O 2 fraction up to 5% but then declines at higher O 2 densities. The yield increases with an increase in the input voltage when the O 2 fraction is above 5%, while it decreases at lower fractions. The optimum O <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> (a 1 Δ g ) yield is achieved at the pressure of 200 Torr, input voltage of 600 V, and O 2 fraction of 5%.