Dongping Liu

O ₃ generation in atmospheric air and O ₂ plasmas poses a significant hindrance to the design and development of atmospheric plasma devices used for the preparation of plasma activated water (PAW). Notably, these devices do not operate in a sealed chamber. In this study, water is activated by N ₂ -based instead of O ₂ -based spark discharge over water to generate PAW at atmospheric pressure. Our analysis demonstrates that the biological reactivity of N ₂ nonthermal PAW (N ₂ -PAW) is much higher than that of O ₂ nonthermal PAW (O ₂ -PAW). E. coli, with their concentration of 1.8 × 10 ⁶ CFU ml ⁻¹ , are completely inactivated in N ₂ -PAW following a plasma treatment time of ⩾40 s. The short-lived species, such as OH, N, H, N ₂ ⁺ , OH ⁺ , H ₂ O ⁺ , and H ⁻ , can be formed due to collisions of N ₂ and H ₂ O with electrons in the N ₂ spark discharge over water. They are essential intermediates for the subsequent gas-phase reactions and the dissolution of reactive nitrogen species (RNS). Fourier-transform infrared spectrometer analysis reveals that O ₃ is not detectable in the gas-phase N ₂ spark discharge over water. Our LC–MS measurements reveal that the functional groups in the side chains of amino acids, such as –SS–, –SH, and aromatic rings, are either oxidized, hydroxylated, or nitrated in N ₂ -PAW, which constitutes permanent chemical modifications of amino acids in proteins. The tertiary and quaternary structures of proteins in E. coli can be severely destroyed in N ₂ -PAW, where hydrophobic interactions, supplemented by auxiliary forces, such as hydrogen bonds, salt bridges (ionic bonds), van der Waals forces, disulfide bonds, and ester bonds, become unstable due to their interactions with aqueous RNS under weakly acidic conditions.

Plasma activated water (PAW) and mist (PAM) have gained widespread application as disinfectants in various fields. However, PAW exhibits limited storage stability and bulk activation efficiency, while PAM demands continuous energy supply and shows non-uniform droplet coverage. This study proposes an innovative approach that forms thin water films via in-situ evaporation–condensation during plasma treatment. The contact of high-temperature, high-humidity gas with a cold sample leads to surface condensation, releasing latent heat that increases the sample temperature. This temperature variation subsequently influences water layer formation, creating a dynamic coupling between heat transfer and condensation processes. The process of dynamic liquid film formation incorporates dissolution of plasma-generated gaseous reactive oxygen and nitrogen species (RONS), thereby enhancing both sterilization efficacy and activation efficiency. A numerical model is established to investigate dynamic liquid film formation through evaporation condensation and its interactions with plasma, while incorporating a hierarchical bacterial inactivation framework to evaluate sterilization efficacy. It also examines how variables such as sample temperature, gas temperature, and relative humidity influence liquid film thickness, RONS concentrations and antimicrobial performance in the liquid film. Results indicate that optimizing key parameters of dynamic water films (thickness, temperature, and surface retention time) may serve as critical design criteria for plasma-based surface disinfection systems.

As an essential part of the neutral beam injection system for the magnetic confinement fusion devices, radio-frequency (RF) ion source is responsible for generating negative hydrogen ions. The addition of argon and xenon is an alternative approach to enhance the volume generation of negative hydrogen ions. A 3D fluid model of RF negative hydrogen ion source is developed to investigate the effects of adding argon and xenon gases on the spatial distributions of electron density, electron temperature and negative hydrogen ion density. The 3D fluid model is validated by comparing the numerical results with experimental results in pure hydrogen discharge. With the addition of argon and xenon, the electron density increases and the electron temperature reduces due to their low ionization threshold energy and large electron impact ionization cross section. Additionally, the results show that the largest enhancement of negative hydrogen ion density at the bottom of expansion chamber is achieved under the xenon admixture fraction of 20% and gas pressure of 0.3 Pa, exhibiting an increase of 1.6 times compared to pure hydrogen discharge. The model can be applied for the optimization of RF negative hydrogen ion source.

In this study, the production and destruction pathways of the most important biomedically active species (H ₂ O _2aq , O _3aq , ONOOH _aq , OH _aq and NO _{x aq} ) in a water film exposed to a cold atmospheric pressure plasma (CAP) were investigated using a comprehensive kinetic model. The primary focus is on the regulatory mechanism of aqueous phase RONS density with a small amount of NO _x fed into the air Dielectric Barrier Discharge (DBD) plasma. The model is experimentally verified by measuring the concentrations of NO _3aq ⁻ and NO _2aq ⁻ , along with the pH value in the liquid phase. The results indicate that the addition of NO increases the density of HNO _2aq in the water film, which facilitates the formation of ONOOH _aq . The addition of NO ₂ promotes an increase in the density of HNO _3aq in the water film, thereby enhancing the acidity of the solution. In addition, the NO/NO ₂ inlet ratio and the distance between the discharge outlet and the sample have been studied as parameters for optimizing the discharge source. The results show that these two factors can be adjusted to regulate the gas-phase HNO _x and N _x O _y densities, thereby influencing the density of RONS in the water film. Such distinct control mechanisms provide valuable insights for the design of atmospheric pressure plasma sources for sterilization applications.

The propagation of the ionization wave (IW) from a pulsed helium atmospheric pressure plasma jet to tilted dielectric and metallic targets grounded was investigated via experiments and simulations. The results indicate that the IW initially propagates along the tube axis but deviates from its original direction around 3 mm above the target surface, eventually striking the tilted surface perpendicularly. This deviation occurs because the enhancement of the induced electric field above the target is not aligned with the original direction of the IW. The deviation values, defined as the distance between the IW strike point and the intersection of the tube axis with the target, ranged from 1.86 to 2.15 mm, while the surface ionization wave (SIW) propagation distance decreased from 7.63 to 1.83 mm as the relative permittivity (εr) increased from 2.55 to 56. Concurrently, the E/N at the IW head increased from 76 to 86 Td at 75 ns as εr increased. The ground potential is the primary factor causing the electric field enhancement observed above the target, while the contribution of the dielectric polarization effect is negligible. Upon striking a metal target, the IW becomes a reflected IW, which generates a secondary IW and propagates back toward the metal surface. ICCD imaging and numerical simulation reveal that a “dark area” emerges between the SIW and the secondary IW following IW reflection.

The radio frequency (RF) negative hydrogen ion source is employed in neutral beam injection (NBI) system for magnetic confined fusion devices. To satisfy the required beam current of negative hydrogen ions in NBI for fusion, the surface production on the plasma grid (PG) surface is introduced to increase the amount of negative hydrogen ions. In this paper, a 2D fluid model is established, including volume production and surface production of negative hydrogen ions. This work focuses on the spatial distributions of negative hydrogen ions with different gas pressures, PG bias potentials, magnetic filter filed (MFF) positions and RF powers. The results show that the density of negative hydrogen ions is enhanced by the surface production. As the gas pressure increases, the increase in the negative hydrogen ion density is dominated by the increase in the conversion of neutral hydrogen atoms at the PG surface. As the bias potential increases, the distribution of negative hydrogen ions is slightly shifted towards the PG due to the reduction of electric field near the PG. As the MFF position shifts towards the PG, the increase in the negative hydrogen ion density is dominated by the increase in the conversion of positive hydrogen ions at the PG surface. Moreover, the negative hydrogen ion density increases with the increased RF power, which is dominated by the increase in the conversion of positive hydrogen ions at the PG surface. The model facilitates the understanding of the negative hydrogen ion distribution and the optimization of negative hydrogen ion production.

Compared to conventional plasma torch, the plasma torch with cascaded anode allows a reduction of the anode surface and a prolongation of the arc column, which improves the stability and specific enthalpy of the plasma jet. A two-dimensional (2D) hydrodynamic model with thermal nonequilibrium and chemical equilibrium is developed to investigate the arc characteristics of a cascaded-anode nitrogen DC plasma torch. The model calculates the current density, the density and temperature of heavy species and electrons, and the flow velocity in the nitrogen plasma torch. It is found that the deviations from the local thermodynamic equilibrium state occur in the fringes of the arc. Moreover, in the central region of arc, e, N⁺ and N are the dominant species, however, at the edge of the arc, N and N₂ appear to be the dominant species. In the nitrogen plasma torch, the density of N₂ ⁺ ions is much lower than the density of N⁺ ions. The gas flow velocity reaches a maximum in the constrictor of the plasma torch and decrease in the diverging section of the plasma torch. In this work, a thorough investigation of the arc characteristics of a cascaded-anode nitrogen DC plasma torch has been carried out. The model is employed to investigate the influence of the gas flow rate on the plasma flow velocity and the differences in the flow and temperature fields in a ‘cascade torch’ compared to that for a ‘standard DC torch’.