The control of plasma instability of weakly ionized plasma in a reactive flow is of great importance in plasma-assisted combustion, catalysis, fuel reforming and material synthesis. In this work, we propose a new concept of plasma chemical instability and analyze the mechanisms and impact of plasma chemical instability on the transition from a uniform discharge to a contracted state of a self-sustained glow discharge in a reactive H ₂ –O ₂ –N ₂ mixture. A one-dimensional numerical model for plasma chemical instability was developed which accounted for convective heat loss, Joule heating of plasma, and major non-equilibrium plasma-assisted combustion kinetic pathways including electron-impact ionization, vibrational energy transfer, electron attachment, combustion reactions, and heat release. The results showed that plasma chemical instability significantly modified the onset plasma current of plasma thermal instability. Specifically, the critical condition of the instability transition was strongly influenced by electron-impact reactant ionization, electron attachment to oxygen, endothermic/exothermic chemical reactions, and the formation of reaction products.

The stability of the weakly ionized plasma and the transition from a stable homogeneous discharge to unstable filaments play an important role in gas laser physics, plasma-assisted combustion, chemical reforming, and material synthesis. Theoretical stability analysis and thermal-chemical mode analysis were performed to understand the mechanism of plasma thermal-chemical instability by using a zero-dimensional plasma system with both simplified and detailed chemical kinetics of H ₂ /O ₂ /N ₂ mixtures. The plasma dynamic and kinetic models accounted for multiple physical mechanisms in the chemically-reactive weakly ionized plasma, including ionization, attachment/detachment, recombination, vibrational and electronic energy relaxation, convective and diffusive species/heat removal, Joule heating, and detailed chemical kinetics. An analytical criterion and the explosive mode species/temperature pointers were formulated while the representative active species were identified for different thermal-chemical modes. The results showed that in addition to the classical thermal-ionization mechanism, various chemical modes from chemical heat imbalance and elementary kinetics significantly modified the time dynamics and the stability of the weakly ionized plasma. The present analysis provides insights and guidance to control plasma instability using chemical kinetics.

Precise classification to distinguish malignancy of tumors is of great significance to accomplish high specificity diagnosis and avert unnecessary biopsy. Photoacoustic tomography (PAT) is a burgeoning new imaging modality, which combines optical contrast and ultrasound penetrating in a deep medium. However, it has not been fully exploited on the capability of PAT to discriminate tumors malignancy. In this paper, a multistatic classification approach in photoacoustic is proposed, which could discriminate malignant/benign tumors based on a feature in clinical diagnosis that tumors show different shape irregularity. The multistatic photoacoustic waves extract two different features to differentiate the two types of tumor models with high accuracy in three different scenarios using Support Vector Machines. In addition, two conventional PAT image reconstructing algorithm are also performed to reconstruct images as a comparative study, which unfortunately cannot differentiate their malignancy precisely because of limited detector bandwidth and strong acoustic distortion. We performed the feasibility study in this paper with both simulation and experimental results, which shows that the proposed multistatic photoacoustic classification to distinguish between malignant and benign tumors models works well, and could be easily applied for state-of-art array-based PAT system to ameliorate the diagnostic accuracy.