Session KT1: Dielectric Barrier Discharges and Low Temperature Jets II

Live

Plasma-enhanced packed bed reactor (PE-PBR) has multiple micro and mesoscopic voids/cavities where the plasma is generated. The plasma propagation in Packed Bed Reactors (PBRs) is complex due to the existence of multiple length scales i.e. the size of the plasma reactor typically in macroscale, size of pellets macro-mesoscale and the size of the void/cavity between the pellets and pores on the surface meso-microscale. In this work, plasma propagation at these length scales~have been experimentally investigated using phase and space resolved optical emission spectroscopy in a simple design with 55 12 mm diameter hemispherical pellets arranged in an organized structure operated in helium gas. It was observed that plasma is generated as only filamentary microdischarges at the position of minimum gap at low voltages while at higher voltages, surface microdischarges are also generated in the void between the pellets. Zooming in the void between the pellet, each surface microdischarge is found to be generated as multiple microscopic bright structures. Increasing the voltage amplitude increased the sharpness of these surface microdischarge structures. In the complete reactor, strong interaction between adjacent cavities results in wave-like emission propagating from the center to the edges.   

Live

Airborne pathogens are responsible for most of the infectious diseases and the vast majority of which are uniquely adapted to spread in the indoor environments. An effective and economical sterilization method is required for living spaces, offices and healthcare facilities. Dielectric Barrier Discharge (DBD) treatment is a promising technology for fast and effective sterilization of surfaces, waterflow, and airflow. A compact DBD system is designed and developed for decontamination of airborne bacteria. It is capable of sterilizing at a volumetric flow rate of 2.206\texttimes 10-3 m3/s. The active DBD volume consists of dielectric tubes arranged in hexagonal pattern, also known as staggered grid arrangement. Tubes have a gap of 1.3 mm for carrier gas/liquid the flow. Self-mixing of airflow while passing through the tubes is one of the main characteristics. The maximum air velocity between the electrode tubes of discharge area is 2 m/sec. The designed system is capable of achieving 5-log reduction (99.999{\%}) in the concentration of the airborne bacteria after single airflow pass through the plasma. The DBD treatment requires relatively shorter exposure time (milliseconds) for rapid inactivation of microorganisms in bio-aerosols. Additionally, DBD treatment is safer compared to toxic chemical and ultraviolet radiations.   

Live

The uniform propagation of positive-polarity surface plasmas in air at atmospheric pressure has been achieved using a multi-layer structure, consisting of a silicon wafer covered by a 1-micron layer of SiO$_{\mathrm{2}}$ as a propagation surface. Instead of the branched streamers typically observed on bulk dielectric surfaces, the plasma exhibits a homogenous ring-shaped structure with a high degree of reproducibility and stability. The plasma is generated by applying nanosecond positive voltage pulses to a tungsten wire touching the dielectric surface. The propagation of an ionization front with a region of high N$_{\mathrm{2}}^{\mathrm{+}}$* emission has been imaged in single shot operation at high spatial resolution with an ultraviolet reflective microscope coupled with a fast ICCD camera. We discuss the origin of the ring-shaped ionization wave, considering the role of the Si-SiO$_{\mathrm{2}}$ interface and the effect of illumination by an external light source. The ring ionization wave may result from branching inhibition, due to a photoelectric effect at the interface created by the photons emitted by the plasma. To investigate the underlying mechanism, we compare ICCD imaging and electrical measurements for additional structures such as Si-Al$_{\mathrm{2}}$O$_{\mathrm{3}}$, Si-Si$_{\mathrm{3}}$N$_{\mathrm{4}}$, and Si-SiO$_{\mathrm{2}}$-UNCD. We also demonstrate the generation of planar waves.   

Live

Air ionizers are a class of air purifier relying on the generation of ions by application of an electric field between two metallic electrodes of unequal radius of curvature. The ions are accelerated by the electric field and thanks to ion-neutral collisions momentum is transferred from ions to neutral molecules, thereby creating a so-called ``ionic wind''. Ionic wind can be used for air purification purpose: dust and particles in suspension in the ambient air collect electrons when crossing the discharge area. These negatively charged particles then deposit onto grounded surfaces, where they can be cleaned up in the usual way. We present here a model for ionic wind generation. The model couples the Navier-Stokes equations for the flow momentum with the drift-diffusion equations for the electron, negative ions and positive ions densities, and with the Poisson equation for the electric potential. Source terms involve ionization, attachment, detachment, and recombination. The model is validated against a laboratory experiment using two cylindrical electrodes of different radii, where the gas velocity profile has been measured accurately. The predicted current-voltage characteristics are also compared with experimental ones.   

Live

RF-driven atmospheric pressure plasmas in argon and helium are of particular interests due to the production of highly excited and reactive species enabling numerous applications. Due to their long lifetimes, the atoms and molecules in the excited states of Ar and He are excellent reservoir of energy. In this contribution, broadband absorption spectroscopy is employed for the first time to measure the absolute densities of Ar atoms in metastable and resonant states, as well as the absolute densities of He atom and dimer metastables in a RF-driven capacitively coupled glow discharge at atmospheric pressure. The density profiles of these species across the plasma gap correlate well with the sheath structure of the plasma operated in the $\alpha $-mode. The electron temperature and density in both discharges are also determined by fitting the measured absolute emission spectra with the neutral bremsstrahlung radiation. This study provides the first detailed analysis of all key parameters including gas and electron temperatures, and densities of electronically excited species in metastable and resonant states.   

On Demand

We report our recent experimental results on the high-resolution high-sensitivity electrical and optical analysis of the surface barrier discharge (SBD) operated in atmospheric pressure air by sinusoidal voltage waveform. The transferred charge of the low-power discharge is carefully evaluated and it is shown that after the complete voltage period the total charge balance is approaching zero. This rather obvious result is mechanically assumed in the literature, yet it is experimentally not understood in detail. We show that continuous and pulsed micro-ampere currents measured during the negative polarity are responsible for the gradual renewal of the charge transfer equilibrium abruptly distorted by strong electrical pulses caused by positive streamers during the opposite half-period. We evidence also exceptionally large discharging events if operated at high over-voltage and reveal their sub-nanosecond development. The SBD discharging mechanisms are reviewed and updated.