The Transition to Paschen's Law for Microscale Gas Breakdown at Subatmospheric Pressure

A computational study on the effects of fast-rising voltage on ionization fronts initiated in sub-mm air and CO[Formula: see text] gaps

Gas discharge and breakdown phenomena have become increasingly important for the development of an ever-growing number of applications. The need for compact and miniaturized systems within power, pulsed power, semiconductor, and power electronic industries has led to the imposing of significant operating electric field stresses on components, even within applications with low operating voltages. Consequently, the interest in gas discharge processes in sub-millimeter and microscale gaps has grown, as the understanding of their initiation and propagation is critical to the further optimization of these technologies. In this work, a computational study of primary ionization fronts has been conducted, which systematically investigated the role of voltage rate-of-rise in point-plane and point-point electrode geometries with an inter-electrode gap maintained at 250 [Formula: see text]m and a needle radius of 80 [Formula: see text]m. Using the hydrodynamic approach with the local mean energy approximation, along with simplified plasma chemistry, simulations have been performed under positive and negative ramp voltages, rising at 50, 25, 16.67, 12.5, and 10 kV/ns in synthetic air and in pure CO[Formula: see text]. Results on the developed electric field, electron densities, and propagation velocities are presented and discussed. Effects on the cathode sheath thickness scaling with voltage rate-of-rise have been additionally analyzed, the mechanisms behind these effects and their potential impacts are discussed. The work conducted in this study contributes towards an increased understanding of the gas discharge process, under fast-transients and nonuniform electric fields, with relevance to microelectromechanical, power, and pulsed power system design.

A High Kinetic Energy Ion Mobility Spectrometer for Operation at Higher Pressures of up to 60 mbar

High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) are usually operated at absolute pressures around 20 mbar in order to reach high reduced electric field strengths of up to 120 Td for influencing reaction kinetics in the reaction region. Such operating points significantly increase the linear range and limit chemical cross sensitivities. Furthermore, HiKE-IMS enables ionization of compounds normally not detectable in ambient pressure IMS, such as benzene, due to additional reaction pathways and fewer clustering reactions. However, operation at higher pressures promises increased sensitivity and smaller instrument size. In this work, we therefore study the theoretical requirements to prevent dielectric breakdown while maintaining high reduced electric field strengths at higher pressures. Furthermore, we experimentally investigate influences of the pressure, discharge currents and applied voltages on the corona ionization source. Based on these results, we present a HiKE-IMS that operates at a pressure of 60 mbar and reduced electric field strengths of up to 105 Td. The corona experiments show shark fin shaped curves for the total charge at the detector with a distinct optimum operating point in the glow discharge region at a corona discharge current of 5 μA. Here, the available charge is maximized while the generation of less-reactive ion species like NO_x⁺ is minimized. With these settings, the reactant ion population, H₃O⁺ and O₂⁺, for ionizing and detecting nonpolar substances like n-hexane is still available even at 60 mbar, achieving a limit of detection of just 5 ppb_V for n-hexane.

The Nanosecond Impulsive Breakdown Characteristics of Air, N2 and CO2 in a Sub-mm Gap

The present paper investigates the breakdown characteristics—breakdown voltage, with breakdown occurring on the rising edge of the applied HV impulses, and time to breakdown—for gases of significance that are present in the atmosphere: air, N2 and CO2. These breakdown characteristics have been obtained in a 100 µm gap between an HV needle and plane ground electrode, when stressed with sub-µs impulses of both polarities, with a rise time up to ~50 ns. The scaling relationships between the reduced breakdown field Etip/N and the product of the gas number density and inter-electrode gap, Nd, were obtained for all tested gases over a wide range of Nd values, from ~1020 m−2 to ~1025 m−2. The breakdown field-time to breakdown characteristics obtained at different gas pressures are presented as scaling relationships of Etip/N, Nd, and Ntbr for each gas, and compared with data from the literature.

Numerical Modeling of the Effects of Pore Characteristics on the Electric Breakdown of Rock for Plasma Pulse Geo Drilling

Drilling costs can be 80% of geothermal project investment, so decreasing these deep drilling costs substantially reduces overall project costs, contributing to less expensive geothermal electricity or heat generation. Plasma Pulse Geo Drilling (PPGD) is a contactless drilling technique that uses high-voltage pulses to fracture the rock without mechanical abrasion, which may reduce drilling costs by up to 90% of conventional mechanical rotary drilling costs. However, further development of PPGD requires a better understanding of the underlying fundamental physics, specifically the dielectric breakdown of rocks with pore fluids subjected to high-voltage pulses. This paper presents a numerical model to investigate the effects of the pore characteristics (i.e., pore fluid, shape, size, and pressure) on the occurrence of the local electric breakdown (i.e., plasma formation in the pore fluid) inside the granite pores and thus on PPGD efficiency. Investigated are: (i) two pore fluids, consisting of air (gas) or liquid water; (ii) three pore shapes, i.e., ellipses, circles, and squares; (iii) pore sizes ranging from 10 to 150 μm; (iv) pore pressures ranging from 0.1 to 2.5 MPa. The study shows how the investigated pore characteristics affect the local electric breakdown and, consequently, the PPGD process.

Application of multi-component fluid model in studies of the origin of skin burns during electrosurgical procedures

This paper reports on safety challenges regarding spark created when the applied electric field exceeds the dielectric breakdown strength as a source of complication during electrosurgery. Despite the unquestionable benefits of electrosurgery, such as minimal chances of infection and fast recovery time, the interaction of the electrosurgical tool with the tissue may result in tissue damage and force feedback to the tool. Some risks of complications often depend on a surgeon's knowledge of instruments and safety aspects of technical equipment that can be eliminated by clarifying the causation and conditions of their development. Current trends in electrosurgery include computational algorithms and methods to control the effect of delivered energy to the patient. For this study, calculations were performed by using the COMSOL simulation package based on a multi-component plasma fluid model. The emphasis is put on conditions that lead to the breakdown of the dielectric medium. It was found that breakdown occurs most easily when both electrodes are cylindrical. For configurations with one or two spherical electrodes, breakdown voltages are higher up to 25% and 48%, respectively. With decreasing the cathode radius, the breakdown voltage may decrease even to 41%. On the other hand, the temperature increase lowers the breakdown voltage. Also, electrical asymmetries appear to be a response to the non-symmetry of the electric field between the electrodes causing differences in the breakdown voltage between 36% and 70%. The results presented here could be very useful for the design of surgical devices to prevent potential complications of electrosurgical procedures.

Electrohydrodynamic Atomization for Minimum Quantity Lubrication (EHDA-MQL) in End Milling Ti6Al4V Titanium Alloy

Titanium alloy Ti6Al4V is a difficult-to-machine material which is extensively used in the aerospace and medical industries. Machining titanium is associated with a short tool life and low productivity. In this paper, a new cooling-lubrication system based on electrohydrodynamic atomization was designed, manufactured and tested and the relevant theory was developed. The major novelty of the system lies within the use of electrohydrodynamic atomization (EHDA) and a three-electrode setup for generating lubricant droplets. The system was tested and compared with that of flood, minimum quantity lubrication (MQL) and compressed air machining. The proposed system can extend the tool life by 6 and 22 times when compared with MQL and flood cooling, respectively. This is equivalent to more than 170 min tool life at 120 m/min cutting speed allowing for significant productivity gains in machining Ti6Al4V.