Plasma Ignition Research Articles

Effects of inter-pulse coupling on nanosecond pulsed high frequency discharge ignition in a flowing mixture

This work numerically investigates the effects of non-equilibrium nanosecond plasma discharge pulse repetition frequency, pulse number, and flow velocity on the critical ignition volume, minimum ignition energy, and chemistry in a plasma-assisted H2/air flow at 300 K and 1 atm using a multi-scale adaptive reduced chemistry solver for plasma assisted combustion (MARCS-PAC). The interactions between discharges/ignition kernels spanning decoupled, partially-coupled and fully-coupled regimes in a pulse train are studied. For a single pulse discharge, increased flow velocity increases the minimum ignition energy required due to the increase of convective heat loss and flame stretch. The results show that the minimum ignition kernel propagation speed at the critical ignition kernel volume increases with the flow velocity. The minimum critical ignition volume decreases with the increase of plasma discharge energy. For sequential two-pulse discharges, ignition fails at both decoupled and partially-coupled regimes even when the total discharge energy is above the minimum ignition energy, but succeeds only in the fully-coupled regime at a shorter inter-pulse time. Overlap of the OH radical pool between the sequential two-pulse discharges and the increase of the chemistry effect due to the increase of reduced electric field in the fully-coupled regime contribute to the ignition enhancement. In addition, for two-pulse discharges in the fully-coupled discharge regime, the mixture can be ignited at a total energy below the minimum ignition energy of a single pulse with the same flow conditions. Moreover, for a given total discharge energy with multiple pulsed discharges, the enhancement of the ignition kernel volume has a non-monotonic dependence on discharge frequency and pulse number. The effective ignition enhancement can be achieved with an optimal pulse repetition frequency and pulse number. This work provides a new understanding of the mechanism for repetitive plasma ignition and insights for the optimization of plasma ignition in a reactive flow.

Application of Spectroscopic Analysis for Plasma Polymerization Deposition onto the Inner Surfaces of Silicone Tubes

In the current study, plasma-polymerized methyl methacrylate (PP-MMA) generation on the inner surface of a silicone tube was performed in a capacitively coupled discharge reactor. The possibility of generating plasma inside the tube was analyzed and calculated by using optical emission spectroscopy (OES). A hollow cathode model was first proposed to determine whether plasma discharge would be generated inside the tube in the low-pressure regime. Since the ignition of plasma inside the tube is necessary for the initiation of polymerization processes, the sheath thickness was calculated analytically. To achieve the goal, the electron temperature and density of plasma should be determined beforehand. In this study, the electron temperature and plasma density were measured and calculated according to OES spectra using both the modified Boltzmann plot and the line-ratio method. The results reveal that the occurrence of plasma inside the tube can be achieved if the tube’s inner diameter is greater than two times the thickness of the sheath. The effect of methyl methacrylate (MMA) monomer concentration on sheath thickness, and, hence, plasma generation and deposition, was investigated in the presence of argon plasma and MMA monomer. According to the study, one could control the ignition of plasma discharges inside the tube followed by plasma polymerization deposition. The OES method was also applied to identify the presence of the excited species related to the fragmented monomer. The deposition of PP-MMA films on the inner surface of the tube was confirmed via attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy.

Propagation of nanosecond plasmas in liquids—Streamer velocities and streamer lengths

Nanosecond plasmas in liquids are often generated by applying a short high voltage pulse to an electrode immersed in a liquid for biomedical or environmental applications. The plasmas appear as streamers that propagate through the liquid. The understanding of the ignition of these nanosecond plasmas in liquids, however, is an open question. The occurrence of any traditional gas phase ignition mechanism is unlikely, because the formation of a gas bubble prior to ignition is suppressed by the inertia of the liquid. Therefore, either electron multiplication inside nanopores that are induced by an electric field pressure gradient or field effects at the tip and at the ionization front of the liquid streamer may act as electron generation mechanisms. A deeper understanding can be achieved by comparing the velocity and dynamic of the plasma propagation with modeling, where the individual mechanisms and transport coefficients can be analyzed. Here, we are using intensified charge-coupled device imaging to investigate the time dependence of the streamer dynamic and compare this with a 1D fluid code for negative voltages. It is shown that the maximum streamer length scales with the applied electric field, indicating that an electric stability field in the liquid streamer channel is important, as known for gas streamers. The 1D fluid code can reproduce the proper streamer velocities, if transport coefficients for hydrated electrons are chosen. The model suggests that the propagation of liquid streamers is dominated by the local ionization rate at the ionization front rather than by advection or diffusion of electrons as in gases. This also explains the finding that positive and negative streamers exhibit almost identical electron densities.

Outside Front Cover: Plasma Process. Polym. 6/2022

Inside Front Cover: For a wide range of plasma processes, stable plasma ignition of remote plasma sources (RPSs) is essential to ensure the stability of the discharge, stable operating conditions, and high plasma density. An RPS operating at a gas pressure greater than 1 Torr relies on the reaction pathways leading to the high-level excited state to sustain the power balance and energy cascade at a high input power level. Further details can be found in the article by Cheongbin Cheon, Jung Hwan Yoon, Sanghyun Jo, Ho Jun Kim, and Hae June Lee (e2100251).

Effect of shock wave formation on propellant ignition in capillary discharge

In order to study the effect of shock wave formation on propellant ignition in capillary discharge, the shock wave formation process was analyzed using experimental and theoretical methods; the plasma jet temperature was measured, and closed bomb and 30 mm gun experiments were carried out. The results show that the first shock wave has a smaller value and larger range of influence, while the second shock wave has a larger value and smaller range of influence. A plasma jet can generate a shock wave at the nozzle according to the calculated plasma pressure and velocity, which is well confirmed by experiments and calculations. The plasma jet temperature is high during the formation of a shock wave and then decreases sharply. Plasma ignition can increase the burning rate of a propellant by about 30% by increasing the burning surface area of the propellant. Compared to conventional ignition, the average maximum chamber pressure and average muzzle velocity of plasma ignition are increased by 9.1 MPa and 29.3 m·s−1 (∼3%), respectively, in a 30 mm gun. Plasma ignition has strong ignition ability and short ignition delay time due to the generation of a shock wave. By increasing the burning rate of the propellant, the muzzle velocity can be greatly improved when the maximum chamber pressure increases a little. The characteristics of the shock wave can be applied in the application of the capillary discharge plasma. For example, it can be applied in fusion, launching and combustion.

Multi-channel gliding arc plasma-assisted ignition in a kerosene-fueled model scramjet engine

A multi-channel gliding arc (MCGA) plasma was utilized to ignite a kerosene-fueled model scramjet engine with an inflow of Ma2.92. High-speed photography from the top and side view of the scramjet combustor, high-speed CH* chemiluminescence imaging coupled with the electrical measurements were used to investigate the ignition process. A three-dimensional RANS simulation was used to show the distributions of velocity, temperature, and local equivalence ratio of the supersonic cold flow. In the ignition process, the formation of the flame kernel mainly depends on the discharge characteristics of the MCGA plasma, whereas the flame propagation process is mainly determined by the cavity flow. The MCGA plasma is more likely to produce a large area of the flame kernel in the spark-type discharge with the long arc column and overlaps of each channel gliding arc. The larger flame can be generated subsequently by the merging of the flame kernels ignited by the MCGA plasma, which develops into the resident flame. The resident flame is unable to be formed in a relatively high local equivalence ratio, and it is hard to spread to the back of the cavity along the shear layer due to the large velocity gradient with a relatively low temperature. The resident flame is formed several times by the MCGA plasma ignition, which increases the possibility of the flame spreading to the back of the cavity to form the mainstream flame. The local equivalence ratio can significantly affect the cavity ignition of the kerosene-fueled scramjet combustor.

Flame kernel development with radiofrequency oscillating plasma ignition

In this paper a radiofrequency oscillating plasma discharge is investigated under various initial pressures up to 5 atm in a constant-volume combustion chamber. The oscillating plasma is suppressed by elevated pressure, both in length and branch number. The ignition performance under elevated background pressure is investigated, and the results are compared with spark events with a similar ignition energy. Under ambient conditions, the oscillating plasma discharge generates multiple streamers that are much longer than a spark gap, resulting in a much bigger initial flame kernel. Under elevated background pressures, fewer streamers with much smaller sizes are observed, thus the advantage of an oscillating plasma discharge over a spark discharge is compromised. Prolonged duration of an oscillating plasma discharge consistently demonstrates a positive impact on flame propagation speed, but neither prolonged duration nor enhanced discharge current has a noticeable impact on flame kernel growth for the spark ignition cases. Both oscillating plasma and spark are used to treat non-combustible propane–air mixtures under background pressures from 1 to 5 atm.

Foundations of plasmas for medical applications

Plasma medicine refers to the application of nonequilibrium plasmas at approximately body temperature, for therapeutic purposes. Nonequilibrium plasmas are weakly ionized gases which contain charged and neutral species and electric fields, and emit radiation, particularly in the visible and ultraviolet range. Medically-relevant cold atmospheric pressure plasma (CAP) sources and devices are usually dielectric barrier discharges and nonequilibrium atmospheric pressure plasma jets. Plasma diagnostic methods and modelling approaches are used to characterize the densities and fluxes of active plasma species and their interaction with surrounding matter. In addition to the direct application of plasma onto living tissue, the treatment of liquids like water or physiological saline by a CAP source is performed in order to study specific biological activities. A basic understanding of the interaction between plasma and liquids and bio-interfaces is essential to follow biological plasma effects. Charged species, metastable species, and other atomic and molecular reactive species first produced in the main plasma ignition are transported to the discharge afterglow to finally be exposed to the biological targets. Contact with these liquid-dominated bio-interfaces generates other secondary reactive oxygen and nitrogen species (ROS, RNS). Both ROS and RNS possess strong oxidative properties and can trigger redox-related signalling pathways in cells and tissue, leading to various impacts of therapeutic relevance. Dependent on the intensity of plasma exposure, redox balance in cells can be influenced in a way that oxidative eustress leads to stimulation of cellular processes or oxidative distress leads to cell death. Currently, clinical CAP application is realized mainly in wound healing. The use of plasma in cancer treatment (i.e. plasma oncology) is a currently emerging field of research. Future perspectives and challenges in plasma medicine are mainly directed towards the control and optimization of CAP devices, to broaden and establish its medical applications, and to open up new plasma-based therapies in medicine.

Experimental study on direct ignition of blast furnace gas by plasma igniter in a gas turbine combustor

Direct ignition of a gas turbine combustor using blast furnace gas fuel has become a critical issue at present. As an alternative technology of traditional spark plug ignition, plasma-assisted ignition has excellent performance in the gas turbine combustor. In this paper, the ignition system is designed to conduct a direct ignition experiment using the plasma and spark plug igniter. Then, the plasma jet characteristics, ignition limit, and flame propagation of the blast furnace gas combustor are analyzed. The results show that, using blast furnace gas fuel in the gas turbine combustor, the direct ignition of spark plug igniter fails and direct ignition of the plasma igniter succeeds. The jet from the plasma igniter is conical with a length of 30 mm and a diameter of 8 mm, which contains various active species (O、, O*、, O+、, O2+、, N2*, and, N2+). With the increase of air flow rate in the combustor, the equivalence ratio of plasma ignition limit increases from 0.7 to 0.83, and the time required for successful ignition increases from 860 ms to 4300 ms. Meanwhile, the flame propagates from a unidirectional mode to a bidirectional mode in the combustor. The laminar flame characteristics and ignition delay time of BFG/air mixtures were investigated by CHEMKIN package using the Gri-Mech 3.0 model. The active species in the plasma jet can shorten the ignition delay time from 1E-4s to 1E-5s and improve the laminar burning velocities from 7.58 cm/s to 10.90 cm/s. A large-scale initial flame kernel with concentrated energy is formed rapidly so that the flame can successfully propagate to the recirculation zone and burn stably. Finally, the blast furnace gas fuel was successfully ignited directly by the plasma igniter.

Time-resolved ion energy distribution in pulsed inductively coupled argon plasma with/without DC bias

Pulsed plasmas have emerged as promising candidates as a means for precise control of ion energy/angle dependent surface processes and surface chemistry during the plasma process, which are key to 3 nm and beyond device fabrication. The ion energy distribution functions (IEDFs) and ion fluxes over a pulsed period are important to understand as they directly influence the feature profile, damage, and selectivity. We have developed an advanced plasma diagnostics (APD) system with advanced pulsing capability, including source, bias, and synchronous pulsing. It is a compact inductively coupled plasma system with a RF source frequency of 13.56 MHz intended to diagnose the general behavior of biased high density plasmas. We report the effect of the pulse frequency (2–10 kHz), RF duty cycle (25%–75%), DC duty cycle (5%–50%), phase lag (50–60 μs), RF power (120–180 W), DC bias voltage (0–150 V), and discharge pressure (20–80 mTorr) on the IEDFs and ion flux over a pulse period on the APD system. The time-resolved IEDFs and ion flux were measured using a retarding field energy analyzer. The ion energy transitions in a pulsed period from a plasma ignition stage to a stable stage and from plasma in a glow period to an afterglow period are studied. The results indicate that the ion energy and ion flux are tailored by RF pulsing and RF-DC pulsing. The time-resolved IEDF demonstrates the merits of pulsing to precisely control ion energy and flux, and the ion energy spread was narrowed by the pulsed plasma.

Plasma-electrolytic liquefaction of human waste for biofuels production and recovery of ammonium, chlorine and metals

Sustainable production of value-added fuels and chemicals from biomass faces enormous challenges for integration in zero-carbon-emissions technologies and circular economy. In this work, green and renewable solvents, polyethylene glycol 200 and glycerol, were used in a plasma-electrolytic liquefaction (PEL) system to electrically liquefy human feces in the absence of a catalyst. It is demonstrated that the plasma ignition during the liquefaction accelerated the conversion of feces (85.67% liquefaction yield within 12 min), and the intrinsic heating features of the PEL greatly reduced the energy consumption. The obtained biocrude oil at the optimized conditions contained 10.53% aldehydes, 5.39% ketones, 18.48% cyclic oxygen-containing compounds and others, with a high heating value of 28.10 MJ/kg. Further analysis of the PEL-converted products reveals that most of the chlorine in the feces was gasified, the metals were extracted into the solid residue, while ammonia was released. An evaluation of the developed plasma electrolytic feces liquefaction process was carried out and favorably compared with common liquefaction methods. Overall, the proposed approach may provide a new avenue for clean chemical production and sustainable resource recovery from human waste under benign process conditions.

H- ion source operational performance and latest development at the Spallation Neutron Source

The U.S. Spallation Neutron Source (SNS) at Oak Ridge National Laboratory is the world’s most powerful accelerator-driven pulsed neutron source. An H- injector feeds the accelerator with high current (>50 mA), time structured (1ms, 60 Hz) H- beam. The injector consists of an RF-driven, Cs-enhanced H- ion source and an electrostatic low energy beam transport section. In the recent three run cycles, an H- ion source operated ∼4 months for each run with excellent availability near 99.9%. The presently ongoing run is also on track to finish another ∼4 months run with similar high availability. Lately, we have tested a solid-state RF system for the ion source to replace the existing vacuum-tube type RF amplifier for the sake of improving the ease of operation and maintenance. Progress has also been made on the continued development of an external antenna RF H- ion source with a focus on improvement to its plasma ignition scheme.

Plasma commissioning in a high power external RF-coil volume-type H- ion source

A high power, high duty cycle, negative hydrogen ion source is in development at ISIS. It will operate in pure volume-production mode and is driven by a 52-turn RF-coil mounted external to the plasma chamber. A solid-state amplifier with a maximum output of 100 kW in 50 Hz, 1 ms pulses delivers RF power to the coil via an impedance-matching network. The amplifier has a relatively wide bandwidth, able to deliver full power from 1.8-4.0 MHz. This flexibility allowed straightforward commissioning of the matching network into an inductively-coupled plasma. Striking of the pulsed plasma is facilitated by a compact microwave ignition gun, requiring only 10 W of power at 2.45 GHz to deliver 1 mA seed pulses of electrons. Experiments have shown that it is vital to encapsulate the RF-coil properly to mitigate high voltage sparking. In addition, the location of the coil relative to the ion source’s permanent magnets has a critical effect on the ease of plasma ignition. The result of commissioning work is that a full duty-factor (50 Hz, 800 ^s) plasma has been achieved at nominal operating power (30 kW) and detailed optical studies have begun.

Importance of higher‐level excited species in argon remote plasma sources: Numerical modeling with consideration of detailed chemical reaction pathways

AbstractStable plasma ignition of remote plasma sources (RPSs) for various plasma processes is essential to ensure the stability of the discharge, steady operating conditions, and high plasma density. In an RPS in which the gas pressure exceeds 1 Torr, the reaction paths leading to the high‐level excited state play an essential role in the power balance and energy cascade at high applied input power. In this study, 97 chemical reactions in a detailed reaction set involving argon discharge are investigated in the steady‐state to understand the extent to which the dominant reactions are affected by variations in the process parameters. We observed that the stepwise ionization process of the argon 4p excited state significantly contributes to sustaining high‐density plasmas in the near 1 Torr regime.

Class DE Switch-Mode Power Amplifier Using GaN Power HEMTs: High-Efficiency Power Amplifier for 13.56 MHz

High-efficiency power amplifiers (PAs) are critical for various applications that require high power density, portability, and effortless thermal management. Historically, these types of PAs, operating in the tens-of-megahertz range in the industrial, scientific, and medical bands, were equipped for transmitters for fixed radiocommunication services, RF heating, and plasma ignition <xref ref-type="bibr" rid="ref1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[1]</xref> , <xref ref-type="bibr" rid="ref2" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[2]</xref> . Today, applications have expanded to wireless power transmission for biomedical devices and charging for consumer electronics and smart vehicles <xref ref-type="bibr" rid="ref3" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[3]</xref> . In this article, we present the first-place design for the Student Design Competition (SDC) High-Efficiency Power Amplifier for 13.56 MHz in the 2021 IEEE Microwave Theory and Techniques Society International Microwave Symposium (IMS) in Atlanta, Georgia.

Design and Manufacturing of Angular DC Magnetron Co- Sputtering System to Provide Multilayer Films

The research aims to design and produced a new vacuum chamber set of the angular multi-target magnetron sputtering system was supplied by three patterns of the process gas emissions in one system. One pattern supplies the gas directly in front of the target surface, while another pattern supplies the gas beside the chamber, and another pattern supplies the gas to the bottom of the lid. The vacuum chamber set was designed as a top-down sputtering process which has six major components is new designed and produced, consists of the base, the chamber with a window, the feedthrough for the anode pole, the lid, the magnetron target-holder gun, and the rotate substrate-holder set. When the system is completely assembled, then try to test the leakage and plasma ignition until both passes tested, and then a preliminary test of the sputtering process to provide a thin fi lm with sputtered one copper target which was mounted on one gun onto glass slide substrates for the gas emissions of three patterns. After that run sputtering process to provide the multilayers fi lms with three target materials include copper, aluminum, and brass, which was mounted on three guns onto the rotate glass slide substrates. The results of the thin fi lm thickness from the sputtering process was found this system can be used to build a thin fi lm for changing various sputtering voltages, including the system can be successfully used to provide multilayers fi lm and generate argon plasma which was supplied three patterns of the process gas emissions in one system.

Optimization of Silicon Heterojunction Interface Passivation on p‐ and n‐Type Wafers Using Optical Emission Spectroscopy

To increase the efficiency in p‐type wafer‐based silicon heterojunction (SHJ) technology, one of the most crucial challenges is the achievement of excellent surface passivation. Herein, chemical passivation techniques known for n‐type technology are successfully applied on p‐type float–zone (FZ) wafers, and wafer surface passivation quality is correlated with parameters from plasma diagnostics, namely crystallization rate and electron temperature indices. It is shown that plasma ignition at higher powers than deposition powers enhances effective minority carrier lifetimes τeff fourfold for p‐ (0.6–2.1 ms) and sixfold for n‐type (0.6–3.2 ms) wafers while giving opportunity to process under lower electron temperature indices during the nucleation phase. A subsequent hydrogen plasma treatment has a further beneficial effect on chemical passivation, leading to high effective minority carrier lifetimes of 4.5 and 3.1 ms, and implies open‐circuit voltages, i‐VOC, of 735 and 720 mV for p‐ and n‐type wafers, respectively. In particular, cell precursors built on p‐type wafers demonstrate excellent surface passivation with τeff and i‐VOC (4.1 ms and 745 mV). Using these process optimizations, SHJ cells on both p‐ and n‐type wafers are fabricated with efficiencies exceeding 21%.

Non-Thermal Plasma Characteristics and Ignition Behavior of Lean Mixture with Radio Frequency Dielectric Barrier Discharge at High Pressure

Non-Thermal Plasma Characteristics and Ignition Behavior of Lean Mixture with Radio Frequency Dielectric Barrier Discharge at High Pressure

Fundamental Study on Laser Induced Breakdown Plasma and Engine Ignition

Fundamental Study on Laser Induced Breakdown Plasma and Engine Ignition

Laser ignition of iso-octane and n-heptane jets under compression-ignition conditions

This work aims to investigate the effect of laser-induced plasma ignition (LI) on combustion behaviours of iso-octane (a gasoline surrogate) at compression-ignition (CI) conditions. A high-energy laser was used to force the fuel ignition at a quiescent-steady environment inside an optically accessible constant-volume combustion chamber with 900K ambient gas temperature, 22.8kg/m3 ambient gas density and 21 vol.% O2 concentration. A diesel surrogate (n-heptane) was tested at a lower charge temperature of 735K to offset its higher fuel reactivity than the iso-octane, such that the flames of both fuels can have a similar lift-off length. Forced laser ignition was introduced either before or after the natural autoignition timing of the fuels. The laser was focused at the jet axis 15mm and 30mm from the nozzle. High-speed schlieren imaging, heat release analysis and flame luminosity measurement were applied to the flames. The high-speed schlieren imaging was used to monitor the flame structure evolution of the natural ignition and LI cases. Due to laser ignition, the flame lift-off lengths decrease, with which the uncertainties in the lift-off distances reduce by more than 80%. The laser-affected flame bases return back to the natural flame base locations. The uncertainties in the lift-off lengths also increase, as the flame stabilisation locations approach the natural lift-off distances. Under the test conditions of this work, the rates at which the iso-octane flames shift downstream are slower than the n-heptane cases. The heat release rate profiles show high heat release from the flames following the LI events, before transitioning to lower steady values. The flame luminosity measurements indicate a strong correlation between the LI affected lift-off length and increased soot formation. The luminosity levels decrease as the flame base shifts downstream over time.