Scott J. Pendleton

The role of non-thermal transient plasma for enhanced flame ignition in C2H4-air

Transient plasma ignition, involving short ignition pulses (typically 10?50?ns), has been shown to effectively reduce ignition delays and improve engine performance for a wide range of combustion-driven engines relative to conventional spark ignition. This methodology is therefore potentially useful for many engine applications; however, at present there is limited understanding of the underlying physics. Evidence is presented here for two distinct phases of the plasma-ignition process: an initial non-equilibrium plasma phase, wherein energetic electrons transfer energy into electronically excited species that accelerate reaction rates, and a spatially distributed thermal phase, that produces exothermic fuel oxidation reactions that result in ignition. It is shown that ignition kernels are formed at the ends of the spatially separated streamer channels, at the cathode and/or anode depending on the local electric field strength, and that the temperature in the streamer channel is close to room temperature up to 100?ns after the discharge.

Fast gas heating in nitrogen?oxygen discharge plasma: II. Energy exchange in the afterglow of a volume nanosecond discharge at moderate pressures

The process of fast gas heating in air in the near afterglow of a pulsed nanosecond spatially uniform discharge has been investigated experimentally and numerically at moderate (3?9?mbar) pressures and high (200?400?Td) reduced electric fields. The temporal behaviour of discharge current, deposited energy, electric field and temperature was measured. The role of processes with participation of excited and charged species was analysed. It was shown that under the considered conditions the main energy release takes place in reactions of nitrogen and oxygen dissociation by electron impact and quenching of electronically excited nitrogen molecules, such as N2( , B?3?g, C?3?u, ) by oxygen and quenching of excited O(1D) atoms by N2. It was shown that about 24% of the discharge energy goes to fast gas heating during the first tens of microseconds after the discharge.

Vibrational and rotational CARS measurements of nitrogen in afterglow of streamer discharge in atmospheric pressure fuel/air mixtures

The use of nonequilibrium plasma generated by nanosecond discharges to ignite fuel/air mixtures, known as transient plasma ignition (TPI), has been shown to effectively reduce ignition delay and improve engine performance relative to spark ignition for combustion engines. While this method is potentially useful for many engine applications, at present the underlying physics are poorly understood. This work uses coherent anti-Stokes Raman spectroscopy (CARS) to measure the rotational and vibrational excitation of nitrogen molecules in the discharge afterglow in a variety of fuel/air mixtures outside the limits of combustion in order to elucidate the thermal behaviour of TPI. The time evolution of relative populations of vibrationally excited states of nitrogen in the electronic ground state are reported for each gas mixture; it is shown that generation of these vibrationally excited states is inefficient during the discharge in air but that generation occurs at a high rate roughly 5??s following the discharge; with the addition of fuels vibrationally excited states are observed during the discharge but an increase in population is still seen at 5??s. Possible mechanisms for this behaviour are discussed. In addition, rotational temperature increases of at least 500?K are reported for all gas mixtures. The effect of this temperature increase on ignition, reaction rates, and thermal energy pathways are discussed.

Low energy compact power modulators for transient plasma ignition

In this paper recent studies of compact power modulators, used to produce nonequilibrium plasma in the transient, formative phase of an arc, and applied to ignition of a quiescent fuel-air mixture in a constant-volume reactor, are reported. In this work, ignition delays produced by transient plasma were measured and compared in pre-mixed C2H4-air at atmospheric pressure. Two compact power modulators studied included; 1) a 54 ns pseudospark switched line-type power modulator that delivered 365 mJ per pulse, and 2) a 12 ns SCR-switched magnetic compression based power modulator that delivered 75 mJ per pulse. Despite the difference in energy delivered, both systems achieved similar ignition delays across a broad range of fuel-air equivalence ratios, and produced ignition delays up to two times shorter than those produced using traditional spark ignition. The results indicate that lower energy and therefore more compact power modulators may be used for this application.

Compact solid state high repetition rate variable amplitude pulse generator

Presented is a solid state high repetition-rate pulse generator with adjustable output amplitude, together with a resonant LC charger. This pulse generator was designed for transient plasma production for ignition and other aerospace plasma applications. The design of the pulse-forming network makes use of commercially available insulated gate bipolar transistors (IGBT) switching a capacitor bank into a METGLAS transformer together with a Fitch voltage doubling circuit. The capacitor bank is charged to 1 kV by a resonant LC charger, also switched by a commercial IGBT. The output of the pulse generator is controlled by the gate voltage of the IGBTs. Pulses with a width of 40ns can be generated with repetition rates up to 10 kHz. The amplitude can be controlled from 9 kV to 38 kV into a 500Ω load.

Surface Streamer Discharge for Plasma Flow Control Using Nanosecond Pulsed Power

Nonequilibrium plasma discharges have been demonstrated to be effective in a variety of flow control applications. Traditionally, surface dielectric barrier discharge (sDBD) or spark gaps have been used due to the small size, weight, and cost of generating electronics. Nanosecond pulsed discharge in the form of streamers has similar physical properties to sDBD while alleviating its disadvantages, but it has remained relatively untested due to pulse generation requirements. New developments in compact pulsed power technology, however, have enabled the generation of surface streamer discharge using small and inexpensive pulse generators. Images of surface streamer discharge generated for plasma flow control are presented.

Pulsed high-density hydrogen plasma source for wakefield accelerator applications

Summary form only given. Reported are new results for pulsed power-driven hollow cathode hydrogen-based plasma source development, needed for plasma-based accelerator experiments, including particle-beam-driven plasma wakefield accelerators. Capillary sources have been constructed of transparent cylindrical borosilicate glass tubes in lengths up to 15 cm and inner diameters up to 2 mm. The plasma discharge is presently driven by a thyratron-switched pulse forming network and step-up transformer. Uniform plasma densities of over 1018 cm-3 have been demonstrated, and the length and density can be readily varied for optimal performance. The pulsed power requirements for increasing capillary size and optimization of solid-state pulsed power switching for the purpose of increased flexibility, energy minimization and long life will be discussed. Time dependence of plasma density, and other variations of the device parameters for fine-tuning of accelerator applications is analyzed and discussed, including the use of this discharge at accelerator facilities for plasma wakefield experiments.

Temporal temperature evolution of atmospheric pressure streamer discharge in air

The use of streamers for the ignition of fuels, also known as transient plasma ignition (TPI), has been shown in a variety of engines to improve combustion through decreased ignition delay and increased energy release relative to conventional spark ignition. The mechanisms behind these improvements, however, remain poorly understood. Of particular interest are the relative roles of thermal effects of the streamer afterglow compared to electronically excited species in initiating combustion. Here we report investigation of gas heating due to dissociation, relaxation, and recombination of active plasma species and present temperature measurements in the afterglow of the streamers by using a second probing discharge in conjunction with optical emission spectroscopy. Experimental results are presented and compared to modeling of the streamers. Future experiments are proposed to better understand the role of streamer kinetics in TPI.

Optical diagnostics on transient plasma ignition

Plasma in a formative or transient phase, when used to ignite fuel/air mixtures, has been shown to improve combustion compared to traditional spark ignition. The mechanisms behind the improvements of transient plasma ignition (TPI), however, are not yet well understood. To further this understanding optical experiments, including OES and LIF, were carried out on TPI to measure plasma properties and active species concentrations. Results are presented discussed for air and combustible mixtures and future experiments are proposed to better the understanding of the mechanisms behind TPI.

Fast gas heating in fast ionization wave discharge: Experiment and modelling

The fast ionization wave (FIW) resulting from nanosecond pulsed discharge at very high overvoltage provides an excellent high reduced electric field (E/N) example discharge due to its spatial uniformity. A comprehensive model of FIW would provide insight into other high E/N discharges used for plasma ignition applications, such as DBD and streamer discharge1. To that end the mechanism of fast gas heating due to dissociation, relaxation, and recombination is investigated2. Spectroscopic methods were used to measure temperature during and after the discharge. Preliminary results are presented and the agreement between modeling and experiment is discussed. Future experiments are proposed in order to better understand FIW and to apply the results to other discharges in plasma ignition applications.