High-power Microwave Discharge Research Articles

Benchmark of the KGMf with a coupled Boltzmann equation solver

The Kinetic Global Model framework (KGMf) is an open-source general-purpose global model (spatially averaged) simulation code developed to explore the reaction kinetics and pathways in plasma discharge systems. It contains species continuity and electron energy balance equations, with a time-dependent evaluated electron energy distribution function (EEDF) for electron impact reactions. The EEDF is utilized to determine the rate coefficients for electron impact reactions, which can have profound impact on the temporal evolutions of plasma parameters. Previously, the EEDF was commonly assumed as Maxwellian or an analytical function of the effective electron temperature. In this work, the KGMf is coupled with a Boltzmann equation (BE) solver to self-consistently compute the EEDF. The EEDF evolution frequency is determined based on relative changes of the reduced electric field. The KGMf is benchmarked with the ZDPlasKin code based on high-pressure low-temperature argon plasma discharge cases. The temporal evolutions of reduced electric field, electron temperature, EEDF, reaction rates, and species densities, are obtained and compared under different discharge conditions, showing good agreement between the KGMf and the ZDPlasKin simulations. The application of the KGMf for predicting breakdown times in high power microwave discharges is also presented, which shows qualitative agreement with particle-in-cell simulations. The KGMf can be further applied for more complicated plasma discharge systems, where the reaction kinetics are more intricate (e.g., plasma-assisted combustion systems).

Energy Balance and Gas Thermalization in a High Power Microwave Discharge in Mixtures

Energy Balance and Gas Thermalization in a High Power Microwave Discharge in Mixtures

Self consistent multi-fluid FDTD simulations of a nanosecond high power microwave discharge in air

FDTD simulations of the Maxwell equations are combined with the multi-fluid plasma equations to study the dynamics of a high power microwave discharge in air. The breakdown takes place in a short time of a few nanoseconds and the concentrations of electrons, ions, excited species, and the dissociation products are quickly enhanced. The breakdown time decreases with decreasing of the pressure and the pulse amplitude, while increases with increasing of the pulse width. N2+ and O2+ are the most important positive ions, whereas O− is the most populated negative ion. For a single microwave pulse, the electron number density is large up to 1 μs, and the dissociation and excitation continue to increase the small radicals and excited species. Then the electron number density drops and the population of excited species declines. The ozone production becomes important after 1 μs when the three body association of O and O2 dominates over the dissociation processes. The ozone number density continues to grow up to 5 ms, and then saturates at a value of 1022 m−3. Quenching of electronically excited nitrogen molecules by O2 molecules and the subsequent dissociation to atomic oxygen and generation of NO, are found to be important and can play a significant role in the ultrafast gas heating.

Diagnostic of ultrafast temporal plasma evolution in high-power microwave discharge

For the most advanced multi-frame camera in the world, the shortest response time between two frames is no shorter than 1.5 ns. Thus, there is no effective way to diagnose the ultrafast sub-nanosecond dynamic of a microwave-driven plasma discharge in a single pulse. Different-length multi-sub-beam optical fibers, together with a spectrometer and an EMICCD camera, are proposed and designed to detect the nanosecond discharge spectra in a single pulse, like a real-time multi-frame spectral camera. This novel method could realize a time interval between two consecutive frames shorter than 0.1 ns by a length difference of 2 cm for sub-fibers, achieving the measurement of ultrafast plasma dynamics. Temporal evolution of electron density as well as energy of electrons and ions during nanosecond microwave discharge is further studied by de-convolving the Stark broadening and thermal Doppler broadening and by calculating the ratio of emission coefficients.

Nanosecond Snapshots of High-Power Microwave Discharge in Waveguides

Using the same bend waveguide with two different E-field polarization directions, the breakdown phenomena in a curved waveguide with different power capacities were studied. For the case of lower power capacity, breakdown happens at the curved arm; with increasing incident power, the reflected peak amplitude increases, the breakdown delay time becomes shorter, and the reflected pulse gradually overlaps with the incident one, forming a slower falling edge at the end of incident pulse. For the case of higher power capacity, discharge may occur at the connected flanges without the choke shot. Using the four-frame intensified-charged-coupled device diagnosing the nanosecond light emission in each of the vertical and horizontal polarization cases, the intensities in both of the surface layer and the space beyond were found to first become bright during the microwave pulse, and then get dark after the pulse. The multiple bright spots on the wall illustrate the nonuniform plasma development and local high-density plasma at the microprotrusions with field enhancement, triggering the intense high-field emission or explosive emission.

Ultrafast high-power microwave window breakdown: Nonlinear and postpulse effects

The time- and space-dependent optical emissions of nanosecond high-power microwave discharges near a dielectric-air interface have been observed by nanosecond-response four-framing intensified-charged-coupled device cameras. The experimental observations indicate that plasma developed more intensely at the dielectric-air interface than at the free-space region with a higher electric-field amplitude. A thin layer of intense light emission above the dielectric was observed after the microwave pulse. The mechanisms of the breakdown phenomena are analyzed by a three-dimensional electromagnetic-field modeling and a two-dimensional electromagnetic particle-in-cell simulation, revealing the formation of a space-charge microwave sheath near the dielectric surface, accelerated by the normal components of the microwave field, significantly enhancing the local-field amplitude and hence ionization near the dielectric surface. The nonlinear positive feedback of ionization, higher electron mobility, and ultraviolet-driven photoemission due to the elevated electron temperature are crucial for achieving the ultrafast discharge. Following the high-power microwave pulse, the sheath sustains a glow discharge until the sheath collapses.

Nonself-Sustained Microwave Discharge in a System for Hydrocarbon Decomposition and Generation of Carbon Nanotubes

This paper deals with the investigation of a method for sustainment of high-power microwave discharge in the installation for natural-gas decomposition. The essence of the method is to provide a generation of auxiliary discharge plasma in the area where the main microwave plasma torch burns. The design of the electrode system of auxiliary discharge resembles that for a coaxial plasmatron that consumes an average current of about 0.1 A. Then, the nonself-sustained microwave discharge with a frequency of 2.45 GHz has been obtained at a power level from 1 to 3 kW. Such a discharge has been used in the installation for natural-gas decomposition and generation of the carbon nanotubes. With a typical gas flow of 1 m3/h, the natural-gas conversion achieves 40%-80%. Three types of nanotubes are contained in the final product: multilayer tubes, single-layer tubes, and onion-type tubes.

Ignition of a combustible gas mixture by a laser spark excited in the reactor volume

Ignition of a stoichiometric CH4: O2 mixture by a laser spark excited in the reactor volume is studied experimentally. It is found that the spark initiates a feebly radiating incomplete-combustion wave, which is much faster than the combustion wave, but is substantially slower than the detonation wave. With a time delay of 500–700 μs, a bright optical flash occupying the entire chamber volume is observed, which indicates fast (involving branching chain reactions) ignition of the gas mixture. A conclusion is drawn regarding the common nature of the process of ignition of a combustible gas mixture by a laser spark excited in the reactor volume and the previously investigated initiation of combustion by laser sparks excited at solid targets, high-power microwave discharges, and high-current gliding discharges.

Microwave excited CO2 laser with multiple units using orthogonal electric fields

To realize a CO2 laser oscillator using a fast-axial-flow high-power microwave discharge excitation, we devised a method of making microwave discharge uniform by varying the oscillation direction of an electric field with time and verified the effectiveness of this method. When five orthogonal electric fields units were arranged in series for one laser tube, a maximum laser output power of 609 W and a laser efficiency of 17.0% were achieved for a microwave input power of 3590 W. In addition, using eight laser tubes, namely, 40 orthogonal electric fields units, we achieved a maximum laser output power of 3.6 kW and a laser efficiency of 13% for a microwave input power of 27 kW. Thus, a high-power microwave-discharge-excited CO2 laser has become feasible.

Metal vapour lasers: a review of recent progress

A brief review of metal vapour lasers is presented with an emphasis on the recent progress made in advancing metal vapour laser technology and applications. It is anticipated that new types of metal vapour lasers will be developed which take advantage of recently observed behaviours of high pressure copper vapour lasers. For example, collisional excitation energy transfer may make it possible to develop efficient single wavelength copper vapour lasers at high pressures. Also, use of new discharge schemes such as high power microwave discharges may significantly advance the metal vapour laser technology. A variety of new applications should also be realized in the years to come.

Spontaneous Raman scattering as a high-resolution XUV radiation source

The paper describes a new type of high-resolution XUV radiation source which is based upon spontaneous anti-Stokes scattering of tunable incident laser radiation from atoms excited to metastable levels. The theory of the source is summarized and two sets of experiments using He ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1s2s</tex> ) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> S atoms, produced in a CW hollow cathode and in a pulsed high-power microwave discharge, are discussed. The radiation source is used to examine transitions originating from the 3p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> shell of potassium. The observed features include four previously unreported absorption lines and several sharp interferences of closely spaced autoionizing lines. A source linewidth of about 1.9 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> at 185 000 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> is demonstrated.

High power microwave discharge as an excitation source for spectroscopic experiments

In order to obtain high radiation intensity per steradian per square centimetre, the axial radiant flux of a long capillary discharge tube (50 cm length, 4 mm i.d.) was investigated which served as the 'inner conductor' of the coaxial line in one microwave circuit. The maximum input power of 800 W was supplied without an isolator to the capillary discharge tube, filled with Xe at pressures below 400 torr; the voltage standing wave ratio was about 1.5. In the near ultraviolet region, this xenon discharge had as intense a continuum as the Hitachi type 4141 hydrogen discharge lamp.

Microwave Burst at Triggered Instability in a Hot Electron Plasma

A hot electron plasma is produced in a magnetic-mirror field by high-power microwave discharges. When an instability is triggered in a hot electron plasma, it is observed that the instability is accompanied by a burst of energetic electrons escaping from the magnetic mirror, a fast rising x-ray signal, a sudden increase of light intensity, and an intense burst of microwaves composed of harmonics of 2.1 GHz. This frequency cannot be explained in terms of the electron cyclotron frequencies associated with the magnetic field intensity within the plasma chamber.