Effective Electron Collision Frequency Research Articles

Power transfer efficiency in an air-breathing radio frequency ion thruster

Abstract Due to a series of challenges such as low-orbit maintenance of satellites, the air-breathing electric propulsion has got widespread attention. Commonly, the radio frequency ion thruster is favored by low-orbit missions due to its high specific impulse and efficiency. In this paper, the power transfer efficiency of the radio frequency ion thruster with different gas composition is studied experimentally, which is obtained by measuring the radio frequency power and current of the antenna coil with and without discharge operation. The results show that increasing the turns of antenna coils can effectively improve the radio frequency power transfer efficiency, which is due to the improvement of Q factor. In pure N2 discharge, with the increase of radio frequency power, the radio frequency power transfer efficiency first rises rapidly and then exhibits a less steep increasing trend. The radio frequency power transfer efficiency increases with the increase of gas pressure at relatively high power, while declines rapidly at relatively low power. In N2/O2 discharge, increasing the N2 content at high power can improve the radio frequency power transfer efficiency, but the opposite was observed at low power. In order to give a better understanding of these trends, an analytic solution in limit cases is utilized, and a Langmuir probe was employed to measure the electron density. It is found that the evolution of radio frequency power transfer efficiency can be well explained by the variation of plasma resistance, which is related to the electron density and the effective electron collision frequency.

Determination of the Effective Collision Frequency of Electrons in the E and D Regions of the High-Latitude Ionosphere from Analysis of Radio Occultation Measurements

Determination of the Effective Collision Frequency of Electrons in the E and D Regions of the High-Latitude Ionosphere from Analysis of Radio Occultation Measurements

Determination of the Effective Collision Frequency of Electrons in the E and D Regions of the High-Latitude Ionosphere from Analysis of Radio Occultation Measurements

Collisions between electrons and neutral molecules are of special interest for the physics of the Earth’s ionosphere, in particular, for determining the ionospheric conductivity and current systems in the lower ionosphere of the planet, as well as elucidating the role they play in attenuating radio waves propagating inside the D and E regions of the ionosphere. The effective collision frequency of electrons can be estimated from laboratory studies of electron mobility in atmospheric gases in combination with rocket measurements of temperature and particle density in the Earth’s upper atmosphere, or it can be determined independently from analysis of radio occultation data. We have developed a method for reconstructing the vertical profiles of the absorption coefficient of decimeter (wavelength ~19 cm) radio waves by solving the inverse problem of signal absorption in the D and E regions of the Earth’s ionosphere. Based on the analysis of radio occultation data from the FORMOSAT-3/COSMIC satellites, the altitude profiles of the absorption coefficient of decimeter (DM) radio waves in the planet’s ionosphere during the geomagnetic storm on June 22–23, 2015, were determined. It is known that the absorption coefficient at a given fixed frequency is directly proportional to both the electron density and the collision frequency of electrons with ions and neutrals. Using the data on the vertical profiles of the absorption coefficient of DM radio waves and the electron density reconstructed from the analysis of FORMOSAT-3/COSMIC radio occultations, we estimated the effective collision frequency of electrons in the D and E regions of the Earth’s high-latitude ionosphere. The practical significance of studying the frequency of electron collisions and the effects of radio wave absorption in the D and E regions of the planet’s ionosphere is associated with maintaining the uninterrupted operation of space radio communication and navigation systems.

Which factor determines the optical losses in refractory tungsten thin films at high temperatures?

Refractory tungsten (W) plays an important role in high temperature photonic/plasmonic applications. Previously room temperature bulk single/poly-crystalline optical constants were extensively used to calculate the optical properties of W nanostructures at high temperatures. This might lead to a significant deviation between the predicted and measured optical properties due to the exclusion of electron–phonon interaction, as well as grain-boundary and surface-scattering.Herein, we show experimentally, how film thickness and temperature affects the optical losses in W. A drastic increase in the effective electron collision frequency is observed with decreasing the film thickness down to 5 nm, due to the grain-boundary and surface-scattering mechanisms. At sufficiently high temperatures (greater than 200 °C for W), the electron–phonon interaction eventually becomes the dominant mechanism, linearly increasing collision frequency with temperature, and it is independent of the geometry of the thin film structure. The impact of thickness and temperature-dependent optical properties of W is showcased with a hyperbolic 1D metamaterial structure acting as a thermophotovoltaic emitter. This work opens new directions in accurate prediction of the optical properties of nanostructures and design of efficient devices in various applications, such as aerospace, energy-efficient lighting, radiative cooling and energy harvesting, by incorporating thickness and temperature-dependent optical constants.

Transient non-classical transport in the hollow cathode plume II: evaluation of models for the anomalous collision frequency

The ability of fluid-based closure models to describe the non-classical electron collision frequency in the plume of a hollow cathode is experimentally investigated. Six models—all predicated on the assumption that the non-classical collision frequency can be attributed to ion acoustic turbulence (IAT)—are considered. Experimental measurements of the time-resolved plasma properties in the cathode plume (Georgin M P, Jorns B A and Gallimore A D 2020 PlasmaSources Sci. Technol, 29 105010) are used to evaluate each closure model and compare it to experimental measurements of the effective electron collision frequency. Though more than one of the considered closures can predict the time-average behavior of the plasma in the cathode plume, it is found that only one model accurately predicts the measurements in both space and time for the cathode and operating conditions that were studied. This new highest fidelity model is derived using a single-equation approach based on modeling the average frequency of the IAT as it evolves in space and time. The implications of the success of this model are discussed in the context of the understanding of the dynamics of the IAT in the cathode plume as well as on-going fluid-based modeling efforts related to cathode plumes.

Modeling the electron collision frequency during solid-to-plasma transition of polystyrene ablator for direct-drive inertial confinement fusion applications

Within the inertial confinement fusion (ICF) framework, the solid-to-plasma transition of the ablator arouses increasing interest, in particular due to the laser-imprint issue. Phase evolution of the ablator is linked to the evolution of the electron collision frequency, which is of crucial importance since it drives electron heating by laser energy absorption and lattice-ion heating due to collisions between electrons and the lattice-ion system. Thus, an accurate description of electron collisions over the whole temperature range occurring in ICF, starting from a few tens of kelvins (solid state) up to tens of millions of kelvins (plasma state), is necessary. In this work, a polystyrene ablator is considered, and a model of chemical fragmentation is presented to describe the heated polystyrene evolution. Electron collisions are described by electron–phonon collisions in the solid state and by electron–ion and electron–neutral collisions in the plasma state. An effective electron collision frequency valid over the whole range of temperatures reached in ICF experiments is established and discussed. Thermal conductivity is also deduced from collisions in the plasma state and shows a good agreement with the one evaluated by ab initio calculations.

Wave-driven non-classical electron transport in a low temperature magnetically expanding plasma

The presence of instabilities in a low density, low temperature plasma expanding through an axially symmetric magnetic nozzle is investigated in the context of non-classical electron cross field transport. Electrostatic probes are used to characterize the background plasma properties and instabilities. The measurements show a primarily azimuthally propagating mode with a broad, incoherent power spectrum that appears linear at low frequencies. It is demonstrated that the observed dispersion is consistent with the lower hybrid drift instability. The energy and linear growth rate of this wave are related through quasilinear theory to an effective electron collision frequency that is shown to be dominant over classical collisions.

Resonant microwaves probing the spatial afterglow of an RF plasma jet

The electron density and effective electron collision frequency in the spatial afterglow of a pulsed radio frequency driven atmospheric-pressure plasma jet are obtained by using microwave cavity resonance spectroscopy in a temporal manner with an ∼1 μs resolution. During the “plasma on” phase, values of 1.7 ± 0.3×1018 m−3 for the electron density and 0.12 ± 0.01 THz for the electron collision frequency were found. These values and standard deviations represent the collective measurement set with repetition rates ranging from 125 to 8000 Hz. The spread in the plasma parameters during this phase within one repetition frequency is smaller than 3%. It is observed that remnant species, e.g., metastables, of previous discharges influence the decay of the plasma. The work reported is enabled by recent developments in the applied diagnostic with respect to the resolution in the plasmas' permittivity. Moreover, a multiplying probe is used for the electrical characterization of the plasma and the presence of the cavity did not influence the plasma impedance. This strongly suggests that the cavity did not affect the discharge.

Influence of the excitation frequency on the RF power transfer efficiency of low pressure hydrogen ICPs

The influence of the excitation frequency on the RF power transfer of inductively heated hydrogen plasmas is investigated in the pressure range between 0.3 and 10 Pa. The experiments are conducted at a cylindrical ICP at frequencies in the range between 1 and 4 MHz and RF powers up to 1 kW. By applying a subtractive method which quantifies the transmission losses within the plasma coil and the RF network, the RF power transfer efficiency is determined. The key plasma parameters of the discharges are measured via optical emission spectroscopy and a double probe. By increasing the frequency from 1 to 4 MHz at a moderate RF power of 520 W, a significant enhancement of the RF power transfer efficiency is observed. It is most prominent at the presently considered low and high pressure limits and allows to reach high efficiencies of up to 95% at pressures between 3 and 5 Pa. While the AC loss resistance of the coil and the RF circuit only displays a relatively weak variation with the applied frequency due to the skin effect, the observed increase of the power transfer efficiency at higher frequencies is dominated by a considerable enhancement of the plasma equivalent resistance. This increased capability of the plasma to absorb the provided power is discussed against the background of collisional and collisionless heating of electrons. Thereby it is demonstrated that the observed behaviour can most likely be attributed to a decreasing difference between the angular excitation frequency and the effective electron collision frequencies. If the RF power is increased however, the RF power transfer efficiency increases globally while frequency induced differences tend to get less pronounced, as the plasma is generally capable of absorbing most of the provided power due to an increasing electron density.

Investigation of the power transfer efficiency in a radio-frequency driven negative hydrogen ion source

The radio frequency power transfer efficiency is experimentally and numerically investigated in an inductively coupled negative hydrogen ion source. The discharge is operated in a low pressure range of 0.1–3 Pa at a driving frequency of 2 MHz and an applied power of up to 6 kW. In the experiment, the power transfer efficiency value is determined by measuring the applied power and current through the antenna coil both with and without discharge operation. Fundamental properties, such as electron density and effective electron temperature, are obtained by means of a Langmuir probe. The effect of the antenna coil turns, N, is also studied in a range of 5–9 turns. It is found that more coil turns can significantly enhance the power transfer efficiency due to the remarkably increasing quality factor of the system. Moreover, the experimental results show that the power transfer efficiency first increases and then reaches the maximum with increasing applied power, while it first increases quickly and then rises at a slower rate with increasing gas pressure. In order to give a comprehensive knowledge of the power absorption mechanism, a self-consistent hybrid model is developed. It is found that the numerical results are in reasonable agreement with that measured in the experiment. The numerical results and the analytic solutions in the limit cases of low and high pressures can well explain the various trends of the power transfer efficiency obtained in the experiment. These trends mainly depend on the quality factor Q, the electron density, and the effective electron collision frequency.

Threshold of parametric instability in the ionospheric heating experiments

Many observations in the ionospheric heating experiment, by a powerful high frequency electromagnetic wave with ordinary polarization launched from a ground-based facility, is attributed to parametric instability (PI). In this paper, the general dispersion relation and the threshold of the PI excitation in the heating experiment are derived by considering the inhomogeneous spatial distribution of pump wave field. It is shown that the threshold of PI is influenced by the effective electron and ion collision frequencies and the pump wave frequency. Both collision and Landau damping should be considered in the PI calculation. The derived threshold expression has been used to calculate the required threshold for excitation of PI for several ionospheric conditions during heating experiments conducted employing EISCAT high frequency transmitter in Tromsø, Norway, on 2nd October 1998, 8th November 2001, 19th October 2012 and 7th July 2014. The results indicate that the calculated threshold is in good agreement with the experimental observations.

Nonlinear transmission and reflection of a strong terahertz pulse by a metal film.

Nonlinear interaction of the powerful terahertz radiation pulse with metal nanofilm is investigated. Quantitative analysis of the nonlinear interaction is illustrated by an example of an aluminum film of a thickness of several tens of nanometers. Rapid heating of the conduction electrons and the lattice causes significant rise of the effective electron collision frequency. This implies a noticeable change in reflectivity and an increase of transmissivity by an order of magnitude.

Analytic non-Maxwellian electron velocity distribution function in a Hall discharge plasma

The electron velocity distribution function in the low-pressure discharges with the crossed electric and magnetic fields, which occur in magnetrons, plasma accelerators, and Hall thrusters with a closed electron drift, is not Maxwellian. A deviation from equilibrium is caused by a large electron mean free path relative to the Larmor radius and the size of the discharge channel. In this study, we derived in the relaxation approximation the analytical expression of the electron velocity distribution function in a weakly ionized Lorentz plasma with the crossed electric and magnetic fields in the presence of the electron density and temperature gradients in the direction of the electric field. The solution was obtained in the stationary approximation far from boundary surfaces, when diffusion and mobility are determined by the classical effective collision frequency of electrons with ions and atoms. The moments of the distribution function including the average velocity, the stress tensor, and the heat flux were calculated and compared with the classical hydrodynamic expressions. It was shown that a kinetic correction to the drift velocity stems from a contribution of the off-diagonal component of the stress tensor. This correction becomes essential if the drift velocity in the crossed electric and magnetic fields would be comparable to the thermal velocity of electrons. The electron temperature has three different components at a nonzero effective collision frequency and two different components in the limit when the collision frequency tends to zero. It is shown that, in the presence of ionization collisions, the components of the heat flux have additives that are not related to the temperature gradient, and arise because of the electron drift.

Features of HF Radio Wave Attenuation in the Midlatitude Ionosphere Near the Skip Zone Boundary

We briefly describe the history of studying the decameter radio wave attenuation by different methods in the midlatitude ionosphere. A new method of estimating the attenuation of HF radio waves in the ionospheric F region near the skip zone boundary is presented. This method is based on an analysis of the time structure of the interference field generated by highly stable monochromatic X-mode radio waves at the observation point. The main parameter is the effective electron collision frequency νeff, which allows for all energy losses in the form of equivalent heat loss. The frequency νeff is estimated by matching the assumed (model) and the experimentally observed structures. Model calculations are performed using the geometrical-optics approximation. The spatial attenuation caused by the influence of the medium-scale traveling ionospheric disturbances is taken into account. Spherical shape of the ionosphere and the Earth’s magnetic field are roughly allowed for. The results of recording of the level of signals from the RWM (Moscow) station at a frequency of 9.996 MHz at point Rostov are used.

Eigen electromagnetic waves of a coaxial waveguiding structure filled by a non‐uniform dissipative plasma with azimuthal magnetic field

This paper studies the propagation and spatial attenuation of high‐frequency eigen‐symmetric and dipolar electromagnetic waves along a coaxial plasma–metal waveguiding structure that contains a slightly axial and strong radial non‐uniform cylindrical plasma slab in an external azimuthal non‐uniform magnetic field. The influence of such parameters as the effective electron collision frequency, the direct current value producing the external azimuthal magnetic field, parameters that characterize plasma density radial profile, and waveguide geometric parameters on the dispersion, spatial attenuation, and radial field structure of the waves is considered. The regions of waveguiding structure parameters where the electromagnetic wave properties can be effectively controlled are studied and analyzed.

Electrostatic turbulence in electron temperature jumps of the solar atmosphere

We discuss the connection of the formation and properties of solar atmosphere transition region characterized by a steep electron temperature gradient with electrostatic turbulence, which provides a high effective electron collision frequency and a low thermal conductivity of the medium. A simple dependence of the noise electric field in the transition region on the effective collision frequency has been derived. Based on known experimental models of the solar atmosphere, we have estimated the height dependence of the thermal conductivity and the strength of the noise electric fields for a tube with a relatively weak magnetic field passing from the chromosphere into the corona.

Sustaining mechanism and spatial structure of high-density ring-shaped hollow cuspate magnetized rf plasma for low-pressure plasma processing

The ring-shaped hollow cuspate magnetized rf plasma with the hollow trench and magnets has been developed to improve the low-density of conventional hollow cathode discharge under a low-pressure, around 1 Pa. From the point of view of the sustaining mechanism, magnetic characteristics of plasma particles such as electrons and ions have been investigated. The effective electron–neutral collision frequency in the presence of a magnetic field has the magnitude of THz, which is approximately 100 000 times higher than the collision frequency without the magnetic field. The effective electron mean free path in the presence of the magnetic field is less than a micrometer, so that the electrons can collide with neutral atoms more frequently in the ring-shaped groove. It is found that the plasma density at the distance of 5 mm under the hollow groove attains more than 2.5 × 1011 cm−3 at a power density of 0.14 W cm−2 and an argon pressure of 1 Pa. The sustaining mechanism is based both on the huge effective electron collision frequency and the strong magnetic confinement of electrons across the transverse magnetic field in the exit of the hollow groove. The spatial distribution of the plasma density has become uniform at the distance of 70 mm from the electrode surface and the uniformity and the averaged plasma density are 18% and 6 × 1010cm−3. It is indicated that the uniformity of the plasma density is controlled by gas pressure.

Polaron-induced parametric interactions in semiconductors: influence of band nonparabolicity and carrier heating

The modified nonlinear wave dynamics due to band nonparabolicity and carrier heating, arising from parametrically interacting electron–longitudinal optical phonons in a polar semiconductor is studied both theoretically and numerically. Expressions for threshold pump required for the onset of polaron-induced parametric interaction and amplification characteristics are explicitly derived. Polaronic effects along with nonparabolicity are found to be additive and result in tremendous increment in the parametric gain. At smaller magnetic field and moderate carrier concentrations, nonlinearity in energy-dependent effective electron mass and collision frequency, affects threshold and amplification characteristics strongly, which can be utilized for the construction of optical switches.

Nanofilm thickness measurement by resonant frequencies

We report a theoretical investigation of monochromatic laser light – thin metal film interaction. The dependences of transmission, reflection and absorption coefficients of an electromagnetic wave on the incidence angle, layer thickness and effective electron collision frequency are obtained. The above coefficients are analysed in the region of resonant frequencies. The resulting formula for the transmission, reflection and absorption coefficients are found to be valid for any angles of incidence. The case of mirror boundary conditions is considered. A formula is derived for contactless measurement of the film thickness by the observed resonant frequencies.

Excitation of surface waves by a short laser pulse in a conductor

We have studied the possibility of exciting surface waves in a conductor, which is irradiated by a focused femtosecond laser pulse incident along the normal to the surface. The time-dependent ponderomotive force is shown to lead to the excitation of surface waves in the terahertz frequency range. It is also shown that the total energy and the pulse amplitude of the surface waves increases with increasing effective electron collision frequency.