Electron-neutral Collisions Research Articles

Background pressure effects on ion dynamics in a low-power magnetic nozzle thruster

The effect of background neutral density on the ion acceleration region of a magnetic nozzle thruster is investigated through a combination of theoretical models and experiment. Ion velocities are measured in the magnetic nozzle using laser induced fluorescence as the background pressure is raised from 0.98 to 26 μTorr-Xe. It is found that the ion velocity exiting the acceleration region decreases from roughly 12 800 to 10 200 m s−1 as background pressure is increased corresponding to a drop in accelerated ion energy of 37%. Global models are employed to determine discharge properties and to calculate the power consumed by collisions in the plume. It is found that charge exchange collisions in the plume cannot explain the decrease in performance. As an alternative, inelastic electron–neutral collisions in the plume are proposed as the mechanism responsible for the decrease in ion energy, consuming between 1.6% and 39% of the power incident on the plume as background pressure is increased over the range tested. The findings suggest that facility effects can significantly alter plume dynamics and thruster performance even at relatively low background pressures. However, with the validated global models it is possible to make predictions of on orbit ion velocities using data taken at finite background pressures.

Generalized treatment of skew-cosh-Gaussian lasers for bifocal terahertz radiation

A generalized treatment of skew-cosh-Gaussian lasers with skewness parameters n and s is provided for obtaining bifocal terahertz radiation in a realistic plasma having electron-neutral collisions. In order to find and control the focusing of the radiation, a relation is established between the position of the peak of the radiation field and the parameters n and s. The significant role of these parameters in getting bifocal radiation or its conversion to unifocal radiation (or vice-versa) with a proposal for enhanced efficiency of the scheme is discussed in detail for its medical applications.

Evidence for neutrals carrying ion-acoustic wave momentum in a partially ionized plasma

An experimental study of Ion Acoustic (IA) wave propagation is performed to investigate the effect of neutral density for Argon plasma in an unmagnetized linear plasma device. The neutral density is varied by changing the neutral pressure, which, in turn, allows the change in ion-neutral, and the electron-neutral collision mean free path. The collisions of plasma species with neutrals are found to modify the IA wave characteristics such as the wave amplitude, the velocity, and the propagation length. Unlike the earlier reported work where neutrals tend to heavily damp the IA wave in the frequency regime ω<νin (where ω is the ion-acoustic mode frequency and νin is the ion-neutral collision frequency), the experimental study of the IA wave presented in this paper suggests that the collisions support the wave to propagate for longer distances as the neutral pressure increases. A simple analytical model is shown to qualitatively support the experimental findings.

Radial variation of electron temperature gradient in a double plasma laboratory device

Radial control of the electron temperature gradient is demonstrated in a double plasma device by making use of segmented grid biasing. The plasma produced in the source region is allowed to diffuse into the target region through a single grid as well as through the cassette of multiple grid assembly, under different grid bias conditions. Both electron heating and cooling are observed radially at one location in the target region when a single grid is used. The electrons are cooled down to a temperature of 3.3 eV from 5.1 eV when the grid bias is raised from −25 to 0 V. Similarly, during heating, the electron temperature increases from 4.8 to 7.3 eV when the grid bias is varied between 0 and +20 V. Two different transparencies of grids, 45% transparency (mess-size, m = 0.8 mm ∼ λDe) and 75% transparency (mess-size, m = 2.4 mm > λDe), are used, where the value of λDe ≈ 0.8 mm. The obtained electron energy distribution function suggests that a grid with less transparency is more effective in cooling the electrons because of insignificant energetic electron–neutral collisions in the target region as a sheath in the close vicinity of grid allows only the high energetic electrons to pass through it. The higher transparent grid, on the other hand, produces electron heating as it exerts a negligible influence on the free flow of accelerated high energy electrons to target plasma due to insignificant thermalization. We expanded this concept and, for novelty, applied it to a radially segmented grid assembly of electrically isolated grids, for effectively charging different plasma regions with differently various potentials for exerting a radial control on electron temperature. The results obtained show that a significantly sharp electron temperature gradient is obtained with a typical gradient scale length of LTe∼10 cm in the target plasma region. The outcome of this study may be useful both in plasma processing applications and for studying plasma turbulence in unmagnetized plasmas.

Experimental investigation of electron heating modes in capacitively coupled radio-frequency oxygen discharge

A Langmuir probe has been used to investigate electron heating mechanisms in a capacitively coupled oxygen discharge over a wide pressure range (50–800 mTorr) at a fixed applied power (200 W). Evidence presented here from experimentally obtained electron energy distribution functions (EEDFs) illustrates discharge transition from a collisionless (stochastic) to a collisional (Ohmic) dominant regime with increasing oxygen pressure. The discharge exhibited a bi-Maxwellian EEDF in the collisionless regime, dominated by stochastic heating whereas Druyvesteyn-like EEDFs were exhibited in the collisional dominant regime. Moreover, in the transition between these two regimes, parameters such as electron density, effective electron temperature and electron-neutral collision frequency exhibited significant variations.

DETERMINATION OF DISCHARGE GAP CONDUCTION IN THE PLASMA SWITCH

Operational analysis and the obtained results are presented for nanosecond pulsed current/voltage generators involving use of plasma switches, shortly PCS-PCI. The electrical conduction in the plasma-filled discharge gap was determined by calculations versus the plasma electron temperature, the concentration of multicharged ions and their ionization state number. Some factors affecting the conduction value of the PCS-PCI discharge gap have been established, such as the cross-section for electron scattering by the intrinsic atomic field and the electron-neutral collisions, which lead to early reduction in the PCS-PCI discharge-gap conduction before the onset of the current cut off phase.

Modification of the Coulomb Logarithm due to Electron-Neutral Collisions.

We demonstrate that in partially ionized plasmas, Coulomb scattering can be significantly perturbed by electron collisions with neutral gas particles, and that this effect can be incorporated in the Coulomb collision terms of the Boltzmann equation by a modification of the classical Coulomb logarithm. We show that Boltzmann transport calculations using this modified Coulomb logarithm are in excellent agreement, for a sensitive model problem and a wide range of conditions, with particle simulations describing the many-body Coulomb interactions from first principles.

Role of laser absorption and equation-of-state models on ns laser induced ablative plasma and shockwave dynamics in ambient air: Numerical and experimental investigations

Ablative plasma and a shock wave (SW) in ambient air were experimentally produced using Nd:YAG laser pulses of ∼7 ns width and a wavelength of 532 nm. The numerical simulations of the experiments were performed using a two-dimensional axis-symmetric radiation-hydrodynamics code. The numerical approach to simulate the experimental observations was not straightforward due to the complex behavior of the laser-air interaction and the associated processes, such as plasma formation and SW evolution, that occur concurrently. Hence, the modeling was attempted based on the combination of two laser absorption coefficients and two equations-of-state (EOSs). One form of absorption coefficient was taken from Zel'dovich and Raizer [Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena (Dover Publications/Academic Press Inc., New York, 2012)], which is the sum of photoionization and inverse bremsstrahlung (IB) due to electron-ion collisions, and the other was taken from DeMichelis [IEEE J. Quantum Electron. 5(4), 188 (1969)] that considers the IB due to electron-ion and electron-neutral collisions. Similarly, the two EOSs, namely the ideal gas EOS and the chemical equilibrium application [S. Gordon and B. J. McBride, NASA Ref. Publ. 1311, 1 (1994)] EOS, are considered. The simulated results obtained using four models were compared with each other and with the experimental observations. These models enabled understanding the transient behavior of the laser-induced air plasma and the SW evolution. The results showed that the absorption coefficient and the EOS play a key role in modeling the dynamics of air plasma and SW. We present the results of this study and the models which validate the experimental results the best in terms of the asymmetric plasma expansion, formation of hot spots, plasma splitting and rolling, SW external dynamics such as the transition from a tear-drop to a spherical shape, and shock front velocity.

Numerical model for the determination of the reduced electric field in a CO2 microwave plasma derived by the principle of impedance matching

Three dimensional electromagnetic modelling of a free-standing CO2 microwave plasma has been performed, by describing the plasma as a dielectric medium. The relative permittivity and conductivity of the medium are parametrised. The waveguide geometry from the experiment, including the tuner, is put into the model, knowing that this corresponds to maximum power transfer of the microwave generator to the plasma under plasma impedance matching conditions. Two CO2 plasma discharge regimes, differing mainly in pressure, input power and temperature, have been studied. The model’s validity has been checked through study of materials of known conductivity. From measurements of the neutral gas temperature and the plasma electron density profile, the reduced electric field is determined. From the parametrisation of the dielectric properties, a range for the effective electron-neutral collision frequency for momentum transfer is estimated. The results for the reduced electric field and the range of the electron neutral collision frequency obtained, are consistent as verified by simulations using BOLSIG+. In addition, from this comparison it is possible to narrow down the range of the collision frequencies, and to estimate the electron temperature. The reduced electric field lies between 80 and 180 Td for the relatively low pressure, low input power, the so-called ‘diffuse’ regime. For the relatively high pressure, high input power (‘contracted’) regime it lies between 10 and 60 Td. The normalised collision frequency lies between 1.6 and 2.3 for the diffuse regime, while for the contracted regime it lies between 2 and 3. The electron temperature ranges from 2 to 3 eV for the diffuse regime, and from 0.5 to 1 eV for the contracted regime.

Modeling of intense terahertz wave generation with controlled field distribution

We report the results of a theoretical model and numerical investigations for an efficient intensity distribution-tunable terahertz (THz) radiation generation by two color Hermite cosh Gaussian lasers pumped in periodic density gas plasma. The effect of various laser parameters like b (decentered parameter of the cosh function), s (mode index of the Hermite function), δ (initial phase difference between two lasers), and νen (electron neutral collision frequency) on the THz field distribution, amplitude, and efficiency is investigated. The largest peak of the THz field is obtained for s=1, b=0, δ=0, π, 2π, and ω≈ωp (resonant excitation) at x=0 (forward emission in the direction of the laser propagation axis). It is found that THz radiation is highly intense for s=1, b=0, νen=0, and ω≈ωp compared to any other set of laser and plasma parameters. This study also reveals that the THz peak field and efficiency decrease monotonically with electron collision frequency. The THz field of ∼G V m−1 with an efficiency of ∼3% for the optimum laser and plasma parameters can be obtained in this scheme.

Dark hollow beams originating terahertz radiation in corrugated plasma under magnetic field

In the present process, two laser beams having the same doughnut profiles but different frequencies are explored in space-periodic nonlinear plasma to produce nonlinear oscillatory current that resonantly excites the terahertz (THz) radiation. The interactions are assumed to be under the effect of external magnetic field and electron-neutral collisions. The intensity gradient of the considered dark hollow beams has a doughnut-shaped distribution, which is responsible for the multifocal field profile of the emitted THz radiation. The present scheme is capable of producing laser-to-THz energy conversion efficiency up to ∼10−3 with the optimization of various laser and plasma parameters even in the presence of electron-neutral collisions.

Temperature-dependent instabilities in weakly ionized dusty plasmas

Considering the effect of self-gravity and gravitational field of dust particles on electrons and ions, instabilities of a weakly ionized dusty plasma in the presence of an external constant electric field are investigated. It is assumed that ionization and electron-neutral collision frequencies and recombination coefficient depend on electron temperature. Taking into account these assumptions, it is shown that most of the instabilities investigated are stipulated by the sign change of derivative of kinetic coefficients with respect to electron temperature. Comparing the obtained results with the results of previous works, it is found that the presence of the dust particles causes the appearance of new instabilities or modifies the growth rate of the plasma instabilities under different conditions.

A transmission line model of the cutoff probe

The previously proposed circuit model has proven to be a powerful tool to analyze and improve the characteristics of the cutoff probe. This model revealed the mechanism of formation of the transmission microwave frequency spectrum of the cutoff probe, and allows for a more precise measurement of electron density by taking into account electron-neutral collision frequency and sheath width. However, a recent study has found that there is a discrepancy between the circuit model and 3D full E/M wave (EMW) simulation, in that the circuit model is applicable to the measurement of low-density plasma but not high-density plasma; this indicates that the basis of the circuit model for the cutoff probe seems to be incomplete (Kim et al 2015 2015 KPS Fall Meeting). To resolve this problem, the current paper proposes a new model of the cutoff probe, named the transmission line (TL) model, based on TL theory. Results of the TL model are compared with EMW simulation, with the TL model found to reproduce the results of EMW simulation in both high-density as well as low-density plasma.

Frequency dependence of plasma characteristics at different pressures in cylindrical inductively coupled plasma source

The effects of driving frequency on plasma parameters and electron heating efficiency are studied in cylindrical inductively coupled plasma (ICP) source. Measurements are made in an Ar discharge for driving frequency at 13.56/2 MHz, and pressures of 0.4–1.2 Pa. In 13.56 MHz discharge, higher electron density (ne) and higher electron temperature (Te) are observed in comparison with 2 MHz discharge at 0.6–1.2 Pa. However, slightly higher ne and Te are observed in 2 MHz discharge at 0.4 Pa. This observation is explained by enhanced electron heating efficiency due to the resonance between the oscillation of 2 MHz electromagnetic field and electron-neutral collision process at 0.4 Pa. It is also found that the variation of Te distribution is different in 13.56 and 2 MHz discharge. For ICP at 13.56 MHz, Te shows an edge-high profile at 0.4–1.2 Pa. For 2 MHz discharge, Te remains an edge-high distribution at 0.4–0.8 Pa. However, the distribution pattern involves into a center-high profile at 0.9–1.2 Pa. The spatial profiles of ne remain a center-high shape in both 13.56 and 2 MHz discharges, which indicates the nonlocal kinetics at low pressures. Better uniformity could be achieved by using 2 MHz discharge. The effects of gas pressure on plasma parameters are also examined. An increase in gas pressure necessitates the rise of ne in both 13.56 and 2 MHz discharges. Meanwhile, Te drops when gas pressure increases and shows a flatter distribution at higher pressure.

A kinetic model for investigating the dielectric properties of rocket exhaust dusty plasmas

The electron kinetic model of rocket exhaust dusty plasma is developed based on the Boltzmann equation. Additional electron-dust elastic and inelastic collision processes are included in the kinetic equation except for the electron-neutral collisions. The dust surface potential and electron density are calculated according to the dust charging balance equation and the quasineutrality condition. The electron energy distribution function (EEDF) is calculated by solving the kinetic equation numerically. It shows that the obtained EEDF results for different dust densities deviate from the Maxwellian distribution obviously. In addition, the dielectric properties of rocket exhausts based on the obtained non-Maxwellian EEDFs are analyzed for different dust and plasma parameters. It is shown that the relative permittivity based on the EEDFs obtained from the kinetic model is larger than that based on Maxwellian distribution, while for the conductivity and attenuation constant, they become smaller. As for the influence of dust particles on the dielectric properties, a high ratio of dust and neutral density (10−9) leads to a smaller absolute value of relative permittivity, electrical conductivity, and attenuation constant for both Maxwellian distribution and EEDF obtained from the kinetic model. When the ratio is low (10−10), the dust particles almost have no influence on the dielectric properties. Meanwhile, it can be seen that the existence of dust particles makes the difference in dielectric properties obtained from the calculated EEDFs and Maxwellian distribution smaller.

A Time-Domain Impedance Probe for Fast Measurements of Electron Plasma Parameters in the Ionosphere

A new time-domain impedance probe is presented in this paper. The new instrument is able to make measurements of absolute electron density and electron-neutral collision frequency in the ionosphere at temporal and spatial resolutions not previously attained. A single measurement is made in $100~\mu \text{s}$ , which yields an instantaneous spatial resolution of 0.1 m for sounding rocket experiments. A prototype of this instrument was integrated into the payload of a NASA Undergraduate Student Instrument Program sounding rocket launched out of Wallops Island on March 1, 2016. Here, we describe the instrument, and present the data obtained from the sounding rocket experiment. A 6-V amplitude Gaussian derivative excitation was applied to a dipole probe structure, and the current through the probe terminals measured with a balanced active bridge circuit. The time-domain current response was sampled at 5 MS/s, at 12-bit resolution. In the course of the flight, the instrument measured a highly nonlinear response of the plasma because of the large input voltage signal applied. The linear theory cannot explain this response, which obscured interpretation of the data. As a result, we used time- and frequency-domain trend analysis to obtain the variation of electron density over the upleg and downleg of the rocket trajectory. The obtained time and fast Fourier transform trends showed enhanced electron densities in the F layer, which confirmed that the instrument was able to measure the density variations during a significant portion of the flight.

A washer gun plasma system for microwave-plasma interaction experiments.

A washer-gun based plasma system has been developed to enable high power microwave (HPM)-plasma interaction in a system for microwave plasma experiments. The critical pre-requisites of the plasma are density, ne ∼ (1-10) × 1017 m-3, uniformity over a radial extent ≈10 cm and axial extent ≈20-30 cm, an axial density gradient of scale-length Ln ≈ wavelength of HPM, and ambient pressure low enough to maintain electron-neutral collision frequency much less than plasma frequency. The system developed deploys a ten stage pulse forming network, discharged to the washer-gun to produce pulsed (τpulse ∼ 100 μs) discharges that get ejected into an experimental chamber. The system is capable of generating ne ∼ 1018 m-3 and Te ∼ 10 eV. Temporal and spatial regimes are identified to obtain the required extents of radial and axial ne uniformity of 10 cm and 20 cm, respectively, and a steep axial gradient Ln ≈ 10 cm. Based on the desired frequency of the interacting HPM (in the range 3-5 GHz) planned for a particular experimental campaign, the density and spatial density profiles of the plasma can be tailored. The present paper presents an account of the plasma source and characterization of the plasma.

Surface-wave-sustained argon plasma kinetics from intermediate to atmospheric pressure

Surface-wave-sustained discharges (SWDs) can operate across a wide range of discharge conditions. From low to atmospheric pressure, plasma sustained by travelling electromagnetic wave is strongly non-equilibrium: the electron energy distribution function (EEDF) is non-Maxwellian; the gas temperature Tg (the translational temperature of the heavy particles) is much lower than the electron temperature Te, i.e. . In this paper, kinetic models of surface-wave-sustained argon plasma based on the Boltzmann equation and its momenta are presented for various discharge conditions (intermediate and high gas pressure and various discharge tube radii). The difference between the models is in the energy level diagrams and the elementary processes taken into account for the various conditions. The models give the EEDF, transport and rate coefficients, mean electron energy, electron–neutral collision frequency for momentum transfer, mean power required to sustain an electron–ion pair in the discharge, and densities of considered atomic and molecular species as a function of the electron density ne. Since the surface-wave plasma is spatially inhomogeneous, all these plasma characteristics change along the plasma column in the axial direction as the wave power absorbed by the electrons and electron density change. The goal is to compare the results obtained from the models with the experimental data and to determine their applicability depending on the SWD conditions.

Optimal waveforms for capacitively coupled ionization in nanosecond plasma discharges

Optimal voltage waveforms for capacitively coupled plasma discharges are numerically investigated using a particle-in-cell kinetic model. The focus is on nanosecond time scales where the transient plasma response and charge separation in the plasma can be exploited with precise voltage timing. Square wave and sinusoidal waveforms with a dynamic frequency equal to half of the instantaneous plasma frequency lead to higher plasma density in less time. Using such optimal waveforms, the maximum plasma densities can be increased from 2 to 9 times higher over a standard RF waveform. Conversely, the same density can be achieved in the same duration using half the voltage amplitude as compared to an RF waveform. Plasma density based waveform optimization is applicable in cases where system size is larger than electron mean free path and dominated by electron-neutral collisions.

Particle-in-cell simulation for effect of anode temperature on discharge characteristics of a Hall effect thruster

Propellant gas flow has an important impact on the ionization and acceleration process of Hall effect thrusters (HETs). In this paper, a particle-in-cell numerical method is used to study the effect of the anode temperature, i.e., the flow speed of the propellant gas, on the discharge characteristics of a HET. The simulation results show that, no matter the magnitude of the discharge voltage, the calculated variation trends of performance parameters with the anode temperature are in good agreement with the experimental ones presented in the literature. Further mechanism analysis indicates that the magnitude of the electron temperature is responsible for the two opposing variation laws found under different discharge voltages. When the discharge voltage is low, the electron temperature is low, and so is the intensity of the propellant ionization; the variation of the thruster performance with the anode temperature is thereby determined by the variation of the neutral density that affects the propellant utilization efficiency. When the discharge voltage is high, the electron temperature is large enough to guarantee a high degree of the propellant utilization no matter the magnitude of the anode temperature. The change of the thruster performance with the anode temperature is thus dominated by the change of the electron temperature and consequently the electron-neutral collisions as well as the electron cross-field mobility that affect the current utilization efficiency.