Electron-neutral Particle Collisions Research Articles

Screened Collision-Induced Quantum Interference in Collisional Plasmas

Abstract The effects of neutral particle collisions on the quantum interference in electron-electron collisions are investigated in collisional plasmas. The effective potential model taking into account the electronneutral particle collision effects is employed in order to obtain the electron-electron collision cross section including the total spin states of the collision system. It is found that the collision effects significantly enhance the cross section. In addition, the collision-induced quantum interference effects are found to be significant in the singlet spin state. It is shown that the quantum interference effects decrease with increasing the thermal energy of the plasma. It is also shown that the quantum interference effects increase with an increase of the collision energy

Nonintrusive measurement of ionisation in vegetation fire plasma

Vegetation fires are slightly ionised gaseous medium. Omnipresent alkali metal species in plant's organic structure are the main source of thermally produced electrons in the fires. In the flames, electron-neutral particle collisions dominate other modes of particle interaction. The collision regime absorbs some of the incident energy when the fire is illuminated with electromagnetic waves. The rate of electromagnetic wave absorption in the vegetation fires has implications on the safety of fire-fighters. During wildfire suppression, radio communication blackout at vhf/uhf has been experienced. This may be partly due to thermal ionisation in the fire. In the experiment, the extent of ionisation in vegetation fires is measured using a 2-port vector network analyser. X-band microwaves are caused to propagate combustion zones of eucalyptus bark and guinea grass fires with maximum temperatures of 1114 and 1054 K respectively. Alkali content in the vegetation fuel was different. Measurements show maximum ionisation in flames produced from guinea grass, which had almost twice much potassium as that of eucalyptus bark, to be 2.63 × 10 16 m −3 while that produced in eucalyptus bark flame was 1.46 × 10 16 m −3 .

Estimation of the cusp loss width in negative-ion sources

To estimate loss width in the cusp magnetic field in multicusp negative-ion sources, diffusion of plasma across magnetic field is investigated analytically. The transport process of plasma depends on various plasma conditions. Diffusion coefficients are classified by degrees of plasma ionization. In weakly ionized plasma, where a case of electron-neutral particle collision is dominant as compared with electron-ion collision, there is a possibility that ambipolar diffusion or electron short circuit occurs in plasma. This strongly depends on the conditions of the plasma, the system length, and the wall material. On the other hand, in fully ionized plasma, only ambipolar diffusion occurs automatically due to momentum conservation of electron and ion. In not fully but strongly ionized plasma, plasma diffusion depends on electron-neutral particle collision, ion-neutral particle collision, and electron-ion collision. Being based on the classification of the diffusion coefficients, we derive expressions of loss widths for various plasma conditions.

Electron concentration behind a sphere of an aluminum-magnesium alloy in a hypersonic flow

Results of microwave measurements of the electron concentration and the frequency of electron-neutral particle collisions in the wakes behind models of an aluminum-magnesium alloy flying in air- xenon mixtures at Mach numbers of 10–15 are reported. For comparison, similar measurements for models without and with aluminum decomposition are presented. The burn-up of Mg particles in the wake is found to change the form of the distribution and to increase appreciably the electron concentration in the wake. The attachment coefficients for electrons in the experiment with Al entrainment and in previous studies are shown to be in good agreement.

Production of RF Plasma Using a Magnetic Line-Cusp Field

A large-volume cylindrical rf plasma source with a magnetic line-cusp field has been developed for large-scale plasma processing. In this type of plasma source, a capacitively-coupled 13.56 MHz rf plasma is produced in the presence of a magnetic line-cusp field. Two versions of the plasma source have been designed and tested. The first version has peripheral rf electrodes placed outside the ionization chamber, and is suitable for preparing a large-volume uniform plasma. This plasma source can attain a useful area of uniform plasma over a 30-cm-diameter region within 10% nonuniformity. The second version features doughnut-shaped parallel plate electrodes which form part of the chamber wall and serve as high-current sources. The electron density in such a version is proportional to the rf power, being equal to 4×1010 cm-3 at 400 W. The measured electron-energy distribution function showed a Maxwellian distribution function due to the electron-neutral particle collisions arising from drift motion in the magnetic line-cusp field.

Remarks concerning effective electron-neutral particle collision cross sections in noble gases

Certain statements in an earlier letter require clarification. They will be elaborated upon in the present note.

Minimization of the effective electron-neutral particle collision cross section in partially ionized noble gases

The effective collision cross sections in partially ionized noble gases are shown to possess a minimum, the profile of which creates optimum conditions for the approximate computation of electrical conductivity in all gases.

Penetration of Intense Electromagnetic Waves in Dense Plasmas

The propagation of an intense Gaussian electromagnetic beam has been studied in a dense plasma when the frequency of the wave (ω) is less than the plasma frequency (ωp). An expression has been obtained for the nonlinear dielectric constant of the medium taking into account the space charge field effects and the appropriate collision mechanism. It is found that the transmittivity of waves increases with the increase in power of the beam for both the electron- ion and electron-neutral particle collision dominated plasmas. At very high power of the beam all waves with ω<ωp penetrate by equal amount. This effect appears at relatively low powers in the electron-ion collision dominated plasma.

Boltzmann-Fokker-Planck model for the electron distribution function in the Earth's ionosphere

Using Boltzmann-Fokker-Planck methods and the diffusion approximation, we derive coupled non-linear equations for the first two angular moments of the electron distribution function in the Earth's ionosphere. The theory includes a phenomenological treatment of photionization of the neutral species by an externally produced photon flux; electron-ion recombination; electronneutral particle attachment; elastic, excitation, deexcitation, and ionizing electron-neutral particle collisions; and elastic electron-electron and electron-ion collisions. At high and low energies, we obtain approximate analytic solutions for the steady-state electron distribution function. Under certain conditions we also obtain the standard continuous slowing-down formulae for the steady-state electron distribution function in the diffusion approximation.

Second harmonic generation using spatially varying static electron number density in a magnetoplasma

The Boltzmann transfer equation for electrons in an r.f. discharge plasma immersed in a stationary magnetic field is solved for the various coefficients of the expansion of the distribution function, expanded in a Taylor series in velocity space. Assuming a spatial variation of static electron number density as the mechanism of harmonic generation, explicit expressions for the inner field and the second harmonic current density are derived. The r.f. electric field is assumed to be spatially uniform. The geometry of the plasma considered is that of a rectangular waveguide, but with the parallel plates of one of the two pairs of the metal plate boundaries of the plasma much more closely spaced than those of the other pair. Cyclotron resonance is studied in a situation where the frequency of electron-neutral particle collisions v is much less than the frequency of the r.f. field ω/2π. The resonance effect is obtained, and is found to be more pronounced at the cyclotron frequency ω = ωc than at 2ω = ωc. For v≫ω/2π, the result is not sensitive to the value of the stationary magnetic field, and the resonance effects are absent. The effect of collisions on the process of harmonic generation is also studied.

On the effect of electron-neutral particle collisions upon the refraction of high-frequency radio waves by the lower atmosphere and ionosphere of Mars

On the effect of electron-neutral particle collisions upon the refraction of high-frequency radio waves by the lower atmosphere and ionosphere of Mars

Radiation from Linear Resonant Antenna in Weakly Ionized Plasma†

ABSTRACT In un earlier work of the author. called part I of the series of studies relating to the radiation from antenna immersed in weakly ionized plasma, a linear resonant centre-fed antenna was used as source. In the present work. called part II of the series, a thin, linear, non-resonant end-fed antenna is investigated under similar conditions. A theoretical study of its radiation resistance for the electromagnetic and electro-acoustic waves is made, taking the propagation constant as complex for both these modes. Also the effect of electron neutral particle collision is taken into consideration wherever effective. It is shown that the effect of plasma is more profound when the source frequency is lower than the plasma frequency. But at frequencies higher than the plasma frequency the effect is found to be not of much consequence. General expressions of radiation resistance for both the electromagnetic and the electro-acoustic waves have been obtained, and from these the behaviour of half-wave linear ...

Collision frequencies and energy transfer electrons

A study is made of the problem of elastic collisions and energy transfer between gases which have separate Maxwellian velocity distributions. It is shown that the expression for the energy transfer rate obtained by Desloge (1) for gases of arbitrary temperature and particle mass can be adapted into a convenient form which involves a ratio of particle masses, the difference in the gas thermal energies, and a collision frequency for energy transfer. An analysis is then made of the collision frequency in terms of an average momentum transfer cross section which is defined for conditions of thermal nonequilibrium. The general equations are next specialized to consider the problem of elastic electron collisions in heavy particle gases. To obtain useful numerical expressions for electron-neutral particle collision frequencies and energy transfer rates, an analysis has been made of the momentum transfer cross sections for N 2, O 2, O, H and He. Calculations have also been made of the Coulomb momentum transfer cross section, collision frequency, and energy transfer rate.

The effective electron collision frequency in the lower F region of the ionosphere

Heating of electrons due to additional energy input into the ionosphere has been considered. The electron temperature profiles for various energy inputs into a model ionosphere were calculated. Using temperature profiles thus obtained, the electron collision frequencies were calculated. It was shown that electron-ion collisions are comparable to electron-neutral particle collisions and cannot be ignored in the lower F region. In view of the large positive ion collision cross section and its temperature dependence, the possibility of ionospheric cross modulation of radio cosmic noise was discussed.

Measurement of Hall Electromagnetic Force in Air

An experiment is reported in which an induced Hall electromagnetic force in air is measured in a shock tube. Agreement is found with theory for values of the product of the electron cyclotron frequency and the mean collision time for electrons over a gas temperature range of 3000° to 4200°K. The theory is based upon a mean collision time for electrons that is determined solely by electron-neutral particle collisions, and an Ohm's law that is valid for slightly ionized gases. The induced Hall electromagnetic force is theoretically independent of the electron concentration of the shock-heated air. The experiment thus provides a technique for determining the collision time that does not depend upon the establishment of equilibrium. Details of the time profile of the measured emf are explained qualitatively using an equivalent circuit.

Nonlinear Interaction of an Electromagnetic Wave with a Plasma Layer in the Presence of a Static Magnetic Field. I. Theory of Harmonic Generation

The theory of electromagnetic wave propagation through an anisotropic ionized layer, including the effects of the nonlinear terms in the Boltzmann transport equation, is presented. The method of solution of the nonlinear equations involves an expansion of all of the dependent variables in a Fourier series in time. The differential equations describing wave propagation are then solved, for each frequency in the series, for plane wave propagation, including all of the reflections within the plasma layer. A solution in closed form has been obtained, under small signal conditions, for the field at the $h\mathrm{th}$ harmonic in the Fourier series. A discussion of the properties of the wave at the second harmonic frequency as a function of the dc magnetic field strength, the electron density, the electron-neutral particle collision frequency, the field strength of the incident wave, and the thickness of the plasma layer is given.