Effect Of Electron-neutral Collisions Research Articles

Terahertz radiation generation by beating of two oblique arbitrary laser beam profiles in a collisional plasma

This paper presents a scheme to achieve THz radiation by the beating of two laser beams in a plasma. Lasers are obliquely incident on an underdense plasma with density ripples. Lasers having different frequencies and wave numbers but the same electric fields exert a ponderomotive force on the plasma electrons at the beating frequency which creates THz waves. The general formulas for the efficiency and THz field amplitude with arbitrary laser beam profiles (such as super-Gaussian and triangular) are derived where the effect of electron-neutral collisions is taken into account. The results show that the efficiency of THz radiation is sensitive to the angle of incidence, beam profile, collision frequency, and beating frequency. The most striking feature of this paper is that with obliquely incident laser beams, higher efficiency can be reached for collisionless and collisional plasma compared to normal incidence.

Improved model for multipactor in low pressure gas

The effect of electron-neutral collisions on the multipactor threshold in a low pressure gas is analyzed using a model that takes into account the friction force from the electron impacts with the neutrals, the thermalization of the electrons, as well as the contribution from impact ionization to the total number of electrons. It is found that the friction force is most important for first-order multipactor for materials with a low first cross-over energy and leads to an increased threshold. For higher-order modes with the same material, the thermalization effect dominates and results in a lowered threshold. The contribution to the total number of electrons from impact ionization for such materials is insignificant, but for materials with a high first cross-over energy this contribution dominates the behavior and a decreased threshold is obtained for all orders of multipactor.

Multipactor in low pressure gas

The effect of electron-neutral collisions on the multipactor threshold in a low pressure gas is analyzed in the case when the ionization energy of the gas particles is larger than the impact energy of the oscillating electrons. It is found that a higher microwave power is required to initiate multipactor, since the collisions tend to slow down the electrons. It is also found that the phase limits, within which multipactor can occur, are affected by the presence of the low density gas. Transition to the regime of corona discharges is also discussed.

Investigation of the Conditions Required for the Formation of a Magnetic Neutral Loop Discharge Plasma

In order to reveal the conditions required for the formation of a neutral loop discharge (NLD) plasma, distributions of excited atoms that are closely related to the distributions of the ionization rates were measured under various discharge conditions. Measurements of the distributions of these excited atoms were performed by laser induced fluorescence spectroscopy and optical emission spectroscopy. It was found that the results of these measurements were consistent with the theoretically predicted dependencies of electron heating on the magnetic field strength, the radio frequency (RF) electric field, and the frequency of the RF source. In addition, it was shown that the effects of electron-neutral collisions and of the finite size of the device set limitations on the formation of the NLD plasma.

Damping and destabilization of drift waves by electron-neutral collisions

The effects of electron-neutral collisions on drift wave in a plasma confined by a magnetic quadrupole are investigated. A new formula for the growth rate is derived which includes nondetrapping as well as detrapping collisions and is not based on the small ion-Larmor-radius approximation. A numerical study has been made of the fundamental drift mode and the next harmonic which have one and two full wavelengths around a closed magnetic field line, respectively. The more numerous non-detrapping collisions are found to have no significant effect on the growth rate of the fundamental mode observed in the UMIST quadrupole. Electron-neutral collisions dampen the next harmonic drift mode for axial wave numbers of the order of 1 cm-1 but tend to destabilize it at somewhat larger or smaller wave numbers.

Effect of electron-neutral collisions on the satellites of the Thomson scattering spectrum in arc plasmas

The effect of electron-neutral collisions on the high frequency spectrum of laser radiation scattered by the free electrons of a plasma is investigated for a partially ionized H2 are plasma at atmospheric pressure. The calculations are carried out along Gorog's theory solving the linearized Boltzmann equation for electrons with a collision term. The collision integral is approximated by a Krook relaxation model with the collision frequency determined from experimental electron-atom scattering data. The collisions influence size, half width, and position of the high frequency satellites. In a H2 arc plasma, the change of the satellites' position is negligible as well as the change in half width within experimentally attainable error limits. The change of size is of minor importance, since the general evaluation procedure uses only normalized scattering intensities. Thus, for laser scattering experiments in are plasmas the collision-free theory can be applied.

Effects of Electron-Neutral Collisions on Propagation of Electron Plasma Waves

The propagation of electron plasma waves was studied experimentally in the regime where electron-neutral collisions dominate. It was found that the effects of collision not only enhance the damping of the wave, but also increase the wave number for a given frequency. These results are compared with some theoretical models.