Particle Density Fluctuations Research Articles

Ambipolar transport noise in a semiconductor in the presence of a magnetic field

The noise spectrum due to fluctuations in the electron-hole pair particle density in a semiconductor in the presence of a magnetic field is presented in this paper. This spectrum was obtained via the Green's function for the macroscopic differential equation which governs the behavior of the conditional average for fluctuations in the electron-hole pair particle density when ambipolar transport and generation-recombination are the important noise processes. The macroscopic differential equation is a generalization of van Roosbroeck's small-signal continuity equation to include the presence of a magnetic field. The resultant spectrum was evaluated numerically and a family of curves corresponding to different values of magnetic field strength generated. This was done for specified values of τ/τa and τ/τd, where τ is the mean volume lifetime of electron-hole pairs, τa is the characteristic ambipolar drift time, and τd is the characteristic ambipolar diffusion time. In the extreme situations when ambipolar drift is important and diffusion is negligible and vice versa, the obtained numerical results are consistent with those obtained previously by the author. For the case treated in this paper when both ambipolar drift and diffusion are equally important, the ``universal 3/2 power law'' still prevails in the high-frequency portion of the spectrum, thus indicating the dominance of ambipolar diffusion in that region of the spectrum.

Fluctuations in Plasma and Nonlinear Susceptibilities

The nonlinear electromagnetic properties of plasma are considered and it is shown, that the nonlinear plasma susceptibilities may be expressed in terms of higher order correlation functions for the charge density fluctuations of noninteracting particles. The generalization of the fluctuation-dissipation theorem for nonlinear electrodynamic systems is obtained.

Ambipolar Drift Noise in a Semiconductor in the Presence of a Magnetic Field

The noise spectrum due to fluctuations of electron-hole pairs in a semiconductor in the presence of uniform and time-independent electric and magnetic fields is presented in this paper. This spectrum is obtained via the Green's function for the macroscopic differential equation which governs the behavior of the conditional average for fluctuations in the electron-hole pair particle density when generation-recombination and ambipolar drift are the only important processes. For the general situation when the electric and magnetic fields are not collinear, the obtained spectrum is a complicated function of both frequency and magnetic field for a fixed value of electric field. From this spectrum the spectra corresponding to the special cases when the electric field is parallel and perpendicular to the magnetic field are obtained. Since the case when the electric field is perpendicular to the magnetic field is the most interesting from an experimental viewpoint, it is discussed in more detail and illustrative plots of the spectrum are presented.

Influence of Collisions on the Ion-Acoustic Instability in a Weakly Ionized Plasma

The influence of collisions on the onset and linear stage of the ion-acoustic instability in a dc-discharge in Helium is studied experimentally. The threshold values of the drift velocity and frequency and the range of the unstable frequency spectrum are in good agreement with the values calculated from the dispersion relation based on the Vlasov equation with inclusion of ionneutral collision. The intensity of charged particle density fluctuation has been measured over 4 orders of magnitude covering the range from thermal level to the fully developed instability.

Laser Interferometry as a Measure of Turbulent Fluctuations

The power spectral density of the integrated neutral particle density fluctuations in the turbulent exhaust of a sonic jet has been measured using a laser interferometer. The measurements are compared with calculations of the power spectral density function.