Branch Oscillations Research Articles

Performance of a transversely excited pulsed HF laser

Operation of a pulsed HF laser with unique spectral and temporal behavior is reported. <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</tex> -branch oscillation in the 1-0 band and peak multiline powers of 700 kW in 200-ns pulses have been observed. The gain of the 75-cm laser was measured to be 235 dB.

Excitonic spectrum of a nonequilibrium electronic solid state plasma

The transversal electromagnetic oscillations and waves (excitons) in a homogeneous electronic solid state plasma, interacting nonlinearly with strong external electric and magnetic fields are investigated. Both the conductivity and complex dielectric constant tensors are determined. Investigation of the dispersion equation, describing the excitons in the electronic solid state plasma, shows the existence of both damping and nondamping oscillations. The spectra and the damping (amplification) of the electromagnetic waves are also determined. It is shown that the nondamping branch of oscillations is related to the occurrence of a negative differential conductivity, resulting from the nonlinear interaction of the nonequilibrium electronic solid state plasma with the external fields.

Kinetic instability of flute oscillations

The author considers the effect of resonance particles drifting under the influence of magnetic field inhomogeneity with a velocity close to the phase velocity of flute oscillations on those oscillations. He investigates the instability of these oscillations at densities below the critical density of the hydrodynamic flute instability Ra/d2 ≪ 1 (R – radius of curvature of the magnetic field lines, a – transverse dimension of the plasma, d – Debye radius).It is shown that, when resonance particles are present, the branch of oscillations with frequency (ω0 – ionic plasma frequency, ωc – ionic cyclotron frequency, k – transverse wave number) is always unstable. For an ion transverse velocity distribution function having the form of a δ-function with an admixture of hot ions, it is shown that the branch of oscillations with a frequency only slightly dependent on density and close to (v0 – mean ion velocity) is also unstable. Both instabilities also occur in the hydrodynamically stable case; i.e. when the magnetic field lines are convex with respect to the plasma.

Surface waves of a magnetoactive plasma

It is proved in the surface potential waves approximation that not oniy a bounded plasma with flows along the magnetostatic field, but also a plasma with a finite ‘longitudinal temperature’ of the electrons can be unstable against axially asymmetric perturbations of magnetohydrodynamic type. The branches of surface oscillations of a cold plasma, and the conditions for their excitation by an admixture of electrons with a sufficiently high ‘longitudinal temperature’ in various geometrical configurations, are also derived in the aper.

ON THE PROPAGATION OF ELECTROMAGNETIC WAVES IN A PLASMA WITH NEGATIVE IONS

Small-amplitude electromagnetic wave propagation has been studied in a collisionless plasma containing electrons and positive and negative ions having a different charge-to-mass ratio. The Chew–Goldberger–Low approximation is used for an unbounded, homogeneous plasma in a uniform magnetic field under the assumption of charge neutrality. The analysis of the dispersion relation shows new branches of oscillations, in the zero-temperature limit as well. The classification of waves is presented using the Clemmow–Mullay–Allis diagrams calculated for several plasmas of interest: the laboratory Cs+ Cl− and Cl+ Cl− plasmas and plasma forming the D layer in the ionosphere. New resonances and stop bands have been found for the right-hand polarized wave and the extraordinary wave in the low-frequency region. The isotropic wave propagation at the crossover frequencies and for degenerate plasma is discussed.The dynamics of particles is discussed and the frequency regions in which the positive and negative ions move in opposite phase are given.

Current instability in an inhomogeneous plasma

The authors investigate the development of instability when a current is passed through a strongly ionized potassium plasma of non-uniform density. The collisionless plasma, having a density of 109 to 1010 cm−3, is subjected to magnetic fields of 400 to 2000 Oe. In accordance with earlier experiments, “universal” instability in the form of drift waves is observed when no current is present. The coefficient of diffusion across the magnetic field is some two orders of magnitude greater than the classical coefficient. When a current is passed through the plasma two oscillation branches are excited: drift and ion-sound waves. With the first branch the oscillation frequency is inversely proportional to the magnetic field and the waves have an azimuthal component, the azimuthal phase velocities of all the harmonics being the same. With the second branch the oscillation frequency is independent of the magnetic field and varies inversely with the length of the system. For practical purposes, the phase velocity of the longitudinal component coincides with the ion-sound velocity. The critical drift velocity of the electrons and of the ions, at which ion-sound waves are excited, is approximately 2 × 106 cm/ sec, an order of magnitude less than the theoretical critical drift velocity for an homogeneous, almost isothermal, plasma. When the current instability is excited, the diffusion coefficient increases by a factor of 1.5 to 2.5, diffusion beginning to increase simultaneously with the increase in oscillation amplitude.

Velocity Space Diffusion from Weak Plasma Turbulence in a Magnetic Field

The quasi-linear velocity space diffusion is considered for waves of any oscillation branch propagating at an arbitrary angle to a uniform magnetic field in a spatially uniform plasma. The space-averaged distribution function is assumed to change slowly compared to a gyroperiod and characteristic times of the wave motion. Nonlinear mode coupling is neglected. An H-like theorem shows that both resonant and nonresonant quasi-linear diffusion force the particle distributions towards marginal stablity. Creation of the marginally stable state in the presence of a sufficiently broad wave spectrum in general involves diffusing particles to infinite energies, and so the marginally stable plateau is not accessible physically, except in special cases. Resonant particles with velocities much larger than typical phase velocities in the excited spectrum are scattered primarily in pitch angle about the magnetic field. Only particles with velocities the order of the wave phase velocities or less are scattered in energy at a rate comparable with their pitch angle scattering rate.

Effect of a cold-plasma component on the stability of an inhomogeneous hot plasma

By applying geometrical optics, the author investigates the effect of a volume component of cold plasma on the low-frequency oscillation spectrum and on the local stability conditions of an inhomogeneous hot plasma contained by a strong magnetic field. Allowance is made for the curvature of the lines of force of the containing magnetic field by introducing an effective gravitational field. The author points to the existence in such a system of a low-frequency oscillation branch, that is “electron” sound, with a frequency proportional to the square root of the ratio of hot to cold plasma densities. For very low cold-plasma densities a hydrodynamic growth of long-wave and short-wave oscillations of the inhomogeneous plasma results from the excitation of an “electron-acoustic” oscillation branch by the gravitational drift of the ions. For higher cold-plasma densities one observes a resonant growth of low-frequency plasma oscillations with frequencies approaching those of the diamagnetic particle drift. Further increases in the cold-plasma density such that the frequency of the electron sound exceeds all the drift frequencies of the plasma lead to an expansion of the stability region of the inhomogeneous plasma.

Oscillations of a Plasma Column

Oscillations in plasma columns exhibit many interesting features not found in infinite uniform plasmas. A study is made on the effects due to retardation of the electromagnetic field which gives rise to a curious behavior of the static mode for long wavelengths and a radiating branch of electrostatic plasma oscillations. The principal aim of this study is to gain an understanding of the physical mechanisms underlying these phenomena. The transition from the finite column to the infinite uniform plasma case is also discussed.

Low-frequectcy plasma oscillations for quasi-transverse propagation

The spectra and damping of some new branches of hot-plasma oscillations propagating perpendicular to or almost perpendicular to the external magnetic field H0 are discussed. Investigation has been performed of (a) an oscillation branch adjoining longitudinal ion-cyclotron oscillations, (b) extraordinary ion-cyclotron and magneto-acoustic waves and (c) ordinary ion-cyclotron and hybrid waves. Phase velocity of these waves along the magnetic field V = ω/k‖ is considerably larger than ion thermal velocity (k‖ is the wave-vector projection in the magnetic-field direction).Waves adjoining longitudinal cyclotron oscillations have a wavelength of the order of the Larmor radius of an ion with thermal velocity and frequencies of the order of ion cyclotron frequency; the phase velocity of these waves along the magnetic field is considerably smaller than thermal electron velocity υe. These waves can be separated from the general dispersion queation for the case of a low-pressure plasma when x ≪ 1, where (n0 is the density; Ti, the plasma ion temperature). For x ≪ cos2 θ ≪1 (θ is the angle between H0 and the propagation direction) this branch changes to longitudinal ion cyclotron oscillations.The dispersion equation for an ordinary wave for V≪ve has two solutions, corresponding to the hybrid wave with frequency ω = ω0 = k‖kVsVA/ωi), where Vs = (Te/mi)1/2, ωi = eH0/mi C and VA = H0/(4πn0)1/2 and the ion-cyclotron wave with ω≈sωi (s = 1, 2,…).For the oscillations pointed out the damping rates which are caused by Cerenkov absorption by plasma electrons and ions and cyclotron absorption by plasma ions have been found. Oscillation excitation by fluxes of low-density charged particles passing through plasma along a magnetic field has also been examined. Cerenkov and cyclotron wave excitation by both a hot and a cold beam, have been investigated.