Ion acoustic parametric decay instability in a plasma with β > 1

It is found that in a plasma with β > 1 a finite amplitude ion acoustic wave is unstable and decays into two shear Alfvén waves propagating in opposite directions. The threshold and growth rate of the instability are determined.     

Observation of the decay instability of a beam plasma wave

The spontaneous decay of a beam plasma wave into an ion acoustic and an electron plasma wave has been observed during the interaction of a monoenergetic electron beam with a helium and a neon plasma.     

Measurement of thomson and rayleigh scattering in a preionizationZ-pinch

A 90° scattering experiment with a preionizationZ-pinch is described. The first part describes conventional measurements of the electron temperature and density (Thomson scattering). In the second part of the experiment Rayleigh scattering from excited hydrogen atoms was observed during very early phases of the discharge. This observation is possible because the Rayleigh scattering cross section of hydrogen atoms in the second and third quantum states is very large because of the small difference in the energy of the photons ofH _α (6563 Å) and the light of a ruby laser (6943 Å). Spectral resolution of this part of the scattered light yields a minimum value for the ion temperature.     

Measurements of nonlinear growth of ion-acoustic waves in two-ion-species plasmas with thomson scattering

We report the first Thomson-scattering measurements of the growth of ion-acoustic waves in well-characterized multi-ion-species plasmas consisting of gold and beryllium. We observe that only the berylliumlike mode grows, verifying linear kinetic theory. In addition, a twofold increase in ion temperature is measured when ion-acoustic waves are excited to large amplitudes by stimulated Brillouin scattering (SBS). This increase in ion temperature is a strong indication of hot ions due to trapping. We explain the measured SBS reflectivity by nonlinear detuning of the SBS instability due to these trapping effects.     

Observation of spontaneous parametric four-photon scattering in aqueous solutions of electrolytes

We have observed spontaneous parametric four-photon scattering in electrolytic solutions (alkali halides and alkaline earth halides in water). The spectral, temporal, and spatial intensity distributions of the scattered light have been measured. Using a Q-switched ruby laser as a pump, and a CuSO₄-solution as a filter we observed signal wavelengths in the range from 350–550 nm with a maximum intensity at 480 nm for all of the investigated solutions. This radiation is emitted into a cone with a half-angle of roughly 60 mrad. In comparison to pure water the scattered radiation in electrolytes has a much higher intensity. Most of the experimental results confirm predictions of the theory of spontaneous four-photon scattering.Parts of this paper were reported at the 2nd. International Conference Laser und ihre Anwendungen, Dresden, 1973.     

On the development time of the parametric instability of polariton-polariton scattering in a planar semiconductor microcavity

It has been shown that the development of the threshold instability of polariton-polariton scattering in a planar semiconductor microcavity under the coherent pulsed excitation above the lower polariton branch requires a minimum pulse length on the order of dozens of picoseconds, needed for the population of polariton modes in the regions of the parametric instability to increase from the noise level to a certain value comparable with the population of the driven mode.     

Experimental evidence of the effect of heat flux on thomson scattering off ion acoustic waves

Thomson self-scattering measurements are performed in a preionized helium gas jet plasma at different locations along the laser propagation direction. A systematic and important variation of the intensity ratio between the blue and the red ion spectral components is observed, depending on whether the location of the probed region is in front of or behind the focal plane. A simple theoretical calculation of Thomson scattering shows that this behavior can be qualitatively understood in terms of a deformation of the electron distribution function due to the return current correlated with the classical thermal heat flux.     

Excitation of low-amplitude solitons in systems with decay instability

A nonlinear equation for the interaction of a pair of opposite-traveling waves with a normally incident decay mode has been derived. It is shown that solutions of the equation are low-amplitude solitons of a new type.     

Non-convective parametric instability associated with whistler wave resonance cone

Experimental results are presented which show that under certain conditions typical of laboratory and ionspheric plasmas, the whistler wave parametric instability is restricted in space to regions of large gradients in amplitude of the pump wave.     

Saturation of the two-plasmon parametric decay instability in experiments on the electron cyclotron plasma heating due to the extraordinary pump wave depletion

The saturation level of the low-threshold parametric two-plasmon instability of the extraordinary pump wave decay to two upper hybrid plasmons is analyzed under conditions when the only efficient saturation mechanism is the pump depletion. A closed system of differential equations describing both the instability excitation and saturation is derived. The system is solved numerically and an analytic expression is obtained for the anomalous absorption coefficient of the pump wave caused by the development of this instability. The saturation level of the two-plasmon decay instability and the related anomalous absorption efficiency are estimated from data obtained in experiments on the electron cyclotron resonance heating of the plasma by an extraordinary wave in the TEXTOR tokamak.     

Observation of the neutron radiative decay

The aim of this work is the experimental observation of and research into a rare neutron mode, the radiative beta decay, where a new particle, the radiative gamma quantum, is formed along with the expected decay products: a beta electron, a recoil proton, and an antineutrino. The discovery of this rare neutron decay mode was conducted through identification of triple-coincidence events: simultaneous registration of a beta electron, a proton, and a radiative gamma quantum. The ordinary neutron decay was registered by double coincidences of a beta electron and a recoil proton. The statistics collected allow one to deduce the branching ratio (BR) BR = (3.2 ± 1.6) × 10⁻³ (90% C.L.) in the gamma energy region greater than 35 keV. This value of BR is consistent with standard electroweak theory. The text was submitted by the authors in English. An erratum to this article is available at .     

PIC modeling of the parametric instability of magnetoactive plasma

The development of parametric instability in a magnetoactive medium with external pumping is studied numerically. Calculations reveal active decay processes and the anomalous transverse heating of the plasma at late (kinetic) stage of instability.     

Nonlinear parametric instability of wind turbine wings

Nonlinear rotor dynamic is characterized by parametric excitation of both linear and nonlinear terms caused by centrifugal and Coriolis forces when formulated in a moving frame of reference. Assuming harmonically varying support point motions from the tower, the nonlinear parametric instability of a wind turbine wing has been analysed based on a two-degrees-of-freedom model with one modal coordinate representing the vibrations in the blade direction and the other vibrations in edgewise direction. The functional basis for the eigenmode expansion has been taken as the linear undamped fixed-base eigenmodes. It turns out that the system becomes unstable at certain excitation amplitudes and frequencies. If the ratio between the support point motion and the rotational frequency of the rotor is rational, the response becomes periodic, and Floquet theory may be used to determine instability. In reality the indicated frequency ratio may be irrational in which case the response is shown to be quasi-periodic, rendering the Floquet theory useless. Moreover, as the excitation frequency exceeds the eigenfrequency in the edgewise direction, the response may become chaotic. For this reason stability of the system has in all cases been evaluated based on a Lyapunov exponent approach. Stability boundaries are determined as a function of the amplitude and frequency of the support point motion, the rotational speed, damping ratios and eigenfrequencies in the blade and edgewise directions.     

Snake instability of one-dimensional parametric spatial solitons

We show that one-dimensional parametric spatial solitons undergo temporal instability that leads to their breakup into spatiotemporal patterns with a characteristic snakelike shape.     

Quenching of the nonlocal electron heat transport by large external magnetic fields in a laser-produced plasma measured with imaging thomson scattering

We present a direct measurement of the quenching of nonlocal heat transport in a laser-produced plasma by applying large external magnetic fields (>10 T). The temporally resolved Thomson-scattering measurements of the electron temperature profile show that the heat front propagation transverse to a high-power laser beam is slowed resulting in extremely strong local heating. We find agreement with hydrodynamic modeling when including a magnetic field model that self-consistently evolves the fields in the plasma.