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.     

On a new scenario for the saturation of the low-threshold two-plasmon parametric decay instability of an extraordinary wave in the inhomogeneous plasma of magnetic traps

The most probable scenario for the saturation of the low-threshold two-plasmon parametric decay instability of an electron cyclotron extraordinary wave has been analyzed. Within this scenario two upperhybrid plasmons at frequencies close to half the pump wave frequency radially trapped in the vicinity of the local maximum of the plasma density profile are excited due to the excitation of primary instability. The primary instability saturation results from the decays of the daughter upper-hybrid waves into secondary upperhybrid waves that are also radially trapped in the vicinity of the local maximum of the plasma density profile and ion Bernstein waves.     

Parametric Mechanism of Anomalous Microwave Power Absorption in Experiments on Electron Cyclotron Heating in Toroidal Magnetic Traps

The nonlinear stage of the parametric decay instability of an extraordinary wave is analyzed in the presence of a nonmonotonic density profile. The decay excites an electron Bernstein wave, which is localized in the vicinity of a local density maximum, and an ion Bernstein wave, which leaves a nonlinear interaction region and is absorbed by ions in the vicinity of the harmonics of the ion cyclotron frequency. The main mechanism of instability saturation is considered to be a cascade of decays of a primary daughter electron Bernstein wave, which leads to the excitation of localized secondary electron Bernstein waves and ion cyclotron (Bernstein) waves. The localization of electron Bernstein waves causes a significant decrease in the secondary- decay excitation threshold, which is thought to provide saturation of the primary instability at the lowest level. The saturation of the primary parametric decay instability of a pump wave and the anomalous absorption of the pump power are analytically estimated. A numerical simulation is performed using the parameters that are typical of the experiments on the electron cyclotron resonance heating of plasma at the second resonance harmonic in TCV tokamak.     

Low-threshold parametric decay instabilities in the experiments on the electron cyclotron resonance heating in tokamaks and stellarators

The experimental conditions have been analyzed for a significant reduction of the threshold of the reflective parametric decay instabilities under electron cyclotron resonance heating (ECRH) of a plasma in magnetic devices in the absence of the upper hybrid resonance for the pump wave. The role of the nonmonotonic profile of the plasma density near the O point of the magnetic island, which allows for the localization of ion Bernstein waves in the direction of the density gradient and the suppression of convective losses from the decay region has been discussed. It has been shown that the threshold of the instability of the induced backscattering near the local maximum of the density profile is decreased by four orders of magnitude and is easily exceeded in present-day ECRH experiments at a power of several hundred kilowatts.     

Low-threshold absolute parametric decay instabilities in experiments on electron cyclotron resonance heating in tokamaks

We have analyzed experimental conditions for the excitation of absolute parametric decay instabilities accompanying the electron cyclotron resonance heating (ECRH) of plasma at the second harmonic of resonance in tokamaks. It has been shown that, in the case of a nonmonotonic radial profile of the plasma density, when a heating beam passes near the equatorial plane of a tokamak, the parametric excitation of resonances of ion Bernstein waves accompanied by the generation of the backscattered microwave radiation can occur. The threshold of absolute instability thus developed is determined by the dissipation of an ion Bernstein wave and can be exceeded in current experiments on the ECRH of a plasma in tokamaks.     

Parametric excitation of circularly polarized and ion waves in a magnetized plasma

Threshold intensities and growth rates are calculated for the parametric decay instability in which a circularly polarized pump wave excites a low frequency electrostatic ion wave and an electromagnetic circularly polarized wave in a magnetized plasma.     

On the low-threshold parametric mechanism of the anomalous power absorption in electron cyclotron resonance heating experiments in toroidal devices

The experimental conditions that facilitate the excitation of parametric decay instabilities upon the electron cyclotron resonance heating of a plasma at the second harmonic extraordinary wave in tokamaks and stellarators and, as a result, make anomalous absorption of microwave power possible have been analyzed. It has been shown that, in the case of a nonmonotonic radial profile of the plasma density, when the beam of electron cyclotron waves passes near the equatorial plane of a toroidal device, the parametric excitation of electron Bernstein waves, as well as the generation of ion Bernstein waves propagating from the parametric decay region to the nearest ion cyclotron harmonic, where they efficiently interact with ions, is possible. The proposed theoretical model can explain the anomalous generation of accelerated ions observed upon electron cyclotron heating in small and moderate toroidal facilities.     

Parametric Instability of Surface Waves on the Second Harmonic of Electron Cyclotron Frequency in a Plasma Layer

The parametric instability of surface waves on the second harmonic of electron cyclotron frequency (SWCF) in a plasma filled dielectric wave guide is examined in a kinetic approximation. The studied surface waves are extraordinary polarized modes and propagate across the external steady magnetic field. The amplitude of the electrical pump wave is assumed to be small. Simple expressions for increments of the parametric instability of the SWCF are calculated. The otained results can be used in controlled fusion researches in order to avoid undesirable regimes of plasma periphery heating in that fusion devices which use the resonance electron cyclotron heating method.     

Generation of cyclotron harmonic waves in the ionosphericmodification experiments

In the present paper, the parametric decay instability of the pump X-mode into electron Bernstein wave (EBW) near second harmonics of electron cyclotron frequency and IBW at different harmonics (ωci; n=2,3,4) is examined. Expressions are derived for homogenous threshold, growth rate and convective threshold for this instability. Applications and relevances of the present investigation to ionospheric modification experiment in the F-layer of the ionosphere as well as during intense electron cyclotron resonance heating in the upcoming MTX tokamak have been given     

Generation of THz Radiation by Laser Plasma Interaction

This paper presents the three wave parametric decay process to generate the Terahertz (THz) radiations in magnetized plasma. The pump wave (Laser beam) is considered in the extraordinary mode (x‐mode), propagating perpendicular to the background magnetic field. This pump wave decays into an upper hybrid wave and a THz wave which is in magnetosonic mode. The appropriate expressions for the coupling coefficients of the threewave interaction and THz wave amplitude have been derived. Subsequently, the growth rate of this decay instability is also calculated. Various laser and plasma parameters were optimized and we report efficiency of the order of ～1.4 × 10^–2 for current scheme. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Resonant decay of a Langmuir wave in the presence of dust grains in a cylindrical plasma

The resonant parametric decay of a Langmuir wave into a backward propagating Langmuir wave and an ion acoustic (IA) wave is studied in a cylindrical dusty plasma. The analysis shows that the frequency of the IA mode decreases with the parameter δ_c (where δ_c is the ratio of the ion density to the electron density) for negatively charged dust grains. The growth rate of the resonance decay instability (RDI) and the threshold required for its onset also decrease with δ_c and are strongly dependent on the electron to ion temperature ratio for both positively and negatively charged dust grains. The results obtained also illustrate the dependence of the threshold of the resonance decay instability (μ_th) on the plasma cylinder radius.     

Four-magnon decay of magnetostatic surface waves in yttrium iron garnet films

The four-magnon instability of magnetostatic surface waves (MSSWs) in yttrium iron garnet epitaxial films is investigated experimentally. It is shown that four-magnon instability for MSSWs with wave numbers 30–600 cm⁻¹ is a decay instability and develops for values of the wave magnetization close to the threshold level for second-order parametric instability of a homogeneous transverse pump wave. When the supercriticality of the MSSW power is 15–20 dB, the generated parametric spin waves themselves become unstable with respect to the four-magnon interaction, so that kinetic instability develops in the film. It is shown that the pump signal transmitted through the signal and the length of the “nonlinear” part of the film, where a MSSW is capable of exciting parametric spin waves, increase as the pump power is increased. Fiz. Tverd. Tela (St. Petersburg) 38, 330–338 (February 1997)     

Experimental observation of motion of edge dislocations in Ge/Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>/Si(001) (<Emphasis Type="Italic">x</Emphasis> = 0.2–0.6) heterostructures

The rotating magnetohydrodynamic flows of a thin layer of astrophysical and space plasmas with a free surface in a vertical external magnetic field are considered in the shallow water approximation. The presence of a vertical external magnetic field changes significantly the dynamics of wave processes in an astrophysical plasma, in contrast to a neutral fluid and a plasma layer in an external toroidal magnetic field. There are three-wave nonlinear interactions in the case under consideration. Using the asymptotic method of multiscale expansions, we have derived nonlinear equations for the interaction of wave packets: three magneto- Poincare waves, three magnetostrophic waves, two magneto-Poincare and one magnetostrophic waves, and two magnetostrophic and one magneto-Poincare waves. The existence of decay instabilities and parametric amplification is predicted. We show that a magneto-Poincare wave decays into two magneto-Poincare waves, a magnetostrophic wave decays into two magnetostrophic waves, a magneto-Poincare wave decays into one magneto-Poincare and one magnetostrophic waves, and a magnetostrophic wave decays into one magnetostrophic and one magneto-Poincare waves. There are the following parametric amplification mechanisms: the parametric amplification of magneto-Poincare waves, the parametric amplification of magnetostrophic waves, the amplification of a magneto-Poincare wave in the field of a magnetostrophic wave, and the amplification of a magnetostrophic wave in the field of a magneto-Poincare wave. The instability growth rates and parametric amplification factors have been found for the corresponding processes.     

Linear and Nonlinear Wavetransformation and Absorption in the Inhomogeneous Plasma Capacitor

The parametric decay process in inhomogeneous layers existing near the plasma boundaries or in front of antennas and probes in a plasma has been investigated. The linear enhancement of the pump field near ω = ω_p, the threshold fieldstrength, the wavenumber selection rules and the influence of spontaneous low frequency fluctuations are discussed in detail using a one-dimensional model of the inhomogeneous plasma capacitor. According to this model the instabilities appear in the layers with maximum linear transformation and (linear) absorption. In addition, a strong nonlinear part of absorption in the presence of the instability has been observed. The level of the spontaneous low frequency fluctuations influences strongly the spectrum of the parametrically excited ion waves. The experiments show a redistribution of the transferred ion acoustic wave energy over the whole wave continuum up to ω_pi, if a sufficient strong spontaneous fluctuation level exists in the plasma. It is impossible, however, to excite ion acoustic turbulence by the decay of the high frequency pump field under the present conditions. The conditions for the linear field enhancement are disturbed by the action of the ponderomotive forces changing the density profile near the critical point before reaching the strong pump amplitude being necessary for the excitation of a cascade of decay processes.     

Parametric excitation of hybrid mode in magnetised piezoelectric semiconductors

Using the hydrodynamic model of homogeneous plasma, the parametric decay of a laser beam into an acoustic wave and another electromagnetic wave has been studied in heavily dopedn-type piezoelectric semiconductors in the presence of a transverse magnetostatic field. This decay process results in the parametric excitation of the hybrid mode. The threshold electric field necessary for the onset of instability equals to zero. The magnetostatic field couples the acoustic and the electromagnetic waves and in its absence the instability disappears. The growth rate increases with the square of the magnetic field.     

Parametric excitation of upper hybrid and diffusion modes in a magnetized plasma

This paper presents the criteria for the parametric instability in which a pump wave with electric field normal to the external magnetic field excites an upper hybrid mode and a diffusion mode in a magnetized plasma.     

Parametric excitation of “cold” ion-Bernstein waves by whistler waves

The large-amplitude whistler wave propagating at an angle to the external magnetic field is shown to decay parametrically into another whistler wave and a “cold” ion-Bernstein wave. The expressions for threshold pump power and growth rate are obtained. A possible application of this parametric process to the magnetospheric plasma is discussed.     

On a Destabilizing Effect of Plasma Inhomogeneity on the Two-Plasmon Decay Instability

It has been shown that plasma inhomogeneity reduces the threshold of the parametric instability of two-plasmon decay and, in particular, allows the excitation of plasmons with the same frequency by a spatially uniform pump field.     

Backscattering and Absorptive Parametric Instabilities Involving the Lower Hybrid Mode in a Magnetized Plasma

The theories of two decay-type parametric instabilities involving the lower hybrid mode in a magnetized plasma are presented. In the first of these, a pump wave of ordinary polarization causes the growth of a backward propagating transverse wave and a longitudinal lower hybrid wave, all with wave vectors perpendicular to the external magnetic field. The second instability concerns the parametric excitation of the upper hybrid and the lower hybrid modes.     

Parametric excitation of electrostatic whistler wave and electronBernstein wave by extraordinary EM wave

In the present paper, the parametric decay instability of an extraordinary electromagnetic wave (X-wave) into an electron Bernstein wave (EBW) and an electrostatic whistler wave (W-wave) has been studied. Expressions are derived for homogeneous threshold, growth rate, and convective threshold for this instability. The relevance of the present parametric process has been pointed out to explain the generation of whistler mode radiations in the SL-2 experiment, ionospheric modification experiment, in the polar cusp region of the magnetosphere, as well as during intense electron cyclotron resonance heating in the MTX tokamak