Longitudinal Plasma Waves Research Articles

Classification of trapped particle sideband instability regimes

The instability of a finite amplitude longitudinal plasma wave with trapped particles is discussed. Distinct instability regimes are examined within the framework of the simplest model wherein all trapped particles are assumed to be concentrated at the bottom of the potential energy troughs of a sinusoidal wave. The dispersion relation describing the instability is solved in combination with the nonlinear dispersion law of the primary wave. The self-consistent approach allows one to establish the conditions of validity of the model considered and to find, for practical purposes, convenient expressions for growth rates in all limiting cases. In accordance with the analysis carried out the ratio of trapped particle energy flux to wave energy flux is of great importance in the classification of sideband instability regimes.

Interacting Intersecting Masers

An interaction of two masers through plasma longitudinal waves in the Stimulate Raman Scattering has been investigated and applied for the extreme high-velocity (900 km s-1) water masers discovered in NGC 4258. For the stimulated Raman scattering, the frequency down-shifted components are easily created but not the up-shifted components. We use the interacting masers for the explanation for the presence of the up-shifted components. The interaction occurs when the wave vector of the longitudinal plasma wave associated with one incident maser coincides with the wave vector of the other longitudinal wave associated with the other maser. This interaction produces two daughter waves, frequency up-shifted and down-shifted radiations with a shift of roughly a plasma frequency. This interaction occurs when two masers intersect with intersecting angle of about 176 degree in plasma of the electron density of about 5 106 cm-3. The brightness temperature of the original incident masers must be greater than about 1013 K (depending on the line width, and the maser beaming angle, etc.) and the scale length of plasma must be larger than about 1013 cm.

Particle acceleration in a relativistic wave in the adiabatic regime

The adiabatic theory of particle acceleration in a relativistic longitudinal plasma wave of fixed phase velocity is developed. The trapping condition for a low energy particle is found to be more severe than the trapping condition previously obtained for a constant amplitude wave. Intermediate energy particles that are trapped in the wave experience the beam splitting and the beam heating effects, with most of the particles ending in the larger energy beam. The particles that are not trapped experience no acceleration at all in the adiabatic limit. The validity conditions of the adiabatic theory are given for trapped and untrapped particles. Numerical results corresponding to the UCLA (University of California at Los Angeles) beat wave experiment [R. L. Williams, C. E. Clayton, C. Joshi, T. Katsouleas, and W. B. Mori, Laser Part. Beams 8, 427 (1990)] illustrate the theory. They show the important role of the trapping condition.

100% transmission of longitudinal plasma waves through an opacity barrier owing to trapped particles

The propagation of Langmuir waves loaded with trapped particles in a weakly inhomogeneous plasma with a density hump is considered. It is shown that the reversibility of energy exchange between these particles and the wave entails the possibility of wave penetration through an opacity region with transmission coefficient D = 1.

Interaction of highly relativistic short laser pulses with plasmas and nonlinear wake-field generation

Generation of nonlinear plasma oscillations by intense short laser pulses is considered. It is shown that longitudinal plasma waves with relativistic amplitudes and phase velocities close to the speed of light c can be excited in the wake of a strong single laser pulse in a cold plasma. The analytical as well as numerical studies of the relativistic wake-field generation by the laser pulses with realistic shapes are carried out. The nonlinear scheme of the laser wake-field accelerator is suggested. It is found that the maximum amplitude of the excited plasma waves is to be expected at the wave phase velocity vp < c and not at vp ≃ c. A second laser pulse sent after the first one is considered and shown that this trailing pulse can both amplify and "absorb" the wake-field created by the first laser pulse. Applications of our results to the proposed Livermore High-Brightness Lasers are discussed.

Finite-amplitude electrostatic waves im magneto-active plasmas

Weakly nonlinear dispersive longitudinal waves in an infinite homogeneous collisionless plasma in the presence of an external constant and uniform magnetic field are considered. Under specific physical assumptions and for an arbitrary three-dimensional envelope modulation of a plane wave, a purely differential system is derived. Taking into account the effect of wave-wave and wave-particle interaction, the evolution of the modulation is described by a modified nonlinear Schrodinger equation, coupled to the space perturbation charge densities. The generation of a static mode is described. As a particular case the electron waves are discussed and some special solutions, resorting to the theory of the perturbed solitions.

Hydrodynamic description of magnetosphere plasma with due regard to the wave activity of Alfven and fast magnetosonic waves

The interaction between the Alfven (A) and fast magnetosonic (FM) waves and the magnetospheric plasma is analysed. Quasilinear interaction of these waves with electrons and induced scattering on background ions have been considered. Moments of integrals of collisions between waves and particles have been taken into account within the framework of the 16-moment approximation Grad method. The effect of wave activity on the forming of anisotropic temperature and the existence of longitudinal drift waves in an inhomogeneous plasma are discussed.

Differential equations for the energy of a single particle in a helicoidal wiggler with invariant guiding field and travelling plasma wave (non-self-consistent Raman regime)

We are interested in the orbital motion of a single particle in a helically wound wiggler, in the presence of a constant longitudinal guiding magnetic field B g and also of an electrical longitudinally travelling plasma wave field E z of invariant amplitude and phase. This could be a model for a free electron Raman laser, at least for the end of the wiggler, with coherent longitudinal plasma wave owing to neglected transverse electromagnetic travelling waves. Near the axis of the wiggler Bessel functions of the wiggler fields have been drastically neglected [P. Diament, Phys. Rev. A23 (1981) 2537]. In this case and using a rotating frame with a fixed origin but following the helicoidal field, we have been able to deduce a nonalgebraic time-dependent constant of the motion which generalizes the axial invariant of Freund-Drobot and Davidson-Uhm [H.P. Freund and A.T. Drobot, Phys. Fluids 25 (1982) 736]. Further we have deduced a third-order nonlinear differential equation (NLDE) for the energy. This equation can be reduced to a nonlinear second-order one, owing to an integrating factor. Actually, this second-order NLDE may be much more easily directly deduced. Finally we have studied this NLDE from a mathematical point of view, first for the preliminary case E z − E 0 = constant, then for the case E z = E 0 cos Φ = E 0 cos( Kz − ωt + φ 0). However, due to the lack of space, the results of this mathematical study will be published elsewhere [J. Valat, submitted].

High frequency Raman current drive

Current drive in reactor grade tokamak plasmas with a pulsed, high power free electron laser (FEL) is investigated. The stimulated Raman process generates fast, longitudinal electron plasma waves which accelerate electrons to relativistic energies. The fast electrons with vi ≫ v⊥ are not trapped toroidally and the resulting current decays slowly. The paper presents a systematic study on different combinations of Raman forward and backward scattering and FEL polarization. The optimal FEL frequencies fall within the range 2.2-4.0ωp (left circularly polarized case) and 3.8-7.0ωp (right circularly polarized case) in a typical tokamak plasma with Ωe ≃ 1.5ωp In a reactor grade plasma the best current drive efficiency, 5 × 10l9 A/W⋅m2, is obtained by using the stimulated Raman forward process and a left circularly polarized FEL beam. The FEL driven current can be localized to the plasma centre and amplified by the bootstrap effect to further enhance the overall efficiency. Raman current drive in high temperature reactor plasmas is quite different from small scale experiments with Te ∼ 1 keV, where the Raman backward process dominates and the current drive efficiency remains rather low.

Probable mechanism of change in the spectral index of ionospheric irregularities

A probable cause of variability in the spectral index of ionospheric irregularities is discussed, based on the ability of intense longitudinal plasma waves to generate density disturbances with steep fronts. The results are experimentally confirmed.

Lateral and Axial Magnetisation in the Inverse Faraday Effect for the Interaction of Waves in Plasma

We evaluate the static lateral magnetisation from the inverse Faraday effect, due to a circularly polarised electromagnetic wave and either a high frequency longitudinal dispersive plasma wave (case 1) or an ion acoustic wave (case 2), all propagating in the same direction in an unmagnetised plasma. These two lateral magnetisations are found to be in opposite directions. For the interaction of 1�06 /lm circularly polarised electromagnetic radiation having a power flux 5x1016 Wm-2 and pulse width 5 ns in a plasma, in case 2 the induced transverse static magnetic field is about 0�0014 Wbm-2 ? In case I, this static field is about 0�0234 Wb m-2. In a quiescent cesium plasma the magnetisation is found to be insignificant in both cases. These theoretical investigations may help in the understanding of anomalous diffusion of plasmas in some problems, and in the explanation of solar prominences and other such astrophysical effects.

Relativistic wake-field generation by an intense laser pulse in a plasma

Generation of nonlinear plasma oscillations by an intense laser pulse is considered. It is shown that longitudinal plasma waves with relativistic amplitudes and phase velocities close to the speed of light c can be excited in the wake of a strong single laser pulse in a cold plasma. An analytical and numerical study of the relativistic wake-field generation by laser pulses with realistic shapes is carried out. A nonlinear scheme of the laser wake-field accelerator is suggested.

Evaluation of the electric field generated by longitudinal plasma waves

The stabilizing effects of the addition of short‐range collisions to a partially ionized plasma are investigated through a recently developed numerical Laplace transform inversion algorithm. The technique is demonstrated using a two‐stream equilibrium distribution and double Maxwellian equilibrium distributions. The regions of plasma stability obtained in previous studies are verified.

High-frequency noise on antennas in plasmas

Fluctuations in plasmas at frequencies near the electron plasma frequency (ωp ) have been measured with in situ wire antennas. Such observations are important to the understanding of basic plasma properties (discrete versus collective effects) and the use of antennas in plasmas. The experiments are performed in a large (1 m diameter ×2 m) pulsed dc discharge and afterglow plasma (ne&amp;lt;1012 cm−3, kTe&amp;lt;5 eV) with a weak axial magnetic field (B0=5 G). The fluctuations are detected from wire antennas (length L≫radius a≳Debye length λD) with a low-noise microwave receiver. The observations reveal three different physical processes that determine the noise spectra: (i) single particle shot noise, (ii) collective oscillations by bounded sheath–plasma resonances, and (iii) noise enhancements by longitudinal plasma waves. The first phenomenon (shot noise) gives rise to broadband noise (ωωp) on both electric and magnetic antennas. In the evanescent regime (ω&amp;lt;ωp) , the shot noise is induced by random electron motions through distances of, at most, a collisionless skin layer (c/ωp) around the antenna. Recalling that electron transit-time effects cause absorption of waves in a collisionless skin layer (known as ‘‘anomalous’’ skin absorption) the present observation of a collisionless skin emission effect can also be understood by the equivalence of blackbody absorption and emission coefficients. The second phenomenon (sheath–plasma resonance) is observed as a narrow resonant enhancement in the shot noise below cutoff (ω&amp;lt;ωp) on electric antennas surrounded by sheaths or dielectrics. The series sheath–plasma resonance, usually identified from reflection/absorption measurements with incident waves, is established here, for the first time, as a feature of thermal emission spectra from antennas. The third phenomenon (plasma dielectric ε(ω,k)→0) produces a broad noise enhancement at ω≳ωp on electric antennas. It can only be observed in the open-loop voltage of dipoles measured with a high-impedance transformer between antenna and 50Ω transmission line. The enhanced noise is electrostatic, randomly polarized, but not significantly enhanced by the presence of energetic electron tails. Existing theories can describe the new observations qualitatively but not quantitatively.

Effect of plasma noise spectrum on stimulated scattering in inhomogeneous plasma

The flux of radiation emitted from an inhomogeneous plasma by stimulated Raman and Brillouin scattering is calculated with a source that includes both bremsstrahlung and longitudinal plasma wave noise sources. Significant enhancement of the flux above that found for a bremsstrahlung source alone are found for stimulated Brillouin scattering and for nonthermal but stable velocity distributions.

Relativistic excitation of beat waves in a plasma in the presence of an external magnetic field

A theoretical investigation of the nonlinear relativistic excitation of the longitudinal electron plasma wave at the beat frequency of two colinear laser beams has been made in a hot, collisionless and homogeneous plasma in the presence of an external magnetic field. The relativistic Vlasov equation has been solved to obtain the nonlinear response of magnetized electrons. It is observed that the power conversion efficiency of the excited-electron plasma wave at the difference frequency is much higher for the relativistic calculation than that for the nonrelativistic calculations. It is further noticed that the power conversion efficiency of the beat wave excitation increases slowly with the external magnetic field when the colinear laser beams propagate transverse to the direction of the external magnetic field, whereas it is almost unaffected when the incident waves propagate parallel to the direction of the magnetic field.

Nonlinear evolution of longitudinal plasma waves

Long-time nonlinear behavior of single-mode longitudinal plasma waves is studied on the basis of the Vlasov equation with Fokker–Planck collision terms. The resonant layer, trapped island, collisional sublayers, and X-point neighborhoods are analyzed. A nonlinear evolution equation is obtained and the wave-generated current is also evaluated.

Quantum theory of emission of longitudinal waves in a nonsteady plasma

The interaction of longitudinal waves with particles of a nonsteady plasma is investigated. The analysis is made on the basis of the equation of balance between wave emission and absorption by plasma particles. The probabilities of emission and absorption are found from the Schroedinger equation. It is shown that the probability of emission per unit time oscillates with time. The effect of oscillations of the Landau decrement is connected with this. This effect is analyzed for a nonsteady plasma penetrated by a steady beam.

THE TWO MODE CONVERSION IN AN INHOMOGENEOUS MAGNETIZED PLASMA

The field structures in the overdense region above the resonant paint of an inhomogeneous magnetized plasma are studied. Our work indicates that owing to the mutual conversion be-tween the extraordinary electromagnetic wave and the longitudinal plasma wave, the electromagnetic wave will grow again in the overdense plasma.

Plasmons from a gauge invariant effective action for hot gluons

The one-loop effective action for a gluon plasma is calculated in a general covariant background field gauge. The result is expressed in terms of the dielectric constant and the magnetic permeability, which immediately yields dispersion relations for both longitudinal and transverse plasma waves. The plasma frequency is gauge fixing independent and agrees with earlier calculations, while the plasma damping constant is gauge fixing dependent and negative definite. We argue that the result in Landau gauge is of physical significance, and that the calculated exponential growth of the plasmon amplitude is a signal for the breakdown of perturbation theory at scales of order ∼ g 2 T. The relevance of this for the dynamical description of the quark-gluon plasma is discussed.