Multistability and hysteresis of non-linear electron oscillations induced in a rippled plasma

Summary In the framework of the hydrodynamic approximation, we study the Langmuir oscillations of an ion density ripple background plasma under the influence of a single external harmonic r.f. field. An analytic frequency response curve is derived for the late-time behaviour exhibiting multistability and hysteresis.     

Planar and nonplanar ion-acoustic envelope solitary waves in a very dense electron-positron-ion plasma

Ion-acoustic envelope solitary waves in a very dense plasma comprised of the electrons, positrons and ions are investigated. For this purpose, the quantum hydrodynamic model and the Poisson equation are used. A modified nonlinear Schrödinger equation is derived by employing the reductive perturbation method. The effects of the quantum correction and of the positron density on the propagation and stability of the envelope solitary waves are examined. The nonplanar (cylindrical/spherical) geometry gives rise to an instability period. The latter cannot exist for planar case and it affected by the quantum parameters, as well as the positron density. The present investigation is relevant to white dwarfs.     

Ultra-low-frequency electrostatic and electromagnetic modes in self-gravitating magnetized dusty plasmas

A theoretical investigation has been made of ultra-low-frequency dust-electrostatic and dust-electromagnetic modes, propagating perpendicular to the external magnetic field, in a self-gravitating, warm, magnetized, two fluid dusty plasma system. It has been shown that the effects of self-gravitational field and dust thermal pressure significantly modify the dispersion properties of these ultra-low-frequency dust-modes. It is also found that under certain conditions, the self-gravitational effect can destabilize these ultra-low-frequency dust-electrostatic and dust-electromagnetic modes. However, the effects of the external magnetic field and dust and ion thermal pressures are found to play stabilizing role, i.e., these effects make these modes stable and counter the gravitational condensation of the dust grains. The implications of these results to some space and astrophysical dusty plasma systems, especially, to planetary ring-systems and cometary tails, are briefly mentioned. Received 16 December 1999     

Time behaviour of electron oscillations in a plasma

Summary This paper analyses the transient part in the dynamical behaviour of electron oscillations in a partially ionized plasma. The influence of electron-neutral collisions on time evolution is pointed out. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Electron oscillations in a plasma: Effect of collisions on Landau damping

Summary Electron oscillations in a plasma are analysed starting from the integral form of Boltzmann-Vlasov equation. The effects of collisions on Landau damping are shown. The conditions for Landau damping existence are derived.

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Riassunto Onde elettrostatiche in un plasma sono studiate ricorrendo alla forma integrale dell'equazione di Boltzmann-Vlasov. In particolare, sono stati evidenziati gli effetti prodotti sul Landau damping dalle collisioni. Si è ricavato che in certe condizioni le collisioni possono impedire il Landau damping e si sono ottenuti i valori di soglia.

     

Solitary wave propagation in quantum electron-positron plasmas

Existence of large amplitude stationary solitary wave structures in an unmagnetized electron-positron (e-p) plasma is studied using a quantum hydrodynamic (QHD) model that includes the quantum force (tunnelling) associated with the Bohm potential and the Fermi-dirac pressure law. It is found that in a quasi-neutral pair (e-p) plasma, where the dispersion is only due to the the quantum tunnelling effects, the large amplitude stationary solitary structure exists only when the normalized Mach speed,M <√2. Such solitary structures do not exist in absence of the Bohm potential term in an unmagnetized quasineutral pair (e-p) plasma. The system is shown to support only rarefactive stationary solitary waves. For such waves the amplitude, being independent of the quantum parameter H (the ratio of the electron plasmon to electron Fermi energy), decreases with the Mach number M, whereas the width increases with both M and H. The present theory is applicable to analyze the formation of localized coherent solitary structures at quantum scales in dense astrophysical objects as well as in intense laser fields.     

Change of EM-wave polarization in a plasma with uniform magnetic shear

Summary An explicit analytical solution is found for the change of polarization of an electromagnetic wave propagating in a plasma having uniformly sheared magnetic field, a constant field intensity and a constant plasma density. The polarisation transfer matrix is also found.     

Evolution of polarization of electromagnetic waves in a plasma with uniformly sheared magnetic field

Summary An analytic solution is found for the evolution of the state of polarization of an electromagnetic wave propagating in a plasma with uniformly sheared magnetic field. It is the generalization of the one found previously for the special case of propagation perpendicular to the magnetic field.     

Electron excitation by a multi passage laser beam in a plasma

The limits put by optical guiding, and channel guiding mechanisms on the Laser Wakefield Acceleration (LWFA) technique are imposed on the Resonant Laser Wakefield Acceleration (RLWFA) scheme. Energy gained by the electrons in both schemes are calculated and compared. It has presented that in the RLWFA case, the electrons gain more and more energy after each traversal of the laser pulse and the electrons in a plasma gain about 3 GeV after 10 passages of the laser pulse. They gain 13 GeV when the laser light makes 50 passages and 26 GeV after the laser beam traverses the plasma 100 times. Moreover, the channel guiding mechanism is integrated to the RLWFA scheme and together with diffraction guiding a model for electron acceleration is proposed. Received 13 September 2000 and Received in final form 27 October 2000     

Rarefactive ion-acoustic electrostatic solitary structures in nonthermal plasmas

An investigation has been made of ion-acoustic solitary waves in an unmagnetized nonthermal plasma whose constituents are an inertial ion fluid and nonthermally distributed electrons. The properties of stationary solitary structures are briefly studied by the pseudo-potential approach, which is valid for arbitrary amplitude waves, and by the reductive perturbation method which is valid for small but finite amplitude limit. The time evolution of both compressive and rarefactive solitary waves, which are found to coexist in this nonthermal plasma model, is also examined by solving numerically the full set of fluid equations. The temporal behaviour of positive (compressive) solitary waves is found to be typical, i.e., the positive initial disturbance breaks up into a series of solitary waves with the largest in front. However, the behaviour of negative (rarefactive) solitary waves is quite different. These waves appear to be unstable and produce positive solitary waves at a later time. The relevancy of this investigation to observations in the magnetosphere of density depressions is briefly pointed out. Received 12 October 1999     

Streaming instability in quantum dusty plasmas

By using a quantum hydrodynamic (QHD) model, we derive a generalized dielectric constant for an unmagnetized quantum dusty plasma composed of electrons, ions, and charged dust particulates. Neglecting the electron inertial force in comparison with the electron pressure, and the force associated with the electron correlations at a quantum scale, we discuss two classes of electrostatic instabilities that are produced by streaming ions, and dust grains. The effects of the plasma streaming speeds, the thermal speed of electrons, and the quantum parameter are examined on the growth rates. The relevance of our investigation to dense astrophysical plasmas is discussed.     

A micro Z-pinch as a spark of an axial detonation

Summary In order that a Z-pinch in a DT plasma could spark off an axial nuclear detonation wave severe conditions on space and time concentration of electromagnetic energy must be satisfied. Such energy compression could be achieved by a magnetic-field compressor (MFC) in which a fast liner compresses an azimuthal field (B _φ) of a micro Z-pinch. The driver of the MFC could be either a heavy-ion beam or an explosive magnetic-field generator (EMG) destroyed at each shot. In conclusion, some of the major problems associated with this approach to ICF are outlined.     

Relativistic second-harmonic generation in a laser-produced plasma

Summary In this paper the relativistic second-harmonic generation of a high-power laser radiation in a laser-produced plasma has been studied theoretically in the presence of a self-generated magnetic field. The relativistic Vlasov equation has been employed for the nonlinear response of the electrons in the hot magnetized plasma. It is observed that the power conversion efficiency of the generated second harmonic wave is much higher for relativistic calculations than that for nonrelativistic calculations. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Biomagnetic measurements utilising a superparamagnetic marker: a feasibility study

Summary A preliminary study is here reported on a new potential marker for biomagnetic measurements. The marker consists of superparamagnetic polymer microspheres which were detected in the presence of external steady magnetic fields by means of an r.f.-SQUID magnetometer. The particles were prepared in samples differing in the concentration value and immersed in a homogeneous magnetic field of variable intensity. A simple model was taken into account for the distribution of the microspheres in the samples, so that the theoretical values were compared to the marker field values measured by the biomagnetic sensor. The overall sensitivity of the experimental apparatus and the minimum concentration value of the marker were then estimated.     

Energy analysis of a toroidal magnetohydrodynamic plasma with fluid motion

Summary The relaxation process in an ideal magnetohydrodynamic (MHD) plasma with fluid velocity and enclosed in a toroidal vessel has been discussed. The expressions for the field parameters and the energy state of the system have been derived. The expression for the minimum energy state of the system has been deduced. An analysis of the conservation of energy of the system in the presence of weak and strong magnetic fields has also been presented. The author of this paper has agreed to not receive the proofs for correction. This work was commenced and partly completed during author's short stay at I.C.T.P.-Trieste, Italy, in 1988.     

A new lateral distribution function for electrons in extensive air showers (EAS) detected near sea level

Summary A detailed analysis of Extensive Air Showers in the size range 10⁴–10⁶ particles detected near sea level has yielded a new distribution function for the radial distribution of EAS electrons. The goodness-of-fit criteria applied to the present and already existing similar distribution functions confirm that the present function is appropriate in EAS at radial distances beyond 20 m from the shower axis.