Volume 5, Issue 10, October 1998
Index of content:
 LETTERS


Evolution of Langmuir soliton caused by resonant emission of ion sound wave
View Description Hide DescriptionThe slow decay of a Langmuir soliton caused by the resonant emission of an ion sound wave is studied analytically.

Observation of the crossexcitation instability in a relativistic backward wave oscillator
View Description Hide DescriptionExperimental observation of the crossexcitation instability in a relativistic backward wave oscillator is reported. The transition between single frequency operation and the crossexcitation regime is measured. The initial mode excited in the crossexcitation regime has a relatively low efficiency, while the second mode has considerably higher efficiency and a slightly higher frequency. The two modes are observed to be beating for a considerable fraction of the radiated microwave pulse. These results are consistent with earlier theoretical predictions for the onset and voltage scaling for this instability.

 ARTICLES


Dynamic plasma screening effects on semiclassical electron captures from hydrogenic ions by protons in weakly coupled plasmas
View Description Hide DescriptionElectron capture processes by protons from hydrogenic ions in dense plasmas, including dynamic screening effects, are investigated using the semiclassical version of the Bohr and Lindhard model with the straightline trajectory method. The dynamic interaction potential and screened electron radius are obtained by considering the longitudinal component of the plasmadielectric function. The scaled semiclassical electron capture probability is obtained as a function of the impact parameter, Debye length, and projectile velocity. The plasma screening effects on the target system is known to be quite small, so that the plasma screening effects on the capture probability is mainly determined by the screened capture radius. The dynamic screening effect on the scaled semiclassical capture probability is found to be more significant for low projectile velocities. When the projectile velocity is smaller than the electron thermal velocity, the dynamic screening effect is weaker than the static screening effect. However, when the projectile velocity is greater than the electron thermal velocity, the dynamic screening effect leads to the static screening effect. The maximum position of the scaled capture probability approaches to the target nucleus as an increase of the projectile energy.

Influence of profile shape on the diocotron instability in a nonneutral plasma column
View Description Hide DescriptionIn this paper we examine theoretically the influence of density profile shape on the diocotron instability in a cylindrical, lowdensity nonneutral electron plasma column confined radially by a uniform axial magnetic field The analysis assumes electrostaticflute perturbations about an axisymmetric equilibrium density profile where is the radial distance from the column axis. Two classes of density profiles with inverted population in radius are considered. These are the following: (a) a stepfunction density profile with uniform density in the column interior and uniform density in an outer annular region and (b) a continuouslyvarying density profile of the form over the interval Here, and are positive constants, and the dimensionless parameter Δ measures the degree of “hollowness” of the equilibrium density profile Detailed linear stability properties are calculated for a wide range of system parameters, including values of the “filling factor” Δ, radial location of the cylindrical conducting wall, azimuthal mode number etc. As a general remark, in both cases, it is found that small increases in Δ from the value (corresponding to the strongest diocotron instability) can have a large effect on the growth rate and detailed properties of the instability. In addition, for the stepfunction density profile, the instability tends to be algebraic in nature and have a large growth rate in the unstable region of parameter space, whereas for the continuouslyvarying density profile, the instability is typically much weaker and involves a narrow class of resonant particles at radius satisfying the resonance condition Here, is the real oscillation frequency, and is the equilibrium rotation velocity of the plasma column.

Kinetic formulation of neutrino–plasma interactions
View Description Hide DescriptionA kinetic equation is derived for a neutrino gas interacting with a background plasma via the weak force. It is shown how the neutrinofluid equations can be derived from the kinetic equation. The plasmaelectrons are also linked with the neutrino population by a ponderomotive force term, and the neutrino–plasma interaction problem can be described as the coupling of two fluids: the electron and the neutrino fluids (the plasma ions being assumed at rest).

Confinement and structure of electrostatically coupled dust clouds in a direct current plasma–sheath
View Description Hide DescriptionMechanisms for the confinement and the internal structure of an electrostatically coupled dust cloud formed in a dc glow discharge have been investigated from a comparative viewpoint between experimental observations and a simple model. Two kinds of dust clouds with different internal structures are clearly observed, depending on the dispersion of the size distribution of dust particles. The dust cloud can be trapped only in the plasma–sheath boundary area, corresponding to the potential minimum region determined by gravitational and electrostatic forces in the cathodesheath. No dust particles were found deep inside of the sheath, which is consistent with the analysis because the dust particles may be charged positively due to an extreme reduction of the electron density. The internal structure of the electrostatically coupled dust cloud was found to be arranged so that the total potential energy, including the repulsive Coulomb interaction among negative dust particles, may become minimal.

Dust acoustic drift waves in a dusty plasma
View Description Hide DescriptionDust acoustic drift waves (in which the dust particles provide the inertia for the wave, and electrons and ions follow a linearized Boltzmann distribution) are studied in a magnetized inhomogeneous dusty plasma, considering the coupling of the perturbation of charge on the dust particles to the waves selfconsistently. It is found that, when the dust particle density is homogeneous but the electron and ion densities are inhomogeneous, the dust drift wave branch is unstable; while, when the dust particle density is inhomogeneous, the modified dust acoustic wave branch is unstable. Furthermore, the coupling of the perturbation of charge on the dust particles to the waves tends to suppress these instabilities.

Beam tracing in inhomogeneous anisotropic plasmas
View Description Hide DescriptionAn asymptotic method for solving the wave equation in the shortwavelength limit is presented. This method, called beam tracing, takes into account the wave properties, i.e., diffraction and interference. It reduces the full wave equation to a set of ordinary differential equations. In this respect, it differs from all other asymptotic techniques describing diffraction which end up with much more complicated partial differential equations. The resulting system of beam tracing equations is expressed in terms of the same Hamiltonian function as in geometric optics(ray tracing) and, similar to the ray tracing, allows powerful numerical solving algorithms. Thus the beam tracing combines the simplicity of ray tracing with a description of the wave phenomena, which are not included in the ray tracing. The beam tracing technique provides an efficient tool for calculation of wave fields in all problems where the shortwave approximation is applicable such as rf heating,current drive and plasma diagnostics with microwave beams.

Effects of dust temperature and fast ions on gravitational instability in a selfgravitating magnetized dusty plasma
View Description Hide DescriptionA theoretical investigation has been made of effects of dust temperature, fast (nonthermal) ions, and external magnetic field on gravitational instability in a selfgravitating magnetized hot dusty plasma, which consists of extremely massive, negatively charged hot dust fluid, nonthermally distributed ions, and Boltzmann distributed electrons, where the gravitational force is comparable to or greater than the electrostatic force. The effects of dust temperature, fast ions, and external magnetic field have been found to modify the criterion for this gravitational instability. It has been shown that the growth rate of this gravitational instability decreases with dust temperature, fast ions, and external magnetic field, but increases with the number of free electrons, with the ratio of ion temperature to electrontemperature, and with the ratio of dust mass to dust charge. The implications of this result in some astrophysical situations are briefly discussed.

Oneoverpolynomial approximation for linear kinetic dispersion and its application to relativistic cyclotron resonance
View Description Hide DescriptionIn this paper we propose a new method to calculate the plasmadispersion relation for a wide variety of distribution functions with a complicated resonance mechanism. An approximation with a oneoverpolynomial function enables us to apply the residue theorem for integration, and gives an analytical form of the dispersion equation. The solution is highly accurate with typical error less than when applied to the ordinary plasmadispersion function (Z function). Application of this method is not limited to the dispersion functions that can be expressed by the Z function; it can be applied to much wider class of problems with arbitrary velocity distributions. One example is the general solution to the dispersion equation for a weakly relativistic magnetized plasma.

Low frequency modes in strongly coupled dusty plasmas
View Description Hide DescriptionThe influence of strong correlations on low frequency collective modes in a dusty plasma is investigated. The dust dynamics is modeled by the generalized hydrodynamics description. For the well known dust acoustic mode, strong correlations lead to new dispersive corrections, an overall reduction of the frequency and phase velocity and the existence of parameter regions where . A novel result is the possibility of sustaining a low frequency transverse mode—a dust shear mode—in which the correlation energy acts as an effective bulk modulus. The influence of ion streaming and collisional interaction with a background of neutrals on the modes are also studied and it is shown that the longitudinal modes may be driven unstable by ion streaming.

Small amplitude solitons in a magnetized dusty plasma with twoion species
View Description Hide DescriptionSmall amplitude dust acoustic solitons are studied in a magnetized plasma having twoion species (hot and cool), electrons, and negatively charged dust grains. The charged dust grains are magnetized. It is found that both compressive and rarefactive solitons can occur in such a plasma system. The effects of the magnetic field strength, wavepropagation angle, particle densities, and temperatures on the soliton characteristics are investigated.

Nonlinear field line resonances: Dispersive effects
View Description Hide DescriptionA model is presented which describes the nonlinear interaction of dispersive shear Alfvén wave(SAW) field line resonances (FLRs) and ion acoustic waves(IAWs), with applications to the Earth’s magnetosphere. Two limits are considered: In lowβ plasma dispersion is dominated by electron inertia (EI), while for higher β it is dominated by the electron thermal effect. In each case, the ponderomotive force steepens the SAW in the radial direction, taken as earthward in the equatorial plane. Following the time of nonlinear steepening, the dynamics strongly depends on dispersion. In the EI case, standing SAWs excited in FLRs exhibit a parametric decay instability (PDI) into secondary SAWs and IAWs. Nonlinearity and dispersion broaden the FLR in the radial direction, leading to rapid density and parallel electric field fluctuations and scale lengths comparable to the EI length. In warm plasmas, SAWs are stable to the PDI, and in this case the FLR emits short perpendicular scale SAWIAW solitons in the antiearthward direction. Observational consequences of both scenarios are discussed.

Strongly nonlinear magnetosonic waves and ion acceleration
View Description Hide DescriptionThe electromagnetic fields associated with a nonlinear compressional Alfvén wave propagating perpendicular to an arbitrarily strong external magnetic field are derived. While recovering the known properties of nonlinear Alfvén waves in the weakly magnetized regime, new scaling laws are found for the strongly magnetized case. It is shown that the electric field increases only with the first power of the external magnetic field strength for the strongly magnetized and high phase velocity case relevant for ion acceleration to high energies. An experimental setup suitable for the pickup and acceleration of background plasma ions that is not subjected to detrapping of ions due to a acceleration is sketched.

Energy loss of charged projectiles in dusty plasmas
View Description Hide DescriptionThe analytical and numerical results for the slowing down of two heavy projectile ions passing through a multicomponent dusty plasma are presented. Within the linear dielectric approach, the electrostatic potential and the stopping power of the two projectiles are computed for different values of (the normalized effective wave number) and R (the separation between the two projectiles) retaining twoioncorrelation effects. The enhancement in the energy loss is observed, and it is compared with that of a single ion projectile case. These results are useful to explain the crystallization of dust grains in astrophysical and laboratory plasmas.

A comparative study of modified Korteweg–de Vries (MKdV) and Korteweg–de Vries (KdV) solitons in plasmas with negative ions under the influence of electrons’ drift motion
View Description Hide DescriptionIn the plasma under consideration, the existence of modified Korteweg–de Vries (MKdV) solitons has been established for different values of electrons’ drift velocities and of ( negative to positive ion mass ratio). The amplitudes of the MKdV solitons are found to be much smaller (<1) than those of the corresponding amplitudes (>1) of the KdV solitons for when Unlike the existence of both compressive and rarefactive KdV solitons with two unaccountable regions of there is found to exist only compressive MKdV solitons of small amplitudes in the lower but entire range of for 1.12, and 1.15 when It is noteworthy to mention that, the width of the KdV soliton does not depend on i.e., on the concentration of negative ions whereas that of the MKdV soliton depends on

Nonlinear drift tearing modes in the presence of a resistive wall and field errors
View Description Hide DescriptionNonlinear tearing modes with electron diamagnetism and resistive wall are studied, with parallel compression and particle diffusion. The resistive wall mode is linearly unstable, and may be diamagnetically stabilized. Whenever this mode is unstable, it locks nonlinearly to the wall, saturating as without diamagneticeffects. Two important parameters are the drag coefficient ν [Phys. Plasmas 5, 461 (1998)] and sound speed For small, flow cancelling the diamagnetic propagation is spontaneously generated. For ν or large, this flow is decreased, and there is slow propagation. For large, the pressure becomes nearly constant along the flux surfaces, leading to slower propagation and smaller flow. If the resistive wall mode is stabilized by diamagnetism, there is a critical field error for locking and the hysteretic lockingunlocking bifurcation diagram is asymmetric, i.e., diamagnetic plasma rotation is more effective in unlocking or preventing locking. Also, there are bifurcations to slowly propagating locked states.

Kinetic theory of photons in a plasma
View Description Hide DescriptionA kinetic equation for the photon gas in a relativistic plasma, neglecting the ions motion is derived. From this equation the fluid equations for the photon gas are obtained and their main properties are discussed. In order to illustrate the potential use of these equations to the study of new physical phenomena, which eventually can occur in relativistic plasmas, they are applied to three distinct situations. First, it is shown that a photon beam propagating in a plasma can excite a beam–plasma instability, similar to that occurring for the wellknown case of electron beams. Second, it is shown that a bunch of photons, moving with constant velocity across a homogeneous plasma, can emit radiation similar to the Cherenkov effect associated with charged particles. Finally, the filamentation instability of a photon beam is discussed.

Beta limits for the mode in rotatingtoroidalresistive plasmas surrounded by a resistive wall
View Description Hide DescriptionThe stability analysis of a highβ toroidaltokamak plasma is carried out in the presence of toroidalflow, finite plasma resistivity, and a surrounding shell of finite electrical resistivity. The beta limits for the mode are set by the resistivewalltearingmode (RWTM), the idealwalltearing mode (IWTM), and the idealplasmaresistivewall mode (IPRWM). Slow plasma rotation suppresses the RWTM while the IPRWM is not directly affected by slow plasma flow. For small plasma resistivity, the IPRWM is stabilized by fast flow only. For large plasma resistivity, the IPRWM only exists in a plasma rotating faster than the typical tearing mode growth rate, and its instability threshold is a complicated function of the wall position and rotation frequency. Very fast rotation can destabilize the ideal kink through centrifugal effects. Furthermore, for (wall radius/plasma radius) below a critical value, a stationary plasma is stable to the ideal kink and tearing mode for large values of β.

Selfsimilarity of the plasma edge fluctuations
View Description Hide DescriptionThe rescaled range analysis techniques are used to investigate longrange dependence in plasma edge fluctuations [Mandelbrot and Wallis, Water Resources Res. 4, 909 (1969)]. This technology has been applied to data from several confinement devices such as tokamaks, stellarators, and reversedfield pinch. The results reveal the selfsimilar character of the electrostatic fluctuations at the plasma edge with selfsimilarity parameters ranging from 0.62 to 0.72. These results show that the tail of the autocorrelation function decays as a power law for time lags longer than the decorrelation time and as long as times of the order of the confinement time. In cold plasma devices at the core), there is no evidence of algebraic tails in the autocorrelation function. Some other characteristic features of the autocorrelation function and power spectrum have been investigated. All of these features are consistent with plasma transport as characterized by selforganized criticality.
