We employ the Monte Carlo method for the calculation of anomalous transport developed by Punjabi and Boozer to calculate the particle diffusion coefficient for electrons in the stochastic magnetic fields of the reversed-field pinch (RFP). in the Monte Carlo calculations represented here, the transport mechanism is the loss of magnetic surfaces due to resistive perturbations. The equilibrium magnetic fields are represented by the Bessel function model for the RFP. The diffusion coefficient D is calculated as a function of a, the amplitude of the perturbation. We see three regimes as the amplitude of the tearing modes is increased: the Rechester—Rosenbluth regime where D scales as a2 the anomalous regime where D scales more rapidly than a2 and the Mynick—Krornmes regime where D scales more slowly than a2. Inclusion of the effects of loop voltage on the particle drift orbits in the RFP does not affect the intervals in the amplitude a where these regimes operate.

Ion-acoustic solitary waves in a weakly relativistic warm plasma at the critical phase velocity are investigated using reductive perturbation theory. The basic set of fluid equations describing the system is reduced to a renormalized warmion modified Korteweg—de Vries (mKdV) equation for the first-order perturbed potential and a renormalized linear inhomogeneous equation for the second-order perturbed potential. The stationary solution of the coupled equations is obtained. The cold-plasma limit is considered in order to rule comparisons with work of Pakira et al.

Expressions for the field intensities in a warm ionized plasma for two type of modes (electromagnetic and electroacoustic) pertaining to a microstrip matched coaxial termination are obtained using linearized hydrodynamic theory coupled with a vector function technique. The radiation power and directivity for the respective modes are evaluated for different ratios of plasma to source frequencies. It is observed that electromagnetic fields are similar to other microstrip antenna structures, but more directive in a particular direction. The electroacoustic mode exhibits a complete absence of multilobed structure, which was found for other antenna structures. The efficiency of this structure in a plasma is poor.

Fully nonlinear kinetic Alfvén solitary waves are investigated, taking account of the drifting effect of electrons in the direction of the external magnetic field in a low-β plasma (where β = 8πn0T/c2B02 is the ratio of the kinetic pressure to the magnetic pressure). Both compressive and rarefactive Alfvén solitons are found to exist in different ranges of kz (direction of wave propagation), depending on the values of M (the Mach number) and ve (the drift velocity, occurring implicitly in an effective parameter A). The existence domain of kinetic Alfvén solitons is found to be concentrated in the upper regime of β (≪ 1) and lower regime ofM ( < 1), but the values of M in the case of rarefactive solitons are observed to be higher than those of compressive solitons. The range of existence of kinetic Alfvén solitons is substantially narrowed when the initial drift velocity v′e of the electrons is taken into account.

The plasma transport equations for a weakly collisional plasma have previously been derived for four different time scales. This paper is devoted to the derivation of the plasma transport equations for the two other complementary regimes: the intermediately collisional regime (ICR) (i.e. for the case where the transit time w1 is of the same order as the collision time is of the same order as the collision time ), and the strongly collisional regime (SCR) (i.e. for the case of ) for different time scales. It is shown that the lowest-order gyromotion is unperturbed by collisions. On the Alfvén time scale, one merely obtains for both the intermediately collisional case and the strongly collisional case the single-fluid ideal MHD equations, if certain additional requirements are satisfied. On the MHD-collision time scale, one arrives at the full set of transport equations, where in both cases, contrary to the weakly collisional case, no turbulent terms are found. On the resistive diffusion timescale, one ends up with the known transport equations, with the addition of turbulent contributions.

Nonlinear wave structures of large-amplitude ion-acoustic waves in a plasma with a relativistic electron beam are studied using the pseudopotential method. The region of existence of large-amplitude ion-acoustic waves is examined, and it is shown that the condition for their existence depends sensitively on parameters such as the relativistic effect of the electron beam, the ion temperature, the electrostatic potential and the electron beam density. It turns out that the region of existence spreads as the relativistic effect (Mach number) increases and the ion temperature decreases. New properties of large-amplitude ion-acoustic waves in a plasma with a relativistic electron beam are predicted.

Following a previous investigation in which the growth rates of tearing modes at weak instability were determined for a frequently used plasma model with convected resistivity, two further models are considered that appear to be more appropriate for realistic situations. In both of them viscosity is included. One has a self-consistently determined equilibrium resistivity that is kept constant during time evolution. In the other Spitzer resistivity is assumed, which, through its temperature dependence, evolves according to the energy law including thermal conductivity. All calculations are carried out for specific equilibria in a sheet pinch. A salient feature of the two models is the emergence of a stability threshold, which was not present in the previous model. Surprisingly, the results obtained for the two new models are almost the same, thus favouring the much simpler model with constant resistivity.

The Debye length, the characteristic shielding distance in a plasma, is, when the electrons and ions have Maxwellian velocity distributions, determined by the ratio of the temperatures of these components, to the electron (or ion) number density. Plasmas encountered in space, however, commonly exhibit non-Maxwellian velocity distributions, where the evaluation of an appropriate ‘temperature’ from an observed velocity distribution is no longer a recognized procedure. This paper evaluates the shielding distance for a plasma having a modified power-law, or kappa, family of distribuitons characteristic of some space plasmas.

The linear response 4-tensor for a relativistic quantum electron gas may be calculated by reinterpreting the electron propagator in the expression for the vacuum polarization tensor as statistical averages over the electron gas. We apply a similar procedure to two other radiative corrections: the electron self-energy and the vertex correction. When the photon propagator in these expressions is interpreted as a statistical average over a distribution of waves in the medium, these radiative corrections lead to a relativistic quantum expression for the ponderomotive force and to a new class of ‘hybrid’ emission processes.

The viscous damping of surface Alfvén waves in a non-uniform plasma is studied in the context of linear and incompressible MHD. It is shown that damping due to resonant absorption and damping on a true discontinuity are two limiting cases of the continuous variation of the damping rate with respect to the dimensionless number Rg = Δλ2Re, where Δ is the relative variation of the local Alfvén velocity, λ is the ratio of the thickness of the inhomogeneous layer to the wavelength, and Re is the viscous Reynolds number. The analysis is restricted to waves with wavelengths that are long in comparison with the extent of the non-uniform layer (λ ≪ 1), and to Reynolds numbers that are sufficiently large that the waves are only slightly damped during one wave period. The dispersion relation is obtained and first investigated analytically for the limiting cases of very small (Rg ≪ 1) and very large (Rg ≫ 1) values of Rg, For very small values of Rg, the damping rate agrees with that found for a true discontinuity, while for very large values of Rg, it agrees with the damping rate due to resonant absorption. The dispersion relation is subsequently studied numerically over a wide range of values of Rg, revealing a continuous but nonmonotonic variation of the damping rate with respect to Rg.

The results of recent developments in the theory of fluctuations in plasmas show that the previously used theory of bremsstrahlung is incomplete and the exact expressions for bremsstrahlung should include transition bremsstrahlung. The collective effects in bremsstrahlung known previously as Debye screening are changed to a qualitatively different structure, which removes the effect of ion polarization in bremsstrahlung and introduces a new effective polarization which depends on an effective ion charge and electron velocity. The results may be relevant for applications in plasmas when the wavelength is greater than the Debye length. It is shown that for the problem of photon transport in the solar interior the correct collective corrections to the bremsstrahlung change the opacity by only about −0·35%, which is less than was calculated previously when collective effects in bremsstrahlung where estimated without taking recent results of plasma fluctuation theory into account.

A theoretical investigation of radio-frequency (RF) current drive via helicity injection in low aspect ratio tokamaks is carried out. A current-carrying cylindrical plasma surrounded by a helical sheet-current antenna and situated inside a perfectly conducting shell is considered. Toroidal features of low-aspect-ratio tokamaks are simulated by incorporating the following effects: (i) arbitrarily small aspect ratio, Ro/a ≡1/∈; (ii) strongly sheared equilibrium magnetic field; and (iii) relatively large poloidal component of the equilibrium magnetic field. This study concentrates on the Alfvén continuum, i.e. the case in which the wave frequency satisfies the condition , where is an eigenfrequency of the shear Alfven wave (SAW). Thus, using low-β magneto- hydrodynamics, the wave equation with correct boundary (matching) conditions is solved, the RF field components are found, and subsequently current drive, power deposition and efficiency are computed. The results of our investigation clearly demonstrate the possibility of generation of RF-driven currents via helicity injection by Alfvén waves in low-aspect-ratio tokamaks, in the SAW mode. A special algorithm is developed that enables one to select the antenna parameters providing optimal current drive efficiency.

A theoretical investigations is made of the stability of electrostatic waves in a magnetized non-thermal plasma. The Zakharov-Kuznetsov equation (or Korteweg-de Vries equation in three dimensionas) for these solitary waves in this plasma system is derived, and their three-dimensional stability is studied by the small-k (long wavelength plane-wave) perturbation expansion method. The instability criterion and its growth rate depending on the magnetic field and the propagation directions of the solitary wave and its perturbation mode are discussed.

Model equations describing the nonlinear coupling between electrostatic ion-cyclotron and drift waves are derived, taking into account the action of the low-frequency ponderomotive force associated with the ion-cyclotron waves. It is found that this interaction is governed by a pair of equations, which can be used for studying the modulational instability of a constant amplitude ion-cyclotron wave as well as the dynamics of nonlinearly coupled ion-cyclotron and drift waves.