An electrodeless plasma produced in an ion guide ion source by a high-energy heavy ion beam propagating through a noble gas has been studied. The plasma parameters in Ar and He buffer gases have been calculated and analysed for projectile beam intensities from 10$^{9}$ to 10$^{17}$ pps cm$^{-2}$. The plasma is characterized by very low electron and gas temperatures, but with an appreciable number of electrons with energy sufficient to ionize atoms of most chemical elements. The ion guide ion source plasma, combined with laser-enhanced ionization of the studied element, is used to improve the detection limit by enhancing the signal-to-noise ratio.

Magnetosonic–gravity waves in an isothermal non-dissipative atmosphere, with a uniform horizontal external magnetic field have been considered in the literature in two cases: (i) ‘one-dimensional’ magnetosonic–gravity waves, in the case of zero horizontal wavenumber and (ii) ‘two-dimensional’ magnetosonic–gravity waves, in which the horizontal wave vector lies in the plane of gravity and the external magnetic field. In the present paper, an extension of case (i) is considered that is distinct from case (ii). This case (iii) is that of magnetosonic–gravity waves with a horizontal wave vector orthogonal to the plane of gravity and the external magnetic field. Since the wave fields depend only on two spatial coordinates and time, the problem could be called ‘two-and-half’-dimensional. The three-dimensional magnetosonic–gravity wave propagates a magnetic field perturbation parallel to the external magnetic field, and velocity perturbations transverse to it. Elimination for the vertical velocity perturbation leads to a second-order wave equation, with four regular singularities. Three regular singularities specify (a) the wave fields at high altitude, where there are two cut-off frequencies involving the acoustic cut-off frequency; (b) the wave fields in the deep layers, where another two cut-off frequencies appear, involving both the acoustic and gravity cut-off frequencies; and (c) the transition between the two regimes, occurring across a critical layer, where one solution of the wave equation vanishes and the other has a logarithmic singularity in the amplitude and also a phase jump. The whole altitude range can be covered using the three pairs of solutions of the wave equation, obtained by expanding in Frobenius–Fuchs series about each regular singularity. The power series solutions are used to plot the wave fields, for several values of the three dimensionless parameters of the problem, namely the plasma $\beta$, frequency and wavenumber. It is shown that the presence of a horizontal wave vector transverse to the plane of gravity and the external magnetic field, can change the properties of the waves significantly: first, the two cut-off frequencies may cease to exist, in which case the full wave frequency spectrum can propagate; secondly, the critical layer occurs at different altitudes for different frequencies, allowing gradual absorption of the waves (e.g. in the solar transition region).

Advection–diffusion equations occur in a wide variety of fields in many contexts of active and passive transport in fluids and plasmas. The effects of sheared advective flows in the presence of irreversible processes such as diffusion and viscosity are of considerable current interest in tokamak and astrophysical contexts, where they are thought to play a key role in both transport and the dynamical structures characteristic of electromagnetic plasma turbulence. In this paper we investigate the spectral and evolutionary properties of relatively simple, linear, advection–diffusion equations. We apply analytical approaches based on standard Green function methods to obtain insight into the nature of the spectra when the advective and diffusive effects occur separately and in combination. In particular, the physically interesting limit of small (but finite) diffusion is studied in detail. The analytical work is extended and supplemented by numerical techniques involving a direct solution of the eigenvalue problem as well as evolutionary studies of the initial-value problem using a parallel code, CADENCE. The three approaches are complementary and entirely consistent with each other when an appropriate comparison is made. They reveal different aspects of the properties of the advection–diffusion equation, such as the ability of sheared flows to generate a direct cascade to high wave numbers transverse to the advection and the consequent enhancement of even small amounts of diffusivity. The invariance properties of the spectra in the low diffusivity limit and the ability of highly sheared, jet-like flows to ‘confine’ transport to low shear regions are demonstrated. The implications of these properties in a wider context are discussed and set in perspective.

The eddy-damped quasi-normal Markovian (EDQNM) two-point statistical closure model is used to study the non-linear triadic energy transfer processes in three-dimensional, incompressible, isotropic, non-helical magnetohydrodynamic (MHD) turbulence in the primitive variable formulation. The triadic transfer functions, which arise from the closure of the three-point correlations in the kinetic energy and magnetic energy spectrum evolution equations, are calculated for non-helical MHD turbulence as a function of wavenumber $k$ for given values of $(p,q)$ with $q$ fixed and $p$ varied. These functions describe the magnitude of the energy transferred into or out of a given mode with wavenumber $k$ due to all allowed interactions of modes with wavenumbers $p$ and $q$ satisfying ${\bf k}={\bf p}+{\bf q}$, and their integrals over all $p$ and $q$ yield the kinetic energy and magnetic energy transfer spectra. Rather than solving the time-dependent, coupled EDQNM equations using initial, prescribed energy spectra, assumed forms of the kinetic energy spectrum $E_{v}(k)$ and the magnetic energy spectrum $E_{B}(k)$ having both a production subrange spectrum proportional to $k$ and a Kolmogorov inertial subrange spectrum proportional to $k^{-5/3}$ are used to evaluate instantaneous values of the triadic transfer functions. The two cases $r_{A}\,{=}\,1$ and $\frac12$ are considered, where $r_{A}$ is the Alfvén ratio. The individual contributions to the transfer functions are also computed in order to determine the dominant interactions that contribute to the total spectral energy transfers. The triadic transfers exhibit forms similar to those found in previous studies of incompressible, isotropic Navier–Stokes turbulence. The non-local-in-wavenumber triadic interactions dominate the local-in-wavenumber interactions, which indicates that the transfer process in non-helical MHD turbulence is primarily local in scale. As the Alfvén ratio decreases, it was found that the most non-local triadic interactions resulted in kinetic energy input and magnetic energy removal at most wavenumbers $k$. The decomposition of the triadic kinetic and magnetic energy transfer functions into their constituents showed that increasing non-locality of the wavenumber interaction involving modes $p$ and $q$ responsible for the net energy transfer into a given wavenumber $k$ corresponds to a qualitative change in the cascade dynamics; namely, there is less energy removal at smaller $k$ and primarily energy input at nearly all $k$. An increased degree of non-locality also results in more cancellation between constituent triadic transfer functions.