R. A. Cairns

Solitary potentials in dusty plasmas

It is found that a dusty plasma with inertial dust fluid and Boltzmann distributed ions admits only negative solitary potentials associated with nonlinear dust‐acoustic waves. The dynamics of small‐amplitude disturbances is governed by the Korteweg–de Vries (KdV) equation, the stationary solution of which assumes the inverted bell‐shaped secant hyperbolic squared profile. The associated dust and ion density perturbations are, on the other hand, positive. The solitary potentials can be identified as nonlinear structures in low‐temperature dusty plasmas such as those in laboratory and astrophysical environments.

Effects of vortex‐like and non‐thermal ion distributions on non‐linear dust‐acoustic waves

The effects of vortex‐like and non‐thermal ion distributions are incorporated in the study of nonlinear dust‐acoustic waves in an unmagnetized dusty plasma. It is found that owing to the departure from the Boltzmann ion distribution to a vortex‐like phase space distribution, the dynamics of small but finite amplitude dust‐acoustic waves is governed by a modified Kortweg–de Vries equation. The latter admits a stationary dust‐acoustic solitary wave solution, which has larger amplitude, smaller width, and higher propagation velocity than that involving adiabatic ions. On the other hand, consideration of a non‐thermal ion distribution provides the possibility of coexistence of large amplitude rarefactive as well as compressive dust‐acoustic solitary waves, whereas these structures appear independently when the wave amplitudes become infinitely small. The present investigation should help us to understand the salient features of the non‐linear dust‐acoustic waves that have been observed in a recent numerical s...

Stability of solitary waves in a magnetized non-thermal plasma

A theoretical investigation 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 dimensions) 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.

Generation of auroral kilometric radiation by electron horseshoe distributions

High time resolution of rocket and satellite electron distribution functions within the source region of auroral kilometric radiation display a characteristic crescent shaped or horseshoe distribution. Such distribution functions are created by a field aligned electron beam moving into an increasing magnetic field, conservation of the first adiabatic invariant causes an increase of their pitch angle. This produces a broad region on the distribution function where ∂fe/∂v⊥>0, and is a possible source of free energy leading to radio wave emission by the cyclotron maser instability, which is more efficient than the conventional loss-cone maser instability. The stability of these electron horseshoe distribution functions is examined for right-hand extraordinary mode (R–X mode) radiation close to the electron cyclotron frequency propagating perpendicular to the magnetic field.

Integrated plasma physics modelling for the Culham steady state spherical tokamak fusion power plant

Integrated modelling of important plasma physics issues related to the design of a steady-state spherical tokamak (ST) fusion power plant is described. The key is a steady-state current drive, and 92% of this is provided by a combination of bootstrap and diamagnetic currents, both of which have a substantial toroidal component in a ST. The remaining current is to be provided by either neutral beam injection or radio-frequency waves, and various schemes for providing this are discussed and quantified. The desire to achieve a high bootstrap current drives the design to high plasma pressure, ? (normalized to the magnetic field pressure), and high elongation. Both these requirements have implications for ideal magneto-hydrodynamic instability which are discussed. Confinement is addressed both through comparison with the recent scaling laws developed from the conventional tokamak database and self-consistent one-dimensional modelling of the transport processes. This modelling shows that the power required for the current drive (~50?MW) is sufficient to heat the plasma to a regime where more than 3?GW of fusion power is produced, taking into account the dilution due to He ash and prompt ?-particle losses, which are small. A preliminary study of the micro-instabilities, which may be responsible for the turbulent transport is provided. Given assumptions about the particle confinement, we make estimates of the fuelling requirements to maintain the steady state. Finally, the power loading due to the exhaust is derived using theory-based scalings for the scrape-off layer width.

Coherent cyclotron maser radiation from UV Ceti

Recent images in the radio of UV Ceti show two intense emission regions above the magnetic poles of the star. The emission is overwhelmingly right hand circularly polarized with 100% right hand polarization at 3 cm and 6 cm wavelengths during flares. This high degree of polarization rules out gyrosynchrotron emission. In this article we propose that such emission can be produced by a coherent cyclotron maser driven by field aligned electrons moving into an increasing magnetic field. Conservation of the first adiabatic invariant causes the electron beam to increase its pitch angle. This produces a broad region on the distribution function and the possibility of producing a positive slope in the perpendicular velocity space. Such distribution functions are common in magnetic mirror type geometries and are possible sources of free energy leading to electromagnetic emission by a cyclotron maser instability mechanism.

Demonstration of auroral radio emission mechanisms by laboratory experiment

Auroral kilometric radiation occurs in regions of depleted plasma density in the polar magnetosphere. These emissions are close to the electron cyclotron frequency and appear to be connected to the formation of high pitch angle electron populations due to the conservation of the magnetic moment. This results in a horseshoe type distribution function being formed in velocity space where electrons are magnetically compressed as they descend towards the Earths atmosphere. Satellites have observed that radio emissions occur in conjunction with the formation of this distribution and show the radiation to have propagation and polarization characteristics of the extraordinary (X-mode) plasma mode with emission efficiency observed at ~1–2%. To investigate this phenomenon a laboratory experiment, scaled to microwave frequencies and lab dimensions by increasing the cyclotron frequency, was constructed whereby an electron beam propagated through a region of increasing magnetic field created by five independently variable solenoids. Results are presented for two experimental regimes of resonant coupling, 11.7 and 4.42 GHz, achieved by varying the peak magnetic field. Measurements of the experimental radiation frequency, power and efficiency were undertaken as a function of the magnetic compression. Results showed the radiation to be polarized in the near cut-off transverse electric radiation modes, with efficiency of emission ~1–2%, peak power outputs of ~19–30 kW and frequency close to the cyclotron frequency. This represented close correlation between the laboratory radiation efficiency, spectra, polarization and propagation with that of numerical predictions and the magnetospheric observations.

The prospects for electron Bernstein wave heating of spherical tokamaks

Electron Bernstein waves are analyzed as possible candidates for heating spherical tokamaks. An inhomogeneous plane slab model of the plasma with a sheared magnetic field is used to calculate the linear conversion of the ordinary mode (O-mode) to the extraordinary mode (X-mode). A formula for the fraction of the incident O-mode energy which is converted to the X-mode at the O-mode cutoff is derived. This fraction is then able to propagate to the upper hybrid resonance where it is converted to the electron Bernstein mode. The damping of electron Bernstein waves at the fourth harmonic resonance, corresponding to a 60 GHz source on the Mega Amp Spherical Tokamak MAST [A. C. Darke et al., Proceedings of the 16th Symposium on Fusion Energy, Champaign-Urbana, Illinois (IEEE, Piscataway, NJ, 1995, Vol. 2, p. 1456)], is computed. For comparison, results are also presented for a lower frequency source, close to the fundamental electron cyclotron resonance. Both the fundamental and the fourth harmonic are shown to ...

Numerical simulation of auroral cyclotron maser processes

Results are presented from a numerical investigation of radiation emission from an electron beam with a horseshoe-shaped velocity distribution. This process is relevant to the phenomenon of auroral kilometric radiation (AKR) which occurs in the polar regions of the Earths magnetosphere. In these regions of the auroral zone, particles accelerated into the increasing magnetic field of the Earths dipole develop a horseshoe-shaped velocity distribution through conservation of magnetic moment. It has been shown theoretically that this distribution is unstable to a cyclotron maser instability. A 2D particle-in-cell (PIC) code model was constructed to simulate a scaled laboratory experiment in which an electron beam subject to significant magnetic compression may be studied and brought into resonance with TE modes of an interaction waveguide. Results were obtained for electron beam energies of 75-85 keV, magnetic compression factors of up to 30 and electron cyclotron frequencies of 4.42 and 11.7 GHz. At 11.7 GHz, beam-wave coupling was observed with the TE03 mode and an RF output power of 20 kW was obtained corresponding to an RF conversion efficiency of 1.3%. At 4.42 GHz, excitation of the TE01 mode was observed with an RF output power of 35 kW for a cyclotron-wave detuning of 2%. This corresponds to an RF conversion efficiency of 2.6%. In both cases PiC particle velocity distributions show the clear formation of a horseshoe-shaped velocity distribution and subsequent action of a cyclotron maser instability. The RF conversion efficiencies obtained are also comparable with estimates for the AKR generation efficiency. (Abstract from: http://iopscience.iop.org/0741-3335/50/7/074011/)

Cyclotron maser radiation from astrophysical shocks

One of the most popular coherent radio emission mechanisms is electron cyclotron maser instability. In this article we demonstrate that electron cyclotron maser emission is directly associated with particular types of charged particle acceleration such as turbulence and shocks commonly inferred in astrophysical plasmas.