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A cyclotron maser instability with application to space and laboratory plasmas

Cairns, R.A. and Speirs, David and Ronald, K. and Vorgul, I. and Kellett, B.J. and Phelps, A.D.R. and Bingham, R. (2005) A cyclotron maser instability with application to space and laboratory plasmas. Physica Scripta, T116. pp. 23-26. ISSN 0031-8949

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When a beam of electrons moves into a converging magnetic field, the velocity distribution function takes on a horseshoe shape as a result of conservation of magnetic moment. A few years ago it was pointed out that such a distribution is unstable to a cyclotron maser type of instability and it was suggested that this instability might be the source of auroral kilometric radiation (Bingham, R. and Cairns, R. A., Phys. Plasmas 7, 3089 (2000).) and also of emission from certain types of star (Bingham, R., Cairns, R. A. and Kellett, B. J., Astron. Astrophys. 370, 1000 (2001).). Here we present more recent work on this topic. Since the dispersion relation which describes the instability only depends on the factor by which the magnetic field increases and on ratios of the wave plasma and cyclotron frequencies, it is possible to scale the effect to laboratory dimensions. An experiment to do this is being carried out at the University of Strathclyde and is expected to be operational during the summer of 2004. The objective is to investigate whether this mechanism might produce a useful radiation source, as well as to give a laboratory simulation of auroral kilometric radiation. The design of this experiment and its current status are described. On the theoretical side we discuss computer simulations showing the instability and its quasilinear saturation in the geometry of the experiment. We also describe a much more detailed theory of the linear instability than we have presented in previous work. Growth rates have been calculated in cylindrical geometry with various electron beam configurations and different cylindrical mode structures. These, and the simulations, support the conclusion of our earlier work that the instability has a high growth rate.