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New kinetic cyclotron instability for electron beam in time-changing magnetic fields

Published online by Cambridge University Press:  18 May 2020

Irena Vorgul Open the ORCID record for Irena Vorgul [Opens in a new window] ,
M. Ayling Open the ORCID record for M. Ayling [Opens in a new window] ,
C. R. Straub Open the ORCID record for C. R. Straub [Opens in a new window] ,
D. M. MacKay Open the ORCID record for D. M. MacKay [Opens in a new window] ,
J. D. Houghton Open the ORCID record for J. D. Houghton [Opens in a new window]  and
G. A. Lamb Open the ORCID record for G. A. Lamb [Opens in a new window]
Irena Vorgul*
Affiliation:
SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
M. Ayling
Affiliation:
SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
C. R. Straub
Affiliation:
SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
D. M. MacKay
Affiliation:
SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
J. D. Houghton
Affiliation:
SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
G. A. Lamb
Affiliation:
SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
*
†Email address for correspondence: iv4@st-andrews.ac.uk
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Abstract

This paper examines the velocity distribution function and cyclotron resonance conditions for a beam of electrons moving in a magnetic field which gradually changes with time. A spatial gradient of magnetic field is known to result in an unstable horseshoe distribution of electrons. The field gradient in time adds additional effects due to an induced electric field. The resultant anisotropic velocity distribution function, which we call a Luvdisk distribution, has some distinctive properties when compared to the horseshoe. Fitting the cyclotron resonance condition circle shows that the frequency of the resultant emission is under the local cyclotron frequency. While the spatial gradient results in the emission coming almost perpendicularly to the field, the direction of the radiation under a time-changing field has more variability. The Luvdisk distribution also arises when the magnetic field has a gradient both in space and time. The beam can be unstable if those gradients are added or subtracted from each other (if the gradients are of equal or different sign), which occurs even when the total change of magnetic field is negative. While the frequency of the emission is related to the final magnetic field value, its direction is indicative of the field’s history which produced the instability.

Keywords

astrophysical plasmasintense particle beamsplasma instabilities
Type
Research Article
Information
Journal of Plasma Physics , Volume 86 , Issue 3 , June 2020 , 905860303
Copyright
© The Author(s), 2020. Published by Cambridge University Press

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