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Astron. Astrophys. 331, 1147-1156 (1998)


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Electrostatic effects during neutral beam propagation through plasmas

John C. Brown 1, Marian Karlický 2, Andrew J. Conway 1 and Suzanne Martland 1

1 University of Glasgow, Department of Physics and Astronomy, Glasgow G12 8QQ, UK
2 Astronomical Institute, Academy of Sciences of the Czech Republic, CZ-251 65 Ondejov, Czech Republic

Received 8 September 1997 / Accepted 14 November 1997

Abstract

In this paper several aspects of the interaction of 1-D neutral beams with plasmas are considered. In order to clearly understand the dual rôles of a background plasma in collisionally decelerating the beam and its response to the consequently generated E -field, we examine two cases: an artificial unresponsive background plasma, that corresponds closely to the case of an unionized gas; and a realistic background plasma, whose charges respond to the presence of any E -field. In the former case, the electric field results solely in extremely small scale oscillation of the beam electrons about the protons as both decelerate collisionally and, although electron runaway is possible, the numbers involved are found to be negligibly small. However, collisional separation of the beam electrons and protons does occur in the case of a realistic background plasma, since E easily drives a neutralising dense plasma electron current. Despite the charge separation, runaway is found to be impossible in this case because of the masking effect that the background plasma electrons have on the separated charges unless the plasma is extremely tenuous compared to the beam, where the plasma behaviour is close to that of a near-neutral gas. These effects are shown by approximate analytic mean particle solutions and confirmed by simulations from an electrostatic particle code, which can deal with collective effects. Additional numerical simulations are also performed to investigate the generation of Langmuir waves by a neutral beam. The consequences of these results are discussed for flare neutral beams and corresponding radiation signatures. As regards hard X-ray production by runaways, although some very high energy electrons may result, the number is far too small to be relevant to solar flare HXR burst production. The beam and plasma oscillations may however be relevant to radio bursts.

Key words: acceleration of particles – Sun: flares – Sun: X-rays, gamma rays – Sun: radio radiation – plasmas

Send offprint requests to: A. Conway

© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998
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