A simulation approach to neutral beam-plasma systems: analysis of Langmuir waves generation
Abstract
Many authors suggested the acceleration of neutral beams in the primary energy release site of solar flares (Simnett, 1986; Simnett and Heines, 1990; Martens and Young, 1990); such neutral beams would consist of electrons and protons with the same drift velocity, so that the protons carry the bulk of kinetic energy, and this would also overcome most of the problems connected with the propagation of pure proton and pure electron beams (Brown et Al., 1989). Numerical simulations in a preliminary work (Karlicky, 1989) indicate that neutral beams can be the result of current neutralization in accelerated proton beams or, according to the model by Martens and Young (1990), their formation can be due to the non-collisional, direct acceleration of protons and electrons through the action of the electric field in the current sheet formed at the top of a post-flare loop. Actually the real existence of neutral beams in solar flares has to be proved yet and, among the possible signatures, the emission of electromagnetic radiation was considered in the literature. Proton beams were suggested by Benz and Simnett (1986) as exciters of slowly-drifting solar radio bursts and Smith and Benz (1989) proposed a model based on high-energy proton beams to explain non-drifting radio emissions observed in the frequency band 1 - - 3 GHz with a lifetime ranging from 2 to 4 s, but many questions concerning this point remain open (Messerotti, 1990). With regard to that in the present work the generation of Langmuir waves in neutral beam-plasma systems was studies using a 1-D electrostatic and a 1-D electromagnetic particle code. For a weak and monoenergetic neutral beam-plasma system a two-step saturation process of the electrostatic beam instability was found to be operating, firstly driven by beam electrons till their trapping and mixing in the phase space, when beam protons undertake the role. The addition of a thermal spread in the neutral beam resulted in the disappearance of the electron trapping phase. The effect of an external magnetic field was also analyzed and the simulations show that the saturation levels decrease according to a square cosine law in the angle between the considered and the beam propagation direction. Furthermore it was tried to estimate the influence of electromagnetic waves on the generated Langmuir waves using a 1-D electromagnetic relativistic code. Numerical results indicate that such influence is negligible, as the level of em waves corresponds roughly to the noise level of the background plasma. A comparison of the parameters derived from the numerical experiments with those from the theoretical estimations confirm the adequate accuracy of the above results. In particular, the most relevant conclusion is that a real neutral beam with monoenergetic protons can generate Langmuir waves with a saturation level 150 times higher than that due to a pure electron beam with the same density and velocity and this suggests that observable radio emission could be produced.
- Publication:
-
Proceeedings of the 1st Symposium on Plasma Dynamics: Theory and Applications. Consorzio di Magnetofluidodinamica
- Pub Date:
- 1992
- Bibcode:
- 1992pdta.proc..115M