Nonlinear aspects of collective, electromagnetic interactions in magnetized plasmas with anisotropic protons and isotropic alpha particles

Abstract

We use computer simulation experiments to investigate the nonlinear behaviour of plasmas with a mixture of anisotropic protons and isotropie alpha particles, embedded in a static magnetic field. Specifically, we study the linearly predicted ‘stop-band’ for the propagation of the proton-produced electromagnetic ion cyclotron waves in conjunction with the energization of the heavier ions by the same waves. For this, three cases are considered: (1) proton + electron plasma; (2) proton + electron + cold alpha particle plasma, and (3) proton + electron + warm alpha particle plasma. Among the main results obtained we mention the following, (a) In the presence of significant relative He2+ concentrations (either cold or warm) all proton-produced left-polarized waves having frequencies above the alpha-particle gyrofrequency are practically suppressed, during the entire nonlinear evolution of the system, indicating that particle–wave–particle interactions are confined to the low-frequency branch of the waves, (b) The ‘remnant’ wave energy, i.e. that part of the wave energy not transferred to the particles, decreases significantly when going from case 1 to case 3. (c) Nevertheless, in all three cases, the initial proton thermal anisotropy relaxes to the same quasi-equilibrium value (≃ 1·5). (d) The cold alpha particles in case 2 are strongly heated by their non-resonant interaction with the proton-produced ion cyclotron electromagnetic waves, (e) In contrast, the initially warm isotropic alpha particles in case 3 are heated by resonant interaction with the proton-produced waves, resulting in an increase in the perpendicular energy and a decrease in the parallel energy. The physical processes involved in the collisionless interaction of these mixed protons and heavier ions (alpha particles) are discussed.