Simulations and theories of relativistic ion cyclotron instabilities driven by MeV alpha particles in thermal deuterium plasmas

Chen, K. R.
May 2003
Physics of Plasmas;May2003, Vol. 10 Issue 5, p1315
Academic Journal
The harmonic relativistic ion cyclotron instabilities driven by MeV alpha particles in magnetized thermal deuterium plasmas are studied with a perturbation theory, a kinetic theory, and particle-in-cell simulations. Due to the mass deficit of alphas, the resonant harmonic cyclotron frequency of the alphas is not smaller than that of the deuterons such that the low harmonics are linearly stabilized by the deuterons. However, the thermal deuterons behave as a cold-gyro-stream plasma at high harmonics so that, by including relativistic streaming in gyro-phase, the alpha particles can drive the high harmonic electrostatic ion cyclotron instability, that is in a quadratic form, although their Lorentz factor is very close to unity (e.g., γ=1.000 94 for 3.5 MeV alphas). The dielectric of the first deuteron cyclotron harmonic determines the alpha harmonic number threshold for instability; the quadratic instability occurs in the lower-hybrid frequency regime. As in a sharp contrast to the nonresistive thermalization of MeV protons due to the two-gyro-stream instability dominating at low harmonics, the high harmonic interaction between the unstable waves and the alphas becomes selective; only alpha particles with its perpendicular momentum above a threshold are involved and the interaction stops when they are slowed down to the threshold. The resultant energy spectrum is in a shape close to the profile of the theoretical interaction strength between the alpha particles and the dominating wave mode; the alpha particles almost loss no net energy. New-born alpha particles experience a similar selective gyro-broadening process in a shorter time scale. A simple explanation for the selective gyro-broadening based on the wave-alpha interaction calculated from the perturbation theory is given.


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