PIC simulations of relativistic electron flows : The fastest-growing mixed mode and the electromagnetic finite grid instability

• Instabilities driven by two electron beams that stream at the relativistic relative velocity vb are important as a plasma thermalization and magnetic field generation mechanism in astrophysics and in inertial confinement fusion. The time-evolution of such instabilities, which are subdivided into the two-stream, mixed mode and filamentation (beam-Weibel) instability, is multi-dimensional and nonlinear and the instabilities are usually modelled with particle-in-cell (PIC) simulations. We examine the wave spectrum we obtain, if both beams have a density ratio 9 and (1-vb^2/c^2)^(-1/2)=4. The mixed mode dominates and grows fastest for highly relativistic vb. An electromagnetic finite grid instability is shown to generate artificial magnetic fields, imposing a simulation constraint.

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