Analytical theory and nonlinear δf perturbative simulations of temperature anisotropy instability in intense charged particle beams

In plasmas with strongly anisotropic distribution functions (T_∥b/T_⊥b≪1) a Harris-like collective instability may develop if there is sufficient coupling between the transverse and longitudinal degrees of freedom. Such anisotropies develop naturally in accelerators and may lead to a deterioration of beam quality. This paper extends previous numerical studies [ E. A. Startsev, R. C. Davidson and H. Qin Phys. Plasmas 9 3138 (2002)] of the stability properties of intense non-neutral charged particle beams with large temperature anisotropy (T_⊥b≫T_∥b) to allow for nonaxisymmetric perturbations with ∂/∂θ≠0. The most unstable modes are identified, and their eigenfrequencies, radial mode structure, and nonlinear dynamics are determined. The simulation results clearly show that moderately intense beams with s_b=ω̂_pb²/2γ_b²ω_β⊥²≳0.5 are linearly unstable to short-wavelength perturbations with k_z²r_b²≳1, provided the ratio of longitudinal and transverse temperatures is smaller than some threshold value. Here, ω̂_pb²=4πn̂_be_b²/γ_bm_b is the relativistic plasma frequency squared, and ω_β⊥ is the betatron frequency associated with the applied smooth-focusing field. A theoretical model is developed based on the Vlasov-Maxwell equations which describes the essential features of the linear stages of instability. Both the simulations and the analytical theory predict that the dipole mode (azimuthal mode number m=1) is the most unstable mode. In the nonlinear stage, tails develop in the longitudinal momentum distribution function, and the kinetic instability saturates due to resonant wave-particle interactions.