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

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

Aug 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003phrvs...6h4401s&link_type=abstract

Physical Review Special Topics - Accelerators and Beams, vol. 6, Issue 8, id. 084401

Physics

24

Charged-Particle Beams, Nonneutral Plasmas, Space-Charge-Dominated Beams, Beams In Particle Accelerators

Scientific paper

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 sb=ω^2pb/2γ2bω2β⊥≳0.5 are linearly unstable to short-wavelength perturbations with k2zr2b≳1, provided the ratio of longitudinal and transverse temperatures is smaller than some threshold value. Here, ω^2pb=4πn^be2b/γbmb is the relativistic plasma frequency squared, and ωβ⊥ 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.

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