Phys. Rev. ST Accel. Beams 6, 084401 (2003) [15 pages]

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

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Edward A. Startsev, Ronald C. Davidson, and Hong Qin
Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543, USA

Received 8 May 2003; published 15 August 2003

In plasmas with strongly anisotropic distribution functions (Tb/Tb≪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 (TbTb) 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-^ pb2/2γb2ωβ⊥2≳0.5 are linearly unstable to short-wavelength perturbations with kz2rb2≳1, provided the ratio of longitudinal and transverse temperatures is smaller than some threshold value. Here, ω-^ pb2=4πn-^ beb2bmb 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.


©2003 The American Physical Society

URL: http://link.aps.org/doi/10.1103/PhysRevSTAB.6.084401
DOI: 10.1103/PhysRevSTAB.6.084401
PACS: 41.75.–i, 52.27.Jt, 52.59.Sa, 29.27.–a

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