Abstract
Collective effects with strong coupling between the longitudinal and transverse dynamics are of fundamental importance for applications of high-intensity bunched beams. The self-consistent Vlasov-Maxwell equations are applied to high-intensity finite-length charge bunches, and a generalized particle simulation algorithm is developed for bunched beams with or without energy anisotropy. The nonlinear method exhibits minimal noise and accuracy problems in comparison with standard particle-in-cell simulations. Systematic studies are carried out under conditions corresponding to strong 3D nonlinear space-charge forces in the beam frame. For charge bunches with isotropic energy, finite bunch-length effects are clearly evident by the fact that the spectra for an infinitely long coasting beam and a nearly spherical charge bunch have strong similarities, whereas the spectra have distinctly different features when the bunch length is varied between these two limiting cases. For bunched beams with anisotropic energy, there exists no exact kinetic equilibrium because the particle dynamics do not conserve transverse energy and longitudinal energy separately. A reference state in approximate dynamic equilibrium has been constructed theoretically, and a quasi-steady state has been established in the simulations for the anisotropic case. Collective excitations relative to the reference state have been simulated using the generalized algorithm. In particular, the electrostatic Harris instability driven by strong energy anisotropy is investigated for a finite-length charge bunch. The observed growth rates are larger than those obtained for infinitely long coasting beams. However, the growth rate decreases for increasing bunch length to a value similar to the case of a long coasting beam. For long bunches, the instability is axially localized symmetrically relative to the beam center, and the characteristic wavelength in the longitudinal direction is comparable to the transverse dimension of the beam.
Highlights
Collective effects in high-intensity charged particle beams are often manifest by collective excitations with certain dynamical properties such as instabilities and Landau damping [1,2,3,4,5,6]
The neutralized drift compression experiment (NDCX) research program is focused on the capability of compressing heavy ion charge bunches both longitudinally and transversely [10 –12] to reach the high intensity and short pulse length required for creating high energy density matter and heavy ion fusion conditions in the laboratory [13,14]
Collective effects associated with the strong coupling between the longitudinal and transverse particle dynamics are of fundamental importance for applications of highintensity bunched beams
Summary
Collective effects in high-intensity charged particle beams are often manifest by collective excitations with certain dynamical properties such as instabilities and Landau damping [1,2,3,4,5,6]. We report recent progress in developing a generalized nonlinear f simulation method to study collective effects for finite-length charge bunches with nonlinear space-charge fields in both the longitudinal and transverse directions. The present analysis is carried out in the beam frame Another consequence of the coupling effect in a finite-length charge bunch is that the particle dynamics does not conserve transverse energy and longitudinal energy separately, and there exists no exact kinetic equilibrium @=@t 0 which has anisotropic energy in. Previous studies have shown that the nonintegrability induced by the coupling between the longitudinal and transverse dynamics is relatively weak, even for high-intensity beams [34 –36] Based on this fact, we have developed a reference state for beams with anisotropic energy, which is not an exact, but an approximate equilibrium solution of the Vlasov-Maxwell equations.
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