PPPL, Princeton, New Jersey
The Paul Trap Simulator Experiment is a compact laboratory Paul trap that simulates a long, thin charged-particle bunch coasting through a kilometers-long magnetic alternating-gradient (AG) transport system by putting the physicist in the beam's frame-of-reference. The transverse dynamics of particles in both systems are described by the same sets of equations, including all nonlinear space-charge effects. The time-dependent quadrupolar electric fields created by the confinement electrodes of a linear Paul trap correspond to the axially-dependent magnetic fields applied in the AG system. Results are presented from experiments in which the lattice period and strength are changed over the course of the experiment to transversely compress a beam with an initial depressed-tune of 0.9. Instantaneous and smooth changes are considered. Emphasis is placed on determining the conditions that minimize the emittance growth and the number of halo particles produced after the beam compression. The results of PIC simulations performed with the WARP code agree well with the experimental data. Initial results from a newly installed laser-induced fluorescence diagnostic will also be discussed.
Voss Scientific, Albuquerque, New Mexico
PPPL, Princeton, New Jersey
The linear growth of the two-stream instability for a charged particle beam that is longitudinally compressing as it propagates through a background plasma (due to an applied velocity tilt) is examined. Detailed, 1D particle-in-cell simulations are carried out to examine the growth of a wave packet produced by a small amplitude density perturbation in the background plasma. Recent analytic and numerical work by Startsev and Davidson [1] predicted reduced linear growth rates, which are indeed observed in the simulations. Here, small-signal asymptotic gain factors are determined in a semi-analytic analysis and compared with the simulation results in the appropriate limits. Nonlinear effects in the PIC simulations, including wave breaking and particle-trapping, are found to limit the linear growth phase of the instability for both compressing and non-compressing beams.
[1] Phys. Plasmas 13, 62108 (2006)
PPPL, Princeton, New Jersey
The transverse compression and dynamics of intense charged particle beams, propagating through a periodic quadrupole lattice, play an important role in many accelerator physics applications. Typically, the compression can be achieved by means of increasing the focusing strength of the lattice along the beam propagation direction. However, beam propagation through the lattice transition region inevitably leads to a certain level of beam mismatch and halo formation. In this paper we present a detailed analysis of these phenomena using particle-in-cell (PIC) numerical simulations performed with the WARP code. A new definition of beam halo is proposed in this work that provides the opportunity to carry out a quantitative analysis of halo production by a beam mismatch.
USC, Los Angeles, California
PPPL, Princeton, New Jersey
Background plasma can be used as a convenient tool for manipulating intense charge particle beams, for example, for ballistic focusing and steering, because the plasma can effectively reduce the space-charge potential and self-magnetic field of the beam pulse. We previously developed a reduced analytical model of beam charge and current neutralization for an ion beam pulse propagating in a cold background plasma. The reduced-fluid description provides an important benchmark for numerical codes and yields useful scaling relations for different beam and plasma parameters. This model has been extended to include the additional effects of a solenoidal magnetic field and gas ionization. Analytical studies show that a sufficiently large solenoidal magnetic field can increase the degree of current neutralization of the ion beam pulse. The linear system of equations has been solved analytically in Fourier space. For a strong enough applied magnetic field, poles emerge in Fourier space. These poles are an indication that whistler waves and lower hybrid waves are excited by the beam pulse.
PPPL, Princeton, New Jersey
In 3D high-intensity bunched beams, the collective effects associated with strong coupling between the longitudinal and transverse dynamics are of fundamental importance. A direct consequence of this coupling is that the particle dynamics does not conserve transverse energy and longitudinal energy separately, and there exists no exact kinetic equilibrium which has an anisotropic energy in the transverse and longitudinal directions. The strong coupling also introduces a mechanism for the electrostatic Harris-type instability driven by strong temperature anisotropy, which exists naturally in beams that have been accelerated to large velocities. The self-consistent Vlasov-Maxwell equations are applied to high-intensity bunched beams, and a generalized low-noise delta-f particle simulation algorithm is developed for bunched beams with or without energy anisotropy. Systematic studies are carried out that determine the particle dynamics, the approximate equilibrium, and stability properties under conditions corresponding to strong 3D nonlinear space-charge force. Finite bunch-length effects on collective excitations and anisotropy-driven instabilities are also investigated.
PPPL, Princeton, New Jersey
This paper makes use of a one-dimensional kinetic model based on the Vlasov-Maxwell equations to describe nonlinear wave and soliton excitations in coasting charged particle beams. The kinetic description makes use of the recently-developed g-factor model [1] that incorporates self-consistently the effects of transverse density profile shape at moderate beam intensities. The nonlinear evolution of wave and soliton excitations is examined for disturbances both moving faster and moving slower than the sound speed, incorporating the important effects of wave dispersion [2]. Analytical solutions are obtained for nonlinear traveling wave pulses with and without trapped particles, and the results of nonlinear perturabtive particle-in-cell simulations are presented that describe the stability properties and long-time evolution.
[1] R. C. Davidson and E. A. Startsev, Phys. Rev. ST Accel. Beams 7, 024401 (2004).[2] R. C. Davidson, Phys. Rev. ST Accel. Beams 7, 054402 (2004).
PPPL, Princeton, New Jersey
A numerical scheme for the electromagnetic particle simulation of high-intensity charged-particle beams has been developed which is a modification of the Darwin model. The Darwin model neglects the transverse induction current in Ampere?s law and therefore eliminates fast electromagnetic (light) waves from the simulations. The model has been incorporated into the nonlinear delta-f Beam Equilibrium Stability and Transport(BEST) code. As a benchmark, we have applied the model to simulate the transverse electromagnetic Weibel-type instability in a single-species charged-particle beam with large temperature anisotropy. Results are compared with previous theoretical and numerical studies using the eighenmode code bEASt. The nonlinear stage of the Weibel instability is also studied using BEST code, and the mechanism for nonlinear saturation is identified.