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.
PPPL, Princeton, New Jersey
LLNL, Livermore, California
LBNL, Berkeley, California
Voss Scientific, Albuquerque, New Mexico
Intense heavy ion beam pulses need to be compressed in both the transverse and longitudinal directions for warm dense matter and heavy ion fusion applications. Previous experiments and simulations utilized a drift region filled with high-density plasma in order to neutralize the space-charge and current of a 300 keV K+ beam, and achieved transverse and longitudinal focusing separately to a radius < 2 mm and pulse width < 5 ns, respectively. To achieve simultaneous beam compression, a strong solenoid is employed near the end of the drift region in order to transversely focus the beam to the longitudinal focal plane. Simulations of near-term experiments predict that the ion beam can be focused to a sub-mm spot size coincident with the longitudinal focal plane, reaching a peak beam density in the range 1012 - 1013 cm-3, provided that the plasma density is large enough for adequate neutralization. Optimizing the compression under the appropriate experimental constraints offers the potential of delivering higher intensity per unit length of accelerator to the target, thereby allowing more compact and cost-effective accelerators and transport lines to be used as ion beam drivers.
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)
LBNL, Berkeley, California
LLNL, Livermore, California
PPPL, Princeton, New Jersey
UCB, Berkeley, California
Voss Scientific, Albuquerque, New Mexico
This work was supported by the Office of Fusion Energy Sciences, of the U. S. Department of Energy under Contract No. DE-AC02-05CH11231, W-7405-Eng-48, DE-AC02-76CH3073 for HIFS-VNL.
PPPL, Princeton, New Jersey
Installation of a laser-induced fluorescence (LIF) diagnostic system has been completed and initial measurement of the beam density profile has been performed on the Paul trap simulator experiment (PTSX). The PTSX device is a linear Paul trap that simulates the collective processes and nonlinear transverse dynamics of an intense charged particle beam propagating through a periodic focusing quadrupole magnetic configuration. Although there are several visible transition lines for the laser excitation of barium ions, the transition from the metastable state has been considered first mainly because an operating, stable, broadband, and high-power laser system is available for experiments in this region of the red spectrum. The LIF system is composed of a dye laser, fiber optic cables, a line generator, which uses a Powell lens, collection optics, and a CCD camera system. Single-pass mode operation of the PTSX device is employed for the initial tests of the LIF system to make optimum use of the metastable ions. By minimizing the background light level, it is expected that enough signal to noise ratio can be obtained to re-construct the radial density profile of the ion beam.
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.
PPPL, Princeton, New Jersey
LBNL, Berkeley, California
Plasmas are sources of electrons for charge neutralizing ion beams to allow them to focus to small spot sizes and compress their axial pulse length. Sources must operate at low pressures and without strong electric/magnetic fields. To produce meter-long plasmas, sources based on ferroelectric ceramics with large dielectric coefficients were developed. The sources use BaTiO3 ceramic to form plasma. The drift tube inner wall of the Neutralized Drift Compression Experiment (NDCX) is covered with ceramic and ~7 kV is applied across the wall of the ceramics. A 20-cm-long prototype source produced plasma densities of 5·1011 cm-3. It was integrated into the Neutralized Transport Experiment and successfully neutralized the K+ beam. A one-meter-long source comprised of five 20-cm-long sources has been tested and characterized, producing relatively uniform plasma over the length of the source in the 1·1010 cm-3 range. This source was integrated into NDCX for beam compression experiments. Experiments with this source yielded compression ratios ~80. Future work will consider longer and higher plasma density sources to support beam compression and high energy density experiments.
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.
PU, Princeton, New Jersey
PPPL, Princeton, New Jersey
Evaluation of ion-atom charge-changing cross sections is needed for many accelerator applications. The validity of the classical trajectory approximation has been studied by comparing the results of simulations with available experimental data and full quantum-mechanical calculations [1]. Additionally, a theoretical criterion has been developed for the validity of the classical trajectory approximation [2]. For benchmarking purposes, a Classical Trajectory Monte Carlo simulation (CTMC) is used to calculate ionization and charge exchange cross sections for most simple, hydrogen and helium targets in collisions with various ions. The calculated cross sections compare favorably with the experimental results for projectile velocities near the projectile velocity corresponding to the maximum of cross section as a function of projectile velocity. At higher or lower velocities, quantum-mechanical effects become more significant and the CTMC results agree less well with the experimental values of the cross sections.
[1] I. D. Kaganovich, et al., , New Journal of Physics 8, 278 (2006).
[2] Igor D. Kaganovich, et al., Nucl. Instr. and Methods A 544, 91(2005).
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.