Search Results (13 total)

Large-space-scale and long-time-scale plasma flow simulations are executed in order to study the spatial and temporal evolution of plasma parameters for two types of plasma sources used in the neutralized drift compression experiment (NDCX). The results help assess the charge neutralization conditions for ion beam compression experiments and can be employed in more sophisticated simulations, which previously neglected the dynamical evolution of the plasma. Three-dimensional simulations of a filtered cathodic-arc plasma source show the coupling efficiency of the plasma flow from the source to the drift region depends on geometrical factors. The nonuniform magnetic topology complicates the well-known general analytical considerations for evaluating guiding-center drifts, and particle-in-cell simulations provide a self-consistent evaluation of the physics in an otherwise challenging scenario. Plasma flow profiles of a ferroelectric plasma source demonstrate that the densities required for longitudinal compression experiments involving ion beams are provided over the drift length, and are in good agreement with measurements. Simulations involving azimuthally asymmetric plasma creation conditions show that symmetric profiles are nevertheless achieved at the time of peak on-axis plasma density. Also, the ferroelectric plasma expands upstream on the thermal expansion time scale, and therefore avoids the possibility of penetration into the acceleration gap and transport sections, where partial neutralization would increase the beam emittance. Future experiments on NDCX will investigate the transverse focusing of an axially compressing intense charge bunch to a sub-mm spot size with coincident focal planes using a strong final-focus solenoid. In order to fill a multi-tesla solenoid with the necessary high-density plasma for beam charge neutralization, the simulations predict that supersonically injected plasma from the low-field region will penetrate and partially fill the high-field region of the solenoid. Because of the magnetic mirroring effect, the on-axis plasma density in the solenoid depends on the injection velocity and magnetic field strength.

Longitudinal bunching factors in excess of 70 of a 300-keV, 27-mA K⁺ ion beam have been demonstrated in the neutralized drift compression experiment [P. K. Roy , Phys. Rev. Lett. 95, 234801 (2005)] in rough agreement with particle-in-cell source-to-target simulations. A key aspect of these experiments is that a preformed plasma provides charge neutralization of the ion beam in the last one meter drift region where the beam perveance becomes large. The simulations utilize the measured ion source temperature, diode voltage, and induction-bunching-module voltage waveforms in order to determine the initial beam longitudinal phase space which is critical to accurate modeling of the longitudinal compression. To enable simultaneous longitudinal and transverse compression, numerical simulations were used in the design of the solenoidal focusing system that compensated for the impact of the applied velocity tilt on the transverse phase space of the beam. Complete source-to-target simulations, that include detailed modeling of the diode, magnetic transport, induction bunching module, and plasma neutralized transport, were critical to understanding the interplay between the various accelerator components in the experiment. Here, we compare simulation results with the experiment and discuss the contributions to longitudinal and transverse emittance that limit the final compression.

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 similar 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 for 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 by the beam compression process. Both the results of particle-in-cell simulations performed with the warp code and envelope equation solutions agree well with the experimental data.

The transverse compression of a long charge bunch is investigated in the Paul trap simulator experiment (PTSX), which is a linear Paul trap that simulates the nonlinear transverse dynamics of an intense charged particle beam propagating through an equivalent kilometers-long magnetic alternating-gradient (AG) focusing system. Changing the voltage amplitude at fixed focusing frequency in the PTSX device corresponds to changing the field gradient of the quadrupole magnets with fixed axial periodicity in the AG transport system. In this work, we present experimental results on transverse compression of the charge bunch in which the amplitude of the applied oscillatory focusing voltage is changed instantaneously, and adiabatically. The experimental data are also compared with analytical estimates and 2D WARP particle-in-cell simulations.

The Paul Trap Simulator Experiment (PTSX) is a linear Paul trap whose purpose is to simulate the nonlinear transverse dynamics of intense charged particle beam propagation in periodic-focusing quadrupole magnetic transport systems. Externally created cesium ions are injected and trapped in the long central electrodes of the PTSX device. In order to have well-matched one-component plasma equilibria for various beam physics experiments, it is important to optimize the ion injection. From the experimental studies reported in this paper, it is found that the injection process can be optimized by minimizing the beam mismatch between the source and the focusing lattice, and by minimizing the number of particles present in the vicinity of the injection electrodes when the injection electrodes are switched from the fully oscillating voltage waveform to their static trapping voltage.

Heavy ion drivers for heavy ion fusion and high energy density physics applications use space-charge-dominated ion beams which must undergo longitudinal bunch compression in order to meet the requisite beam intensities desired at the target. The Neutralized Drift Compression Experiment-1A (NDCX-1A) at Lawrence Berkeley National Laboratory is used to determine the effective limits of neutralized drift compression, which occurs due to an imposed longitudinal velocity tilt on the drifting beam and subsequent neutralization of the beam’s space charge with background plasma. The accurate and temporally resolved measurement of the ion beam’s current and pulse length, which has been longitudinally compressed to a few nanoseconds duration at its focal plane, is a critical diagnostic. This paper describes the design and experimental results for a fast and accurate ion beam probe, which reliably measures the absolute beam current in the presence of high density plasma at the focal plane as a function of time. A particle-in-cell code has been used to model the propagation of the intense ion beam and to design the diagnostic probe.

Longitudinal compression of a velocity-tailored, intense neutralized K⁺ beam at 300 keV, 25 mA has been demonstrated. The compression takes place in a 1–2 m drift section filled with plasma to provide space-charge neutralization. An induction cell produces a head-to-tail velocity ramp that longitudinally compresses the neutralized beam, enhancing the beam peak current by a factor of 50 and producing a pulse duration of about 3 ns. This measurement has been confirmed independently with two different diagnostic systems.

In heavy-ion inertial-confinement fusion systems, intense beams of ions must be transported from the exit of the final-focus magnet system through the fusion chamber to hit spots on the target with radii of about 2 mm. For the heavy-ion-fusion power-plant scenarios presently favored in the U.S., a substantial fraction of the ion-beam space charge must be neutralized during this final transport. The most effective neutralization technique found in numerical simulations is to pass each beam through a low-density plasma after the final focusing. To provide quantitative comparisons of these theoretical predictions with experiment, the Virtual National Laboratory for Heavy Ion Fusion has completed the construction and has begun experimentation with the neutralized-transport experiment. The experiment consists of three main sections, each with its own physics issues. The injector is designed to generate a very high-brightness, space-charge-dominated potassium beam, while still allowing variable perveance by a beam aperturing technique. The magnetic-focusing section, consisting of four pulsed quadrupoles, permits the study of magnet tuning, as well as the effects of phase-space dilution due to higher-order nonlinear fields. In the final section, the converging ion beam exiting the magnetic section is transported through a drift region with plasma sources for beam neutralization, and the final spot size is measured under various conditions of neutralization. In this paper, we discuss the design and characterization of the three sections in detail and present initial results from the experiment.

The results presented here demonstrate that the Paul trap simulator experiment (PTSX) simulates the propagation of intense charged particle beams over distances of many kilometers through magnetic alternating-gradient (AG) transport systems by making use of the similarity between the transverse dynamics of particles in the two systems. Plasmas have been trapped that correspond to normalized intensity parameters s-^ =ω_p²(0)/2ω_q²≤0.8, where ω_p(r) is the plasma frequency and ω_q is the average transverse focusing frequency in the smooth-focusing approximation. The measured rms radius of the beam is consistent with a model, equally applicable to both PTSX and AG systems. The PTSX device confines one-component cesium ion plasmas for hundreds of milliseconds, which is equivalent to over 10 km of beam propagation.

Small transverse magnetic quadrupole fields sharply degrade the confinement of non-neutral plasmas held in Malmberg-Penning traps. For example, a quadrupole magnetic field of only 0.02   G/cm doubles the diffusion rate in a trap with a 100 G axial magnetic field. Larger quadrupole fields noticeably change the shape of the plasma. The transport is greatest at an orbital resonance. These results cast doubt on plans to use magnetic quadrupole neutral atom traps to confine antihydrogen atoms created in double-well positron/antiproton Malmberg-Penning traps.

The decay of the diocotron rotation was studied in a new regime in which trap asymmetries dominate. Decay within a few diocotron periods was observed, sometimes orders of magnitude faster than predicted by the traditional “rotational pumping” theory. The decay does not conserve angular momentum, and is strongest for small, low-density columns. The new regime appears when “magnetron-like” rotation from the end confinement fields becomes dominant, and appears to be associated with errors in these fields. Transition to decay dominated by rotational pumping was observed for larger and denser columns. The asymmetry-dominated transport was also studied, and found to depend linearly on the line density (and not the density) over nearly 4 orders of magnitude.

We study the stability of small strangelets by employing a simple model of strange matter as a gas of noninteracting fermions confined in a bag. We solve the Dirac equation and populate the energy levels of the bag one quark at a time. We find that for system parameters such that strange matter is unbound in bulk, there may still exist strangelets with A<100 that are metastable. We cannot determine, however, whether the lifetime of these strangelets is sufficient to detect them in current accelerator experiments.