Search Results (17 total)

A combination of analytical calculations and Monte Carlo simulations is used to find the ground state structures in monodisperse ferrofluid monolayers. Taking into account the magnetic dipole-dipole interaction between all particles in the system we observe different topological structures that are likely to exist at low temperatures. The most energetically favored structures we find are rings, embedded rings, and rings side by side, and we are able to derive analytical expressions for the total energy of these structures. A detailed analysis of embedded rings and rings side by side shows that the interring interactions are negligible. We furthermore find that a single ideal ring is the ground state structure for a ferrofluid monolayer. We compared our theoretical predictions to the results of simulated annealing data and found them to be in excellent agreement.

The use of nonlinear lattices with the large betatron tune spreads can increase instability and space charge thresholds by orders of magnitude compared to typical linear accelerator lattices. Unfortunately, strong nonlinear fields create, in general, strong resonances and chaotic motion. This shrinks the dynamic aperture to impractical values, thus erasing all benefits from their use. Previously known examples of stable and regular accelerator motion with special nonlinear lenses were related to one-dimensional motion or round beams. However, no solution has been realized with real 2D transverse magnetic fields to produce stable, close to regular 2D motion with the large dynamic aperture and betatron tune spread comparable to the betatron tune itself. This paper presents possible solutions for such 2D lattices. They consist of straight sections with short linear and nonlinear lenses with transverse magnetic fields.

Thin carbon foils are used as strippers for charge exchange injection into high intensity proton rings. However, the stripping foils become radioactive and produce uncontrolled beam loss, which is one of the main factors limiting beam power in high intensity proton rings. Recently, we presented a scheme for laser stripping an H^- beam for the Spallation Neutron Source (SNS) ring. First, H^- atoms are converted to H⁰ by a magnetic field, then H⁰ atoms are excited from the ground state to the upper levels by a laser, and the excited states are converted to protons by a magnetic field. In this paper we report on the proof-of-principle demonstration of this scheme to give high efficiency (around 90%) conversion of H^- beam into protons at SNS in Oak Ridge. The experimental setup is described, and comparison of the experimental data with simulations is presented.

We present experimental data from the Los Alamos Proton Storage Ring (PSR) showing long-lived linac microbunch structure during beam storage with no rf bunching. Analysis of the experimental data and particle-in-cell simulations of the experiments indicate that space charge, coupled with energy spread effects, is responsible for the sustained microbunch structure. The simulated longitudinal phase space of the beam reveals a well-defined separatrix in the phase space between linac microbunches, with particles executing unbounded motion outside of the separatrix. We show that the longitudinal phase space of the beam was near steady state during the PSR experiments, such that the separatrix persisted for long periods of time. Our simulations indicate that the steady state is very sensitive to the experimental conditions. Finally, we solve the steady-state problem in an analytic, self-consistent fashion for a set of periodic longitudinal space-charge potentials.

Finding self-consistent distributions of beam particles interacting with each other via the space charge force is one of the challenges of accelerator physics. Exactly solvable models are used for simulation benchmarks, instability threshold calculations, etc. Since such distributions have been found only in one and two dimensions (Kapchinsky-Vladimirsky distribution), it is not possible to apply them to a general three dimensional motion. This paper shows how to construct new sets of self-consistent distributions, extending even to the three dimensional case.

This paper presents a scheme for three-step laser-based stripping of an H^- beam for charge exchange injection into a high-intensity proton ring. First, H^- atoms are converted to H⁰ by Lorentz stripping in a strong magnetic field, then neutral hydrogen atoms are excited from the ground state to upper levels by a laser, and the remaining electron, now more weakly bound, is stripped in a strong magnetic field. The energy spread of the beam particles gives rise to a Doppler broadened absorption linewidth, which makes for an inefficient population of the upper state by a narrow-band laser. We propose to overcome this limitation with a “frequency sweeping” arrangement, which populates the upper state with almost 100% efficiency. We present estimates of peak laser power and describe a method to reduce the power by tailoring the dispersion function at the laser-particle beam interaction point. We present a scheme for reducing the average power requirements by using an optical ring resonator. Finally, we discuss an experimental setup to demonstrate this approach in a proof-of-principle experiment.

We present a particle core model study of the space charge effect on high intensity synchrotron beams, with specific emphasis on the Proton Storage Ring (PSR) at Los Alamos National Laboratory. Our particle core model formulation includes realistic lattice focusing and dispersion. We transport both matched and mismatched beams through real lattice structure and compare the results with those of an equivalent uniform-focusing approximation. The effects of lattice structure and finite momentum spread on the resonance behavior are specifically targeted. Stroboscopic maps of the mismatched envelope are constructed and show high-order resonances and stochastic effects that dominate at high mismatch or high intensity. We observe the evolution of the envelope phase-space structure during a high intensity PSR beam accumulation. Finally, we examine the envelope-particle parametric resonance condition and discuss the possibility for halo growth in synchrotron beams due to this mechanism.

Because of a long-range resistive wake, the closed orbit may experience an unstable drift. Unlike the conventional betatron instabilities, this closed orbit instability is not sensitive to the spread of the betatron frequencies. For bunched beams, feedback appears to be the only way to stabilize the closed orbit above threshold. This new instability can be significant for both existing and designed high-intensity rings.

Elliptic flow from nuclear collisions is a hadronic observable sensitive to the early stages of system evolution. We report first results on elliptic flow of charged particles at midrapidity in Au+Au collisions at sqrt[s_NN]  =  130  GeV using the STAR Time Projection Chamber at the Relativistic Heavy Ion Collider. The elliptic flow signal, v₂, averaged over transverse momentum, reaches values of about 6% for relatively peripheral collisions and decreases for the more central collisions. This can be interpreted as the observation of a higher degree of thermalization than at lower collision energies. Pseudorapidity and transverse momentum dependence of elliptic flow are also presented.

Based on experience gained from present machines, a new generation of accelerators with high intensities and low losses is being designed. For example, the design for the Spallation Neutron Source storage ring specifies fractional beam losses of the order of 10^-4, so that even small instabilities or resonances can lead to the violation of this number. The purpose of this paper is to show that there exist potentially important beam fields, missing from standard analyses, that can lead either to instabilities or to large deviations of instability thresholds from their conventional values. Some of these fields and related effects, e.g., “fast damping,” were discovered earlier and are presented here in more standard form as an extension of the wake field's definition. In addition, nonrelativistic collective phenomena are analyzed. It is shown that the nonrelativistic case could be significantly different from the relativistic case.

Numerical calculations for the Spallation Neutron Source accumulator ring indicate that lattice resonances excited by the space-charge potential can increase a mismatch significantly by deforming the beam distribution in phase space. Hence increased mismatch leads to enhanced envelope oscillations that are driving the 2:1 parametric resonance leading to halo formation, even for initially matched beams. We have observed this behavior for the 2ν_x-2ν_y=0 resonance and for the 4ν_y=23 resonance. This mechanism for halo formation peculiar to rings through resonance driven mismatch is very sensitive to the tunes, which emphasizes the importance of a careful choice of operating point in tune space.

Uncontrolled beam losses due to space-charge-induced halo generation are a concern in high intensity rings, which are characterized by high beam intensities and low uncontrolled beam loss requirements. It is therefore important to investigate the dynamics of space charge in high intensity rings. We report here the results of extensive calculations using a particle-tracking approach with a self-consistent particle-in-cell model and alternatively with a particle core model. We find that the inclusion of space charge forces provides agreement between calculated and experimentally observed beam profile shapes in the high intensity proton storage ring. We also confirm computationally the extension to rings of the accepted dynamics of halo generation with rms beam mismatch exciting the parametric resonance. In addition, we propose a new two-stage mechanism for halo production in rings in which space-charge-driven lattice resonances generate beam mismatch that excites the parametric resonance. Because of its dependence on lattice resonances, this mechanism is peculiar to rings and is capable of generating halo even from initially matched beams. It is also very sensitive to the operating point in tune space, as we show in the results of a vertical tune scan simulating injection into the Spallation Neutron Source accumulator ring. Our results extend and enhance the understanding of fundamental space charge physics, which has been developed for linear accelerators, to rings.

The beam-beam interaction in the Tevatron collider sets limits on bunch intensity and luminosity. These limits are caused by a tune spread in each bunch which is mostly due to head-on collisions, but there is also a bunch-to-bunch tune spread due to parasitic collisions in multibunch operation. We propose to compensate these effects with the use of a countertraveling electron beam, and we present general considerations and physics limitations of this technique.

The wake forces produced by a beam bunch can be reduced by making the vacuum chamber cross section axially asymmetric. Furthermore, the asymmetry results in a betatron tune shift for particles in the tail of the bunch. As a result, transverse instabilities of the bunch should be significantly suppressed for an asymmetric vacuum chamber.

This article is devoted to stability analysis of the antiproton beam interacting with an electron beam in an “electron lens” setup for beam-beam compensation in the Tevatron collider. Electron space charge forces cause transverse “head-tail” coupling within antiproton bunch which may lead to a transverse mode coupling instability (TMCI). We present a theory, analytical studies, and numerical simulations of this effect. An estimate of threshold longitudinal magnetic field necessary to avoid the instability is given. Dependence of the threshold on electron and antiproton beam parameters is studied.

Transverse mode coupling instability is one of the major limitations of a single bunch current in storage rings. Until now it has appeared in large electron-positron machines, while its presence in proton colliders has not been observed. This paper describes a theoretical analysis of the effect of longitudinal variation of the betatron tune on the transverse mode coupling instability. This variation can be introduced by an RF quadrupole. In the result, the instability threshold could be significantly increased when a modulation of the betatron frequency is comparable with the synchrotron tune.