Search Results (17 total)

Using a movable Schottky cavity resonant at 2.07 GHz, we have developed a simple method of deriving the beam sizes at the detector. In this report we will explain the theory behind the method, describe the system and the signal processing, and then present the results from experiments using this method. We will also present our plans for using this new technique for obtaining beam emittances during normal operation of the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL).

We show that, for a test ion moving in a collisionless single-species electron plasma, exact analytical solutions can be obtained for certain anisotropic velocity distributions of the electron plasma. By comparing the analytical formula with the numerical results calculated for the more realistic Maxwellian plasma, we demonstrate that plasmas with three different velocity distributions behave similarly for ions moving with velocity smaller than the velocity spread of the electrons. Furthermore, we show that the response of the electron density to a rest ion decays exponentially with distance, provided the anisotropic velocity distribution exhibits elliptical symmetry.

Operational stochastic cooling of 100  GeV/nucleon gold beams has been achieved in the BNL Relativistic Heavy-Ion Collider. We discuss the physics and technology of the longitudinal cooling system and present results with the beams. A simulation algorithm is described and shown to accurately model the system.

Since 2001, the Relativistic Heavy Ion Collider has experienced electron cloud effects, some of which have limited the beam intensity. These include dynamic pressure rises (including pressure instabilities), tune shifts, a reduction of the instability threshold for bunches crossing the transition energy, and possibly incoherent emittance growth. We summarize the main observations in operation and dedicated experiments as well as countermeasures including baking, nonevaporable getter coated warm beam pipes, solenoids, bunch patterns, antigrazing rings, prepumped cold beam pipes, scrubbing, and operation with long bunches.

The four electron stripping stages leading to fully stripped gold ions in the Relativistic Heavy Ion Collider (RHIC) are briefly described. The third stripper, which removes 46 electrons from the Au³¹⁺ ions leading to heliumlike Au⁷⁷⁺, offers the greatest challenges in terms of energy loss and induced energy spread. These problems are described in detail as well as recent advances in the design and performance of this stripper. Measurements performed with several carbon and aluminum strippers show general agreement with a semiempirical model but small systematic deviations suggest that some model adjustments may be in order. The best performance is predicted and obtained with a combined carbon-aluminum foil system. Measurements showing the enhanced performance in the alternating gradient synchrotron are described. The stripper that removes the last two electrons has also been improved and the results of relevant calculations and measurements are presented.

Bunched beam stochastic cooling in the Relativistic Heavy Ion Collider (RHIC) at 100 GeV has been achieved. The longitudinal cooling system is designed for heavy ion operation but was tested using protons. A very low intensity bunch with ∼10⁹ protons was prepared so that cooling times and voltage requirements would be comparable to the heavy ion case. With this bunch a cooling time of the order of an hour was observed through shortening of the bunch length and narrowing of the Schottky lines.

The Brookhaven Relativistic Heavy Ion Collider (RHIC) has been providing collisions of polarized protons at a beam energy of 100 GeV since 2001. Equipped with two full Siberian snakes in each ring, polarization is preserved during acceleration from injection to 100 GeV. However, the intrinsic spin resonances beyond 100 GeV are about a factor of 2 stronger than those below 100 GeV making it important to examine the impact of these strong intrinsic spin resonances on polarization survival and the tolerance for vertical orbit distortions. Polarized protons were first accelerated to the record energy of 205 GeV in RHIC with a significant polarization measured at top energy in 2005. This Letter presents the results and discusses the sensitivity of the polarization survival to orbit distortions.

We present a design of nonscaling fixed field alternating gradient accelerators (FFAG) minimizing the dispersion action function H. The design is considered both analytically and via computer modeling. We present the basic principles of a nonscaling FFAG lattice and discuss optimization strategies so that one can accelerate over a broad range of momentum with reasonable apertures. Acceleration schemes for muons are discussed.

The energy gain and motion of electrons can quantitatively describe the mechanism of electron multipacting in a long-bunched proton machine. Strong multipacting usually happens around the bunches’ tails due to the high energy of electrons when they hit the chamber surface. We investigated several important parameters of electron multipacting, proving that it is sensitive to the beam’s intensity, the shape of its longitudinal profile, its transverse size, the secondary emission yield, and the energy at peak secondary emission yield. Our analyses, simulations, and experiments are all in agreement.

Stable, coherent, longitudinal oscillations have been observed in several accelerators. Within the context of perturbation theory, the beam parameters and machine impedance often suggest these oscillations should be Landau damped. When nonlinear effects are included, long-lived, stable oscillations become possible for low intensity beams. In this paper we report observations of stable humps in bunched beams and present a theoretical framework for their description. Implications for bunched beam stochastic cooling are considered.

Electron cloud instabilities in the Los Alamos Proton Storage Ring and those foreseen for the Oak Ridge Spallation Neutron Source are examined theoretically, numerically, and experimentally.

Intense ion beams in the Brookhaven Relativistic Ion Collider lead to a rise in the vacuum pressure. Electron clouds can contribute to such a process. To measure electron cloud densities the coherent tune shift along the bunch train was observed with different bunch spacings and intensities. From the measured coherent tune shifts, electron cloud densities are computed and compared with densities obtained in electron cloud simulations.

Transverse stability with nonlinear space charge is studied within the context of coasting beams. For bare tune spreads originating from chromaticity or frequency slip, the space charge tune spread has a fairly small effect and the incoherent space charge force is well modeled by a transverse capacitance. For tune spreads due to octupoles or fringe fields, beams are more stable when the bare tune increases with betatron amplitude.

This paper summarizes the low-loss design for the Spallation Neutron Source accumulator ring [“Spallation Neutron Source Design Manual” (unpublished)]. A hybrid lattice consisting of FODO arcs and doublet straights provides optimum matching and flexibility for injection and collimation. For this lattice, optimization focuses on six design goals: a space-charge tune shift low enough (below 0.15) to avoid strong resonances, adequate transverse and momentum acceptance for efficient beam collimation, injection optimized for desired target beam shape and minimal halo development, compensation of magnet field errors, control of impedance and instability, and prevention against accidental system malfunction. With an expected collimation efficiency of more than 90%, the uncontrolled fractional beam loss is expected to be at the 10^-4 level.

A barrier bucket experiment with two dedicated barrier cavities was performed at the Brookhaven AGS. One of the barrier cavities was a magnetic alloy (MA)–loaded cavity and the other was a ferrite-loaded cavity. They generated a single sine wave with a peak voltage of 40 kV at a repetition rate of 351 kHz. A barrier rf system was established with these cavities and five bunches from the AGS booster were accumulated. A total of 3×10¹³ protons were stored without beam loss, and were successfully rebunched and accelerated. The longitudinal emittance growth was observed during accumulation by the barrier bucket, the blowup factor of which was about 3. The longitudinal mismatch between the rf bucket and the beam bunch was the main reason for the emittance growth. The potential distortions by beam loading of the ferrite cavity and the overshooting voltage of the MA cavity disturbed the smooth debunching.

The fast head-tail instability with space charge is studied using series expansion techniques, numerical simulations, and a new formulation which allows for precise estimates of growth rates and thresholds. In regimes where they are reliable, all three techniques predict that space charge suppresses the fast head-tail instability. It is found that the series expansion techniques are unreliable for parameter regimes commonly employed in hadron accelerators. The numerical techniques are less prone to error, but the computational requirements become severe as space charge tune shifts increase. The new model has neither of these problems, but it underestimates the benefits of chromaticity, at least in its simplest form.

The effect of space charge on transverse instabilities in synchrotrons is considered. In addition to the coherent forces produced by image currents on the vacuum chamber walls, there are direct particle-particle forces that can significantly decrease the incoherent tune. Both types of space-charge force are a function of longitudinal position within a bunch, being proportional to the instantaneous current. In the low- and very-high-intensity regimes we find that the space-charge forces tend to reduce growth rates. A technique for the intermediate-intensity range, which includes the effect of incoherent space-charge tune spread, is introduced.