Search Results (31 total)

The luminosity of the ring-ring version of the proposed electron-ion collider eRHIC is limited by the beam-beam effect on the electrons. Once the beam-beam limit is reached, the luminosity no longer increases linearly with the bunch intensity of the ion beam, but begins to saturate and even drops again if the beam-beam tuneshift is increased further. To overcome this limitation we investigate a compensation scheme with an electron lens acting on the electron beam. Using weak-strong simulations we find a possible luminosity increase of about a factor 2.

The beam-beam interaction is one of the dominant sources of emittance growth and luminosity lifetime deterioration. A current-carrying wire has been proposed to compensate long-range beam-beam effects in the LHC and strong localized long-range beam-beam effects are experimentally investigated in the RHIC collider. Tune shift, beam transfer function, and beam loss rate are measured in dedicated experiments. In this paper, we report on simulations to study the effect of beam-wire interactions based on diffusive apertures, beam loss rates, and beam transfer function using a parallelized weak-strong beam simulation code (bbsimc). The simulation results are compared with measurements performed in RHIC during 2007 and 2008.

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.

We report the first observations of beam losses due to bound-free pair production at the interaction point of a heavy-ion collider. This process is expected to be a major luminosity limit for the CERN Large Hadron Collider when it operates with ²⁰⁸Pb⁸²⁺ ions because the localized energy deposition by the lost ions may quench superconducting magnet coils. Measurements were performed at the BNL Relativistic Heavy Ion Collider (RHIC) during operation with 100  GeV/nucleon ⁶³Cu²⁹⁺ ions. At RHIC, the rate, energy and magnetic field are low enough so that magnet quenching is not an issue. The hadronic showers produced when the single-electron ions struck the RHIC beam pipe were observed using an array of photodiodes. The measurement confirms the order of magnitude of the theoretical cross section previously calculated by others.

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.

Molecular desorption coefficients from electron bombardment of the warm Relativistic Heavy Ion Collider beam pipe are derived from measurements for both baked and unbaked stainless steel. For this, we analyze electron detector and pressure gauge signals in the presence of an electron cloud. Finally, we present a comparison between the measured and simulated energy spectrum of the cloud electrons.

Recently, resonance driving terms were successfully measured in the CERN SPS and the BNL RHIC from the Fourier spectrum of beam position monitor (BPM) data. Based on these measurements a new analysis has been derived to extract truly local observables from BPM data. These local observables are called local resonance terms since they share some similarities with the global resonance terms. In this paper we derive these local terms analytically and present experimental measurements of sextupolar global and local resonance terms in RHIC. Nondestructive measurements of these terms using ac dipoles are also presented.

Grazing collisions with the stainless steel beam pipes of gold ions, the so-called “halo scraping,” result in large secondary electron emission and surface molecular desorption yields in the Relativistic Heavy Ion Collider. Here we estimate electron emission yields as function of incidence angle, we show that desorption rates will follow a similar angular dependence at small angles, and we propose a simple approach to mitigate these effects.

The linear one-turn map of a storage ring contains coupling information on which a correction algorithm can be based. In principal, the one-turn matrix can be fitted from turn-by-turn data of beam position monitors. However, the signal-to-noise ratio of the coupling information can be greatly enhanced by fitting maps for larger turn numbers N. Furthermore, by using a number of beam position monitors in a region with only small coupling sources, the determination of the N-turn map can be made robust against failures of individual beam position monitors, and the signal-to-noise ratio of the coupling information is further enhanced. With the so obtained N-turn maps an automated global coupling correction is possible without the need for a tune change. This is demonstrated for the Brookhaven Relativistic Heavy Ion Collider where the implementation of the algorithm allows a global coupling correction within a few seconds at injection.

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.

Polarized neutrons are used to determine the antisymmetric part of the magnetic susceptibility in noncentrosymmetric MnSi. The paramagnetic fluctuations are found to be incommensurate with the chemical lattice and to have a chiral character. We argue that antisymmetric interactions must be taken into account to properly describe the critical dynamics in MnSi above T_C. The possibility of directly measuring the polarization dependent part of the dynamical susceptibility in a large class of compounds by polarized inelastic neutron scattering is outlined as it can yield evidence for antisymmetric interactions such as spin-orbit coupling in metals as well as in insulators.

The two rings of the Relativistic Heavy Ion Collider are equipped with superconducting dipole magnets. At injection, induced persistent currents in these magnets lead to a sextupole component. As the persistent currents decay with time, the horizontal and vertical chromaticities change. From magnet measurements of persistent current decays, chromaticity changes in the machine are estimated and compared with chromaticity measurements.

In order to further understand phenomena observed during studies of adiabatic excitation of longitudinal bunch shape oscillations [M. Bai et al., Phys. Rev. ST Accel. Beams 3, 064001 (2000)], we have developed a simulation using a one-turn map. In this report we will present the physical foundations for the simulation and the methods used in the simulator. We will present simulation results using parameters of actual experiments, along with the corresponding experimental results.

Response is made to J. A. MacLachlan's preceding Comment [Phys. Rev. ST Accel. Beams 4, 017001 (2001)]. We are gratified to see that high quality simulations, as presented by MacLachlan, verify our experimental results.

By modulating the rf voltage at near twice the synchrotron frequency, the longitudinal bunch shape can be modulated. This method can be used to shorten bunches. We show experimentally that the bunch shape can be modulated while preserving the longitudinal emittance when the rf voltage modulation is turned on adiabatically. Experimental measurements will be presented along with theoretical predictions.

We investigate the absorption and emission of photoexcitations in polycrystalline hexaphenyl films. Transient and quasi-steady-state optical modulation spectroscopy is used to identify the photoexcited species and their time evolution. The observed photoinduced absorption spectrum can be assigned to at least three different species: (1) transitions between excited states (S₁→S_n) and (2) transitions between triplet states (T₁→T_n) which are formed by intersystem crossing from the singlet to the triplet regime in the 100 ps time domain. In addition we assign one peak to the (3) formation of aggregates in the oligomer films. The depopulation of the excited singlet states is responsible for both the dynamics of the initial photoinduced absorption and for stimulated emission. The stimulated emission found in the transient optical modulation spectroscopy shows a very limited spectral overlap with the photoinduced absorption. In contrast to doping experiments we find no evidence for long living polaronic states upon photoexcitation.

The dynamic aperture experiment at the CERN Super Proton Synchrotron (SPS) was aimed at finding the relevant effects that limit single-particle stability in hadron storage rings. These effects were studied in the SPS and compared with long-term particle tracking to determine to what extent computer simulations can predict the dynamic aperture under well-known conditions. Such investigations are very important for future hadron colliders such as the Large Hadron Collider as the design of these machines relies heavily on simulations. Besides this practical goal it was of utmost interest to improve the phenomenological understanding of the intricate details of particle motion in phase space. This experiment was carried out by successive teams over a period of ten years. We summarize the techniques, results, and conclusions.

We use a backscattering assembly developed recently to investigate the enhanced backscattering from polycrystalline materials near 180°. The nominal scattering angle can be continuously varied between exactly 180.0° and 178.5°. It is shown that the stochastic model of flux peaking, though its predictions are surprisingly consistent with experimental results, either does not apply to real measurements or does only take account of a negligible fraction of the effect, which cannot be documented by experiment. The energy loss of He projectiles along correlated inward and outward trajectories turns out to be the same as in any random direction. To get the enhancement factor unaffected from detector resolution we measured the integrated backscattering yield from a number of targets with different thickness and deduced the yield by two different methods. For 400 keV He projectiles scattered from partly oxidized Ta a maximum enhancement factor of 2.8 is found.