Beam-beam Interaction Research Articles

The RHIC polarized source upgrade

The RHIC polarized H− ion source is being upgraded to higher intensity and polarization for use in the RHIC polarization physics program at enhanced luminosity RHIC operation. The higher beam intensity will allow reduction of the longitudinal transverse beam emittance at injection to AGS to reduce polarization losses in AGS. There is also a planned RHIC luminosity upgrade by using the electron beam lens to compensate the beam-beam interaction at collision points. This upgrade is also essential for future BNL plans for a high-luminosity electron-proton (ion) Collider eRHIC. The basic limitations on the high-intensity H− ion beam production in charge-exchange collisions of the neutral atomic hydrogen beam in the Na-vapor jet ionizer cell were experimentally studied.

Mitigation of the kink instability in the energy recovery linac based electron-ion collider

Beam-beam effects in eRHIC, the proposed energy recovery linac (ERL)-based electron-ion collider (EIC) at BNL, have several unique features distinguishing them from those in hadron and lepton colliders. Taking the advantage of the fact that the electron beam is used only once, we expect the luminosity to be 10 times that of a ring-ring collision scheme with similar parameters. However, without instituting proper treatments, the quality of electron and hadron beams can undergo degradation or even beam loss, driven by the beam-beam interactions. Kink instability is one of the most important collective effects of the ion ring due to the special beam-beam interaction in an ERL-based EIC. In this article, we present novel treatments for kink instability in ERL-based EICs including a scheme dedicated to suppressing instability.

Correction of beam-beam effects in luminosity measurement in the forward region at CLIC

Procedures for correcting the beam-beam effects in luminosity measurements at CLIC at 3 TeV center-of-mass energy are described and tested using Monte Carlo simulations. The angular counting loss due to the combined Beamstrahlung and initial-state radiation effects is corrected based on the reconstructed velocity of the collision frame of the Bhabha scattering. The distortion of the luminosity spectrum due to the initial-state radiation is corrected by deconvolution. At the end, the counting bias due to the finite calorimeter energy resolution is numerically corrected. To test the procedures, BHLUMI Bhabha event generator, and Guinea-Pig beam-beam simulation were used to generate the outgoing momenta of Bhabha particles in the bunch collisions at CLIC. The systematic effects of the beam-beam interaction on the luminosity measurement are corrected with precision of 1.4 permille in the upper 5% of the energy, and 2.7 permille in the range between 80 and 90% of the nominal center-of-mass energy.

Simulation of beam-beam effects and Tevatron experience

Effects of electromagnetic interactions of colliding bunches in the Tevatron had a variety of manifestations in beam dynamics presenting vast opportunities for development of simulation models and tools. In this paper the computer code for simulation of weak-strong beam-beam effects in hadron colliders is described. We report the collider operational experience relevant to beam-beam interactions, explain major effects limiting the collider performance and compare results of observations and measurements with simulations.

Anomalous beam diffusion near beam-beam synchrobetatron resonances

The diffusion process near low order synchro-betatron resonances driven by beam-beam interactions at a crossing angle is investigated. Macroscopic observables such as beam emittance, lifetime and beam profiles are calculated. These are followed with detailed studies of microscopic quantities such as the evolution of the variance at several initial transverse amplitudes and single particle probability distribution functions. We present evidence to show that the observed diffusion is anomalous and the dynamics follows a non-Markovian continuous time random walk process. We derive a modified master equation to replace the Chapman-Kolmogorov equation in action-angle space and a fractional diffusion equation to describe the density evolution for this class of processes.

Six-dimensional weak-strong simulation of head-on beam-beam compensation in the Relativistic Heavy Ion Collider

To compensate the beam-beam tune spread and beam-beam resonance driving terms in the polarized proton operation in the Relativistic Heavy Ion Collider (RHIC), we will introduce a low energy DC electron beam into each ring to collide head-on with the opposing proton beam. The device to provide the electron beam is called an electron lens. In this article, using a 6D weak-strong beam-beam interaction simulation model, we will investigate the effects of head-on beam-beam compensation with electron lenses on the proton beam dynamics for the RHIC 250 GeV polarized proton operation. Frequency maps, dynamic apertures, and proton beam loss rates are calculated for this study. Key beam parameters involved in this scheme are varied to search for the optimum compensation condition. The sensitivities of head-on beam-beam compensation to beam imperfections and beam offsets are also studied.

Beamstrahlung considerations in laser-plasma-accelerator-based linear colliders

Beam-beam interaction constraints modify the basic plasma density scalings for a linear collider based on laser-plasma accelerators. In the quantum beamstrahlung regime, it is shown that operating at low plasma density increases beamstrahlung effects, owing to the higher bunch charge and longer bunch length. At high plasma density, the bunch charge is limited by beam loading, and the required power is proportional to the square root of the plasma density. At low plasma density, the bunch charge is limited by beamstrahlung, which, for fixed luminosity, requires operation at higher laser repetition rate and, hence, higher power requirements, or the use of multibunch trains. If round beams are used in a multibunch train format with fixed beam loading, and the collider is constrained by beamstrahlung, then the required collider power is independent of plasma density.

A study on dynamic beam-beam effects and their application in BEPCII

Beam-beam interactions cause dynamic behaviour of the charged particle beams in circular colliders, which is directly related to their luminosity. The linear beam-beam force introduces an additional quadrupole that perturbs betatron motion of particles, including dynamic β, dynamic emittance and beam-beam tune-shifts; while the nonlinear force may excite resonances and cause tune-spread. With the weak-strong beam-beam mode, the linear and nonlinear effects are studied, dynamic β and dynamic emittance as functions of tunes are analyzed, and formulas of beam-beam octupole caused tune-spread are derived, starting from beam-beam potential and equations of particle motions, and the measures to increase luminosity of BEPCII are discussed.

Absolute luminosity measurements with the LHCb detector at the LHC

Absolute luminosity measurements are of general interest for colliding-beam experiments at storage rings. These measurements are necessary to determine the absolute cross-sections of reaction processes and are valuable to quantify the performance of the accelerator. Using data taken in 2010, LHCb has applied two methods to determine the absolute scale of its luminosity measurements for proton-proton collisions at the LHC with a centre-of-mass energy of 7 TeV. In addition to the classic ``van der Meer scan'' method a novel technique has been developed which makes use of direct imaging of the individual beams using beam-gas and beam-beam interactions. This beam imaging method is made possible by the high resolution of the LHCb vertex detector and the close proximity of the detector to the beams, and allows beam parameters such as positions, angles and widths to be determined. The results of the two methods have comparable precision and are in good agreement. Combining the two methods, an overal precision of 3.5% in the absolute luminosity determination is reached. The techniques used to transport the absolute luminosity calibration to the full 2010 data-taking period are presented.

Spin tracking studies for polarimetry at the ILC

At the International Linear Collider (ILC), the beam polarization is planned to be measured with a yet unequalled precision of 0.25 % by two Compton polarimeters located 1.8 km upstream and 150 m downstream of the e+e− interaction point (IP). The decisive quantity for the experiments, i.e. the luminosity-weighted average polarization, has to be determined from these measurements. A detailed understanding of the spin tracking and the depolarizing effects is mandatory in order to estimate how precise the polarization at the IP is known from these measurements performed 2 km apart. For that purpose, a beamline simulation including spin tracking has been set up. First results comprise the results of static misalignments, dynamic misalignments and beam-beam interaction are in preparation.

Relativistic Heavy Ion Collider spin flipper commissioning plan

The commissioning of the RHIC spin flipper in the RHIC Blue ring during the RHIC polarized proton run in 2009 showed the detrimental effects of global vertical coherent betatron oscillation induced by the 2-AC dipole plus 4-DC dipole configuration [1]. This global orbital coherent oscillation of the RHIC beam in the Blue ring in the presence of collision modulated the beam-beam interaction between the two RHIC beams and affected Yellow beam lifetime. The experimental data at injection with different spin tunes by changing the snake current also demonstrated that it was not possible to induce a single isolated spin resonance with the global vertical coherent betatron oscillation excited by the two AC dipoles. Hence, RHIC spin flipper was re-designed to eliminate the coherent vertical betatron oscillation outside the spin flipper by adding three additional AC dipoles. This paper presents the experimental results as well as the new design.

The RHIC polarized source upgrade

The RHIC polarized H− ion source is being upgraded to higher intensity (5-10 mA) and polarization for use in the RHIC polarization physics program at enhanced luminosity RHIC operation. The higher beam peak intensity will allow reduction of the transverse beam emittance at injection to AGS to reduce polarization losses in AGS. There is also a planned RHIC luminosity upgrade by using the electron beam lens to compensate the beam-beam interaction at collision points. This upgrade is also essential for future BNL plans for a high-luminosity electron – proton (ion) Collider eRHIC.

Effect of electron disruption in the energy recovery linac based electron ion collider

Beam-beam effects present one of the major factors limiting the luminosity of colliders. In the energy recovery linac (ERL) based eRHIC design, the electron beam, accelerated in a superconducting ERL, collides with the proton beam circulating in the RHIC ring. During such collisions the electron beam undergoes a very strong beam-beam interaction with the protons, which warrants careful examination. We evaluated transverse disruption and linear mismatch effects in the electron beam caused by collisions and considered several countermeasures to mitigate the emittance growth from these interactions. The minimum required aperture of transport lines is calculated that should allow the transport of the electron beam during the deceleration process.

Test of “Crab-Waist” Collisions at theDAΦNEΦFactory

The electron-positron collider DAPhiNE, the Italian Phi factory, has been recently upgraded in order to implement an innovative collision scheme based on large crossing angle, small beam sizes at the crossing point, and compensation of beam-beam interaction by means of sextupole pairs creating a "crab-waist" configuration in the interaction region. Experimental tests of the novel scheme exhibited an increase by a factor of 3 in the peak luminosity of the collider with respect to the performances reached before the upgrade. In this Letter we present the new collision scheme, discuss its advantages, describe the hardware modifications realized for the upgrade, and report the results of the experimental tests carried out during commissioning of the machine in the new configuration and standard operation for the users.

Implementation of depolarization due to beam-beam effects in the beam-beam interaction simulation tool GUINEA-PIG++

Depolarization is a new feature in the beam-beam simulation tool GUINEA-PIG++ (GP++). The results of this simulation are studied and compared with another beam-beam simulation tool, CAIN, considering different beam parameters for the International Linear Collider (ILC) with a centre-of-mass energy of 500 GeV.

Simulations of beam-beam and beam-wire interactions in RHIC

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.

PRESENT STATUS OF THE DAΦNE UPGRADE AND PERSPECTIVES

The DAΦNE collider has been recently upgraded in order to implement a new collision scheme based on large Piwinski angle and cancellation of the synchro-betatron resonances by means of electromagnetic sextupoles (Crab-Waist compensation). The novel approach has proved to be effective in improving beam-beam interaction and collider luminosity. The results and the measurements taken during commissioning as well as the perspectives for the SIDDHARTA run are presented and discussed.

Tevatron electron lenses: Design and operation

The beam-beam effects have been the dominating sources of beam loss and lifetime limitations in the Tevatron proton-antiproton collider [1]. Electron lenses were originally proposed for compensation of electromagnetic long-range and head-on beam-beam interactions of proton and antiproton beams [2]. Results of successful employment of two electron lenses built and installed in the Tevatron are reported in [3,4,5]. In this paper we present design features of the Tevatron electron lenses (TELs), discuss the generation of electron beams, describe different modes of operation and outline the technical parameters of various subsystems.

Experimental Demonstration of Colliding-Beam-Lifetime Improvement by Electron Lenses

We report the successful application of space-charge forces of a low-energy electron beam for improvement of particle lifetime determined by beam-beam interaction at a high-energy collider. In our experiments, an electron lens, a novel instrument developed for the beam-beam compensation, was set on a 980-GeV proton bunch at the Fermilab Tevatron proton-antiproton collider. The proton-bunch losses due to its interaction with the antiproton beam were reduced by a factor of 2 when the electron lens was operating. We describe the principle of electron lens operation and present experimental results.

Chaos and emittance growth due to nonlinear interactions in a circular accelerator

Incoherent emittance growth caused by a strong nonlinear interaction between beam and electron cloud is discussed. This emittance growth arises from nonlinear diffusion related to chaos and resonances, and strongly depends on the number of degrees of freedom of the interacting system. A simple model, in which beam particles interact with a fixed round charge distribution, is used to study the mechanism of the emittance growth. The same discussion can be applied to the emittance growth due to beam-beam interactions in colliders and space charge effects in low energy proton rings.