Study of beam-beam interaction with a large Piwinski angle at LHC

Collision with a large Piwinski angle is one of the update scenarios of LHC toward the luminosity 1035 cm−2s−1. The large Piwinski angle is realized by a small beta function at the collision point and longer bunch length. The Piwinski angle is increased from 0.6 to 2 in the scenario. The bunch population is increased so as to keep the beam-beam parameter. The beam-beam performance is degraded by crossing angle which induces additional nonlinear terms due to a symmetry breaking of the collision especially for the high beam-beam parameter. Effect of crossing angle for the nominal LHC design and the large Piwinski angle scheme are studied

Beam-beam effects under the influence of external noise

Fast external noise, which gives fluctuation into the beam orbit, is discussed in connection with beam-beam effects. Phase noise from crab cavities and detection devices (position monitor) and kicker noise from the bunch by bunch feedback system are the sources. Beam-beam collisions with fast orbit fluctuations with turn by turn or multi-turn correlations, cause emittance growth and luminosity degradation. We discuss the tolerance of the noise amplitude for LHC and HL-LHC.Comment: 6 pages, contribution to the ICFA Mini-Workshop on Beam-Beam Effects in Hadron Colliders, CERN, Geneva, Switzerland, 18-22 Mar 201

Electron cloud effects: codes and simulations at KEK

Electron cloud effects had been studied at KEK-Photon Factory since 1995. e-p instability had been studied in proton rings since 1965 in BINP, ISR and PSR. Study of electron cloud effects with the present style, which was based on numerical simulations, started at 1995 in positron storage rings. The instability observed in KEKPF gave a strong impact to B factories, KEKB and PEPII, which were final stage of their design in those days. History of cure for electron cloud instability overlapped the progress of luminosity performance in KEKB. The studies on electron cloud codes and simulations in KEK are presented.Comment: 6 pages, contribution to the Joint INFN-CERN-EuCARD-AccNet Workshop on Electron-Cloud Effects: ECLOUD'12; 5-9 Jun 2012, La Biodola, Isola d'Elba, Ital

Head-tail instability caused by electron cloud in positron storage rings

In positron or proton storage rings with many closely spaced bunches, a large number of electrons can be generated in the vacuum chamber due to photoemission or secondary emission. The density of this `electron cloud' increases along a bunch train, until the growth saturates under the influence of its own space charge field. In this report, we discuss the possibility of a single-bunch two-stream instability driven by the electron cloud, where any initial head-tail perturbation of the bunch is amplified by the coherent motion of cloud electrons near the beam. Depending on the strength of the beam-electron interaction, the chromaticity and the synchrotron oscillation frequency, this instability either resembles a linac beam break up, or a head-tail instability. We present computer simulations of beam break up and head-tail instabilities for the Low Energy Ring of the KEK B factory, and compare the simulation results with analytical estimates

Topological Excitations in Spinor Bose-Einstein Condensates

We investigate the properties of skyrmion in the ferromagnetic state of spin-1 Bose-Einstein condensates by means of the mean-field theory and show that the size of skyrmion is fixed to the order of the healing length. It is shown that the interaction between two skyrmions with oppositely rotating spin textures is attractive when their separation is large, following a unique power-law behavior with a power of -7/2.Comment: 4 pages, 5 figure

Beam-beam studies for the High-Energy LHC

LHC upgrades are being considered both towards higher luminosity (HL-LHC) and towards higher energy (HE-LHC). In this paper we report initial studies of the beam-beam effects in the HE-LHC [1]. The HE-LHC aims at beam energies of 16.5 TeV, where the transverse emittance decreases due to synchrotron radiation with a 2-hour damping time. As a result of this emittance, shrinkage the beam-beam parameter increases with time, during a physics store. The beam-beam limit in the HE-LHC is explored using computer simulations.Comment: 6 pages, contribution to the EuCARD-AccNet-EuroLumi Workshop: The High-Energy Large Hadron Collider, Malta, 14 -- 16 Oct 201

Increasing the Integrated Luminosity of SLHC by Luminosity Levelling via the Crossing Angle

With an increase of luminosity by a factor of 10, the luminosity lifetime in an upgraded LHC would decrease from 15 hours to two to four hours, depending on the upgrade strategy. If the luminosity increase is achieved primarily by a stronger and more efficient focusing rather than a beam current increase, the luminosity lifetime is in the low bound of this range. We show in this paper that the "early separation" scheme and/or a crab crossing lend itself to a very efficient luminosity levelling. It can be used to counteract the faster luminosity decay and provide a constant luminosity over hours as well as a significant increase of integrated luminosity. This is achieved by adjusting the crossing angle rather than the b* function by means of a bump closed inside the experimental straight section, i.e. operationally simple. The initially large crossing angle reduces the beam-beam tune shift, allowing an increased beam current and higher performance with lower pile-up in the detector and lower energy deposition in the triplet. The impact of the large Piwinski angle required is first investigated

Short Range Wake Field Caused by Electron Cloud in Bending Magnet

A short-range wake field caused by the electron cloud has previously been studied for a drift space. In a bending magnet, the cloud electrons undergo cyclotron motion with a small radius (10 GHz) in the horizontal plane due to the strong magnetic field of order 1 T. In this report, we study the motion of electrons under the combined influence of a strong magnetic dipole field and the electric field of the beam on the time scale of the bunch length, discuss the short-range wake field caused by the electrons, and simulate the emittance growth. As expected, the wake field in a bending magnet is very different from that in a drift space. The dipole field almost completely suppresses any horizontal coherent motion and rms-size blow up, and it also slows down the instability in the vertical direction

Electron cloud studies for KEKB

Electron-cloud build up, incoherent tune spread and electron-induced beam instability are likely to be responsible for the vertical beam-size increase observed at the KEKB Low Energy Ring (LER). We report on recent simulations and analytical estimated, a ddressing the electron-cloud evolution for various magnet configurations, computing the beam size blow up predicted by three different simulation models (micro-bunches, multiparticle tracking with soft Gaussian approximation, particle-in cell) and finally discussing analytical expressions for the instability threshold