Transverse profile of the electron beam for the RHIC electron lenses

Abstract

a b s t r a c tThe transverse profile of the electron beam plays a very important role in assuring the success of the electron lens beam-beam compensation, as well as its application in space charge compensation. To compensate for the beam-beam effect in the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, we recently installed and commissioned two electron lenses. In this paper, we describe, via theory and simulations using the code Parmela, the evolution of the density of the electron beam with space charge within an electron lens from the gun to the main solenoid. Our theoretical analysis shows that the change in the beam transverse density is dominated by the effects of the space charge induced longitudinal velocity reduction, not by those of transverse Coulomb collisions. We detail the transverse profile of RHIC electron-lens beam, measured via the YAG screen and pinhole detector, and also describe its profile that we assessed from the signal of the electron-backscatter detector (eBSD) via scanning the electron beam with respect to the RHIC beam. We verified, in simulations and experiments, that the distribution of the transverse electron beam is Gaussian throughout its propagation in the RHIC electron lens.& 2015 Elsevier B.V. All rights reserved. MotivationDuring the 2013 255 GeV proton run of the Relativistic Heavy Ion Collider, we found that the threshold of the intensity of the proton bunch in RHIC was about 2 Â 10 11 [1], as was predicted by the beam-beam simulations [2]. To further increase the bunch intensity, and therefore, its luminosity via compensating for the large beam-beam tune spread from the proton-proton interactions at IP6 and IP8, we installed two electron lenses in the electronproton interaction region IR10 [3] and commissioned them during the RHIC 2013 and 2014 runs. For long-range compensation of beam-beam tune shifts [4], electron lenses had been installed and operated in the Fermilab Tevatron collider [5][6][7]. They were also used for head-on beam-beam compensation [8], to remove uncaptured particles in the abort gap [9], and as well as to demonstrate halo scraping with hollow electron beams [10].In an earlier paper [2], we specified the requirements of the electron beam in the RHIC electron lens, such as its current, shape, and the distribution of the profile of the transverse beam in the interaction region. For optimum head-on beam-beam compensation, according to theory and simulation, the electron beam in the RHIC electron lens should have the same transverse distribution as does the proton beam, i.e., it should be a Gaussian beam [11].According to theoretical modeling [12, p. 169] of the electron beam, if the effects of space charge dominate the physics of the beam, or if the Debye length is much smaller than the beam radius, then, in a state of thermal equilibrium, the beam tends to be uniform with a sharp radius. If we ignore the space charge, or if the Debye length is much greater than beam radius, then the external focusing field will dominate the p...

Beam physics research with the IOTA electron lens

“…A similar setup is used for the RHIC electron lenses, as described in refs. [109,110]. -20 -In addition to the 21 button BPMs in the ring, 2 BPMs are included in the beam pipe inside the main solenoid for aligning the low-energy electron beam with the circulating beam.…”

Electron lenses for head-on beam-beam compensation in RHIC

“…To avoid unwanted transverse profile distortion [34] and electron beam instabilities, the minimum magnetic field from the electron gun to the interaction region was designed to be ≥0.3 T.…”

“…Here it was assumed that the beam profile distortion between the cathode and the YAG screen is negligible [34]. Therefore, the r=σ ratio, which is measured on the YAG screen, can be used on the cathode side to calculate the beam size with a known cathode radius r.…”

Compensation of head-on beam-beam induced resonance driving terms and tune spread in the Relativistic Heavy Ion Collider

“…The expected total tune spread is based on the BTF measured tune spread ΔQ 0 and the beam-beam generated tune spread ΔQ bb ðaÞ using Eqs. (12) and (13), where ΔQ bb ð∞Þ ¼ jð2ξ Al þ ξ e Þj. The beam-beam parameter ξ Al is obtained by a measurement of the bunch intensity N Al with a direct currentcurrent transformer (DCCT) and the scaled emittance ϵ n with an ionization profile monitor (IPM).…”