Prototype "electron lens" set-up for the Tevatron beam-beam compensation

Abstract

A prototype "electron lens" for the Tevatron beam-beam compensation project is commissioned at Fermilab. We describe the set-up, report results of the first tests of the electron beam, and discuss future plans. SCOPECompensation of beam-beam effects in the Tevatron with electron beams is a promising technique to improve the collider performance [1]. The method implies that an antiproton beam propagates through a countertraveling low-energy high-current electron beam ("electron lens"). An impact of the negative electron space charge can reduce betatron tune spread within antiproton bunch and a bunch-to-bunch tune spread -the effects due to collision with intensive proton beam which limit beam lifetime and luminosity.An experimental R&D program on beam-beam compensation was started at FNAL Beam Division early in 1998. The "electron lens" prototype has been designed, fabricated, assembled in the Linac Lab and commissioned in December 1998. The goal of the set-up is to study feasibility and properties of the electron beam required for the beam-beam compensation. Currently, these studies are under way. Table 1 shows the Tevatron "electron lens" (TEL) design parameters and parameters of the prototype set-up operation to date. "ELECTRON LENS" PROTOTYPEMajor components and systems of the "electron lens" prototype are: electron gun, electron collector, modulator, power supplies, magnetic system, vacuum system, control system. The set-up (in present configuration) is shown in Fig.1. Electrons are thermally emitted from a cathode of the gun and extracted toward the positive potential of an anode U a . Then they propagate through some 2 m long beam pipe which is under potential U p (usually, U p ≈ U a ). Finally, electron beam is absorbed in a high efficiency collector at a smaller potential U c ≤ U a . Some of the electrons can be * on leave from BINP, Novosibirsk, Russia lost and absorbed on the vacuum pipe walls or at other aperture limits, i.e., not in the designated place of the collector. Strong longitudinal magnetic fields (of the order of few kG all along the set-up) helps to keep these losses low.Figure 1: Electrical scheme of the "electron lens" prototype.The high perveance electron gun is made in accordance with a novel approach proposed in [2] based on use of a convex cathode. The gun is immersed in 0.7-2 kG longitudinal magnetic field. It has a uniform current density profile, low transverse beam temperature and high perveance (these conditions can not be met in a standard Pierce geometry with a planar cathode). Fig.2 shows the gun construction. o convex cathode is made of tungsten impregnated with emitting oxides (made by HeatWave Co., CA; see details in [3]). Fig.3 shows the maximum electron current J e vs voltage between the anode and the cathode of the gun U a . The maximum current is limited by negative space charge of the electron cloud near the cathode and follows Child's law J e = PU 3/2 a . Numerical simulations of the gun yielded the perveance P = 4.9 · 10 −6 A/V 3/2 , while a fit of the meas...

Electron lenses: historical overview and outlook

“…Construction of two Tevatron electron lenses was approved by Fermilab shortly after successful demonstration of key technologies at the electron lens test stand [40] (see also Appendix A). The electron lens test setup operated a 10 kV, 2 m long electron beam with total current up to a few Amperes, properly fast-modulated and with the desired transverse current density profiles, propagating in the system of three 1-4 kG solenoid magnets.…”

“…The electron lens prototype setup (see figure 13) was designed, fabricated, and assembled in the FNAL Linac basement (Linac Lab) and commissioned in December 1998. The goal of the setup is to study the feasibility and properties of the electron beam required for the beam-beam compensation in the Tevatron [44]. Major components and systems of the electron lens prototype included a 3 A 10 kV electron gun, electron collector, HV modulator, magnet power supplies, magnetic system of three solenoids and dipole correctors, vacuum system, and control system.…”

Electron Beam Generation in Tevatron Electron Lenses

“…To get desired current distribution, voltage on the "control" electrodes -controlled by a separate power supply -should be the same as cathode voltage. Gun characteristics were measured on a test bench used at Fermilab for prototyping of TEL elements [5]. The test bench consists of the gun immersed into longitudinal magnetic field B gun of 1-2 kG generated by a gun solenoid, a drift tube with diagnostics placed inside 4 kG, 2 m long main solenoid, and a collector, also inside a separate solenoid.…”

“…For these and similar cases, where the gun has to be immersed into a strong longitudinal magnetic field, the perveance can be increased by usage of a convex cathode [4]. All the guns were tested at the "Tevatron Electron Lens" (TEL) prototype set-up at Fermilab [5]. The paper presents results of the gun designs and test measurements.…”

Magnetic field alignment in the beam-beam compensation device

“…However, in our case the requirement becomes less stringent for shorter wavelength λ, in contrast with electron cooling magnetic systems. The problem of the field straightness measurement and its correction is nevertheless an important issue, which inspired development of a suitable technique that has been tested at a prototype of the beam-beam compensation device recently commissioned at Fermilab [2].…”