The observation of plasma focusing of a 28.5 GeV positron beam is reported.The plasma was formed by ionizing a nitrogen jet only 3 mm thick. Simultaneous focusing in both transverse dimensions was observed with effective focusing strengths of order Tesla per micron. The minimum area of the beam spot was reduced by a factor of 2.0 ± 0.3 by the plasma. The longitudinal beam envelope was measured and compared with numerical calculations.

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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...

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Guiding solenoidal magnetic field of the Tevatron beambeam compensation device has to be carefully aligned with respect to a straight trajectory of the antiproton beam. We present in this paper an optical method which allows to measure the direction of the magnetic field, results of magnetic measurements, and results of the field quality improvement with dipole correctors.