Collimation Studies with Hollow Electron Beams

Recent experimental studies at the Fermilab Tevatron collider have shown that magnetically confined hollow electron beams can act as a new kind of collimator for highintensity beams in storage rings. In a hollow electron beam collimator, electrons enclose the circulating beam. Their electric charge kicks halo particles transversely. If their distribution is axially symmetric, the beam core is unaffected. This device is complementary to conventional two-stage collimation systems: the electron beam can be placed arbitrarily close to the circulating beam; and particle removal is smooth, so that the device is a diffusion enhancer rather than a hard aperture limitation. The concept was tested in the Tevatron collider using a hollow electron gun installed in one of the existing electron lenses. We describe some of the technical aspects of hollow-beam scraping and the results of recent measurements. We are studying hollow electron beams as a new kind of collimator for high-intensity beams in storage rings and colliders [1, 2]. In a hollow electron beam collimator (HEBC), electrons enclose the circulating beam (Figure 1). The electron beam is generated by a pulsed electron gun and transported with strong axial magnetic fields, in an arrangement similar to electron cooling or to the existing Tevatron electron lenses [3]. The electric charge of the electrons kicks halo particles transversely. If the hollow distribution is axially symmetric, the core of the circulating beam is unperturbed. For typical parameters, the kick given to 980-GeV protons is of the order of 0.2 μrad. In a conventional two-stage collimation scheme, primary collimators impart random transverse kicks due to multiple scattering. The affected particles have increasing oscillation amplitudes and a large fraction of them is caught by the secondary collimators. These systems offer robust shielding of sensitive components. They are also very efficient in reducing beam losses at the experiments. However, they have limitiations. In high-power accelerators, no material can be placed too close to the beam. The minimum distance is limited by instantaneous loss rates, radiation damage, and by the electromagnetic impedance of the device. Another problem is beam jitter. The orbit of the circulating beam oscillates due to ground motion and other vibrations. Even with active orbit stabilization, the beam centroid may ∗ Fermi Research Alliance, LLC operates Fermilab under Contract No. DE-AC02-07CH11359 with the US Department of Energy. This work was partially supported by the US LHC Accelerator Research Program (LARP). † On leave from Istituto Nazionale di Fisica Nucleare, Sezione di Ferrara, Italy. E-mail: [email protected]. oscillate by tens of microns. This translates into periodic bursts of losses at aperture restrictions. The hollow electron beam collimator addresses these limitations. A magnetically confined electron beam can be −10 −5 0 5 −10 −5 0 5 HORIZONTAL POSITION (mm) V E R T IC A L P O SI T IO N ( m m ) proton core antiproton core hollow electron beam protons antiprotons hollow electron beam TEL2 PICKUP MODULATOR (4 kV/V) COLLECTOR (1 A/V) A13 A14 A15 P1 P2 P3 Figure 1: (top) Transverse beam layout. (center) Tevatron electron lens. (bottom) Example of pulse synchronization: modulator voltage (yellow), shortest possible pulse; electron current at the collector (magenta); beam pickup signal (cyan) showing proton and antiproton bunches (P1–P3 and A13–A15) and the derivative of the electron pulse. FERMILAB-CONF-11-412-AD-APC