A novel concept of controlled halo removal for intense high-energy beams in storage rings and colliders is presented. It is based on the interaction of the circulating beam with a 5-keV, magnetically confined, pulsed hollow electron beam in a 2-m-long section of the ring. The electrons enclose the circulating beam, kicking halo particles transversely and leaving the beam core unperturbed. By acting as a tunable diffusion enhancer and not as a hard aperture limitation, the hollow electron beam collimator extends conventional collimation systems beyond the intensity limits imposed by tolerable losses. The concept was tested experimentally at the Fermilab Tevatron proton-antiproton collider. The first results on the collimation of 980-GeV antiprotons are presented. PACS numbers: 29.20.db, 41.85.Si Keywords: storage rings and colliders; beam collimation; magnetically confined electron beams; beam diffusion In high-energy particle accelerators and storage rings, the collimation system must protect equipment from intentional and accidental beam aborts by intercepting particle losses [1][2][3]. Its functions include controlling and reducing the beam halo, which is continually replenished by various processes such as beam-gas scattering, intrabeam scattering, electrical noise in the accelerating cavities, ground motion, betatron resonances, and beam-beam collisions. Uncontrolled losses of even a small fraction of the circulating beam can damage components, quench superconducting magnets, or produce intolerable experimental backgrounds. Collimators also serve as a diagnostic tool for fundamental machine measurements, such as transverse admittances, beam vibrations, and diffusion rates.Conventional collimation schemes are based on scatterers and absorbers, possibly incorporating several stages. The primary collimators (or targets) are the devices closest to the beam. They generate random transverse kicks mainly via multiple Coulomb scattering. In the Tevatron, the primary collimators are 5-mm tungsten plates positioned about 5 standard deviations (σ ) away from the beam axis. The random multiple-scattering kick has a root mean square (r.m.s.) of 17 µrad for 980-GeV protons. The betatron oscillation amplitude of the affected particles increases, and a large fraction of them is captured by the secondary collimators (or absorbers), suitably placed around the ring. In the Tevatron, the absorbers are 1.5-m steel blocks at 6σ .The conventional two-stage system offers robust shielding of sensitive components and it is very efficient in reducing beam-related backgrounds at the experiments. However, it has limitations. In high-power accelerators, the minimum distance between the collimator and the beam axis is limited by instantaneous loss rates, radiation damage, and by the electromagnetic impedance of the device. Moreover, beam jitter, caused by ground motion and other vibrations and partly mitigated by active orbit feedback, can cause periodic bursts of losses at aperture restrictions.

Bent-crystal channeling is a technique with a potential to increase beam-halo collimation efficiency in high-energy colliders. First measurements at the Tevatron in 2005 have shown that using a thin silicon crystal to deflect the 1-TeV proton beam halo onto a secondary collimator improves the system performance by reducing the machine impedance, beam losses in the collider detectors and irradiation of the superconducting magnets, all in agreement with simulations. Recent results, obtained with an improved goniometer and enhanced beam diagnostics, are reported here for dedicated beam studies and first full collider stores along with simulation results and plans for substantial enhancement of the T-980 experimental setup.

Since the beginning of 2002 an intensive measurement program has been performed at the Fermilab Magnet Test Facility (MTF) to understand dynamic effects in Tevatron magnets. Based on the results of this program a new correction algorithm was proposed to compensate for the decay of the sextupole field during the dwell at injection and for the subsequent field "snapback" during the first few seconds of the energy ramp. Beam studies showed that the new correction algorithm works better than the original one, and improves the Tevatron efficiency by at least 3%. The beam studies also indicated insufficient correction during the first 6s of the injection plateau where an unexpected discrepancy of 0.15 sextupole units of extra drift was observed. This paper reports on the most recent measurements of the Tevatron dipoles field at the beginning of the injection plateau. Results on the field decay and snapback in the Tevatron quadrupoles are also presented.

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During the beginning of Run II of the Tevatron Collider it became apparent that a large skew quadrupole source, or sources, had developed in the superconducting synchrotron. Efforts to locate the current source of coupling were undertaken, with the eventual discovery that the main magnets had developed a systematic skew quadrupole moment over their lifetime. Over the past year, the magnets have been altered in place in an attempt to restore the systematic skew quadrupole moment to zero. Beam observations and their interpretations are presented, and remedial measures are discussed.