Halo dynamics influences global accelerator performance: beam lifetimes,
emittance growth, dynamic aperture, and collimation efficiency. Halo monitoring
and control are also critical for the operation of high-power machines. For
instance, in the high-luminosity upgrade of the LHC, the energy stored in the
beam tails may reach several megajoules. Fast losses can result in
superconducting magnet quenches, magnet damage, or even collimator deformation.
The need arises to measure the beam halo and to remove it at controllable
rates. In the Tevatron and in the LHC, halo population densities and
diffusivities were measured with collimator scans by observing the time
evolution of losses following small inward or outward collimator steps, under
different experimental conditions: with single beams and in collision, and, in
the case of the Tevatron, with a hollow electron lens acting on a subset of
bunches. After the LHC resumes operations, it is planned to compare measured
diffusivities with the known strength of transverse damper excitations. New
proposals for nondestructive halo population density measurements are also
briefly discussed.Comment: 5 pages, 1 figure, 1 table. Submitted to the Proceedings of the 54th
ICFA Advanced Beam Dynamics Workshop on High-Intensity and High-Brightness
Hadron Beams (HB2014), East Lansing, MI, USA, November 10-14, 201