191 research outputs found
Beam-beam compensation studies in the Tevatron with electron lenses
At the Fermilab Tevatron collider, we studied the feasibility of suppressing
the antiproton head-on beam-beam tune spread using a magnetically confined
5-keV electron beam with Gaussian transverse profile overlapping with the
circulating beam. When electron cooling of antiprotons was applied in regular
Tevatron operations, the nonlinear head-on beam-beam effect on antiprotons was
small. Therefore, we first focused on the operational aspects, such as beam
alignment and stability, and on fundamental observations of tune shifts, tune
spreads, lifetimes, and emittances. We also attempted two special collider
stores with only 3 proton bunches colliding with 3 antiproton bunches, to
suppress long-range forces and enhance head-on effects. We present here the
results of this study and a comparison between numerical simulations and
observations. These results contributed to the application of this compensation
concept to RHIC at Brookhaven.Comment: 5 pages, 6 figures. Submitted to the Proceedings of the ICFA
Mini-Workshop on Beam-beam Effects in Hadron Colliders (BB2013), Geneva,
Switzerland, 18-22 March 201
Bunch-by-bunch measurement of transverse coherent beam-beam modes in the Fermilab Tevatron collider
A system for bunch-by-bunch detection of transverse proton and antiproton
coherent oscillations in the Tevatron is described. It is based on the signal
from a single beam-position monitor located in a region of the ring with large
amplitude functions. The signal is digitized over a large number of turns and
Fourier-analyzed offline with a dedicated algorithm. To enhance the signal,
band-limited noise is applied to the beam for about 1 s. This excitation does
not adversely affect the circulating beams even at high luminosities. The
device has a response time of a few seconds, a frequency resolution of 1.6e-5
in fractional tune, and it is sensitive to oscillation amplitudes of 60 nm. It
complements Schottky detectors as a diagnostic tool for tunes, tune spreads,
and beam-beam effects. Measurements of coherent mode spectra are presented to
show the effects of betatron tunes, beam-beam parameter, and collision pattern,
and to provide an experimental basis for beam-beam numerical codes. Comparisons
with a simplified model of beam-beam oscillations are also described.Comment: 21 pages, 13 figures, 1 tabl
Measurements of beam halo diffusion and population density in the Tevatron and in the Large Hadron Collider
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
Effect of pulsed hollow electron-lens operation on the proton beam core in LHC
Collimation with hollow electron beams is currently one of the most promising
concepts for active halo control in the HL-LHC. In order to further increase
the diffusion rates for a fast halo removal as e.g. desired before the squeeze,
the electron lens (e-lens) can be operated in pulsed mode. In case of profile
imperfections in the electron beam the pulsing of the e-lens induces noise on
the proton beam which can, depending on the frequency content and strength,
lead to emittance growth. In order to study the sensitivity to the pulsing
pattern and the amplitude, a beam study (machine development MD) at the LHC has
been proposed for August 2016 and we present in this note the preparatory
simulations and estimates
Resonant and random excitations on the proton beam in the Large Hadron Collider for active halo control with pulsed hollow electron lenses
We present the results of numerical simulations and experimental studies
about the effects of resonant and random excitations on proton losses,
emittances, and beam distributions in the Large Hadron Collider (LHC). In
addition to shedding light on complex nonlinear effects, these studies are
applied to the design of hollow electron lenses (HEL) for active beam halo
control. In the High-Luminosity Large Hadron Collider (HL-LHC), a considerable
amount of energy will be stored in the beam tails. To control and clean the
beam halo, the installation of two hollow electron lenses, one per beam, is
being considered. In standard electron-lens operation, a proton bunch sees the
same electron current at every revolution. Pulsed electron beam operation
(i.e., different currents for different turns) is also considered, because it
can widen the range of achievable halo removal rates. For an axially symmetric
electron beam, only protons in the halo are excited. If a residual field is
present at the location of the beam core, these particles are exposed to
time-dependent transverse kicks and to noise. We discuss the numerical
simulations and the experiments conducted in 2016 and 2017 at injection energy
in the LHC. The excitation patterns were generated by the transverse feedback
and damping system, which acted as a flexible source of dipole kicks. Proton
beam losses, emittances, and transverse distributions were recorded as a
function of excitation patterns and strengths. The resonant excitations induced
rich dynamical effects and nontrivial changes of the beam distributions, which,
to our knowledge, have not previously been observed and studied in this detail.
We conclude with a discussion of the tolerable and achievable residual fields
and proposals for further studies.Comment: 33 pages, 32 figures, 46 references. Revised manuscript submitted to
Phys. Rev. Accel. Beam
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