927 research outputs found
Loss of Landau Damping for Bunch Oscillations
Conditions for the existence, uniqueness and stability of self-consistent
bunch steady states are considered. For the existence and uniqueness problems,
simple algebraic criteria are derived for both the action and Hamiltonian
domain distributions. For the stability problem, van Kampen theory is used. The
onset of a discrete van Kampen mode means the emergence of a coherent mode
without any Landau damping; thus, even a tiny couple-bunch or multi-turn wake
is sufficient to drive the instability. The method presented here assumes an
arbitrary impedance, RF shape, and beam distribution function. Available areas
on the intensity-emittance plane are shown for resistive wall wake and single
harmonic, bunch shortening and bunch lengthening RF configurations. Thresholds
calculated for the Tevatron parameters and impedance model are in agreement
with the observations. These thresholds are found to be extremely sensitive to
the small-argument behaviour of the bunch distribution function. Accordingly, a
method to increase the LLD threshold is suggested. This article summarizes and
extends recent author's publications.Comment: 8 p
Circular Modes for Flat Beams in LHC
Typically x/y optical coupling is considered as unwanted and thus
suppressed--particular exclusions are electron and ionization coolers. Could
some special coupled modes be effectively applied for the LHC complex?
Apparently, the answer is positive: use of the circular modes in the injectors
with their transformation into planar modes in the LHC allows both the space
charge and beam-beam luminosity limitations to be significantly reduced, if not
practically eliminated.Comment: 3 p
Head-Tail Modes for Strong Space Charge
The head-tail modes are described for the space charge tune shift
significantly exceeding the synchrotron tune. A general equation for the modes
is derived. The spatial shapes of the modes, their frequencies, and coherent
growth rates are explored. The Landau damping rates are also found. The
suppression of the transverse mode coupling instability by the space charge is
explained.Comment: 11 pages, 7 figures, submitted to Phys. Rev. ST-Accel. Beam
Bucket shaking stops bunch dancing in Tevatron
Bunches in Tevatron are known to be longitudinally unstable: their collective
oscillations, also called "dancing bunches," persist without any signs of
decay. Typically, a damper is used to stop these oscillations, but recently, it
was theoretically predicted that the oscillations can be stabilized by means of
small bucket shaking. Dedicated measurements in Tevatron have shown that this
method does stop the dancing.Comment: 3 pp. Particle Accelerator, 24th Conference (PAC'11) 2011. 28 Mar - 1
Apr 2011. New York, US
Impedances of Laminated Vacuum Chambers
Longitudinal and transverse impedances are derived for round and flat
laminated vacuum chambers.Comment: 12 p
- …