86 research outputs found
Optimal Axes of Siberian Snakes for Polarized Proton Acceleration
Accelerating polarized proton beams and storing them for many turns can lead
to a loss of polarization when accelerating through energies where a spin
rotation frequency is in resonance with orbit oscillation frequencies.
First-order resonance effects can be avoided by installing Siberian Snakes in
the ring, devices which rotate the spin by 180 degrees around the snake axis
while not changing the beam's orbit significantly. For large rings, several
Siberian Snakes are required.
Here a criterion will be derived that allows to find an optimal choice of the
snake axes. Rings with super-period four are analyzed in detail, and the HERA
proton ring is used as an example for approximate four-fold symmetry. The
proposed arrangement of Siberian Snakes matches their effects so that all
spin-orbit coupling integrals vanish at all energies and therefore there is no
first-order spin-orbit coupling at all for this choice, which I call snakes
matching. It will be shown that in general at least eight Siberian Snakes are
needed and that there are exactly four possibilities to arrange their axes.
When the betatron phase advance between snakes is chosen suitably, four
Siberian Snakes can be sufficient.
To show that favorable choice of snakes have been found, polarized protons
are tracked for part of HERA-p's acceleration cycle which shows that
polarization is preserved best for the here proposed arrangement of Siberian
Snakes.Comment: 14 pages, 16 figure
Strength of Higher-Order Spin-Orbit Resonances
When polarized particles are accelerated in a synchrotron, the spin
precession can be periodically driven by Fourier components of the
electromagnetic fields through which the particles travel. This leads to
resonant perturbations when the spin-precession frequency is close to a linear
combination of the orbital frequencies. When such resonance conditions are
crossed, partial depolarization or spin flip can occur. The amount of
polarization that survives after resonance crossing is a function of the
resonance strength and the crossing speed. This function is commonly called the
Froissart-Stora formula. It is very useful for predicting the amount of
polarization after an acceleration cycle of a synchrotron or for computing the
required speed of the acceleration cycle to maintain a required amount of
polarization. However, the resonance strength could in general only be computed
for first-order resonances and for synchrotron sidebands. When Siberian Snakes
adjust the spin tune to be 1/2, as is required for high energy accelerators,
first-order resonances do not appear and higher-order resonances become
dominant. Here we will introduce the strength of a higher-order spin-orbit
resonance, and also present an efficient method of computing it. Several
tracking examples will show that the so computed resonance strength can indeed
be used in the Froissart-Stora formula. HERA-p is used for these examples which
demonstrate that our results are very relevant for existing accelerators.Comment: 10 pages, 6 figure
Beam-Breakup Instability Theory for Energy Recovery Linacs
Here we will derive the general theory of the beam-breakup instability in
recirculating linear accelerators, in which the bunches do not have to be at
the same RF phase during each recirculation turn. This is important for the
description of energy recovery linacs (ERLs) where bunches are recirculated at
a decelerating phase of the RF wave and for other recirculator arrangements
where different RF phases are of an advantage. Furthermore it can be used for
the analysis of phase errors of recirculated bunches. It is shown how the
threshold current for a given linac can be computed and a remarkable agreement
with tracking data is demonstrated. The general formulas are then analyzed for
several analytically solvable cases, which show: (a) Why different higher order
modes (HOM) in one cavity do not couple so that the most dangerous modes can be
considered individually. (b) How different HOM frequencies have to be in order
to consider them separately. (c) That no optics can cause the HOMs of two
cavities to cancel. (d) How an optics can avoid the addition of the
instabilities of two cavities. (e) How a HOM in a multiple-turn recirculator
interferes with itself. Furthermore, a simple method to compute the orbit
deviations produced by cavity misalignments has also been introduced. It is
shown that the BBU instability always occurs before the orbit excursion becomes
very large.Comment: 12 pages, 6 figure
Beam Based Alignment of Interaction Region Magnets
In conventional beam based alignment (BBA) procedures, the relative alignment
of a quadrupole to a nearby beam position monitor is determined by finding a
beam position in the quadrupole at which the closed orbit does not change when
the quadrupole field is varied. The final focus magnets of the interaction
regions (IR) of circular colliders often have some specialized properties that
make it difficult to perform conventional beam based alignment procedures. At
the HERA interaction points, for example, these properties are: (a) The
quadrupoles are quite strong and long. Therefore a thin lens approximation is
quite imprecise. (b) The effects of angular magnet offsets become significant.
(c) The possibilities to steer the beam are limited as long as the alignment is
not within specifications. (d) The beam orbit has design offsets and design
angles with respect to the axis of the low-beta quadrupoles. (e) Often
quadrupoles do not have a beam position monitor in their vicinity. Here we
present a beam based alignment procedure that determines the relative offset of
the closed orbit from a quadrupole center without requiring large orbit changes
or monitors next to the quadrupole. Taking into account the alignment angle
allows us to reduce the sensitivity to optical errors by one to two orders of
magnitude. We also show how the BBA measurements of all IR quadrupoles can be
used to determine the global position of the magnets. The sensitivity to errors
of this method is evaluated and its applicability to HERA is shown
Thermocurrents and their Role in high Q Cavity Performance
Over the past years it became evident that the quality factor of a
superconducting cavity is not only determined by its surface preparation
procedure, but is also influenced by the way the cavity is cooled down.
Moreover, different data sets exists, some of them indicate that a slow
cool-down through the critical temperature is favourable while other data
states the exact opposite. Even so there where speculations and some models
about the role of thermo-currents and flux-pinning, the difference in behaviour
remained a mystery. In this paper we will for the first time present a
consistent theoretical model which we confirmed by data that describes the role
of thermo-currents, driven by temperature gradients and material transitions.
We will clearly show how they impact the quality factor of a cavity, discuss
our findings, relate it to findings at other labs and develop mitigation
strategies which especially addresses the issue of achieving high quality
factors of so-called nitrogen doped cavities in horizontal test
Planning the Future of U.S. Particle Physics (Snowmass 2013): Chapter 6: Accelerator Capabilities
These reports present the results of the 2013 Community Summer Study of the
APS Division of Particles and Fields ("Snowmass 2013") on the future program of
particle physics in the U.S. Chapter 6, on Accelerator Capabilities, discusses
the future progress of accelerator technology, including issues for high-energy
hadron and lepton colliders, high-intensity beams, electron-ion colliders, and
necessary R&D for future accelerator technologies.Comment: 26 page
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