27,421 research outputs found
Fast Mapping of Terahertz Bursting Thresholds and Characteristics at Synchrotron Light Sources
Dedicated optics with extremely short electron bunches enable synchrotron
light sources to generate intense coherent THz radiation. The high degree of
spatial compression in this so-called low-alpha optics entails a complex
longitudinal dynamics of the electron bunches, which can be probed studying the
fluctuations in the emitted terahertz radiation caused by the micro-bunching
instability ("bursting"). This article presents a "quasi-instantaneous" method
for measuring the bursting characteristics by simultaneously collecting and
evaluating the information from all bunches in a multi-bunch fill, reducing the
measurement time from hours to seconds. This speed-up allows systematic studies
of the bursting characteristics for various accelerator settings within a
single fill of the machine, enabling a comprehensive comparison of the measured
bursting thresholds with theoretical predictions by the bunched-beam theory.
This paper introduces the method and presents first results obtained at the
ANKA synchrotron radiation facility.Comment: 7 pages, 7 figures, to be published in Physical Review Accelerators
and Beam
Deriving Bisimulation Congruences: 2-categories vs precategories
G-relative pushouts (GRPOs) have recently been proposed by the authors as a new foundation for Leifer and Milner’s approach to deriving labelled bisimulation congruences from reduction systems. This paper develops the theory of GRPOs further, arguing that they provide a simple and powerful basis towards a comprehensive solution. As an example, we construct GRPOs in a category of ‘bunches and wirings.’ We then examine the approach based on Milner’s precategories and Leifer’s functorial reactive systems, and show that it can be recast in a much simpler way into the 2-categorical theory of GRPOs
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
A detailed and unified treatment of spin-orbit systems using tools distilled from the theory of bundles
We return to our study \cite{BEH} of invariant spin fields and spin tunes for
polarized beams in storage rings but in contrast to the continuous-time
treatment in \cite{BEH}, we now employ a discrete-time formalism, beginning
with the maps of the continuous time formalism. We then
substantially extend our toolset and generalize the notions of invariant spin
field and invariant frame field. We revisit some old theorems and prove several
theorems believed to be new. In particular we study two transformation rules,
one of them known and the other new, where the former turns out to be an
-gauge transformation rule. We then apply the theory to the dynamics of
spin- and spin- particle bunches and their density matrix functions,
describing semiclassically the particle-spin content of bunches. Our approach
thus unifies the spin-vector dynamics from the T-BMT equation with the
spin-tensor dynamics and other dynamics. This unifying aspect of our approach
relates the examples elegantly and uncovers relations between the various
underlying dynamical systems in a transparent way. As in \cite{BEH}, the
particle motion is integrable but we now allow for nonlinear particle motion on
each torus. Since this work is inspired by notions from the theory of bundles,
we also provide insight into the underlying bundle-theoretic aspects of the
well-established concepts of invariant spin field, spin tune and invariant
frame field. Since we neglect, as is usual, the Stern-Gerlach force, the
underlying principal bundle is of product formso that we can present the theory
in a fashion which does not use bundle theory. Nevertheless we occasionally
mention the bundle-theoretic meaningof our concepts and we also mention the
similarities with the geometrical approach to Yang-Mills Theory
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