66 research outputs found
Experiments and simulation of transverse cooling and heating in ion channeling
Within the theory of ion channeling in single crystals the so-called rule of reversibility claims equal probabilities for de- and rechanneling. However, the experimental study of this work shows a strong violation of this rule. Specially developed computer simulations provide the explanation of this unexpected behavior in terms of a hysteresis of the mean ion charge state during the ion's motion along lattice directions. This sheds new light on the process of charge exchange of heavy ions in solids.In der Theorie des Ionen-Channeling in Einkristallen gibt es eine sog. Reversibilitätsregel, die besagt, dass die Wahrscheinlichkeiten für De- und Rekanalisierung gleich groß sein sollten. Die Experimente dieser Arbeit zeigen eine starke Verletzung dieser Regel. Eigens entwickelte Computersimulationen liefern die Erklärung für dieses unerwartete Verhalten in Form einer Hysterese des mittleren Ladungszustands des Ions während seiner Bewegung entlang von Gitterketten. Dies liefert neue Einsichten in den Prozess des Ladungsaustausch von schweren Ionen in Festkörpern
Evolution of Relativistic Pair Beams: Implications for Laboratory and TeV Astrophysics
Missing cascades from TeV blazar beams indicate that collective plasma
effects may play a significant role in their energy loss. It is possible to
mimic the evolution of such highly energetic pair beams in laboratory
experiments using modern accelerators. The fate of the beam is governed by two
different processes, energy loss through the unstable mode and energetic
broadening of the pair beam through diffusion in momentum space. We chalk out
this evolution using a Fokker-Planck approach in which the drift and the
diffusion terms respectively describe these phenomena in a compact form. We
present particle-in-cell simulations to trace the complete evolution of the
unstable beam-plasma system for a generic narrow Gaussian pair beam for which
the growth rate is reactive. We show that the instability leads to an energetic
broadening of the pair beam, slowing down the instability growth in the linear
phase, in line with the analytical and numerical solutions of the Fokker-Planck
equation. Whereas in a laboratory experiment the change in the momentum
distribution is an easily measured observable as a feedback of the instability,
the consequence of diffusive broadening in an astrophysical scenario can be
translated to an increase in the opening angle of the pair beam.Comment: 20 pages, 10 figure
Collins and Sivers transverse-spin asymmetries in inclusive muoproduction of mesons
The production of vector mesons in deep inelastic scattering is an
interesting yet scarsely explored channel to study the transverse spin
structure of the nucleon and the related phenomena. The COMPASS collaboration
has performed the first measurement of the Collins and Sivers asymmetries for
inclusively produced mesons. The analysis is based on the data set
collected in deep inelastic scattering in using a
beam impinging on a transversely polarized target. The
mesons are selected from oppositely charged hadron pairs, and the
asymmetries are extracted as a function of the Bjorken- variable, the
transverse momentum of the pair and the fraction of the energy carried by
the pair. Indications for positive Collins and Sivers asymmetries are observed
Spin Density Matrix Elements in Exclusive Meson Muoproduction
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard
exclusive meson muoproduction at COMPASS using 160~GeV/ polarised
and beams impinging on a liquid hydrogen target. The
measurement covers the kinematic range 5.0~GeV/ 17.0~GeV/,
1.0 (GeV/) 10.0 (GeV/) and 0.01 (GeV/) 0.5 (GeV/). Here, denotes the mass of the final
hadronic system, the virtuality of the exchanged photon, and
the transverse momentum of the meson with respect to the
virtual-photon direction. The measured non-zero SDMEs for the transitions of
transversely polarised virtual photons to longitudinally polarised vector
mesons () indicate a violation of -channel helicity
conservation. Additionally, we observe a dominant contribution of
natural-parity-exchange transitions and a very small contribution of
unnatural-parity-exchange transitions, which is compatible with zero within
experimental uncertainties. The results provide important input for modelling
Generalised Parton Distributions (GPDs). In particular, they may allow one to
evaluate in a model-dependent way the role of parton helicity-flip GPDs in
exclusive production
A RESTful Extension of OPC UA
RESTful interfaces are a wide-spread architecture style for webservice implementations and are built upon the resource-oriented approach to decentralized architectures (ROA). REST postulates a set of requirements that are not covered by the OPC Unified Architecture (OPC UA) communication protocol per se. We propose a set of simple extensions to the OPC UA binary protocol that enable RESTful communication. The evaluation shows an order of magnitude improvement in the use of communication resources for sporadic service requests. Additionally, RESTful OPC UA allows applications to profit from the advantages of the resource-oriented architecture style, such as caching and loose application coupling
Linearization of the Longitudinal Phase Space without Higher Harmonic Field
Accelerator applications like free-electron lasers, time-resolved electron diffraction, and advanced accelerator concepts like plasma acceleration desire bunches of ever shorter longitudinal extent. However, apart from space charge repulsion, the internal bunch structure and its development along the beam line can limit the achievable compression due to nonlinear phase space correlations. In order to improve such a limited longitudinal focus, a correction by properly linearizing the phase space is required. At large scale facilities like Flash at DESY or the European XFEL, a higher harmonic cavity is installed for this purpose. In this paper, another method is described and evaluated: Expanding the beam after the electron source enables a higher order correction of the longitudinal focus by a subsequent accelerating cavity which is operated at the same frequency as the electron gun. The elaboration of this idea presented here is based on a ballistic bunching scheme, but can be extended to bunch compression based on magnetic chicanes. The core of this article is an analytic model describing this approach, which is verified by simulations, predicting possible bunch length below 1 fs at low bunch charge. Minimizing the energy spread down to σE/E<10−5 while keeping the bunch long is another interesting possibility, which finds applications, e.g., in time resolved transmission electron microscopy concepts
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