Coupling of cytoplasm and adhesion dynamics determines cell polarization and locomotion

Observations of single epidermal cells on flat adhesive substrates have revealed two distinct morphological and functional states, namely a non-migrating symmetric unpolarized state and a migrating asymmetric polarized state. These states are characterized by different spatial distributions and dynamics of important biochemical cell components: F-actin and myosin-II form the contractile part of the cytoskeleton, and integrin receptors in the plasma membrane connect F-actin filaments to the substratum. In this way, focal adhesion complexes are assembled, which determine cytoskeletal force transduction and subsequent cell locomotion. So far, physical models have reduced this phenomenon either to gradients in regulatory control molecules or to different mechanics of the actin filament system in different regions of the cell. Here we offer an alternative and self-organizational model incorporating polymerization, pushing and sliding of filaments, as well as formation of adhesion sites and their force dependent kinetics. All these phenomena can be combined into a non-linearly coupled system of hyperbolic, parabolic and elliptic differential equations. Aim of this article is to show how relatively simple relations for the small-scale mechanics and kinetics of participating molecules may reproduce the emergent behavior of polarization and migration on the large-scale cell level.Comment: v2 (updates from proof): add TOC, clarify Fig. 4, fix several typo

On Landau damping of dipole modes by non-linear space charge and octupoles

The joint effect of space-charge non-linearities and octupole lenses is important for Landau damping of coherent instabilities. The octupole strength required for stabilisation can depend strongly on the sign of the excitation current of the lenses. This note tries to extend results, previously obtained for coasting beams and rigid bunches, to more general head--tail modes

The Status of Stochastic Cooling

Presented at a symposium to celebrate the 30th anniversary of electron cooling, this report is intended to give the status of the companion technique, stochastic cooling, some 28 years after its invention. An overview of past developments reveals the close relationship between the two cooling ideas. Then the report concentrates on the principal ingredients of stochastic cooling in order to discuss the limits encountered and some recent ideas for pushing back these limits

Beam Cooling: Past, Present and Future

To date, four main methods to increase the phase-space density of circulating beams in storage rings are operational: cooling of e+e- -beams by synchrotron radiation, cooling of protons and ions by electrons, stochastic cooling of (anti-)protons and ions, and cooling of special ions by laser light. A fifth method, ionisation cooling of muons, is under intense development. Each of these techniques will be covered in detail in specialised contributions at this workshop. The present paper is intended to introduce the different techniques and put them into perspective: specific features will be outlined, common characteristics will be sketched, and potential future applications will be mentioned

Production of Low-Energy Antiprotons

The production, collection and deceleration of antiprotons is reviewed with the aim of establishing guidelines for the design of a simple yet efficient source of stopped antiprotons. A high-energy (20-100 GeV) high-intensity (~ 1013 protons/pulse) proton accelerator is needed to produce antiprotons in copious numbers. A 'passive' conversion-target consisting of a thin iridium rod embedded in graphite, and a magnetic-horn type lens to collect the antiproton flux from the target represent a good compromise between yield and reliability. To transport the flux to low energy a large-acceptance cooling and deceleration ring working up to an energy equal to one-eighth to one-tenth of the primary proton energy is required. Stochastic cooling (at high energy) and electron cooling (at lower energy) are indispensable for providing low-energy beams of useful density

LEAR, history and early achievements

This report retraces the early history of the Low Energy Antiproton Ring (LEA) and recalls some of its unusual design features

The mid-Holocene extinction of silver fir ( Abies alba ) in the Southern Alps: a consequence of forest fires? Palaeobotanical records and forest simulations

Pollen records suggest that Abies alba played a dominating role in both the montane and lowland forests at the border of the Southern Alps between ca. 8500 and 5700 years ago. Two major declines in fir, at about 7300-7000 cal b.p. and at ca. 6000 cal b.p., followed by the local extinction of the species are characteristic of the area below ca. 1000 m a.s.l. In order to test the impact of fire on the population dynamics of silver fir, a dynamic model (DisCForm) with a fire module was applied to simulate the early- and mid-Holocene forest development. Simulation outputs based on different fire scenarios were compared with the pollen record from Lago di Annone (226ma.s.l.). The marked Abies decreases shown in the pollen record can be simulated with very intensive fire scenarios, but they do not result in an extinction of silver fir in the model. Low charcoal influx values related to the Abies declines in the palaeobotanical record suggest that fire was not the only reason for the extinction of silver fir. Human impact, as well as Holocene climatic changes leading to temporary moisture deficits and reduced adaptability due to low genetic variation may have had a significant impact on the Abies forest

Stochastic cooling in hadron colliders

Stochastic cooling in big hadron colliders is hampered by the high particle density, the bunch structure and by an unexpectedly large "RF-activity" up to very high frequencies. The more modest goal of tail cleaning in the LHC is made difficult due to the high discrimination required for pick-ups which selectively observe the beam halo only. This paper reviews sources of these difficulties, which have so far frustrated attempts to apply stochastic cooling to bunched beams at high energy