6,235 research outputs found
The Baum-Connes Conjecture via Localisation of Categories
We redefine the Baum-Connes assembly map using simplicial approximation in
the equivariant Kasparov category. This new interpretation is ideal for
studying functorial properties and gives analogues of the assembly maps for all
equivariant homology theories, not just for the K-theory of the crossed
product. We extend many of the known techniques for proving the Baum-Connes
conjecture to this more general setting
Direction for the Future - Successive Acceleration of Positive and Negative Ions Applied to Space Propulsion
Electrical space thrusters show important advantages for applications in
outer space compared to chemical thrusters, as they allow a longer mission
lifetime with lower weight and propellant consumption. Mature technologies on
the market today accelerate positive ions to generate thrust. The ion beam is
neutralized by electrons downstream, and this need for an additional
neutralization system has some drawbacks related to stability, lifetime and
total weight and power consumption. Many new concepts, to get rid of the
neutralizer, have been proposed, and the PEGASES ion-ion thruster is one of
them. This new thruster concept aims at accelerating both positive and negative
ions to generate thrust, such that additional neutralization is redundant. This
chapter gives an overview of the concept of electric propulsion and the state
of the development of this new ion-ion thruster.Comment: 10 pages, contribution to the CAS-CERN Accelerator School: Ion
Sources, Senec, Slovakia, 29 May - 8 June 2012, edited by R. Bailey. appears
in CERN Yellow Report CERN-2013-007, pp.575-58
Boosted tops: Physics and reconstruction
The production of boosted top quarks in the high centre-of-mass energy collisions at the LHC is a probe to new phenomena at the TeV scale. Numerous extensions to the Standard Model (SM) predict massive particles whose couplings to top quarks are enhanced. Such massive particles, referred as t¯t resonances, will distort the t¯t invariant mass spectrum relative to the SM expectation. This presentation is dedicated to studies performed by the ATLAS and CMS Collaborations. New techniques were developed to reconstruct boosted top quarks as single objects, both as a top-tagged jet with a substructure in the hadronic decay mode and a jet with a lepton inside in the leptonic decay mode. These techniques were applied in dedicated analyses to search for high mass t¯t resonances. Expected limits and the potential for observation are presented for a luminosity of few hundreds of pb−1
Plasma Arc Cutting - Reversed Swirl Ring, Electrode Thread and Cut Direction Effects on Kerf Geometry
Plasma arc cutting is used to cut any conductive material. It consists in blowing pressurized gas and feed current to an arc, leading to a thin plasma dart able to melt down the material and blow it away, creating a kerf. Its quality depends on its shape. This paper shows, through experimental measurements, how the inner geometry of the torch can affect the cut quality. It appears that one side of the kerf is much more oblique and sensitive to factors variation than the other. A theory based on a computational fluid dynamics model is proposed to investigate the causes of these phenomena
Chromatic correlations at injection and related ejection problems in separated sector cyclotrons
International audienceInjection into a cyclotron, in order to preventemittance and phase spread dilution, requires propercouplings in the matching. One must first introduce aAP/r' associated with an r/AW coupling (through simplecticconditions) ; according to the angle of theaccelerating dees and the choice of harmonic numberthe(r,r') acceptance may also have to be tilted. Allthese effects are investigated in the case of theGANIL SSC's. At extraction corresponding correlationsexist. For a resonant system, extraction may be difficultwhen the energy spread is large because of thelarge coupling induced by resonance. A precessionalextraction which has been studied might in this casebe more efficient. Other ways for making extractioneasier are also considered. Moreover a new method ofphase compression at injection into the SSC is presentlyunder study at GANIL
Periodic formation and propagation of double layers in the expanding chamber of an inductive discharge operating in Ar/SF₆ mixtures
It has previously been shown [Tuszewski et al., Plasma Sources Sci. Technol.12, 396 (2003)] that inductive discharges in electronegative gases are subject to two types of instability: the sourceinstability related to the E to H transition and a transport instability, occurring downstream when an expanding chamber is present. These two types of instability are observed in our “helicon” reactor operated without a static magnetic field in low-pressure Ar∕SF6 mixtures. Temporally and spatially resolved measurements show that, in our experiment, the downstream instability is a periodic formation and propagation of a double layer. The double layer is born at the end of the source tube and propagates slowly to the end of the expansion region with a velocity of 150ms⁻¹
Equilibrium model for two low-pressure electronegative plasmas connected by a double layer
Plihon et al. [J. Appl. Phys.98, 023306 (2005)] have recently shown that double layers usually form during the expansion of a low pressure electronegative plasma. These double layers act as permeable internal boundaries between the source (upstream) plasma and the downstream expanding plasma; positive ions flow from upstream to downstream whereas negative ions flow in the opposite direction. So far, the detailed physical mechanisms leading to their formation have not been identified. In this paper, we develop a model for the two plasma equilibria, upstream and downstream, assuming that the double layer exists and couples the two plasmas. At very low pressure, typically 0.5mTorr, the coupling is strong and acts both ways. The negative ions created downstream contributes to the upstream equilibrium as well as the upstream positive ions contribute to the downstream equilibrium. As the pressure increases, the situation becomes asymmetric. The sourceplasma is not affected by the negative ions flowing from downstream, whereas the positive ions coming from the source control the downstream plasma equilibrium, where local ionization is negligible.This work has been supported by the European Space
Agency, under Ariadna Study Contract No. ACT-04-3101.
One of the authors A.J.L. acknowledges the hospitality of
the LPTP, where the collaboration was begun
Electron heating mechanisms in dual frequency capacitive discharges
We discuss electron heating mechanisms in the sheath regions of dual-frequency capacitive discharges, with the twin aims of identifying the dominant mechanisms and supplying closed-form expressions from which the heating power can be estimated. We show that the heating effect produced by either Ohmic or collisionless heating is much larger when the discharge is excited by a superposition of currents at two frequencies than if either current had acted alone. This coupling effect occurs because the lower frequency current, while not directly heating the electrons to any great extent, strongly affects the spatial structure of the discharge in the sheath regions
Simulations of electromagnetic effects in high frequency capacitively coupled discharges using the Darwin approximation
The Darwin approximation is investigated for its possible use in simulation
of electromagnetic effects in large size, high frequency capacitively coupled
discharges. The approximation is utilized within the framework of two different
fluid models which are applied to typical cases showing pronounced standing
wave and skin effects. With the first model it is demonstrated that Darwin
approximation is valid for treatment of such effects in the range of parameters
under consideration. The second approach, a reduced nonlinear Darwin
approximation-based model, shows that the electromagnetic phenomena persist in
a more realistic setting. The Darwin approximation offers a simple and
efficient way of carrying out electromagnetic simulations as it removes the
Courant condition plaguing explicit electromagnetic algorithms and can be
implemented as a straightforward modification of electrostatic algorithms. The
algorithm described here avoids iterative schemes needed for the divergence
cleaning and represents a fast and efficient solver, which can be used in fluid
and kinetic models for self-consistent description of technical plasmas
exhibiting certain electromagnetic activity
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