475 research outputs found
A constant of quantum motion in two dimensions in crossed magnetic and electric fields
We consider the quantum dynamics of a single particle in the plane under the
influence of a constant perpendicular magnetic and a crossed electric potential
field. For a class of smooth and small potentials we construct a non-trivial
invariant of motion. Do to so we proof that the Hamiltonian is unitarily
equivalent to an effective Hamiltonian which commutes with the observable of
kinetic energy.Comment: 18 pages, 2 figures; the title was changed and several typos
corrected; to appear in J. Phys. A: Math. Theor. 43 (2010
Magnetic and electric field effect on the photoelectron emission from prototype LHC bean screen material.
This paper describes experimental studies of the effect of a dipole field on the photoelectron emission and on the photon reflectivities from LHC beam screen material. These studies were performed using synchrotron radiation from the VEPP-2M storage ring at BINP (Novosibirsk). The particular surface roughness and geometry of the prototype LHC beam screen material requires dedicated experimental measurements. The experiments were performed under conditions close to those expected in the LHC. An important result obtained is that a dipole magnetic field attenuates the photoelectron emission from surface by more than two orders of magnitude with the magnetic field aligned parallel to the surface. The measurements of photon reflectivities, forward scattered and diffuse, and the azimuthal distribution of emitted photoelectrons from the same material are reported. These experimental results are important input for the final design of the LHC beam screen
Stem cell bioprocessing: The bioengineering of lung epithelium in 3D from embryonic stem cells
Stem cell therapies and tissue engineering strategies are required for the
clinical treatment of respiratory diseases. Previous studies have established protocols
for the differentiation of airway epithelium from stem cells but have involved costly
and laborious culture methods. The aim of this thesis was to achieve efficient and
reproducible maintenance and differentiation of embryonic stem cells to airway
epithelium, in 2D and 3D culture, by developing appropriate bioprocessing
technology.
Firstly, the 2D differentiation process of human and murine ES cells into
pulmonary epithelial cells was addressed. The main finding in was that the
proportion of type II pneumocytes, the major epithelial component of the gas-exchange
area of lung, differentiated with this method was higher than that obtained
in previous sudies, 33% of resultant cell expressed the specific marker surfactant
protein C (SPC) compared with up to 10%.
Secondly, the maintenance and differentiation was carried out in 3D. A
protocol was devised that maintained undifferentiated human ES cells in culture for
more than 200 days encapsulated in alginate without any feeder layer or growth
factors. For ES cell differentiation in 3D, a method was devised to provide a
relatively cheap and simple means of culture and use medium conditioned by a
human pneumocyte tumour cell line (A549). The differentiation of human and murine
ES cells into pulmonary epithelial cells, particularly type II pneumocytes, was found
to be upregulated by culture in this conditioned medium, with or without embryoid
body formation.
The third step was to test whether this differentiation protocol was amenable
to scale-up and automation in a bioreactor using cell encapsulation. It was possible to
show that encapsulated murine ES cells cultured in static, co-culture or rotating wall
bioreactor (HARV) systems, differentiate into endoderm and, predominantly, type I
and II pneumocytes. Flow cytometry revealed that the mean yield of differentiated
type II pneumocytes was around 50% at day 10 of cultivation.
The final stage of the work was to design and produce a perfusion system
airlift bioreactor to mimic the pulmonary microenvironment in order to achieve large
scale production of biologically functional tissue. The results of these studies thus
provide new protocols for the maintenance of ES cells and their differentiation
towards pulmonary phenotypes that are relatively simple and cheap and can be
applied in bioreactor systems that provide for the kind of scale up of differentiated
cell production needed for future clinical applications
Heat-kernels and functional determinants on the generalized cone
We consider zeta functions and heat-kernel expansions on the bounded,
generalized cone in arbitrary dimensions using an improved calculational
technique. The specific case of a global monopole is analysed in detail and
some restrictions thereby placed on the coefficient. The computation
of functional determinants is also addressed. General formulas are given and
known results are incidentally, and rapidly, reproduced.Comment: 26p,LaTeX.(Cosmetic changes and eqns (9.8),(11.2) corrected.
The Dirac system on the Anti-de Sitter Universe
We investigate the global solutions of the Dirac equation on the
Anti-de-Sitter Universe. Since this space is not globally hyperbolic, the
Cauchy problem is not, {\it a priori}, well-posed. Nevertheless we can prove
that there exists unitary dynamics, but its uniqueness crucially depends on the
ratio beween the mass of the field and the cosmological constant
: it appears a critical value, , which plays a role
similar to the Breitenlohner-Freedman bound for the scalar fields. When
there exists a unique unitary dynamics. In opposite, for
the light fermions satisfying , we construct several asymptotic
conditions at infinity, such that the problem becomes well-posed. In all the
cases, the spectrum of the hamiltonian is discrete. We also prove a result of
equipartition of the energy.Comment: 33 page
The super-LHC
We review here the prospects of a long-term upgrade programme for the Large
Hadron Collider (LHC), CERN laboratory's new proton-proton collider. The
super-LHC, which is currently under evaluation and design, is expected to
deliver of the order of ten times the statistics of the LHC. In addition to a
non-technical summary of the principal physics arguments for the upgrade, I
present a pedagogical introduction to the technological challenges on the
accelerator and experimental fronts, and a review of the current status of the
planning.Comment: To appear in Contemporary Physic
Detecting chaos in particle accelerators through the frequency map analysis method
The motion of beams in particle accelerators is dominated by a plethora of
non-linear effects which can enhance chaotic motion and limit their
performance. The application of advanced non-linear dynamics methods for
detecting and correcting these effects and thereby increasing the region of
beam stability plays an essential role during the accelerator design phase but
also their operation. After describing the nature of non-linear effects and
their impact on performance parameters of different particle accelerator
categories, the theory of non-linear particle motion is outlined. The recent
developments on the methods employed for the analysis of chaotic beam motion
are detailed. In particular, the ability of the frequency map analysis method
to detect chaotic motion and guide the correction of non-linear effects is
demonstrated in particle tracking simulations but also experimental data.Comment: Submitted for publication in Chaos, Focus Issue: Chaos Detection
Methods and Predictabilit
Parasitic Energy Loss in the LEP Superconducting Cavities
The energy loss of bunches in the LEP superconducting (SC) cavities has been determined by measuring the closed orbit as a function of current with the beam position monitors located at finite dispersion. This method has already been used in earlier experiments to determine the distribution of the longitudinal impedance of different parts of LEP. In the present experiment the energy loss in two straight sections, containing only SC cavities, was compared with that in sections having both copper cavities and SC cavities. The results confirm the impedance calculations for the two types of cavities. The accuracy of the measurements was considerably improved by determining simultaneously the orbits of bunches with different currents. At the same time with these beam-based impedance measurements, the power dissipation was observed directly by local temperature monitors in different elements: the inter-cavity bellows inside the cryostat, the warm intermodule bellows, and Ferrite absorbers which were installed in two places to reduce the energy leaking out of cavities. These observations were correlated with the change of cryogenics power consumption, and showed an unexpected dependence of energy loss on beam energy
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