8,590 research outputs found
Economical and accurate methods for measuring the multipole field coefficients of bending magnets and quadrupoles
On the recombination in high-order harmonic generation in molecules
We show that the dependence of high-order harmonic generation (HHG) on the
molecular orientation can be understood within a theoretical treatment that
does not involve the strong field of the laser. The results for H_2 show
excellent agreement with time-dependent strong field calculations for model
molecules, and this motivates a prediction for the orientation dependence of
HHG from the N_2 3s_g valence orbital. For both molecules, we find that the
polarization of recombination photons is influenced by the molecular
orientation. The variations are particularly pronounced for the N_2 valence
orbital, which can be explained by the presence of atomic p-orbitals.Comment: 6 pages 7 figure
Electrical Transport in High Quality Graphene pnp Junctions
We fabricate and investigate high quality graphene devices with contactless,
suspended top gates, and demonstrate formation of graphene pnp junctions with
tunable polarity and doping levels. The device resistance displays distinct
oscillations in the npn regime, arising from the Fabry-Perot interference of
holes between the two pn interfaces. At high magnetic fields, we observe
well-defined quantum Hall plateaus, which can be satisfactorily fit to
theoretical calculations based on the aspect ratio of the device.Comment: to appear in a special focus issue in New Journal of Physic
Black carbon ageing in the Canadian Centre for Climate modelling and analysis atmospheric general circulation model
International audienceBlack carbon (BC) particles in the atmosphere have important impacts on climate. The amount of BC in the atmosphere must be carefully quantified to allow evaluation of the climate effects of this type of aerosol. In this study, we present the treatment of BC aerosol in the developmental version of the 4th generation Canadian Centre for Climate modelling and analysis (CCCma) atmospheric general circulation model (AGCM). The focus of this work is on the conversion of insoluble BC to soluble/mixed BC by physical and chemical ageing. Physical processes include the condensation of sulphuric and nitric acid onto the BC aerosol, and coagulation with more soluble aerosols such as sulphates and nitrates. Chemical processes that may age the BC aerosol include the oxidation of organic coatings by ozone. Four separate parameterizations of the ageing process are compared to a control simulation that assumes no ageing occurs. These simulations use 1) an exponential decay with a fixed 24h half-life, 2) a condensation and coagulation scheme, 3) an oxidative scheme, and 4) a linear combination of the latter two ageing treatments. Global BC burdens are 2.15, 0.15, 0.11, 0.21, and 0.11TgC for the control run, and four ageing schemes, respectively. The BC lifetimes are 98.1, 6.6, 5.0, 9.5, and 4.9 days, respectively. The sensitivity of modelled BC burdens, and concentrations to the factor of two uncertainty in the emissions inventory is shown to be greater than the sensitivity to the parameterization used to represent the BC ageing, except for the oxidation based parameterization. A computationally efficient parameterization that represents the processes of condensation, coagulation, and oxidation is shown to simulate BC ageing well in the CCCma AGCM. As opposed to the globally fixed ageing time scale, this treatment of BC ageing is responsive to varying atmospheric composition
Investigation of a direction sensitive sapphire detector stack at the 5 GeV electron beam at DESY-II
Extremely radiation hard sensors are needed in particle physics experiments
to instrument the region near the beam pipe. Examples are beam halo and beam
loss monitors at the Large Hadron Collider, FLASH or XFEL. Currently artificial
diamond sensors are widely used. In this paper single crystal sapphire sensors
are considered as a promising alternative. Industrially grown sapphire wafers
are available in large sizes, are of low cost and, like diamond sensors, can be
operated without cooling. Here we present results of an irradiation study done
with sapphire sensors in a high intensity low energy electron beam. Then, a
multichannel direction-sensitive sapphire detector stack is described. It
comprises 8 sapphire plates of 1 cm^2 size and 525 micro m thickness,
metallized on both sides, and apposed to form a stack. Each second metal layer
is supplied with a bias voltage, and the layers in between are connected to
charge-sensitive preamplifiers. The performance of the detector was studied in
a 5 GeV electron beam. The charge collection efficiency measured as a function
of the bias voltage rises with the voltage, reaching about 10 % at 950 V. The
signal size obtained from electrons crossing the stack at this voltage is about
22000 e, where e is the unit charge.
The signal size is measured as a function of the hit position, showing
variations of up to 20 % in the direction perpendicular to the beam and to the
electric field. The measurement of the signal size as a function of the
coordinate parallel to the electric field confirms the prediction that mainly
electrons contribute to the signal. Also evidence for the presence of a
polarisation field was observed.Comment: 13 pages, 7 figures, 3 table
Image transmission through a stable paraxial cavity
We study the transmission of a monochromatic "image" through a paraxial
cavity. Using the formalism of self-transform functions, we show that a
transverse degenerate cavity transmits the self-transform part of the image,
with respect to the field transformation over one round-trip of the cavity.
This formalism gives a new insight on the understanding of the behavior of a
transverse degenerate cavity, complementary to the transverse mode picture. An
experiment of image transmission through a hemiconfocal cavity show the
interest of this approach.Comment: submitted to Phys. Rev.
Why the oxygen isotopic composition of sea water changes with time
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94773/1/grl4282.pd
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