281 research outputs found
Engineering of spin-lattice relaxation dynamics by digital growth of diluted magnetic semiconductor CdMnTe
The technological concept of "digital alloying" offered by molecular-beam
epitaxy is demonstrated to be a very effective tool for tailoring static and
dynamic magnetic properties of diluted magnetic semiconductors. Compared to
common "disordered alloys" with the same Mn concentration, the spin-lattice
relaxation dynamics of magnetic Mn ions has been accelerated by an order of
magnitude in (Cd,Mn)Te digital alloys, without any noticeable change in the
giant Zeeman spin splitting of excitonic states, i.e. without effect on the
static magnetization. The strong sensitivity of the magnetization dynamics to
clustering of the Mn ions opens a new degree of freedom for spin engineering.Comment: 9 pages, 3 figure
Dynamic Control of Laser Produced Proton Beams
The emission characteristics of intense laser driven protons are controlled
using ultra-strong (of the order of 10^9 V/m) electrostatic fields varying on a
few ps timescale. The field structures are achieved by exploiting the high
potential of the target (reaching multi-MV during the laser interaction).
Suitably shaped targets result in a reduction in the proton beam divergence,
and hence an increase in proton flux while preserving the high beam quality.
The peak focusing power and its temporal variation are shown to depend on the
target characteristics, allowing for the collimation of the inherently highly
divergent beam and the design of achromatic electrostatic lenses.Comment: 9 Pages, 5 figure
A Bright Spatially-Coherent Compact X-ray Synchrotron Source
Each successive generation of x-ray machines has opened up new frontiers in
science, such as the first radiographs and the determination of the structure
of DNA. State-of-the-art x-ray sources can now produce coherent high brightness
keV x-rays and promise a new revolution in imaging complex systems on nanometre
and femtosecond scales. Despite the demand, only a few dedicated synchrotron
facilities exist worldwide, partially due the size and cost of conventional
(accelerator) technology. Here we demonstrate the use of a recently developed
compact laser-plasma accelerator to produce a well-collimated,
spatially-coherent, intrinsically ultrafast source of hard x-rays. This method
reduces the size of the synchrotron source from the tens of metres to
centimetre scale, accelerating and wiggling a high electron charge
simultaneously. This leads to a narrow-energy spread electron beam and x-ray
source that is >1000 times brighter than previously reported plasma wiggler and
thus has the potential to facilitate a myriad of uses across the whole spectrum
of light-source applications.Comment: 5 pages, 4 figure
Synchrotron x-ray radiation from laser wakefield accelerated electron beams in a plasma channel
Synchrotron x-ray radiation from laser wakefield accelerated electron beams was characterized at the HERCULES facility of the University of Michigan. A mono-energetic electron beam with energy up to 400 MeV was observed in the interaction of an ultra-short laser pulse with a super-sonic gas jet target. The experiments were performed at a peak intensity of 5×1019 W/cm2 by using an adaptive optic. The accelerated electron beam undergoes a so called "betatron" oscillation in an ion channel, where plasma electrons have been expelled by the laser ponderomotive force, and, therefore, emits synchrotron radiation. We observe broad synchrotron x-ray radiation extending up to 30 keV. We find that this radiation is emitted in a beam with a divergence angle as small as 12×4 mrad2 and can have a source size smaller than 3 microns and a peak brightness of 1022 photons/mm2/mrad2/second/0.1% bandwidth, which is comparable to currently existing 3rd generation conventional light sources. This opens up the possibility of using laser-produced "betatron" sources for many applications that currently require conventional synchrotron sources.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85402/1/jpconf10_244_042026.pd
CLT in Functional Linear Regression Models
International audienceWe propose in this work to derive a CLT in the functional linear regression model to get confidence sets for prediction based on functional linear regression. The main difficulty is due to the fact that estimation of the functional parameter leads to a kind of ill-posed inverse problem. We consider estimators that belong to a large class of regularizing methods and we first show that, contrary to the multivariate case, it is not possible to state a CLT in the topology of the considered functional space. However, we show that we can get a CLT for the weak topology under mild hypotheses and in particular without assuming any strong assumptions on the decay of the eigenvalues of the covariance operator. Rates of convergence depend on the smoothness of the functional coefficient and on the point in which the prediction is made
Hot Electron and X-ray Production from Intense Laser Irradiation of Wavelength-Scale Polystyrene Spheres
Hot electron and x-ray production from solid targets coated with polystyrene-spheres which are irradiated with high-contrast, 100 fs, 400 nm light pulses at intensity up to 2×1017 W/cm2 have been studied. The peak hard x-ray signal from uncoated fused silica targets is an order of magnitude smaller than the signal from targets coated with submicron sized spheres. The temperature of the x-rays in the case of sphere-coated targets is twice as hot as that of uncoated glass. A sphere-size scan of the x-ray yield and observation of a peak in both the x-ray production and temperature at a sphere diameter of 0.26 μm, indicate that these results are consistent with Mie enhancements of the laser field at the sphere surface and multipass stochastic heating of the hot electrons in the oscillating laser field. These results also match well with particle-in-cell simulations of the interaction
- …