194 research outputs found
Observation of longitudinal and transverse self-injections in laser-plasma accelerators
Laser-plasma accelerators can produce high quality electron beams, up to
giga-electronvolts in energy, from a centimeter scale device. The properties of
the electron beams and the accelerator stability are largely determined by the
injection stage of electrons into the accelerator. The simplest mechanism of
injection is self-injection, in which the wakefield is strong enough to trap
cold plasma electrons into the laser wake. The main drawback of this method is
its lack of shot-to-shot stability. Here we present experimental and numerical
results that demonstrate the existence of two different self-injection
mechanisms. Transverse self-injection is shown to lead to low stability and
poor quality electron beams, because of a strong dependence on the intensity
profile of the laser pulse. In contrast, longitudinal injection, which is
unambiguously observed for the first time, is shown to lead to much more stable
acceleration and higher quality electron beams.Comment: 7 pages, 7 figure
Angular momentum evolution in laser-plasma accelerators
The transverse properties of an electron beam are characterized by two
quantities, the emittance which indicates the electron beam extend in the phase
space and the angular momentum which allows for non-planar electron
trajectories. Whereas the emittance of electron beams produced in laser- plasma
accelerator has been measured in several experiments, their angular momentum
has been scarcely studied. It was demonstrated that electrons in laser-plasma
accelerator carry some angular momentum, but its origin was not established.
Here we identify one source of angular momentum growth and we present
experimental results showing that the angular momentum content evolves during
the acceleration
Effect of experimental laser imperfections on laser wakefield acceleration and betatron source
International audienceLaser pulses in current ultra-short TW systems are far from being ideal Gaussian beams. The influence of the presence of non-Gaussian features of the laser pulse is investigated here from experiments and 3D Particle-in-Cell simulations. Both the experimental intensity distribution and wavefront are used as input in the simulations. It is shown that a quantitative agreement between experimental data and simulations requires to use realistic pulse features. Moreover, some trends found in the experiments, such as the growing of the X-ray signal with the plasma length, can only be retrieved in simulations with realistic pulses. The performances on the electron acceleration and the synchrotron X-ray emission are strongly degraded by these non-Gaussian features, even keeping constant the total laser energy. A drop on the X-ray photon number by one order of magnitude was found. This clearly put forward the limitation of using a Gaussian beam in the simulations
Observation of beam loading in a laser-plasma accelerator
Beam loading is the phenomenon which limits the charge and the beam quality
in plasma based accelerators. An experimental study conducted with a
laser-plasma accelerator is presented. Beam loading manifests itself through
the decrease of the beam energy, the reduction of dark current and the increase
of the energy spread for large beam charge. 3D PIC simulations are compared to
the experimental results and confirm the effects of beam loading. It is found
that, in our experimental conditions, the trapped electron beams generate
decelerating fields on the order of 1 GV/m/pC and that beam loading effects are
optimized for trapped charges of about 20 pC.Comment: 10 pages,4 figure
Application des recommandations dans la prise en charge du cancer de l’endomètre en pratique clinique. Étude rétrospective bretonne
National audienceObjective - To assess the use of French Cancer Institute recommendations for the diagnosis and treatment of endometrial cancer. Methods - Retrospective observational study involving 137 patients with endometrial cancer between 2011 and 2013. Results - Both MRI and pathological assessment with correct report as recommended were used for 66.4% of patients with endometrial cancer. For patients with correct preoperative assessment, 44.9% of patients were uncorrectly classified and upgraded on final pathological analysis of hysterectomy concerning lymph node involvement risk. These patients did not have confident surgical assessment, according this risk. Conclusion - To improve relevance of preoperative assessment in endometrial cancer, radiological and pathological expertise is required. However, even performed optimally, preoperative assessment does not allow correct risk stratification of lymph node involvement in endometrial cancer. This ineffective stratification leads to propose sentinel lymph node biopsy with hysterectomy in case of preoperative low risk endometrial cancer assessment
Phenotype of autosomal dominant spastic paraplegia linked to chromosome 2
Summary We report the clinical features of 12 families with autosomal dominant spastic paraplegia (ADSP) linked to the SPG4 locus on chromosome 2p, the major locus for this disorder that accounts for ∼40% of the families. Among 93 gene carriers, 32 (34%) were unaware of symptoms but were clinically affected. Haplotype reconstruction showed that 90% of the asymptomatic gene carriers presented increased reflexes and/or extensor plantar responses independent of age at examination. The mean age at onset was 29 years, ranging from 1 to 63 years. Intra- as well as inter-familial variability of age at onset was important, but did not result from anticipation. Phenotype—genotype correlations and comparison with SPG3 and SPG5 families indicated that despite the variability of age at onset, SPG4 is a single genetic entity but no clinical features distinguish individual SPG4 patients from those with SPG3 or SPG5 mutation
Les immunothérapies spécifiques dans le traitement des cancers
The offer of anti-cancer drugs has recently been disrupted by the introduction of checkpoint inhibitors on the market. Currently, one anti-CTLA-4, two anti-PD-1 and two anti-PD-L1 are authorized in the European Union, in seven different types of cancer. The clinical development of these therapies is still in full swing: in July 2017, more than 1 500 clinical trials were evaluating anti-PD-1, anti-PD-L1 and anti-CTLA-4 drugs in about twenty different locations and this number continues to increase. In the short term in France, other immunotherapies, the CAR-T cells, will complete this therapeutic arsenal. These immunotherapies appear as a real revolution in the treatment of some cancers. Nevertheless, many issues are associated with these therapies, particularly regarding the identification of good responders, the proper use of these drugs including the management of therapeutic strategies and safety profile, as well as the organization of care. In addition, the expenses associated with ipilimumab, nivolumab and pembrolizumab are substantial and almost tripled in one year, going from 120 million euros in 2015 to more than 340 million euros in 2016. This raises the question of the ability of the current healthcare system to maintain equitable access to innovation and best treatments for all patients. For all these reasons, the French National Cancer Institute decided to dedicate its thematic annual report on these innovative immunotherapies, targeting in particular checkpoint inhibitors and CAR-T cells, in order to produce an inventory of current data and an analysis regarding the different issues associated with these therapies
Spatiotemporal dynamics of ultrarelativistic beam-plasma instabilities
An electron or electron-positron beam streaming through a plasma is
notoriously prone to micro-instabilities. For a dilute ultrarelativistic
infinite beam, the dominant instability is a mixed mode between longitudinal
two-stream and transverse filamentation modes, with a phase velocity oblique to
the beam velocity. A spatiotemporal theory describing the linear growth of this
oblique mixed instability is proposed, which predicts that spatiotemporal
effects generally prevail for finite-length beams, leading to a significantly
slower instability evolution than in the usually assumed purely temporal
regime. These results are accurately supported by particle-in-cell (PIC)
simulations. Furthermore, we show that the self-focusing dynamics caused by the
plasma wakefields driven by finite-width beams can compete with the oblique
instability. Analyzed through PIC simulations, the interplay of these two
processes in realistic systems bears important implications for upcoming
accelerator experiments on ultrarelativistic beam-plasma interactions
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