1,298 research outputs found
Anticorrelation between Ion Acceleration and Nonlinear Coherent Structures from Laser-Underdense Plasma Interaction
In laser-plasma experiments, we observed that ion acceleration from the
Coulomb explosion of the plasma channel bored by the laser, is prevented when
multiple plasma instabilities such as filamentation and hosing, and nonlinear
coherent structures (vortices/post-solitons) appear in the wake of an
ultrashort laser pulse. The tailoring of the longitudinal plasma density ramp
allows us to control the onset of these insabilities. We deduced that the laser
pulse is depleted into these structures in our conditions, when a plasma at
about 10% of the critical density exhibits a gradient on the order of 250
{\mu}m (gaussian fit), thus hindering the acceleration. A promising
experimental setup with a long pulse is demonstrated enabling the excitation of
an isolated coherent structure for polarimetric measurements and, in further
perspectives, parametric studies of ion plasma acceleration efficiency.Comment: 4 pages, 5 figure
Case-control survival analysis with a general semiparametric shared frailty model--a pseudo full likelihood approach
In this work we deal with correlated failure time (age at onset)
data arising from population-based case-control studies, where case
and control probands are selected by population-based sampling and
an array of risk factor measures is collected for both cases and con-
trols and their relatives. Parameters of interest are e®ects of risk
factors on the failure time hazard function and within-family depen-
dencies among failure times after adjusting for the risk factors. Due
to the retrospective sampling scheme, large sample theory for existing
methods has not been established. We develop a novel technique for
estimating the parameters of interest under a general semiparamet-
ric shared frailty model. We also present a simple, easily computed,
and non-iterative nonparametric estimator for the cumulative base-
line hazard function. We provide rigorous large sample theory for the
proposed method. We also present simulation results and a real data
example for illustrating the utility of the proposed method
Short Intense Laser Pulse Collapse in Near-Critical Plasma
It is observed that the interaction of an intense ultra-short laser pulse
with an overdense gas jet results in the pulse collapse and the deposition of a
significant part of energy in a small and well localized volume in the rising
part of the gas jet, where the electrons are efficiently accelerated and
heated. A collisionless plasma expansion over 150 microns at a sub-relativistic
velocity (~c/3) has been optically monitored in time and space, and attributed
to the quasistatic field ionization of the gas associated to the hot electron
current. Numerical simulations in good agreement with the observations suggest
the acceleration in the collapse region of relativistic electrons, along with
the excitation of a sizeable magnetic dipole that sustains the electron current
over several picoseconds. Perspectives of ion beam generation at high
repetition rate directly from gas jets are discussed
Clinical Spectrum and Genetic Diagnosis of 54 Consecutive Patients Aged 0-25 with Bilateral Cataracts
Childhood cataract affects 2.5–3.5 per 10,000 children in the UK, with a genetic mutation identified in 50–90% of bilateral cases. However, cataracts can also manifest in adolescence
and early adulthood in isolation, as part of a complex ocular phenotype or with systemic features
making accurate diagnosis more challenging. We investigate our real-world experience through
a retrospective review of consecutive bilateral cataract patients (0–25 years) presenting to the ocular genetics service at Moorfields Eye Hospital between 2017 and 2020. Fifty-four patients from
44 unrelated families were identified, with a median age of 13.5 years (range 1 to 68 years) and a
median age at diagnosis of 43.9 months IQR (1.7–140.3 months); 40.7% were female and 46.3% were
Caucasian. Overall, 37 patients from 27 families (61.4%) were genetically solved (50%) or likely solved
(additional 11.4%), with 26 disease-causing variants (8 were novel) in 21 genes; the most common
were crystallin genes, in 8 (29.6%) families, with half occurring in the CRYBB2 gene. There was no
significant difference in the molecular diagnostic rates between sporadic and familial inheritance
(P = 0.287). Associated clinical diagnoses were retinal dystrophies in five (18.5%) and aniridia in three
(11.1%) families. Bilateral cataracts were the presenting feature in 27.3% (6/22) of either complex
or syndromic cases, and isolated cataract patients were 11.5 years younger (rank-sum Z = 3.668,
P = 0.0002). Prompt genetic investigation with comprehensive panel testing can aid with diagnosis
and optimise management of cataract patient
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
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