1,630 research outputs found
Improved local-constant-field approximation for strong-field QED codes
The local-constant-field approximation (LCFA) is an essential theoretical
tool for investigating strong-field QED phenomena in background electromagnetic
fields with complex spacetime structure. In our previous work
[Phys.~Rev.~A~\textbf{98}, 012134 (2018)] we have analyzed the shortcomings of
the LCFA in nonlinear Compton scattering at low emitted photon energies for the
case of a background plane-wave field. Here, we generalize that analysis to
background fields, which can feature a virtually arbitrary spacetime structure.
In addition, we provide an explicit and simple implementation of an improved
expression of the nonlinear Compton scattering differential probability that
solves the main shortcomings of the standard LCFA in the infrared region, and
is suitable for background electromagnetic fields with arbitrary spacetime
structure such as those occurring in particle-in-cell simulations. Finally, we
carry out a systematic procedure to calculate the probability of nonlinear
Compton scattering per unit of emitted photon light-cone energy and of
nonlinear Breit-Wheeler pair production per unit of produced positron
light-cone energy beyond the LCFA in a plane-wave background field, which
allows us to identify the limits of validity of this approximation
quantitatively.Comment: 15 pages, 3 figure
Implementing nonlinear Compton scattering beyond the local constant field approximation
In the calculation of probabilities of physical processes occurring in a
background classical field, the local constant field approximation (LCFA)
relies on the possibility of neglecting the space-time variation of the
external field within the region of formation of the process. This
approximation is widely employed in strong-field QED as it allows to evaluate
probabilities of processes occurring in arbitrary electromagnetic fields
starting from the corresponding quantities computed in a constant
electromagnetic field. Here, we demonstrate in the case of nonlinear single
Compton scattering that the LCFA is quantitatively and qualitatively
insufficient for describing the low-energy part of the emitted photon
probability. In addition, we provide a simple recipe to implement an improved
expression of the photon emission probability beyond the LCFA in numerical
codes, which are an essential tool to interpret present and upcoming
experiments in strong-field QED.Comment: 12 pages, 3 figur
Pair production in a strong slowly varying magnetic field: the effect of a background gravitational field
The production probability of an pair in the presence of a strong,
uniform and slowly varying magnetic field is calculated by taking into account
the presence of a background gravitational field. The curvature of the
spacetime metric induced by the gravitational field not only changes the
transition probabilities calculated in the Minkowski spacetime but also primes
transitions that are strictly forbidden in absence of the gravitational field.Comment: 56 pages, no figure
TAVI nel trattamento della stenosi aortica degenerativa: stato dell’arte e prospettive
Degenerative aortic stenosis is the most common form of heart valve disease in developed countries and predominantly affects the elderly. Aortic valve replacement (AVR) has been the gold standard, but recently, transcatheter aortic valve implantation has emerged as an effective therapeutic alternative to conventional AVR for high-risk patients. This review analyzed the literature about AVR, with the objective of evaluating the outcomes of transcatheter aortic valve implantation in patients who are not eligible for surgery showing an improvement in quality of life and middle-term outcomes. The crucial point is the lack of studies with long-term follow-up that could give therapeutic importance to percutaneous valve replacemen
Modeling and performance assessment of the split-pi used as a storage converter in all the possible dc microgrid scenarios. Part i: Theoretical analysis
The integration of an electrical storage system (ESS) into a DC microgrid using a bidirectional DC/DC converter provides substantial benefits but requires careful design. Among such converter topologies, the Split-pi converter presents several merits at the cost of non-isolated operation. However, the few works in the literature on the Split-pi presented only closed-loop control with a single control loop; furthermore, they neglected the reactive components’ parasitic resistances and did not perform any experimental validation. This work aimed at investigating the use of the Split-pi converter as a power interface between an ESS and a DC microgrid. Five typical microgrid scenarios are presented, where each of which requires a specific state-space model and a suitable control scheme for the converter to obtain high performance. In this study, two different state-space models of the converter that consider the parasitic elements are presented, the control schemes are discussed, and criteria for designing the controllers are also given. Several simulations, as well as experimental tests on a prototype realized in the lab, were performed to validate the study. Both the simulation and experimental results will be presented in part II of this work. The proposed approach has general validity and can also be followed when other bidirectional DC/DC converter topologies are employed to interface an ESS with a DC microgrid
Modeling and Experimental Validation of a Voltage-Controlled Split-Pi Converter Interfacing a High-Voltage ESS with a DC Microgrid
The Split-pi converter can suitably interface an energy storage system (ESS) with a DC microgrid when galvanic isolation is not needed. Usually, the ESS voltage is lower than the grid-side voltage. However, limitations in terms of the ESS current make the use of a high-voltage ESS unavoidable when high power levels are required. In such cases, the ESS voltage can be higher than the microgrid voltage, especially with low microgrid voltages such as 48 V. Despite its bidirectionality and symmetry, the Split-pi exhibits a completely different dynamic behavior if its input and output ports are exchanged. Thus, the present work aims to model the Split-pi converter operating with an ESS voltage higher than the grid-side voltage in three typical microgrid scenarios where the controlled variable is the converter's output voltage. The devised state-space model considers the parasitic elements and the correct load model for each scenario. Furthermore, it is shown that the presence of the input LC filter can make the design of the loop controllers more complicated than in the case of a lower ESS voltage than the grid-side voltage. Finally, the study is validated through simulations and experimental tests on a lab prototype, and a robustness analysis is performed
Swallowing evaluation with videofluoroscopy in the paediatric population
Paediatric swallowing disorders can have several causes, from prematurity and congenital anomalies to gastro-oesophageal reflux and
infective or inflammatory pathologies of the upper digestive tract. In neonates, the swallowing process is reflexive and involuntary. Later
in infancy, the oral phase comes under voluntary control, while the pharyngeal phase and oesophageal phases remain involuntary. Swallowing difficulties can severely compromise pulmonary health and nutritional intake of paediatric patients. Videofluoroscopic Swallow Study
(VFSS) is a radiographic procedure that provides a dynamic view of the swallowing process and is frequently considered to be definitive
evaluation for objective assessment of dysphagia in paediatric patients. This review focuses on the different possible aetiologies of paediatric swallowing disorders and related videofluoroscopic swallowing study procedures and appearances
Strong signatures of radiation reaction below the radiation dominated regime
The influence of radiation reaction (RR) on multiphoton Thomson scattering by
an electron colliding head-on with a strong laser beam is investigated in a new
regime, in which the momentum transferred on average to the electron by the
laser pulse approximately compensates the one initially prepared. This
equilibrium is shown to be far more sensitive to the influence of RR than
previously studied scenarios. As a consequence RR can be experimentally
investigated with currently available laser systems and the underlying widely
discussed theoretical equations become testable for the first time.Comment: 4 pages, 3 figure
FILTRI EMI COMPATTI NEI CONVERTITORI ELETTRONICI DI POTENZA
The goal of the research recently started is to create efficient EMI filters
compact that allow to obtain converters with low dimensions and economically
competitive
PROGETTAZIONE OTTIMIZZATA DI FILTRI EMI AD ELEVATA POWER DENSITY PER CONVERTITORI ELETTRONICI DI POTENZA
In molti dei campi di impiego dei convertitori elettronici di potenza (aeronautico, aerospaziale, automotive, etc.) è richiesta la realizzazione di sistemi di conversione che, in relazione a stringenti vincoli di progetto, presentino, a parità di potenza, dimensioni sempre più compatte e pesi ridotti. In tale ottica, l’ottimizzazione della densità di potenza (power density) del convertitore diventa un obiettivo essenzial
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