Development of Magnetohydrodynamic Modes During Sawteeth in Tokamak Plasmas

A dynamical analysis applied to a reduced resistive magnetohydrodynamics model is shown to explain the chronology of the nonlinear destabilization of modes observed in tokamak sawteeth. A special emphasis is put on the nonlinear self-consistent perturbation of the axisymmetric m = n = 0 mode that manifests through the q-profile evolution. For the very low fusion-relevant resistivity values, the q-profile is shown to remain almost unchanged on the early nonlinear timescale within the central tokamak region, which supports a partial reconnection scenario. Within the resistive region, indications for a local flattening or even a local reversed-shear of the q-profile are given. The impact of this ingredient in the occurrence of the sawtooth crash is discussed.Fil: Firpo, Marie C.. Ecole Polytechnique; Francia. Centre National de la Recherche Scientifique; FranciaFil: Ettoumi, W.. Ecole Polytechnique; Francia. Centre National de la Recherche Scientifique; FranciaFil: Farengo, R.. Comision Nacional de Energia Atomica. Gerencia del Area de Investigaciones y Aplicaciones no Nucleares. Gerencia de Fisica (CAB); Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Ferrari, Hugo Emilio. Comision Nacional de Energia Atomica. Gerencia del Area de Investigaciones y Aplicaciones no Nucleares. Gerencia de Fisica (CAB); Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Garcia Martinez, Pablo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Lifschitz, A. F.. Ecole Polytechnique; Francia. Centre National de la Recherche Scientifique; Franci

Very High Energy gamma-rays from electron/positron Pair Halos

In this paper we study the formation of giant electrons-positron pair halos around the powerful high energy extragalactic sources. We investigate the dependence of radiation of pair halos, in particular the spectral and angular distributions on the energy spectrum of the primary gamma-rays, the redshift of the source, and the flux of the extragalactic background light.Comment: 17 pages, 17 figures, published in Volume No. 18, Issue No. 06 of "International Journal Of Modern Physics D

Ring sequence decomposition of an accretion disk: the viscoresistive approach

We analyze a two dimensional viscoresistive magnetohydrodynamical (MHD) model for a thin accretion disk which reconciles the crystalline structure outlined in [Coppi(2005), Coppi and Rousseau(2006)] with real microscopic and macroscopic features of astrophysical accreting systems. In particular, we consider small dissipative effects (viscosity and resistivity, characterized by a magnetic Prandtl number of order unity), poloidal matter fluxes and a toroidal component of the magnetic field. These new ingredients allow us to set up the full equilibrium profile including the azimuthal component of the momentum conservation equation and the electron force balance relation. These two additional equations, which were identically satisfied in the original model, permit us to deal with non-zero radial and vertical matter fluxes, and the solution we construct for the global equilibrium system provides a full description of the radial and vertical dependence within the plasma disk. The main issue of our analysis is outlining a modulation of the matter distribution in the disk which corresponds to the formation of a ring-like sequence, here associated with a corresponding radial oscillation of the matter flux.Comment: 6 pages, 2 figure

Temporal and Spectral Variabilities of High Energy Emission from Blazars Using Synchrotron Self-Compton Models

Multiwavelength observations of blazars such as Mrk 421 and Mrk 501 show that they exhibit strong short time variabilities in flare-like phenomena. Based on the homogeneous synchrotron self-Compton (SSC) model and assuming that time variability of the emission is initiated by changes in the injection of nonthermal electrons, we perform detailed temporal and spectral studies of a purely cooling plasma system. One important parameter is the total injected energy E and we show how the synchrotron and Compton components respond as E varies. We discuss in detail how one could infer important physical parameters using the observed spectra. In particular, we could infer the size of the emission region by looking for exponential decay in the light curves. We could also test the basic assumption of SSC by measuring the difference in the rate of peak energy changes of synchrotron and SSC peaks. We also show that the trajectory in the photon-index and flux plane evolves clockwise or counter-clockwise depending on the value of E and observed energy bands.Comment: 35 pages, 18 figures, accepted by the Astrophysical Journa

Absorption of 10--200 Gev Gamma Rays by Radiation from BLR in Blazars

In this paper, we study the photon-photon pair production optical depth for gamma-rays with energies from 10 to 200 GeV emitted by powerful blazars due to the diffuse radiation field of broad line region (BLR). There are four key parameters in the BLR model employed to determine the $\gamma-\gamma$ attenuation optical depth of these gamma-rays. They are the gamma-ray emitting radius $R_{\gamma}$, the BLR luminosity $L_{\rm{BLR}}$, the BLR half thickness $h$ and the ratio $\tau_{\rm{BLR}}/f_{\rm{cov}}$ of the Thomson optical depth to the covering factor of BLR. For FSRQs, on average, it is impossible for gamma-rays with energies from 10 to 200 GeV to escape from the diffuse radiation field of the BLR. If $\it GLAST$ could detect these gamma-rays for most of FSRQs, the gamma-ray emitting region is likely to be outside the cavity formed by the BLR. Otherwise, the emitting region is likely to be inside the BLR cavity. As examples, we estimate the photon-photon absorption optical depth of gamma-rays with energies from 10 to 200 GeV for two powerful blazars, HFSRQ PKS 0405$-$123 and FSRQ 3C 279. Comparing our results with $\it GLAST$ observations in the future could test whether the model employed and the relevant assumptions in this paper are reliable and reasonable, and then limit constraints on the position of the gamma-ray emitting region relative to the BLR and the properties of the BLR.Comment: 8 figure

Complex Spectral Variability from Intensive Multi-wavelength Monitoring of Mrk421 in 1998

We conducted a multi-frequency campaign for the TeV blazar Mrk~421 in 1998 April. The campaign started from a pronounced high amplitude flare recorded by SAX and Whipple; ASCA observation started three days later. In the X-ray data, we detected multiple flares, occuring on time scales of about one day. ASCA data clearly reveal spectral variability. The comparison of the data from ASCA, EUVE and RXTE indicates that the variability amplitudes in the low energy synchrotron component are larger at higher photon energies. In TeV Gamma-rays, large intra-day variations -- which were correlated with the X-ray flux -- were observed when results from three Cherenkov telescopes are combined. The RMS variability of TeV Gamma--rays was similar to that observed in hard X-rays, above 10 keV. The X-ray light curve reveals flares which are almost symmetric for most of cases, implying the dominant time scale is the light crossing time through the emitting region. The structure function analysis based on the continuous X-ray light curve of seven days indicates that the characteristic time scale is ~0.5 day. The analysis of ASCA light curves in various energy bands appears to show both soft (positive) and hard (negative) lags. These may not be real, as systematic effects could also produce these lags, which are all much smaller than an orbit. If the lags of both signs are real, these imply that the particle acceleration and X-ray cooling time scales are similar.Comment: 10 pages, 4 figures, accepted for publication in ApJ Letter

Foreign body ingestion during the COVID-19 pandemic: a retrospective single centre review

Introduction: During the COVID-19 pandemic in 2020, there were many changes in the provision of healthcare as well as home and educational environments for children. We noted an apparent increase in the number of children presenting with ingested foreign bodies and due to the potential impact of injury from this, further investigated this phenomenon. Method: Using a prospective electronic record, data were retrospectively collected for patients referred to our institution with foreign body ingestion from March 2020 to September 2020 and compared with the same period the year prior as a control. Results: During the 6-month pandemic period of review, it was observed that 2.5 times more children were referred with foreign body ingestion (n=25) in comparison to the control period (n=10). There was also a significant increase in the proportion of button battery and magnet ingestions during the COVID-19 pandemic (p 0.04). Conclusion: These findings raise concerns of both increased frequency of foreign body ingestion during the COVID-19 pandemic and the nature of ingested foreign bodies linked with significant morbidity. This may relate to the disruption of home and work environments and carries implications for ongoing restrictions. Further awareness of the danger of foreign body ingestion, especially batteries and magnets, is necessary (project ID: 2956)

Parents’ experiences of feeding children born with oesophageal atresia/tracheo-oesophageal fistula

Background: Feeding difficulties are widely acknowledged following oesophageal atresia/tracheo-oesophageal fistula repair. However, little is understood about the nature and severity of these difficulties. This study explored feeding in children with oesophageal atresia/tracheo-oesophageal fistula from the parent perspective. Methods: In collaboration with a patient support group, data were collected using a research-specific online discussion forum. Thematic analysis was used to code the data and identify themes. Results: One hundred and twenty-seven parents registered for the online forum, of whom 83 (65%) provided demographic data. Seventy-four (89%) of responders were mothers, 75 (90%) were of white ethnicity, 65 (78%) were from the UK. Six key themes were identified: feeding is a traumatic experience, feeding my child is scary, feeding is isolating and filled with uncertainty, feeding outside of the home is difficult, feeding associated emotions, developing coping strategies. Parents described features of medical, nutritional, feeding skill and psychosocial dysfunction across all stages of eating/drinking development. They described how their child's feeding difficulties had an impacted their own well-being. An interactional model of feeding difficulties in OA/TOF is proposed. Conclusions: Exploring parent experiences provides rich data from which to expand understanding of the complex nature of feeding difficulties in OA/TOF. Feeding should be viewed as a dyadic process, occurring within a family system. Intervention for feeding difficulties should be family-centred, addressing parental anxiety, trauma and uncertainty, as well as the child's underlying medical/surgical needs to optimise outcome. Further study of clinical correlates with parental experience is required