arXiv subjects

U. Bhardwaj

Publications and source records attributed to U. Bhardwaj.

10 recordsLinked to original sources

arXiv2025

Molecular Dynamics Simulations of Collision Cascades in Niobium: Comparing Interatomic Potentials

Radiation damage in structural materials is a major challenge for advanced nuclear energy systems, and niobium is of particular interest due to its high melting point, mechanical strength, and corrosion resistance. To better understand its radiation response, we carried out large-scale molecular dynamics simulations of collision cascades in pure niobium at 300 K over a primary knock-on atom (PKA) energy range of 1-75 keV, employing four interatomic potentials: an embedded atom method (EAM), two Finnis-Sinclair models (FS-1 and FS-2), and a machine learning-based spectral neighbor analysis potential (SNAP) we developed. All reproduce the general features of cascade formation but differ significantly in defect production, clustering, and morphology. At low energies, defect generation follows trends governed by threshold displacement energy (TDE) and the stiffness-to-range ratio (|S/R|). At higher energies, subcascade formation makes defect evolution dependent on the combined effects of |S/R|, average TDE, and other material-specific factors. Vacancy clustering dominates over interstitial clustering across all cases: EAM produces the largest vacancy clusters and the highest clustering fraction, while SNAP shows the strongest interstitial clustering. Morphological analysis indicates that EAM forms a balanced mix of 1/2<111>, 1/2<110> loops, C15 rings, and hybrid structures; FS-2 favors extended 1/2<111> dumbbells, crowdions, and dislocation loops; whereas FS-1 and SNAP generate more compact or disordered clusters, with SNAP produces a high fraction of C15-like rings (maximum size up to nine atoms) that may evolve into dislocation loops of 1/2<111> and <100>. These findings give clear insights into how niobium reacts when exposed to irradiation, especially at high energies.

cond-mat.mtrl-sci↗

arXiv2024

Statistical Study of the Defect Cluster Morphology in the Primary Damage of Tungsten from Collision Cascades from Five Inter-atomic potentials

The size and morphology of defect clusters formed during primary damage play a crucial role in the subsequent microstructural evolution of irradiated materials. Molecular dynamics (MD) simulations of collision cascades in tungsten (W) were performed using five interatomic potentials (IAPs): the quantum-accurate machine-learned Spectral Neighbor Analysis Potential (W-SNAP), the machine learning-based tabGAP potential, and three embedded-atom method (EAM) potentials. A total of 3,500 MD simulations were conducted with primary knock-on atoms (PKAs) at energies of 5, 10, 20, 50, 75, 100, and 150 keV. PKAs were launched in 100 random directions at each energy to ensure statistical validity. Analysis was performed using CSaransh , a web-based tool for large-scale collision cascade databases, to quantify: (i) the number of defects (isolated and clustered), (ii) defect cluster morphologies, (iii) defect cluster size distributions and (iv) the number of sub-cascades formed. We show that the difference in the formation energy of self interstitial atom dumbells along the <1 1 0> and <1 1 1> directions critically influence defect cluster morphology. Our results indicate that IAP stiffness and interaction range independently do not affect defect count. However, these parameters combined with defect formation energies, threshold displacement energies, and other factors significantly influence defect production.

cond-mat.mtrl-sci↗

arXiv2023

Ready for O4 II: GRANDMA Observations of Swift GRBs during eight-weeks of Spring 2022

We present a campaign designed to train the GRANDMA network and its infrastructure to follow up on transient alerts and detect their early afterglows. In preparation for O4 II campaign, we focused on GRB alerts as they are expected to be an electromagnetic counterpart of gravitational-wave events. Our goal was to improve our response to the alerts and start prompt observations as soon as possible to better prepare the GRANDMA network for the fourth observational run of LIGO-Virgo-Kagra (which started at the end of May 2023), and future missions such as SM. To receive, manage and send out observational plans to our partner telescopes we set up dedicated infrastructure and a rota of follow-up adcates were organized to guarantee round-the-clock assistance to our telescope teams. To ensure a great number of observations, we focused on Swift GRBs whose localization errors were generally smaller than the GRANDMA telescopes' field of view. This allowed us to bypass the transient identification process and focus on the reaction time and efficiency of the network. During 'Ready for O4 II', 11 Swift/INTEGRAL GRB triggers were selected, nine fields had been observed, and three afterglows were detected (GRB 220403B, GRB 220427A, GRB 220514A), with 17 GRANDMA telescopes and 17 amateur astronomers from the citizen science project Kilonova-Catcher. Here we highlight the GRB 220427A analysis where our long-term follow-up of the host galaxy allowed us to obtain a photometric redshift of $z=0.82\pm0.09$, its lightcurve elution, fit the decay slope of the afterglows, and study the properties of the host galaxy.

astro-ph.HE↗

arXiv2023

Virgo Detector Characterization and Data Quality: results from the O3 run

The Advanced Virgo detector has contributed with its data to the rapid growth of the number of detected gravitational-wave (GW) signals in the past few years, alongside the two Advanced LIGO instruments. First during the last month of the Observation Run 2 (O2) in August 2017 (with, most notably, the compact binary mergers GW170814 and GW170817), and then during the full Observation Run 3 (O3): an 11-months data taking period, between April 2019 and March 2020, that led to the addition of about 80 events to the catalog of transient GW sources maintained by LIGO, Virgo and now KAGRA. These discoveries and the manifold exploitation of the detected waveforms require an accurate characterization of the quality of the data, such as continuous study and monitoring of the detector noise sources. These activities, collectively named {\em detector characterization and data quality} or {\em DetChar}, span the whole workflow of the Virgo data, from the instrument front-end hardware to the final analyses. They are described in details in the following article, with a focus on the results achieved by the Virgo DetChar group during the O3 run. Concurrently, a companion article describes the tools that have been used by the Virgo DetChar group to perform this work.

gr-qc↗

arXiv2023

Virgo Detector Characterization and Data Quality: tools

Detector characterization and data quality studies -- collectively referred to as {\em DetChar} activities in this article -- are paramount to the scientific exploitation of the joint dataset collected by the LIGO-Virgo-KAGRA global network of ground-based gravitational-wave (GW) detectors. They take place during each phase of the operation of the instruments (upgrade, tuning and optimization, data taking), are required at all steps of the dataflow (from data acquisition to the final list of GW events) and operate at various latencies (from near real-time to vet the public alerts to offline analyses). This work requires a wide set of tools which have been developed over the years to fulfill the requirements of the various DetChar studies: data access and bookkeeping; global monitoring of the instruments and of the different steps of the data processing; studies of the global properties of the noise at the detector outputs; identification and follow-up of noise peculiar features (whether they be transient or continuously present in the data); quick processing of the public alerts. The present article reviews all the tools used by the Virgo DetChar group during the third LIGO-Virgo Observation Run (O3, from April 2019 to March 2020), mainly to analyse the Virgo data acquired at EGO. Concurrently, a companion article focuses on the results achieved by the DetChar group during the O3 run using these tools.

gr-qc↗

arXiv2023

The Virgo O3 run and the impact of the environment

Sources of geophysical noise (such as wind, sea waves and earthquakes) or of anthropogenic noise impact ground-based gravitational-wave interferometric detectors, causing transient sensitivity worsening and gaps in data taking. During the one year-long third Observing Run (O3: from April 01, 2019 to March 27, 2020), the Virgo Collaboration collected a statistically significant dataset, used in this article to study the response of the detector to a variety of environmental conditions. We correlated environmental parameters to global detector performance, such as observation range, duty cycle and control losses. Where possible, we identified weaknesses in the detector that will be used to elaborate strategies in order to improve Virgo robustness against external disturbances for the next data taking period, O4, currently planned to start at the end of 2022. The lessons learned could also provide useful insights for the design of the next generation of ground-based interferometers.

gr-qc↗

arXiv2022

Virgo Detector Characterization and Data Quality during the O3 run

The Advanced Virgo detector has contributed with its data to the rapid growth of the number of detected gravitational-wave signals in the past few years, alongside the two LIGO instruments. First, during the last month of the Observation Run 2 (O2) in August 2017 (with, most notably, the compact binary mergers GW170814 and GW170817) and then during the full Observation Run 3 (O3): an 11 months data taking period, between April 2019 and March 2020, that led to the addition of about 80 events to the catalog of transient gravitational-wave sources maintained by LIGO, Virgo and KAGRA. These discoveries and the manifold exploitation of the detected waveforms require an accurate characterization of the quality of the data, such as continuous study and monitoring of the detector noise. These activities, collectively named {\em detector characterization} or {\em DetChar}, span the whole workflow of the Virgo data, from the instrument front-end to the final analysis. They are described in details in the following article, with a focus on the associated tools, the results achieved by the Virgo DetChar group during the O3 run and the main prospects for future data-taking periods with an improved detector.

gr-qc↗

arXiv2022

The GRANDMA network in preparation for the fourth gravitational-wave observing run

GRANDMA is a world-wide collaboration with the primary scientific goal of studying gravitational-wave sources, discovering their electromagnetic counterparts and characterizing their emission. GRANDMA involves astronomers, astrophysicists, gravitational-wave physicists, and theorists. GRANDMA is now a truly global network of telescopes, with (so far) 30 telescopes in both hemispheres. It incorporates a citizen science programme (Kilonova-Catcher) which constitutes an opportunity to spread the interest in time-domain astronomy. The telescope network is an heterogeneous set of already-existing observing facilities that operate coordinated as a single observatory. Within the network there are wide-field imagers that can observe large areas of the sky to search for optical counterparts, narrow-field instruments that do targeted searches within a predefined list of host-galaxy candidates, and larger telescopes that are devoted to characterization and follow-up of the identified counterparts. Here we present an overview of GRANDMA after the third observing run of the LIGO/VIRGO gravitational-wave observatories in $2019-2020$ and its ongoing preparation for the forthcoming fourth observational campaign (O4). Additionally, we review the potential of GRANDMA for the discovery and follow-up of other types of astronomical transients.

astro-ph.IM↗

arXiv2021

Calibration of Advanced Virgo and reconstruction of detector strain h(t) during the Observing Run O3

The three Advanced Virgo and LIGO gravitational wave detectors participated to the third observing run (O3) between 1 April 2019 15:00 UTC and 27 March 2020 17:00 UTC,leading to several gravitational wave detections per month. This paper describes the Advanced Virgo detector calibration and the reconstruction of the detector strain $h(t)$ during O3, as well as the estimation of the associated uncertainties. For the first time, the photon calibration technique as been used as reference for Virgo calibration, which allowed to cross-calibrate the strain amplitude of the Virgo and LIGO detectors. The previous reference, so-called free swinging Michelson technique, has still been used but as an independent cross-check. $h(t)$ reconstruction and noise subtraction were processed online, with good enough quality to prevent the need for offline reprocessing, except for the two last weeks of September 2019. The uncertainties for the reconstructed $h(t)$ strain, estimated in this paper in a 20-2000~Hz frequency band, are frequency independent: 5% in amplitude, 35 mrad in phase and 10 $μ$s in timing, with the exception of larger uncertainties around 50 Hz.

gr-qc↗

arXiv2020

Inclusion and validation of electronic stopping in the open source LAMMPS code

Electronic stopping (ES) of energetic atoms is not taken care of by the interatomic potentials used in molecular dynamics (MD) simulations when simulating collision cascades. The Lindhard-Scharff (LS) formula for electronic stopping is therefore included as a drag term for energetic atoms in the open source large scale atomic molecular massively parallel simulator (LAMMPS) code. In order to validate the ES implementation, MD simulations of collision cascades at primary knock-on atom (PKA) energies of 5, 10 and 20 keV are carried out in W and Fe in 100 random directions. The total ES losses from the MD simulations show energy straggling due to the stochastic nature of the phenomena. Thelosses due to ES are compared with that predicted by theNorgett-Robinson-Torrens (NRT) model to validate our implementation. It is seenthat the root mean square deviation of ES losses from the MD implementation is around 10 \% for both W and Fe compared to the NRT model. The velocity threshold above which electronic stopping is important is explored. The effect of ES on the number of defects in collision cascades is presented for Fe and W.

physics.comp-ph↗