arXiv subjects

S. Jaiswal

Publications and source records attributed to S. Jaiswal.

At least 19 recordsLinked to original sources

arXiv2025

Cataloging the nonlinear waves excited by moving a charged body in the dusty plasma medium

The nonlinear waves excited by the movement of a charged body in the dusty plasma medium are studied. A charged body moving through a dusty plasma medium can generate diverse nonlinear waves, such as precursors and pinned solitons. These wave excitations under weakly nonlinear and dispersive limits are described theoretically by the forced Korteweg-de Vries (fKdV) type equation. We have examined the role of the driver in shaping and evolving these wave excitations. In particular we studied the effect of primarily three source parameters, namely, amplitude, width, and flow speed, on the evolution of nonlinear structures. The driver generates a perturbation in the stable system configuration, which couples with medium characteristics and eventually evolves into propagating excitations. Our finding shows that the excitation of nonlinear structure by a moving body in a plasma medium is not just dictated by the mach number but also the features of the source such as amplitude and width. As a novel finding apart from pinned and precursor solitons, we observe another nonlinear structure that lags behind the source term, maintaining its shape and speed as it propagates. These features are the first ever theoretical depiction of such lagging structures.

physics.plasm-ph↗

arXiv2023

Structural transformation of dusty plasma crystal in DC discharge plasma by changing confinement ring bias

We report the experimental study of the structural transition of a stable complex plasma crystal to a solid-liquid phase coexistence by the controlled adjustment of the confinement potential, while keeping all other parameters constant. The experiments are carried out in a tabletop Linear Dusty Plasma Experimental (LDPEx) device which consists of a circular powered electrode and an extended grounded cathode plate. A stationary crystal of melamine formaldehyde particles is formed in a background of Argon plasma inside a confining ring that is isolated to the cathode by a ceramic cover. The stable crystal structure breaks in the core region and transitions to a coexistent state by carefully changing the confining potential, thereby modifying the sheath structure. The transition is confirmed by evaluating the variation in different characteristic parameters such as the pair correlation function, local bond order parameter, and dust kinetic temperature as a function of confining bias potential. It is found that melting in the core is due to the onset of dust fluctuations in the layers beneath the topmost layer, which grow in amplitude as the confining bias potential is reduced below a threshold value. The present technique of changing confinement provides a unique feature to study structural transitions of plasma crystals without affecting the overall plasma parameters.

physics.plasm-ph↗

arXiv2023

SKA Science Data Challenge 2: analysis and results

The Square Kilometre Array Observatory (SKAO) will explore the radio sky to new depths in order to conduct transformational science. SKAO data products made available to astronomers will be correspondingly large and complex, requiring the application of advanced analysis techniques to extract key science findings. To this end, SKAO is conducting a series of Science Data Challenges, each designed to familiarise the scientific community with SKAO data and to drive the development of new analysis techniques. We present the results from Science Data Challenge 2 (SDC2), which invited participants to find and characterise 233245 neutral hydrogen (Hi) sources in a simulated data product representing a 2000~h SKA MID spectral line observation from redshifts 0.25 to 0.5. Through the generous support of eight international supercomputing facilities, participants were able to undertake the Challenge using dedicated computational resources. Alongside the main challenge, `reproducibility awards' were made in recognition of those pipelines which demonstrated Open Science best practice. The Challenge saw over 100 participants develop a range of new and existing techniques, with results that highlight the strengths of multidisciplinary and collaborative effort. The winning strategy -- which combined predictions from two independent machine learning techniques to yield a 20 percent improvement in overall performance -- underscores one of the main Challenge outcomes: that of method complementarity. It is likely that the combination of methods in a so-called ensemble approach will be key to exploiting very large astronomical datasets.

astro-ph.IM↗

arXiv2021

Effect of surface porosity of catalytic supports on plasma-assisted catalysis for ammonia synthesis

A fundamental understanding of plasma-catalyst interactions is important for understanding reaction mechanisms, optimizing the catalyst, and increasing the efficiency of plasma-assisted catalytic process for ammonia (\ce{NH3}) synthesis. We report on the effect of the surface porosity of the catalyst support on this reaction carried out in a coaxial dielectric barrier discharge (DBD) plasma reactor. The discharge was created using a variable AC applied voltage at room temperature and near atmospheric pressure (550 Torr). Two catalyst supports were compared: porous silica (\ce{SiO2}) ceramic beads and smooth, non-porous soda lime glass beads of almost equal diameter ($\sim$1.5 mm) were used. \ce{N2} conversion and the \ce{NH3} synthesis rate was increased with increasing voltage for both supports, but the energy yield for \ce{NH3} production increased for the \ce{SiO2} beads and decreased for the glass beads. All three of these parameters were always higher when using the \ce{SiO2} beads, which suggests that porosity can be a small advantage for plasma assisted \ce{NH3} synthesis. Discharge and plasma properties were estimated from Lissajous plots and using calculations with the BOLSIG+ software. The effect of different catalyst supports on the physical properties of the discharge was negligible. High resolution optical emission spectra (OES) were used to explore the evolution of gas phase active species, \ce{N2+}, atomic N, electronically excited \ce{N2}, and atomic H (H$_α$, H$_β$), in the plasma in the presence of both supports. The relative concentration of these species was lower in the case of the porous \ce{SiO2} beads for all applied voltages, which suggests that surface reactions are more significant than gas phase reactions for the formation of \ce{NH3} in plasma assisted \ce{NH3} synthesis.

physics.plasm-ph↗

arXiv2020

Square Kilometre Array Science Data Challenge 1: analysis and results

As the largest radio telescope in the world, the Square Kilometre Array (SKA) will lead the next generation of radio astronomy. The feats of engineering required to construct the telescope array will be matched only by the techniques developed to exploit the rich scientific value of the data. To drive forward the development of efficient and accurate analysis methods, we are designing a series of data challenges that will provide the scientific community with high-quality datasets for testing and evaluating new techniques. In this paper we present a description and results from the first such Science Data Challenge (SDC1). Based on SKA MID continuum simulated observations and covering three frequencies (560 MHz, 1400MHz and 9200 MHz) at three depths (8 h, 100 h and 1000 h), SDC1 asked participants to apply source detection, characterization and classification methods to simulated data. The challenge opened in November 2018, with nine teams submitting results by the deadline of April 2019. In this work we analyse the results for 8 of those teams, showcasing the variety of approaches that can be successfully used to find, characterise and classify sources in a deep, crowded field. The results also demonstrate the importance of building domain knowledge and expertise on this kind of analysis to obtain the best performance. As high-resolution observations begin revealing the true complexity of the sky, one of the outstanding challenges emerging from this analysis is the ability to deal with highly resolved and complex sources as effectively as the unresolved source population.

astro-ph.IM↗

arXiv2020

Stability study and time resolution measurement of Straw Tube detectors

Straw tube detectors are single wire proportional counters that are widely used as a tracking device. We have carried out R$\&$D with a straw tube detector prototype. The motivation of this work is to study the stability of the performance in terms of gain and energy resolution of the straw tube detectors under high rate radiation. Two different methods are incorporated to perform this study. The gain and energy resolution of the detector are studied along with its variation with ambient temperature and pressure. X-ray from a radioactive source is used to irradiate the detector and the same source is also used to monitor the energy spectra simultaneously for calculation of gain. Variation of the gain and energy resolution of the straw tube detector under X-ray irradiation in Ar/CO$_2$ gas mixture is discussed in this article. We have also estimated the time resolution of the straw tube detectors that can be best achieved with cosmic rays as trigger for the same gas mixture. The details of the measurement process and the experimental results are presented in this article.

physics.ins-det↗

arXiv2020

J1110+4817 -- a compact symmetric object candidate revisited

Compact symmetric objects (CSOs) are radio-emitting active galactic nuclei (AGNs) typically with a double-lobed radio structure confined to within 1 kpc. CSOs represent the earliest evolutionary phase of jetted AGNs. Some of them may eventually evolve into large-scale extended double sources, while others stall within the host galaxy and die out, depending on the longevity of nuclear activity, the jet power, and parameters of the surrounding galactic environment. Studying CSOs is a useful tool for understanding the evolution of the galaxies and the interactions between the jets and the medium of the host galaxy. Based on milliarcsec-resolution imaging observations using very long baseline interferometry (VLBI), it is not always straightforward to distinguish between a compact double-lobed or a core-jet structure. The quasar J1110+4817 was considered a CSO candidate in the literature earlier, but because of the lack of clear evidence, it could not be securely classified as a CSO. Here we present a comprehensive analysis of archival multi-frequency VLBI observations combined with accurate Gaia optical astrometric information. Lower-frequency VLBI images reveal an extended radio feature nearly perpendicular to the main structural axis of the source, apparently emanating from the brighter northern feature, that is rare among the known CSOs. While the presence of a binary AGN system cannot be fully excluded, the most plausible explanation is that J1110+4817 is a CSO.

astro-ph.GA↗

arXiv2020

Melting transition of two-dimensional complex plasma crystal in the DC glow discharge

The formation of self-consistent dust crystal and its melting is a well known phenomenon in rf generated plasma but remains challenging in DC glow discharge plasma. Here, we report the melting of a two dimensional dusty plasma crystal, suspended in the cathode sheath of a DC glow discharge plasma. The experiments are carried out in a $Π-$shaped Dusty Plasma Experimental (DPEx) device where a stationary crystal of melamine formaldehyde particles is formed between the confining strips in a background of Argon plasma. The stable structure breaks and leads to a fluid state on reducing the neutral pressure.% and even reaches to a gas like states at very low pressure. The neutral pressure range where this melting transition is observed is an order of magnitude less than what reported in rf discharge plasma. The transition is confirmed by evaluating the variation in different characteristic parameters such as the pair correlation function, voronoi diagram, local bond order, defect fraction and dust temperature as a function of background neutral pressure. The transition is attributed to an increase in effective particle temperature which we believe is occurred due to increase in charge fluctuation and ion streaming. The special feature of the device that helps formation of dust crystal and its melting in DC glow discharge can be implemented to study various phenomena associated with dust crystal in dc glow discharge plasma devices.

physics.plasm-ph↗

arXiv2018

Optical spectroscopy of star-forming regions in dwarf Wolf-Rayet galaxies

We present here spatially-resolved optical spectroscopic observations of four nearby dwarf Wolf-Rayet (WR) galaxies. The ages of the most recent starburst events in these galaxies are found between 3 and 10 Myr. The gas-phase metallicities [12+log(O/H)] for the spatially-resolved star-forming regions are derived using several indicators. The star-forming regions within the galaxies are found chemically homogeneous within the uncertainties in the estimates. Nitrogen-enrichment as expected in the WR regions is not detected. This implies that metal-enrichment due to supernovae explosions in the most recent star-forming episode is not being detected here. It is suggested that the newly synthesized metals still reside in hot gas-phase. The metals from the previous episodes, cooled by now and well mixed across the whole extent of galaxies, are making galaxies chemically homogeneous with normal N/O ratio. These galaxies are residing in dense environments with galaxy density in the range of $8-80$ Mpc$^{-3}$.

astro-ph.GA↗

arXiv2018

Femtosecond formation dynamics of the spin Seebeck effect revealed by terahertz spectroscopy

Understanding the transfer of spin angular momentum is essential in modern magnetism research. A model case is the generation of magnons in magnetic insulators by heating an adjacent metal film. Here, we reveal the initial steps of this spin Seebeck effect with <27fs time resolution using terahertz spectroscopy on bilayers of ferrimagnetic yttrium-iron garnet and platinum. Upon exciting the metal with an infrared laser pulse, a spin Seebeck current $j_\textrm{s}$ arises on the same ~100fs time scale on which the metal electrons thermalize. This observation highlights that efficient spin transfer critically relies on carrier multiplication and is driven by conduction electrons scattering off the metal-insulator interface. Analytical modeling shows that the electrons' dynamics are almost instantaneously imprinted onto $j_\textrm{s}$ because their spins have a correlation time of only ~4fs and deflect the ferrimagnetic moments without inertia. Applications in material characterization, interface probing, spin-noise spectroscopy and terahertz spin pumping emerge.

cond-mat.mes-hall↗

arXiv2018

Experimental investigation of dynamical structures formation due to flowing complex plasma past an obstacle

We report the experimental observation of dynamical behavior of flowing complex plasma past a spherical obstacle. The experiment has been carried out in a $Π$-shaped DC glow discharge experimental device using kaolin particles as the dust component in a background of Argon plasma. A stationary dust cloud is formed by maintaining the pumping speed and gas flow rate. A spherical obstacle vertically mounted on the cathode tray acts as an obstacle to the flow of dust particles. The controlled dust flow is generated by reducing the mass flow of the neutrals through a mass flow controller. The flowing dust particles are repelled by the electrostatic field of the negatively charged sphere and a microparticle free region (dust void) is formed surrounding the obstacle. The far particles are attracted towards the floating obstacle and reflected back when they have arrived at a minimum distance, causing a ring shaped structure around the obstacle. We characterize the shape of this structure over a range of dust flow speeds and obstacle biases. For a supersonic flow of dust fluid around a negatively biased obstacle, a bow shock is formed on the upstream side of the sphere, while the generation of wave structures is observed on the downstream side for a particular range of flow velocities. Reynolds numbers in this case is estimated as $R_e \gtrsim 50$. This wave structure reminds of the beginning of the formation of a Von-Kármán vortex street. A physical picture for the observed structure based on ion-drag, neutral streaming and electric forces is discussed.

physics.plasm-ph↗

arXiv2018

Dust density waves in a dc flowing complex plasma with discharge polarity reversal

We report on the observation of the self-excited dust density waves in the dc discharge complex plasma. The experiments were performed under microgravity conditions in the Plasmakristall-4 facility on board the International Space Station. In the experiment, the microparticle cloud was first trapped in an inductively coupled plasma, then released to drift for some seconds in a dc discharge with constant current. After that the discharge polarity was reversed. DC plasma containing a drifting microparticle cloud was found to be strongly non-uniform in terms of microparticle drift velocity and plasma emission in accord with [Zobnin et.al., Phys. Plasmas 25, 033702 (2018)]. In addition to that, non-uniformity in the self-excited wave pattern was observed: In the front edge of the microparticle cloud (defined as head), the waves had larger phase velocity than in the rear edge (defined as tail). Also, after the polarity reversal, the wave pattern exhibited several bifurcations: Between each of the two old wave crests, a new wave crest has formed. These bifurcations, however, occurred only in the head of the microparticle cloud. We show that spatial variations of electric field inside the drifting cloud play an important role in the formation of the wave pattern. Comparison of the theoretical estimations and measurements demonstrate the significant impact of the electric field on the phase velocity of the wave. The same theoretical approach applied to the instability growth rate, showed agreement between estimated and measured values.

physics.plasm-ph↗

arXiv2017

Tidal interaction, star formation and chemical evolution in blue compact dwarf galaxy Mrk 22

The optical spectroscopic and radio interferometric HI 21 cm-line observations of the blue compact dwarf galaxy Mrk 22 are presented. The Wolf-Rayet (WR) emission line features corresponding to high ionization lines of HeII $λ$4686 and CIV $λ$5808 from young massive stars are detected. The ages of two prominent star forming regions in the galaxy are estimated as $\sim$10 Myr and $\sim$ 4 Myr. The galaxy has non-thermal radio deficiency, which also indicates a young star-burst and lack of supernovae events from the current star formation activities, consistent with the detection of WR emission lines features. A significant N/O enrichment is seen in the fainter star forming region. The gas-phase metallicities [12 + log(O/H)] for the bright and faint regions are estimated as 7.98$\pm$0.07 and 7.46$\pm$0.09 respectively. The galaxy has a large diffuse HI envelop. The HI images reveal disturbed gas kinematics and HI clouds outside the optical extent of the galaxy, indicating recent tidal interaction or merger in the system. The results strongly indicate that Mrk 22 is undergoing a chemical and morphological evolution due to ongoing star formation, most likely triggered by a merger.

astro-ph.GA↗

arXiv2017

Effect of magnetic field on the phase transition in a dusty plasma

The formation of self-consistent crystalline structure is a well-known phenomenon in complex plasmas. In most experiments the pressure and rf power are the main controlling parameters in determining the phase of the system. We have studied the effect of externally applied magnetic field on the configuration of plasma crystals, suspended in the sheath of a radio-frequency discharge using the Magnetized Dusty Plasma Experiment (MDPX) device. Experiments are performed at a fixed pressure and rf power where a crystalline structure is formed within a confining ring. The magnetic field is then increased from 0 to 1.28 T. We report on the breakdown of the crystalline structure with increasing magnetic field. The magnetic field affects the dynamics of the plasma particles and first leads to a rotation of the crystal. At higher magnetic field, there is a radial variation (shear) in the angular velocity of the moving particles which we believe leads to the melting of the crystal. This melting is confirmed by evaluating the variation of the pair correlation function as a function of magnetic field.

physics.plasm-ph↗

arXiv2017

Investigation of the Dzyaloshinskii-Moriya interaction and room temperature skyrmions in W/CoFeB/MgO thin films and microwires

Recent studies have shown that material structures, which lack structural inversion symmetry and have high spin-orbit coupling can exhibit chiral magnetic textures and skyrmions which could be a key component for next generation storage devices. The Dzyaloshinskii-Moriya Interaction (DMI) that stabilizes skyrmions is an anti-symmetric exchange interaction favoring non-collinear orientation of neighboring spins. It has been shown that material systems with high DMI can lead to very efficient domain wall and skyrmion motion by spin-orbit torques. To engineer such devices, it is important to quantify the DMI for a given material system. Here we extract the DMI at the Heavy Metal (HM) /Ferromagnet (FM) interface using two complementary measurement schemes namely asymmetric domain wall motion and the magnetic stripe annihilation. By using the two different measurement schemes, we find for W(5 nm)/Co20Fe60B20(0.6 nm)/MgO(2 nm) the DMI to be 0.68 +/- 0.05 mJ/m2 and 0.73 +/- 0.5 mJ/m2, respectively. Furthermore, we show that this DMI stabilizes skyrmions at room temperature and that there is a strong dependence of the DMI on the relative composition of the CoFeB alloy. Finally we optimize the layers and the interfaces using different growth conditions and demonstrate that a higher deposition rate leads to a more uniform film with reduced pinning and skyrmions that can be manipulated by Spin-Orbit Torques.

cond-mat.mtrl-sci↗

arXiv2017

Ultrabroadband single-cycle terahertz pulses with peak fields of 300 kV cm$^{-1}$ from a metallic spintronic emitter

To explore the capabilities of metallic spintronic thin-film stacks as a source of intense and broadband terahertz electromagnetic fields, we excite a W/CoFeB/Pt trilayer on a large-area glass substrate (diameter of 7.5 cm) by a femtosecond laser pulse (energy 5.5 mJ, duration 40 fs, wavelength 800 nm). After focusing, the emitted terahertz pulse is measured to have a duration of 230 fs, a peak field of 300 kV cm$^{-1}$ and an energy of 5 nJ. In particular, the waveform exhibits a gapless spectrum extending from 1 to 10 THz at 10% of amplitude maximum, thereby facilitating nonlinear control over matter in this difficult-to-reach frequency range and on the sub-picosecond time scale.

cond-mat.mtrl-sci↗

arXiv2016

Efficient metallic spintronic emitters of ultrabroadband terahertz radiation

Terahertz electromagnetic radiation is extremely useful for numerous applications such as imaging and spectroscopy. Therefore, it is highly desirable to have an efficient table-top emitter covering the 1-to-30-THz window whilst being driven by a low-cost, low-power femtosecond laser oscillator. So far, all solid-state emitters solely exploit physics related to the electron charge and deliver emission spectra with substantial gaps. Here, we take advantage of the electron spin to realize a conceptually new terahertz source which relies on tailored fundamental spintronic and photonic phenomena in magnetic metal multilayers: ultrafast photo-induced spin currents, the inverse spin-Hall effect and a broadband Fabry-Pérot resonance. Guided by an analytical model, such spintronic route offers unique possibilities for systematic optimization. We find that a 5.8-nm-thick W/CoFeB/Pt trilayer generates ultrashort pulses fully covering the 1-to-30-THz range. Our novel source outperforms laser-oscillator-driven emitters such as ZnTe(110) crystals in terms of bandwidth, terahertz-field amplitude, flexibility, scalability and cost.

cond-mat.mtrl-sci↗

arXiv2016

Flowing dusty plasma experiments: Generation of flow and measurement techniques

A variety of experimental techniques for the generation of subsonic/supersonic dust fluid flows and means of measuring such flow velocities are presented. The experiments have been carried out in a $Π-$shaped Dusty Plasma Experimental (DPEx) device with micron size kaolin/Melamine Formaldehyde (MF) particles embedded in a background of Argon plasma created by a direct current (DC) glow discharge. A stationary dust cloud is formed over the cathode region by precisely balancing the pumping speed and gas flow rate. A flow of dust particles/fluid is generated by additional gas injection from a single or dual locations or by altering the dust confining potential. The flow velocity is then estimated by three different techniques, namely, by super Particle Identification (sPIT) code, Particle Image Velocimetry (PIV) analysis and the excitation of Dust Acoustic Waves (DAWs). The results obtained from these three different techniques along with their merits and demerits are discussed. An estimation of the neutral drag force responsible for the generation as well as the attenuation of the dust fluid flow is made. These techniques can be usefully employed in laboratory devices to investigate linear and non-linear collective excitations in a flowing dusty plasma.

physics.plasm-ph↗