arXiv subjects

T. Sun

Publications and source records attributed to T. Sun.

8 recordsLinked to original sources

arXiv2026

Features of spherical torus p 11B burning plasmas

A spherical torus (ST) p B11 plasma model that satisfies multi-magnetofluid force balance is developed, which includes small fractions of suprathermal ions with temperatures around 0.5 MeV and suprathermal electrons in the MeV range. Alongside the primary thermal plasma with ion temperatures exceeding 100 keV and densities above 10E20 m-3, these components enhance fusion reaction rates by leveraging the p B11 double-peak fusion cross section. Suprathermal ions and strong toroidal rotation driven by neutral beam injection have been observed in devices such as START, MAST, NSTX, Globus-M2, and ST40. Central-solenoid-free plasma initiation, ramp-up, and sustainment were tested on EXL-50 and replicated on EXL-50U with partial central induction, demonstrating efficient current drive and consistent with the multi-magnetofluid equilibrium model. Motivated by ENN's aneutronic commercial fusion roadmap, this paper presents a rotating, thermally un-equilibrated ST p B11 plasma with unique properties: fluid components experience separate balance under centripetal, electrostatic, and Lorentz forces with common electric and magnetic fields, leading to large rotation speed differences between thermal boron ions and suprathermal protons; a large outboard region with magnetic well and omnigeneity is created, affecting neoclassical transport and gradient-driven turbulence; suprathermal charged particles can extend beyond the last closed flux surface and be limited by plasma-facing components, influencing recycling and pedestal conditions; and the superposition of these plasma components modifies sources and sinks of free energy, prompting renewed evaluation of stability, turbulence, transport, heating, current drive, and flux diffusion. Challenges and opportunities for sustained burn are discussed for a compact p B11 ST with 1.4-meter major radius, 13-MA current, and 3-T toroidal field.

physics.plasm-ph↗

arXiv2024

Deep Learning Approach to Forecasting COVID-19 Cases in Residential Buildings of Hong Kong Public Housing Estates: The Role of Environment and Sociodemographics

Introduction: The current study investigates the complex association between COVID-19 and the studied districts' socioecology (e.g. internal and external built environment, sociodemographic profiles, etc.) to quantify their contributions to the early outbreaks and epidemic resurgence of COVID-19. Methods: We aligned the analytic model's architecture with the hierarchical structure of the resident's socioecology using a multi-headed hierarchical convolutional neural network to structure the vast array of hierarchically related predictive features representing buildings' internal and external built environments and residents' sociodemographic profiles as model input. COVID-19 cases accumulated in buildings across three adjacent districts in HK, both before and during HK's epidemic resurgence, were modeled. A forward-chaining validation was performed to examine the model's performance in forecasting COVID-19 cases over the 3-, 7-, and 14-day horizons during the two months subsequent to when the model for COVID-19 resurgence was built to align with the forecasting needs in an evolving pandemic. Results: Different sets of factors were found to be linked to the earlier waves of COVID-19 outbreaks compared to the epidemic resurgence of the pandemic. Sociodemographic factors such as work hours, monthly household income, employment types, and the number of non-working adults or children in household populations were of high importance to the studied buildings' COVID-19 case counts during the early waves of COVID-19. Factors constituting one's internal built environment, such as the number of distinct households in the buildings, the number of distinct households per floor, and the number of floors, corridors, and lifts, had the greatest unique contributions to the building-level COVID-19 case counts during epidemic resurgence.

cs.CY↗

arXiv2023

Morphological stability of solid-liquid interfaces under additive manufacturing conditions

Understanding rapid solidification behavior at velocities relevant to additive manufacturing (AM) is critical to controlling microstructure selection. Although in-situ visualization of solidification dynamics is now possible, systematic studies under AM conditions with microstructural outcomes compared to solidification theory remain lacking. Here we measure solid-liquid interface velocities of Ni-Mo-Al alloy single crystals under AM conditions with synchrotron X-ray imaging, characterize the microstructures, and show discrepancies with classical theories regarding the onset velocity for absolute stability of a planar solid-liquid interface. Experimental observations reveal cellular/dendritic microstructures can persist at velocities larger than the expected absolute stability limit, where banded structure formation should theoretically appear. We show that theory and experimental observations can be reconciled by properly accounting for the effect of solute trapping and kinetic undercooling on the velocity-dependent solidus and liquidus temperatures of the alloy. Further theoretical developments and accurate assessments of key thermophysical parameters - like liquid diffusivities, solid-liquid interface excess free energies, and kinetic coefficients - remain needed to quantitatively investigate such discrepancies and pave the way for the prediction and control of microstructure selection under rapid solidification conditions.

cond-mat.mtrl-sci↗

arXiv2022

Probing into emission mechanisms of GRB 190530A using time-resolved spectra and polarization studies: Synchrotron Origin?

Multi-pulsed GRB 190530A, detected by the GBM and LAT onboard \fermi, is the sixth most fluent GBM burst detected so far. This paper presents the timing, spectral, and polarimetric analysis of the prompt emission observed using \AstroSat and \fermi to provide insight into the prompt emission radiation mechanisms. The time-integrated spectrum shows conclusive proof of two breaks due to peak energy and a second lower energy break. Time-integrated (55.43 $\pm$ 21.30 \%) as well as time-resolved polarization measurements, made by the Cadmium Zinc Telluride Imager (CZTI) onboard \AstroSat, show a hint of high degree of polarization. The presence of a hint of high degree of polarization and the values of low energy spectral index ($\alpha_{\rm pt}$) do not run over the synchrotron limit for the first two pulses, supporting the synchrotron origin in an ordered magnetic field. However, during the third pulse, $\alpha_{\rm pt}$ exceeds the synchrotron line of death in few bins, and a thermal signature along with the synchrotron component in the time-resolved spectra is observed. Furthermore, we also report the earliest optical observations constraining afterglow polarization using the MASTER (P $<$ 1.3 \%) and the redshift measurement ($z$= 0.9386) obtained with the 10.4m GTC telescopes. The broadband afterglow can be described with a forward shock model for an ISM-like medium with a wide jet opening angle. We determine a circumburst density of $n_{0} \sim$ 7.41, kinetic energy $E_{\rm K} \sim$ 7.24 $\times 10^{54}$ erg, and radiated $\gamma$-ray energy $E_{\rm \gamma, iso} \sim$ 6.05 $\times 10^{54}$ erg, respectively.

astro-ph.HE↗

arXiv2017

Pre-melting hcp to bcc Transition in Beryllium

Beryllium (Be) is an important material with wide applications ranging from aerospace components to X-ray equipments. Yet a precise understanding of its phase diagram remains elusive. We have investigated the phase stability of Be using a recently developed hybrid free energy computation method that accounts for anharmonic effects by invoking phonon quasiparticles. We find that the hcp to bcc transition occurs near the melting curve at 0<P<11 GPa with a positive Clapeyron slope of 41 K/GPa. The bcc phase exists in a narrow temperature range that shrinks with increasing pressure, explaining the difficulty in observing this phase experimentally. This work also demonstrates the validity of this theoretical framework based on phonon quasiparticle to study structural stability and phase transitions in strongly anharmonic materials.

cond-mat.mtrl-sci↗

arXiv2012

Mass measurements near the $r$-process path using the Canadian Penning Trap mass spectrometer

The masses of 40 neutron-rich nuclides from Z = 51 to 64 were measured at an average precision of $\delta m/m= 10^{-7}$ using the Canadian Penning Trap mass spectrometer at Argonne National Laboratory. The measurements, of fission fragments from a $^{252}$Cf spontaneous fission source in a helium gas catcher, approach the predicted path of the astrophysical $r$ process. Where overlap exists, this data set is largely consistent with previous measurements from Penning traps, storage rings, and reaction energetics, but large systematic deviations are apparent in $\beta$-endpoint measurements. Differences in mass excess from the 2003 Atomic Mass Evaluation of up to 400 keV are seen, as well as systematic disagreement with various mass models.

nucl-ex↗

arXiv2011

Investigation of the photosensitivity, temperature sustainability and fluorescence characteristics of several Er-doped photosensitive fibers

Three different types of Er doped photosensitive fibers, germanium/erbium (Ge/Er) fiber, tin/germanium/erbium fiber (Sn/Er) and antimony/germanium/erbium fiber (Sb/Er) have been manufactured and studied for use in optical sensor systems. Their characteristics of photosensitivity, the temperature sustainability of fiber Bragg gratings (FBGs) written into these fibers and the fluorescence emission from the Er dopant were investigated and compared. It has been shown in this work that these fibers all show a satisfactory degree of photosensitivity to enable the fabrication of FBGs and a significant level of fluorescence emission within the 1550 nm band for sensor use. The high temperature sustainability of the FBGs written into these fibers was investigated and seen to be quite significant at temperatures as high as 850 ^{\circ}C, in particular for the Sn/Er and Sb/Er fibers. A fiber laser using the Sb/Er fiber as the gain medium was demonstrated, giving evidence of the strong fluorescence emission from the Er dopant. These fibers are all suitable for use in a variety of sensing applications for the simultaneous measurement of temperature and strain by means of monitoring both the fluorescence characteristics and the peak wavelength shift of the FBGs formed in fiber laser sensor application.

physics.optics↗

arXiv2009

Double-beta decay Q values of 130Te, 128Te, and 120Te

The double-beta decay Q values of 130Te, 128Te, and 120Te have been determined from parent-daughter mass differences measured with the Canadian Penning Trap mass spectrometer. The 132Xe-129Xe mass difference, which is precisely known, was also determined to confirm the accuracy of these results. The 130Te Q value was found to be 2527.01(32) keV which is 3.3 keV lower than the 2003 Atomic Mass Evaluation recommended value, but in agreement with the most precise previous measurement. The uncertainty has been reduced by a factor of 6 and is now significantly smaller than the resolution achieved or foreseen in experimental searches for neutrinoless double-beta decay. The 128Te and 120Te Q values were found to be 865.87(131) keV and 1714.81(125) keV, respectively. For 120Te, this reduction in uncertainty of nearly a factor of 8 opens up the possibility of using this isotope for sensitive searches for neutrinoless double-electron capture and electron capture with positron emission.

nucl-ex↗