arXiv subjects

E. Williams

Publications and source records attributed to E. Williams.

15 recordsLinked to original sources

arXiv2025

The CCOR Compact Coronagraphs for the Geostationary Operational Environmental Satellite-19 (GOES-19) and the Space Weather Follow On (SWFO) Missions

The CCOR Compact Coronagraph is a series of two operational solar coronagraphs sponsored by the National Oceanic and Atmospheric Administration (NOAA). They were designed, built, and tested by the U.S. Naval Research Laboratory (NRL). The CCORs will be used by NOAA's Space Weather Prediction Center to detect and track Coronal Mass Ejections (CMEs) and predict the Space Weather. CCOR-1 is on board the Geostationary Operational Environmental Satellite -U (GOES-U, now GOES-19/GOES-East). GOES-U was launched from Kennedy Space Flight Center, Florida, on 25 June 2024. CCOR-2 is on board the Space Weather Follow On at Lagrange point 1 (SWFO-L1). SWFO-L1 is scheduled to launch in the fall of 2025. SWFO will be renamed SOLAR-1 once it reaches L1. The CCORs are white-light coronagraphs that have a field of view and performance similar to the SOHO LASCO C3 coronagraph. CCOR-1 FOV spans from 4 to 22 Rsun, while CCOR-2 spans from 3.5 to 26 Rsun. The spatial resolution is 39 arcsec for CCOR-1 and 65 arcsec for CCOR-2. They both operate in a band-pass of 470 - 740 nm. The synoptic cadence is 15 min and the latency from image capture to the forecaster on the ground is less than 30 min. Compared to past generation coronagraphs such as the Large Angle and Spectrometric Coronagraph (LASCO), CCOR uses a compact design; all the solar occultation is done with a single multi-disk external occulter. No internal occulter is used. This allowed a substantial reduction in size and mass compared to SECCHI COR-2, for example, but with slightly lower signal-to-noise ratio. In this article, we review the science that the CCORs will capitalize on for the purpose of operational space weather prediction. We give a description of the driving requirements and accommodations, and provide details on the instrument design. In the end, information on ground processing and data levels is provided.

astro-ph.IM↗

arXiv2019

SLAC UED LLRF System Upgrade

The SLAC mega-electron-volt (MeV) ultrafast electron diffraction (UED) instrument is a powerful "electron camera" for the study of time-resolved, ultrafast atomic & molecular dynamics in chemical and solid-state systems. The UED laser timing synchronization is upgraded to SLAC (Advanced Telecommunications Computing Architecture) ATCA based femtosecond laser timing synchronization system. UED radio frequency (RF) gun cavity low level RF (LLRF) control is also being upgraded to SLAC ATCA based LLRF system to provide improved performance and maintainability. The new laser timing synchronization and LLRF control are implemented in one ATCA crate located in the laser room. A 2nd ATCA crate is set up to monitor the Klystron and sub-booster performance. With the ATCA based RF control system, the laser timing jitter has achieved 10 fs. The UED operation beam rate is being upgraded from 180 Hz to 360 Hz, which paves the way for a further upgrade of operation beam rate to 1 kHz in the future. In this paper, we present design details and characterization results of the implementation.

physics.acc-ph↗

arXiv2017

Exploring Zeptosecond Quantum Equilibration Dynamics: From Deep-Inelastic to Fusion-Fission Outcomes in $^{58}$Ni+$^{60}$Ni Reactions

Energy dissipative processes play a key role in how quantum many-body systems dynamically evolve towards equilibrium. In closed quantum systems, such processes are attributed to the transfer of energy from collective motion to single-particle degrees of freedom; however, the quantum many-body dynamics of this evolutionary process are poorly understood. To explore energy dissipative phenomena and equilibration dynamics in one such system, an experimental investigation of deep-inelastic and fusion-fission outcomes in the $^{58}$Ni+$^{60}$Ni reaction has been carried out. Experimental outcomes have been compared to theoretical predictions using Time Dependent Hartree Fock and Time Dependent Random Phase Approximation approaches, which respectively incorporate one-body energy dissipation and fluctuations. Excellent quantitative agreement has been found between experiment and calculations, indicating that microscopic models incorporating one-body dissipation and fluctuations provide a potential tool for exploring dissipation in low-energy heavy ion collisions.

nucl-ex↗

arXiv2017

Evidence for the role of proton shell closure in quasi-fission reactions from X-ray fluorescence of mass-identified fragments

The atomic numbers and the masses of fragments formed in quasi-fission reactions have been simultaneously measured at scission in 48 Ti + 238 U reactions at a laboratory energy of 286 MeV. The atomic numbers were determined from measured characteristic fluorescence X-rays whereas the masses were obtained from the emission angles and times of flight of the two emerging fragments. For the first time, thanks to this full identification of the quasi-fission fragments on a broad angular range, the important role of the proton shell closure at Z = 82 is evidenced by the associated maximum production yield, a maximum predicted by time dependent Hartree-Fock calculations. This new experimental approach gives now access to precise studies of the time dependence of the N/Z (neutron over proton ratios of the fragments) evolution in quasi-fission reactions.

nucl-ex↗

arXiv2017

Entrance Channel Effects on the Quasifission Reaction Channel in Cr + W Systems

Background: Formation of a fully equilibrated compound nucleus is a critical step in the heavy-ion fusion reaction mechanism but can be hindered by orders of magnitude by quasifission, a process in which the dinuclear system breaks apart prior to full equilibration. To provide a complete description of heavy-ion fusion it is important to characterize the quasifission process. In particular, the impact of changing the neutron-richness of the quasifission process is not well known. A previous study of Cr + W reactions at a constant 13 % above the Coulomb barrier concluded that an increase in neutron-richness leads to a decrease in the prominence of the quasifission reaction channel. Purpose: The interplay between the fusion-fission and quasifission reaction channels, with varying neutron-richness, was explored at a constant excitation energy, closer to the interaction barrier than the previous work, to see if the correlation between neutron-richness and quasifission is valid at lower energies. Methods: Mass distributions were determined for eight different combinations of Cr + W reactions at the Australian National University at 52.0 MeV of excitation energy in the compound nucleus. Results: A curvature parameter was determined for the fission-like fragment mass distributions and compared to various reaction parameters known to influence quasifission. Conclusions: The present work demonstrates that at energies near the interaction barrier the deformation effects dominate over the neutron-richness effects in the competition between quasifission and compound nucleus formation in these Cr + W systems and is an important consideration for future with heavy and superheavy element production reactions.

nucl-ex↗

arXiv2015

Reduced quasifission competition in fusion reactions forming neutron-rich heavy elements

Measurements of mass-angle distributions (MADs) for Cr + W reactions, providing a wide range in the neutron-to-proton ratio of the compound system, (N/Z)CN, have allowed for the dependence of quasifission on the (N/Z)CN to be determined in a model-independent way. Previous experimental and theoretical studies had produced conflicting conclusions. The experimental MADs reveal an increase in contact time and mass evolution of the quasifission fragments with increasing (N/Z)CN, which is indicative of an increase in the fusion probability. The experimental results are in agreement with microscopic time-dependent Hartree-Fock calculations of the quasifission process. The experimental and theoretical results favor the use of the most neutron-rich projectiles and targets for the production of heavy and superheavy nuclei.

nucl-ex↗

arXiv2014

Interplay between quantum shells and orientation in quasi-fission

The quasi-fission mechanism hinders fusion in heavy systems through breakup within zeptoseconds into two fragments with partial mass equilibration. Its dependence on the structure of both the collision partners and the final fragments is a key question. Our original approach is to combine an experimental measurement of the fragments' mass-angle correlations in $^{40}$Ca$+^{238}$U with microscopic quantum calculations. We demonstrate an unexpected interplay between the orientation of the prolate deformed $^{238}$U with quantum shell effects in the fragments. In particular, calculations show that only collisions with the tip of $^{238}$U produce quasi-fission fragments in the magic $Z=82$ region, whilst collisions with the side are the only one which may result in fusion.

nucl-th↗

arXiv2014

Microscopic study of the effect of intrinsic degrees of freedom on fusion

Fusion cross-sections are computed for the $^{40}$Ca$+^{40}$Ca system over a wide energy range with two microscopic approaches where the only phenomenological input is the Skyrme energy density functional. The first method is based on the coupled-channels formalism, using the bare nucleus-nucleus potential calculated with the frozen Hartree-Fock technique and the deformation parameters of vibrational states computed with the time-dependent Hartree-Fock (TDHF) approach. The second method is based on the density-constrained TDHF method to generate nucleus-nucleus potentials from TDHF evolution. Both approaches incorporate the effect of couplings to internal degrees of freedoms in different ways. The predictions are in relatively good agreement with experimental data.

nucl-th↗

arXiv2013

Microscopic approach to coupled-channels effects on fusion

Background: Near-barrier fusion can be strongly affected by the coupling between relative motion and internal degrees of freedom of the collision partners. The time-dependent Hartree-Fock (TDHF) theory and the coupled-channels (CC) method are standard approaches to investigate this aspect of fusion dynamics. However, both approaches present limitations, such as a lack of tunnelling of the many-body wave function in the former and a need for external parameters to describe the nucleus-nucleus potential and the couplings in the latter. Method: A method combining both approaches is proposed to overcome these limitations. CC calculations are performed using two types of inputs from Hartree-Fock (HF) theory: the nucleus-nucleus potential calculated with the frozen HF method, and the properties of low-lying vibrational states and giant resonances computed from the TDHF linear response. Results: The effect of the couplings to vibrational modes is studied in the $^{40}$Ca$+^{40}$Ca and $^{56}$Ni$+^{56}$Ni systems. This work demonstrates that the main effect of these couplings is a lowering of the barrier, in good agreement with the fusion thresholds predicted by TDHF calculations. Conclusions: As the only phenomenological inputs are the choice of the internal states of the nuclei and the parameters of the energy density functional used in the HF and TDHF calculations, the method presented in this work has a broad range of possible applications, including studies of alternative couplings or reactions involving exotic nuclei.

nucl-th↗

arXiv2009

Regularities and symmetries of subsets of collective 0+ states

The energies of subsets of excited 0+ states in geometric collective models are investigated and found to exhibit intriguing regularities. In models with an infinite square well potential, it is found that a single formula, dependent on only the number of dimensions, describes a subset of 0+ states. The same behavior of a subset of 0+ states is seen in the large boson number limit of the Interacting Boson Approximation (IBA) model near the critical point of a first order phase transition, in contrast to the fact that these 0+ state energies exhibit a harmonic behavior in all three limiting symmetries of the IBA. Finally, the observed regularities in 0+ energies are analyzed in terms of the underlying group theoretical framework of the different models.

nucl-th↗

arXiv2007

APOLLO: the Apache Point Observatory Lunar Laser-ranging Operation: Instrument Description and First Detections

A next-generation lunar laser ranging apparatus using the 3.5 m telescope at the Apache Point Observatory in southern New Mexico has begun science operation. APOLLO (the Apache Point Observatory Lunar Laser-ranging Operation) has achieved one-millimeter range precision to the moon which should lead to approximately one-order-of-magnitude improvements in the precision of several tests of fundamental properties of gravity. We briefly motivate the scientific goals, and then give a detailed discussion of the APOLLO instrumentation.

astro-ph↗

arXiv2006

Evidence for Differential Rotation on a T Tauri Star

Five years of photometric monitoring of the T Tauri star HBC 338 in NGC 1333 has revealed that it is a periodic variable, but the period has changed significantly with time. From 2000-2003, a period near 5.6 days was observed, while in the last two seasons, the dominant period is near 4.6 days. No other T Tauri star has been seen to change its period by such a large percentage. We propose a model in which a differentially rotating star is seen nearly equator-on and a high latitude spot has gradually been replaced by a low latitude spot. We show that this model provides an excellent fit to the observed shapes of the light curves at each epoch. The amplitude and sense of the inferred differential rotation is similar to what is seen on the Sun. This may be surprising given the likely high degree of magnetic surface activity on the star relative to the Sun but we note that HBC 338 is clearly an exceptional T Tauri star.

astro-ph↗

arXiv2005

On the transmission of light through tilted interference filters

The transmission characteristics of commercial interference filters are not quite what we expected from the specifications provided by manufacturers. The unexpectedly sharp dependences of the transmission coefficient on the incidence angle, wavelength, and on the position where the light beam falls on the filter may be important in the analysis of systematic effects in experiments incorporating interference filters.

physics.optics↗

arXiv2004

Kerr effect in liquid helium at temperatures below the superfluid transition

The electro-optical Kerr effect induced by a slowly-varying applied electric field in liquid helium at temperatures below the $λ$-point is investigated. The Kerr constant of liquid helium is measured to be $(1.43\pm 0.02^{(stat)} \pm 0.04^{(sys)})\times 10^{-20}$ (cm/V)$^2$ at $T=1.5$ K. Within the experimental uncertainty, the Kerr constant is independent of temperature in the range $T=1.5$ K to 2.17 K, which implies that the Kerr constant of the superfluid component of liquid helium is the same as that of normal liquid helium. Our result also indicates that pair and higher correlations of He atoms in the liquid phase account for about 23% of the measured Kerr constant. Liquid nitrogen was used to test the experimental set-up, the result for the liquid nitrogen Kerr constant is $(4.38\pm 0.15)\times 10^{-18} $(cm/V)$^2$. The knowledge of the Kerr constant in these media allows the Kerr effect to be used as a non-contact technique for measuring the magnitude and mapping out the distribution of electric fields inside these cryogenic insulants.

physics.optics↗

arXiv1998

A Dirac Sea and thermodynamic equilibrium for the quantized three-wave interaction

The classical version of the three wave interaction models the creation and destruction of waves; the quantized version models the creation and destruction of particles. The quantum three wave interaction is described and the Bethe Ansatz for the eigenfunctions is given in closed form. The Bethe equations are derived in a rigorous fashion and are shown to have a thermodynamic limit. The Dirac sea of negative energy states is obtained as the infinite density limit. Finite particle/hole excitations are determined and the asymptotic relation of energy and momentum is obtained. The Yang-Yang functional for the relative free energy of finite density excitations is constructed and is shown to be convex and bounded below. The equations of thermal equilibrium are obtained.

solv-int↗