arXiv subjects

T. Johnson

Publications and source records attributed to T. Johnson.

At least 37 records · Page 2Linked to original sources

arXiv2021

Inductive Power Transfer Through Saltwater

We investigated inductive power transfer (IPT) through a rectangular slab of saltwater. Our inductively-coupled transmitters and receivers were made from loop-gap resonators (LGRs) having resonant frequencies near 100 MHz. Electric fields are confined within the narrow gaps of the LGRs making it possible to strongly suppress the power dissipation associated with electric fields in a conductive medium. Therefore, the power transfer efficiency in our system was limited by magnetic field dissipation in the conducting medium. We measured the power transfer efficiency as a function of both the conductivity of the water and the resonant frequency of the LGRs. We also present an equivalent circuit model that can be used to model IPT through a conductive medium. Finally, we show that using dividers to partition the saltwater volume provides another means of enhancing power transfer efficiency.

physics.app-ph↗

arXiv2021

Moment-Preserving and Mesh-Adaptive Reweighting Method for Rare-Event Sampling in Monte-Carlo Algorithms

We present novel roulette schemes for rare-event sampling that are both structure-preserving and unbiased. The boundaries where Monte Carlo markers are split and deleted are placed automatically and adapted during runtime. Extending existing codes with the new schemes is possible without severe changes because the equation of motion for the markers is not altered. These schemes can also be applied in the presence of nonlinear and nonlocal coupling between markers. As an illustrative application, we have implemented this method in the ASCOT-RFOF code, used to simulate fast-ion generation by radio-frequency waves in fusion plasmas. In this application, with this method the Monte-Carlo noise level for typical fast-ion energies can be reduced at least of one order of magnitude without increasing the computational effort.

physics.comp-ph↗

arXiv2021

First Measurement of Coherent Elastic Neutrino-Nucleus Scattering on Argon

We report the first measurement of coherent elastic neutrino-nucleus scattering (\cevns) on argon using a liquid argon detector at the Oak Ridge National Laboratory Spallation Neutron Source. Two independent analyses prefer \cevns over the background-only null hypothesis with greater than $3σ$ significance. The measured cross section, averaged over the incident neutrino flux, is (2.2 $\pm$ 0.7) $\times$10$^{-39}$ cm$^2$ -- consistent with the standard model prediction. The neutron-number dependence of this result, together with that from our previous measurement on CsI, confirms the existence of the \cevns process and provides improved constraints on non-standard neutrino interactions.

nucl-ex↗

arXiv2021

Development of a $^{83\mathrm{m}}$Kr source for the calibration of the CENNS-10 Liquid Argon Detector

We report on the preparation of and calibration measurements with a $^{83\mathrm{m}}$Kr source for the CENNS-10 liquid argon detector. $^{83\mathrm{m}}$Kr atoms generated in the decay of a $^{83}$Rb source were introduced into the detector via injection into the Ar circulation loop. Scintillation light arising from the 9.4 keV and 32.1 keV conversion electrons in the decay of $^{83\mathrm{m}}$Kr in the detector volume were then observed. This calibration source allows the characterization of the low-energy response of the CENNS-10 detector and is applicable to other low-energy-threshold detectors. The energy resolution of the detector was measured to be 9$\%$ at the total $^{83\mathrm{m}}$Kr decay energy of 41.5 keV. We performed an analysis to separately calibrate the detector using the two conversion electrons at 9.4 keV and 32.1 keV

physics.ins-det↗

arXiv2020

Permittivity of waste-activated sludge by an open-ended coaxial line

The complex permittivity of thickened waste activated sludge (WAS) was measured from 3 MHz to 40 GHz. The solid content of the thickened WAS sample was varied from 4.5% to 18% by weight. The permittivity spectra exhibit features typical of biological tissues that have a high water content. At high frequencies, a Debye-type dispersion is observed with a relaxation rate of 19 GHz characteristic of the bulk water in the sample ($γ$-dispersion). At lower frequencies, the solid content of the samples determines the properties of the permittivity. The onset of the so-called $β$-dispersion, attributed to the charging of cell membranes, occurs between 10-100 MHz. For samples with higher solid concentrations, a weak dispersion of the real part of the permittivity, characteristic of bound water, was observed at intermediate frequencies ($δ$-dispersion).

physics.app-ph↗

arXiv2020

COHERENT Collaboration data release from the first detection of coherent elastic neutrino-nucleus scattering on argon

Release of COHERENT collaboration data from the first detection of coherent elastic neutrino-nucleus scattering (CEvNS) on argon. This release corresponds with the results of "Analysis A" published in Akimov et al., arXiv:2003.10630 [nucl-ex]. Data is shared in a binned, text-based format representing both "signal" and "backgrounds" along with associated uncertainties such that the included data can be used to perform independent analyses. This document describes the contents of the data release as well as guidance on the use of the data. Included example code in C++ (ROOT) and Python show one possible use of the included data.

nucl-ex↗

arXiv2020

Permittivity measurements of biological samples by an open-ended coaxial line

We previously reported on the complex permittivity and dc conductivity of waste-activated sludge. The measurements, spanning a frequency range of 3 MHz to 40 GHz, were made using an open-ended coaxial transmission line. Although this technique is well established in the literature, we found that it was necessary to combine methods from several papers to use the open-ended coaxial probe to reliably characterize biological samples having a high dc conductivity. Here, we provide a set of detailed and practical guidelines that can be used to determine the permittivity and conductivity of biological samples over a broad frequency range. Due to the electrode polarization effect, low frequency measurements of conducting samples require corrections to extract the intrinsic electrical properties. We describe one practical correction scheme and verify its reliability using a control sample.

physics.app-ph↗

arXiv2019

Sensitivity of the COHERENT Experiment to Accelerator-Produced Dark Matter

The COHERENT experiment is well poised to test sub-GeV dark matter models using low-energy recoil detectors sensitive to coherent elastic neutrino-nucleus scattering (CEvNS) in the $π$-DAR neutrino beam produced by the Spallation Neutron Source. We show how a planned 750-kg liquid argon scintillation detector would place leading limits on scalar light dark matter models, over two orders of magnitude of dark matter mass, for dark matter particles produced through vector and leptophobic portals in the absence of other effects beyond the standard model. The characteristic timing structure of a $π$-DAR beam allows a unique opportunity for constraining systematic uncertainties on the standard model background in a time window where signal is not expected, enhancing expected sensitivity. Additionally, we discuss future prospects, further increasing the discovery potential of CEvNS detectors. Such methods would test the calculated thermal dark matter abundance for all couplings $α'\leq1$ within the vector portal model over an order of magnitude of dark matter masses.

hep-ex↗

arXiv2019

The DRAO Synthesis Telescope

The DRAO Synthesis Telescope (ST) is a forefront telescope for imaging large-scale neutral hydrogen and polarized radio continuum emission at arcminute resolution. Equipped for observations at 1420 and 408 MHz, the ST completed the Canadian Galactic Plane Survey, providing pioneering measurements of arcminute-scale structure in HI emission and self-absorption and of the diffuse polarized emission, using a fine grid of Rotation Measures to chart the large-scale Galactic magnetic field, and advancing the knowledge of the Galactic rotation curve. In this paper we describe a plan for renewal of the Synthesis Telescope that will create a forefront scientific instrument, a testbed for new radio astronomy technologies, and a training ground for the next generation of Canadian radio astronomers and radio telescope engineers. The renewed telescope will operate across the entire range 400 to 1800 MHz. Collaborations between DRAO and university partners have already demonstrated a novel feed antenna to cover this range, low-noise amplifiers, and a new GPU-based correlator of bandwidth 400 MHz. The renewed ST will provide excellent sensitivity to extended HI, covering the Galactic disk and halo, spectro-polarimetry with unprecedented resolution in angle and in Faraday depth, the ability to search for OH masers in all four 18-cm lines simultaneously, and sensitive recombination-line observations stacked over as many as forty transitions. As a testbed the renewed ST will evaluate low-cost digital clocking and sampling techniques of wide significance for the ngVLA, SKA, and other future telescopes, and a prototype of the digital correlator developed at DRAO for SKA-mid.

astro-ph.IM↗

arXiv2019

First Constraint on Coherent Elastic Neutrino-Nucleus Scattering in Argon

Coherent elastic neutrino-nucleus scattering (CEvNS) is the dominant neutrino scattering channel for neutrinos of energy $E_ν< 100$ MeV. We report a limit for this process using data collected in an engineering run of the 29 kg CENNS-10 liquid argon detector located 27.5 m from the Oak Ridge National Laboratory Spallation Neutron Source (SNS) Hg target with $4.2\times 10^{22}$ protons on target. The dataset yielded $< 7.4$ observed CEvNS events implying a cross section for the process, averaged over the SNS pion decay-at-rest flux, of $<3.4 \times 10^{-39}$ cm$^{2}$, a limit within twice the Standard Model prediction. This is the first limit on CEvNS from an argon nucleus and confirms the earlier CsI non-standard neutrino interaction constraints from the collaboration. This run demonstrated the feasibility of the ongoing experimental effort to detect CEvNS with liquid argon.

hep-ex↗

arXiv2019

Measurement of the neutron capture cross-section on argon

The use of argon as a detection and shielding medium for neutrino and dark matter experiments has made the precise knowledge of the cross section for neutron capture on argon an important design and operational parameter. Since previous measurements were averaged over thermal spectra and have significant disagreements, a differential measurement has been performed using a Time-Of-Flight neutron beam and a $\sim$4$π$ gamma spectrometer. A fit to the differential cross section from $0.015-0.15$\,eV, assuming a $1/v$ energy dependence, yields $σ^{2200} = 673 \pm 26 \text{ (stat.)} \pm 59 \text{ (sys.)}$\,mb.

nucl-ex↗

arXiv2019

The GMRT High Resolution Southern Sky Survey for pulsars and transients -II. New discoveries, timing and polarization properties

We have been conducting the GMRT High Resolution Southern Sky (GHRSS) survey for the last four years and have discovered 18 pulsars to date. The GHRSS survey is an off-Galactic-plane survey at 322 MHz in a region of the sky (declination range -40 degrees to -54 degrees) complementary to other ongoing low-frequency surveys. In this paper we report the discovery of three pulsars, PSRs J1239-48, J1516-43 and J1726-52. We also present timing solutions for three pulsars previously discovered with the GHRSS survey: PSR J2144-5237, a millisecond pulsar with a period P=5 ms in a 10 day orbit around a < 0.18 Msun companion; PSR J1516-43, a mildly recycled P=36 ms pulsar in a 228 day orbit with a companion of mass ~0.4 Msun; and the P=320 ms PSR J0514-4408 which we show is a source of pulsed $γ$-ray emission. We also report radio polarimetric observations of three of the GHRSS discoveries, PSRs J0418-4154, J0514-4408 and J2144-5237.

astro-ph.HE↗

arXiv2019

Spatially Resolved Outflows in a Seyfert Galaxy at z = 2.39

We present the first spatially resolved analysis of rest-frame optical and UV imaging and spectroscopy for a lensed galaxy at z = 2.39 hosting a Seyfert active galactic nucleus (AGN). Proximity to a natural guide star has enabled high signal-to-noise VLT SINFONI + adaptive optics observations of rest-frame optical diagnostic emission lines, which exhibit an underlying broad component with FWHM ~ 700 km/s in both the Balmer and forbidden lines. Measured line ratios place the outflow robustly in the region of the ionization diagnostic diagrams associated with AGN. This unique opportunity - combining gravitational lensing, AO guiding, redshift, and AGN activity - allows for a magnified view of two main tracers of the physical conditions and structure of the interstellar medium in a star-forming galaxy hosting a weak AGN at cosmic noon. By analyzing the spatial extent and morphology of the Ly-alpha and dust-corrected H-alpha emission, disentangling the effects of star formation and AGN ionization on each tracer, and comparing the AGN induced mass outflow rate to the host star formation rate, we find that the AGN does not significantly impact the star formation within its host galaxy.

astro-ph.GA↗

arXiv2018

Non-Maxwellian fast particle effects in gyrokinetic GENE simulations

Fast ions have recently been found to significantly impact and partially suppress plasma turbulence both in experimental and numerical studies in a number of scenarios. Understanding the underlying physics and identifying the range of their beneficial effect is an essential task for future fusion reactors, where highly energetic ions are generated through fusion reactions and external heating schemes. However, in many of the gyrokinetic codes fast ions are, for simplicity, treated as equivalent-Maxwellian-distributed particle species, although it is well known that to rigorously model highly non-thermalised particles, a non-Maxwellian background distribution function is needed. To study the impact of this assumption, the gyrokinetic code GENE has recently been extended to support arbitrary background distribution functions which might be either analytic, e.g. slowing down and bi-Maxwellian, or obtained from numerical fast ion models. A particular JET plasma with strong fast-ion related turbulence suppression is revised with these new code capabilities both with linear and nonlinear gyrokinetic simulations. It appears that the fast ion stabilization tends to be less strong but still substantial with more realistic distributions, and this improves the quantitative power balance agreement with experiments.

physics.plasm-ph↗

arXiv2017

The Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas (MEGaSaURA) I: The Sample and the Spectra

We introduce Project MEGaSaURA: The Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas. MEGaSaURA comprises medium-resolution, rest-frame ultraviolet spectroscopy of N=15 bright gravitationally lensed galaxies at redshifts of 1.68$<$z$<$3.6, obtained with the MagE spectrograph on the Magellan telescopes. The spectra cover the observed-frame wavelength range $3200 < λ_o < 8280$ Å; the average spectral resolving power is R=3300. The median spectrum has a signal-to-noise ratio of $SNR=21$ per resolution element at 5000 Å. As such, the MEGaSaURA spectra have superior signal-to-noise-ratio and wavelength coverage compared to what COS/HST provides for starburst galaxies in the local universe. This paper describes the sample, the observations, and the data reduction. We compare the measured redshifts for the stars, the ionized gas as traced by nebular lines, and the neutral gas as traced by absorption lines; we find the expected bulk outflow of the neutral gas, and no systemic offset between the redshifts measured from nebular lines and the redshifts measured from the stellar continuum. We provide the MEGaSaURA spectra to the astronomical community through a data release.

astro-ph.GA↗

arXiv2017

The Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas (MEGaSaURA) II: Stacked Spectra

We stack the rest-frame ultraviolet spectra of N=14 highly magnified gravitationally lensed galaxies at redshifts 1.6<z<3.6. The resulting new composite spans $900< λ_{rest} < 3000$ Å, with a peak signal-to-noise ratio of 103 per spectral resolution element ($\sim$100 km/s). It is the highest signal-to-noise ratio, highest spectral resolution composite spectrum of $z\sim2$--3 galaxies yet published. The composite reveals numerous weak nebular emission lines and stellar photospheric absorption lines that can serve as new physical diagnostics, particularly at high redshift with the James Webb Space Telescope (JWST). We report equivalent widths to aid in proposing for and interpreting JWST spectra. We examine the velocity profiles of strong absorption features in the composite, and in a matched composite of $z\sim0$ COS/HST galaxy spectra. We find remarkable similarity in the velocity profiles at $z\sim 0$ and $z\sim2$, suggesting that similar physical processes control the outflows across cosmic time. While the maximum outflow velocity depends strongly on ionization potential, the absorption-weighted mean velocity does not. As such, the bulk of the high-ionization absorption traces the low-ionization gas, with an additional blueshifted absorption tail extending to at least $-2000$ km/s . We interpret this tail as arising from the stellar wind and photospheres of massive stars. Starburst99 models are able to replicate this high-velocity absorption tail. However, these theoretical models poorly reproduce several of the photospheric absorption features, indicating that improvements are needed to match observational constraints on the massive stellar content of star-forming galaxies at $z \sim 2$. We publicly release our composite spectra.

astro-ph.GA↗

arXiv2015

The veto system of the DarkSide-50 experiment

Nuclear recoil events produced by neutron scatters form one of the most important classes of background in WIMP direct detection experiments, as they may produce nuclear recoils that look exactly like WIMP interactions. In DarkSide-50, we both actively suppress and measure the rate of neutron-induced background events using our neutron veto, composed of a boron-loaded liquid scintillator detector within a water Cherenkov detector. This paper is devoted to the description of the neutron veto system of DarkSide-50, including the detector structure, the fundamentals of event reconstruction and data analysis, and basic performance parameters.

physics.ins-det↗

arXiv2014

Electromagnetic stabilization of tokamak microturbulence in a high-$β$ regime

The impact of electromagnetic stabilization and flow shear stabilization on ITG turbulence is investigated. Analysis of a low-$β$ JET L-mode discharge illustrates the relation between ITG stabilization, and proximity to the electromagnetic instability threshold. This threshold is reduced by suprathermal pressure gradients, highlighting the effectiveness of fast ions in ITG stabilization. Extensive linear and nonlinear gyrokinetic simulations are then carried out for the high-$β$ JET hybrid discharge 75225, at two separate locations at inner and outer radii. It is found that at the inner radius, nonlinear electromagnetic stabilization is dominant, and is critical for achieving simulated heat fluxes in agreement with the experiment. The enhancement of this effect by suprathermal pressure also remains significant. It is also found that flow shear stabilization is not effective at the inner radii. However, at outer radii the situation is reversed. Electromagnetic stabilization is negligible while the flow shear stabilization is significant. These results constitute the high-$β$ generalization of comparable observations found at low-$β$ at JET. This is encouraging for the extrapolation of electromagnetic ITG stabilization to future devices. An estimation of the impact of this effect on the ITER hybrid scenario leads to a 20% fusion power improvement.

physics.plasm-ph↗