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J. Kern

Publications and source records attributed to J. Kern.

7 recordsLinked to original sources

Nonlinear suppression of dispersion broadening of ultrashort spin-wave pulses in thin YIG films

We study experimentally the nonlinear propagation of short pulses of forward volume spin waves in nanometer-thick YIG films. We show that nonlinearity of the spin system can efficiently counteract dispersion broadening of the pulses, leading to the formation of envelope solitons. We demonstrate that in microscopic YIG systems, microwave powers of the order of one milliwatt are sufficient to reach the soliton formation threshold. At powers slightly above this threshold, we achieve transmission of 3-ns spin-wave pulses over distances of up to 50 micrometers without increase in their temporal width. Our results demonstrate a promising way towards high-rate transmission of information in microscopic spin-wave circuits unaffected by detrimental dispersion effects.

cond-mat.mes-hall

An Algorithm Architecture for Radio Interferometric Data Processing

We present a foundational, scalable algorithm architecture for processing data from aperture synthesis radio telescopes. The analysis leading to the architecture is rooted in the theory of aperture synthesis, signal processing and numerical optimization keeping it scalable for variations in computing load, algorithmic complexity, and accommodate the continuing evolution of algorithms. It also adheres to scientific software design principles and use of modern performance engineering techniques providing a stable foundation for long-term scalability, performance, and development cost. We first show that algorithms for both calibration and imaging algorithms share a common mathematical foundation and can be expressed as numerical optimization problems. We then decompose the resulting mathematical framework into fundamental conceptual architectural components, and assemble calibration and imaging algorithms from these foundational components. For a physical architectural view, we used a library of algorithms implemented in the LibRA software for the various architectural components, and used the Kokkos framework in the compute-intensive components for performance portable implementation. This was deployed on hardware ranging from desktop-class computers to multiple super-computer class high-performance computing (HPC) and high-throughput computing (HTC) platforms with a variety of CPU and GPU architectures, and job schedulers (HTCondor and Slurm). As a test, we imaged archival data from the NSF's Karl G. Jansky Very Large Array (VLA) telescope in the A-array configuration for the Hubble Ultra Deep Field. Using over 100 GPUs we achieve a processing rate of ~2 Terabyte per hour to make one of the deepest images in the 2 -- 4 GHz band with an RMS noise of ~1 microJy/beam.

astro-ph.IM

Review of Core Process Representation in Power System Operational Models: Gaps, Challenges, and Opportunities for Multisector Dynamics Research

Power grid operations increasingly interact with environmental systems and human systems such as transportation, agriculture, the economy, and financial markets. Our objective is to discuss the modelling gaps and opportunities to advance the science for multisector adaptation and tradeoffs. We focus on power system operational models, which typically represent key physical and economic aspects of grid operations over days to a year and assume a fixed power grid infrastructure. Due to computational burden, models are typically customized to reflect regional resource opportunities, data availability, and applications of interest. While there are model intercomparison papers, there is however no model-agnostic characterization and systematic overview of the state-of-the-art process representations in operational power system models. To address our objective, we conceptualize power system operational models with four core processes: physical grid assets (generation, transmission, loads, and storage), model objectives and purpose, institutions and decision agents, and performance metrics. We taxonomize the representations of these core processes based on a review of 23 existing open-source and commercial models. As we acknowledge the computational burden of certain representations, we leverage this taxonomy to describe tradeoffs in process fidelity and tractability that have been adopted by the research community to address interactions between the power grid and hydrometeorological uncertainties, global change, and/or technological innovation. The core process taxonomy along with the existing computational tradeoffs are used to identify technical gaps and recommend future model development needs and research directions to better represent power grid operations as part of integrated multisector dynamics modeling and interdisciplinary research.

eess.SY

An Overview of the 2014 ALMA Long Baseline Campaign

A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ~15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from September to late November 2014, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ~350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.

astro-ph.IM

Hydraulic External Pre-Isolator System for LIGO

The Hydraulic External Pre-Isolator (HEPI) is the first 6 degrees of freedom active seismic isolation system implemented at the Laser Interferometer Gravitational Wave Observatory (LIGO). Implementation was first completed at the LIGO Livingston Observatory (LLO) prior to LIGO's 5th science run, successfully cutting down the disturbance seen by LLO's suspended optics in the two most prominent seismic disturbance bands, the microseism (0.1-0.3Hz) and the anthropogenic (1-3Hz) bands, by a factor of a few to tens. The improvement in seismic isolation contributed directly to LLO's much improved duty cycle of 66.7% and LIGO's triple coincident duty cycle of 53%. We report the design, control scheme, and isolation performance of HEPI at LLO in this paper. Aided with this success, funding for incorporating HEPI into the LIGO Hanford Observatory was approved and installation is currently underway.

gr-qc

Dynamics of four-photon photoluminescence in gold nanoantennas

Two-pulse correlation is employed to investigate the temporal dynamics of both two-photon photoluminescence (2PPL) and four-photon photoluminescence (4PPL) in resonant and nonresonant nanoantennas excited at a wavelength of 800 nm. Our data are consistent with the same two-step model being the cause of both 4PPL and 2PPL, implying that the first excitation step in 4PPL is a three-photon sp->sp direct interband transition. Considering energy and parity conservation, we also explain why 4PPL behavior is favored over three-and five-photon photoluminescence in the power range below the damage threshold of our antennas. Since sizeable 4PPL requires larger peak intensities of the local field, we are able to select either 2PPL or 4PPL in the same gold nanoantennas by choosing a suitable laser pulse duration. We thus provide a first consistent model for the understanding of multiphoton photoluminescence generation in gold nanoantennas, opening new perspectives for applications ranging from the characterization of plasmonic resonances to biomedical imaging.

physics.optics

The Brightest Pulses in the Universe: Multifrequency Observations of the Crab Pulsar's Giant Pulses

We analyze the Crab pulsar at ten frequencies from 0.43 to 8.8 GHz using data obtained at the Arecibo Observatory. Giant pulses occur only in the main and interpulse components manifest from radio frequencies to gamma-ray energies. Individual giant pulses reach brightness temperatures of at least $10^{32}$K in our data, which do not resolve the narrowest pulses, and are known to reach $10^{37}$K in nanosecond-resolution observations (Hankins et al 2003). Giant pulses are therefore the brightest known in the observable universe and represent an important milestone for theories of the pulsar emission mechanism to explain. Their short durations allow them to serve as especially sensitive probes of the Crab Nebula and the interstellar medium. We analyze frequency structure in individual giant pulses using a scintillating, amplitude-modulated,polarized shot-noise model. The frequency structure associated with multipath propagation decorrelates on a time scale of 25 sec at 1.5 GHz, which requires that multipath propagation be strongly influenced by material within the Crab Nebula. Additional frequency structure decorrelates faster than one spin period, as would be expected from the shot-noise pattern of nanosecond duration pulses emitted by the pulsar. Taking into account the Crab pulsar's locality inside a bright supernova remnant, we conclude that the brightest pulse in a typical 1-hour observation would be most easily detectable in our lowest frequency band (0.43 GHz) to a distance of 1.6 Mpc. We also discuss the detection of such pulses using future instruments such as LOFAR and the SKA.

astro-ph