W Plus Multiple Jets at the LHC with High Energy Jets

We study the production of a W boson in association with n hard QCD jets (for n>=2), with a particular emphasis on results relevant for the Large Hadron Collider (7 TeV and 8 TeV). We present predictions for this process from High Energy Jets, a framework for all-order resummation of the dominant contributions from wide-angle QCD emissions. We first compare predictions against recent ATLAS data and then shift focus to observables and regions of phase space where effects beyond NLO are expected to be large.Comment: 19 pages, 9 figure

An Enhanced Operational Definition of Dielectric Breakdown for DC Voltage Step-up Tests

The imprecise definition of breakdown in the ASTM D3755-14 standard can misidentify breakdown. If the recommended test circuit current sensing element threshold is set too high, breakdown may occur undetected. Conversely, false positives may result from designating a low current threshold. An operational definition of breakdown much less sensitive to these pitfalls is outlined herein. This enhanced definition of breakdown is based on the average rate of change of the leakage current with increasing voltage, rather than a simple current threshold, avoiding ambiguous association with anomalies in current traces. For tests that continuously monitor leakage current, breakdown can be detected by a transition from negligible current to an ohmic slope defined by the circuit’s current limiting resistors. In practice, a fixed current threshold is inadequate to define dielectric breakdown. Field-enhanced conductivity, partial discharge, surface flashovers, incomplete breakdowns, and other phenomena may further obscure the characteristic dielectric breakdown signature. Pre-breakdown anomalies in current traces can also now be clearly identified and studied, in addition to the breakdown itself

Physics-Driven Dual-Defect Model Fits of Voltage Step-Up to Breakdown Data in Spacecraft Polymers

Overly conservative estimates of breakdown strength can increase the mass and cost of spacecraft electrostatic discharge (ESD) mitigation methods. Improved estimates of ESD likelihood in the space environment require better models of ESD distributions. The purpose of this work is to evaluate our previously proposed dual-defect model of voltage step-up-to-breakdown tests with a case study across four dielectric materials. We predicted that materials best fit by mixed Weibull distributions would exhibit better fits with the dual-defect model compared to a mean field single defect theory. Additional data for biaxially oriented polypropylene (BOPP), polyimide (PI or Kapton) from three sources, and polyether ether ketone (PEEK) are compared to the previous study on low-density polyethylene (LDPE). Except in one case, the dual-defect model is a better fit to bimodal distributions of tests results

Highly Accelerated Test Method for Characterizing Likelihood of Breakdown in HVDC Dielectric Materials

Increasing application and development of HVDC technologies emphasizes the need for improved characterization of candidate insulating materials. Accurately predicting the lifetime to breakdown of dielectric materials by means of accelerated voltage step-up to breakdown tests can be prohibitively time consuming. Step-up to breakdown tests with sufficiently slow voltage ramp rates that continuously monitor leakage current have detected a distribution of DC partial discharge (DCPD) events occurring prior to breakdown, which increase with increasing field. These DCPD distributions are shown to correlate strongly with the likelihood of breakdown for four common polymers. Given that hundreds of DCPD events are typically observed in a single destructive, low-ramp rate, step-up test, measuring the distribution of the DCPD can potentially accelerate the characterization of the breakdown likelihood in candidate insulators by orders of magnitude in time. This relationship is discussed in the context of a dual-defect model of breakdown and thermally recoverable defects

Muon Contribution to Cathodoluminescence Tests?

Tests of composites incorporating highly disordered insulating materials that were bombarded with low-flux keV electron beams exhibited three distinct forms of light emission: short-duration (\u3c\u3c1 s), high intensity luminous electrostatic discharges between the insulator and ground--termed “arcs”; intermediate-duration (10-100 s), intense surface emissions—termed “flares”; and lower intensity, continuous surface cathodoluminescent “glow”. During long-duration experiments at temperatures \u3c150 K, relatively intense flare events occurred at rates of ~2 per min. Rapid increase in photon emission and electron displacement current were observed, with long exponential decay times \u3e1 min. We propose that the source of the flares is the interactions of high energy muons—of cosmic ray origin—with the highly-charged insulating components of the composite materials, which trigger avalanche electrostatic discharge and subsequent recharging along with concomitant light emission. We review evidence from the insulator conductivity at low temperatures, the rates and magnitude of surface charging, the flare frequency, and the magnitude and time-dependence of currents and light emission with regard to this muon hypothesis. Finally, a muon coincidence detection experiment using scintillation detectors is proposed to investigate the potential correlation between incident muons and the observed flares

Electrostatic Discharge Breakdown Analyses

Electrostatic discharge (ESD) and the associated material breakdown is the primary cause for spacecraft damage due to space environment interactions. This phenomenon occurs when the space plasma fluxes charge a craft to high voltages where insulating materials then break down and conduct current. This can damage or destroy onboard electrical systems. This project deals with how suspect materials break down under high voltage exposure. The USU Material Physics Group has acquired hundreds of samples that underwent ESD. The ESD damage sites of these samples have been analyzed for parameters including breakdown size, shape, coloring, and location and material characteristics such as thickness and polymer type. The results have been recorded in an ESD Quality Summary Table. Initial trends within this data set are being indentified and sorted within the matrix based on experimental parameters and material type in order to locate trends. Information that we have already found; as well as correlations which we hope to find, will aid in predicting which materials are best suited for use in high voltage scenarios

Temperature Dependency of Electrostatic Breakdown in LDPE and PEEK

Electrostatic breakdown is a leading cause of many of the anomalies and failures attributed to spacecraft interactions with the space environment. It is therefore critical to understand how the electrostatic field strength varies due to changing environmental conditions, including temperature and radiation dose. Standard step-up to electrostatic discharge (ESD) tests were performed on two polymers, low density polyethylene (LDPE) and polyetheretherketone (PEEK). Tests were done at room temperature and at other temperatures ranging from ~130 K to ~350 K. Preliminary analysis found that samples tested at a higher temperature had lower average breakdown field strength and a narrower distribution of breakdown field values. These results are considered with respect to a proposed dual-defect theory for electrostatic breakdown, which incorporates both lower energy recoverable defect modes that can be generated and annihilated through thermal annealing and higher energy irrecoverable defect modes such as those created by radiation damage. The model predicts that at lower electric field strengths, an annealing process occurs due to the higher temperature which limits the density of low energy defects in the material. This means that while the overall breakdown field strength decreases, the minimum field strength required to breakdown the material would increase, thereby narrowing the breakdown distribution. This work was supported by a NASA Space Technology Research Fellowship

Electrostatic Discharge Properties of Fused Silica Coatings

The electric field value at which electrostatic discharge (ESD) occurs was studied for thin coatings of fused silica (highly disordered SiO2/SiOx) on conductive substrates, such as those encountered as optical coatings and in Si microfabrication. The electrostatic breakdown field was determined using an increasing voltage, while monitoring the leakage current. A simple parallel-plate capacitor geometry was used, under medium vacuum and at temperatures down to ~150 K using a liquid N2 reservoir. The breakdown field, pre-breakdown arcing and I-V curves for fused silica samples are compared for ~60 nm and ~80 μm thick, room and low temperature, and untreated and irradiated samples. Unlike typical I-V results for polymeric insulators, the thin film silica samples did not exhibit pre-breakdown arcing, displayed transitional resistivity after initial breakdown, and in many cases showed evidence of a second discontinuity in the I-V curves. This diversity of observed discharge phenomena is discussed in terms of breakdown modes and defect generation on a microscopic scale