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Thermal island destabilization and the Greenwald limit
Magnetic reconnection is ubiquitous in the magnetosphere, the solar corona, and in toroidal fusion research discharges. A magnetic island saturates at a width which produces a minimum in the magnetic energy of the configuration is evident in a fusion device. At saturation, the modified current density profile, a function of the flux in the island, is essentially flat, the growth rate proportional to the difference in the current at the O-point and the X-point. Furthermore, modification of the current density profile in the island interior causes a change in the island stability and additional growth or contraction of the saturated island. Because field lines in an island are isolated from the outside plasma, an island can heat or cool preferentially depending on the balance of Ohmic heating and radiation loss in the interior, changing the resistivity and hence the current in the island. A simple model of island destabilization due to radiation cooling of the island is constructed, and the effect of modification of the current within an island is calculated. In addition destabilization effect is described, and it is shown that a small imbalance of heating can lead to exponential growth of the island. A destabilized magnetic island near the plasma edge can lead to plasma loss, and because the radiation is proportional to plasma density and charge, this effect can cause an impurity dependent density limit.
Experimental Evaluation of PV Inverter Anti-Islanding with Grid Support Functions in Multi-Inverter Island Scenarios
As PV and other DER systems are connected to the grid at increased penetration levels, island detection may become more challenging for two reasons: 1.) In islands containing many DERs, active inverter-based anti-islanding methods may have more difficulty detecting islands because each individual inverter's efforts to detect the island may be interfered with by the other inverters in the island. 2.) The increasing numbers of DERs are leading to new requirements that DERs ride through grid disturbances and even actively try to regulate grid voltage and frequency back towards nominal operating conditions. These new grid support requirements may directly or indirectly interfere with anti-islanding controls. This report describes a series of tests designed to examine the impacts of both grid support functions and multi-inverter islands on anti-islanding effectiveness. Crucially, the multi-inverter anti-islanding tests described in this report examine scenarios with multiple inverters connected to multiple different points on the grid. While this so-called 'solar subdivision' scenario has been examined to some extent through simulation, this is the first known work to test it using hardware inverters. This was accomplished through the use of power hardware-in-the-loop (PHIL) simulation, which allows the hardware inverters to be connected to a real-time transient simulation of an electric power system that can be easily reconfigured to test various distribution circuit scenarios. The anti-islanding test design was a modified version of the unintentional islanding test in IEEE Standard 1547.1, which creates a balanced, resonant island with the intent of creating a highly challenging condition for island detection. Three common, commercially available single-phase PV inverters from three different manufacturers were tested. The first part of this work examined each inverter individually using a series of pure hardware resistive-inductive-capacitive (RLC) resonant load based anti-islanding tests to determine the worst-case configuration of grid support functions for each inverter. A grid support function is a function an inverter performs to help stabilize the grid or drive the grid back towards its nominal operating point. The four grid support functions examined here were voltage ride-through, frequency ride-through, Volt-VAr control, and frequency-Watt control. The worst-case grid support configuration was defined as the configuration that led to the maximum island duration (or run-on time, ROT) out of 50 tests of each inverter. For each of the three inverters, it was observed that maximum ROT increased when voltage and frequency ride-through were activated. No conclusive evidence was found that Volt-VAr control or frequency-Watt control increased maximum ROT. Over all single-inverter test cases, the maximum ROT was 711 ms, well below the two-second limit currently imposed by IEEE Standard 1547-2003. A subsequent series of 244 experiments tested all three inverters simultaneously in the same island. These tests again used a procedure based on the IEEE 1547.1 unintentional islanding test to create a difficult-to-detect island condition. For these tests, which used the two worst-case grid support function configurations from the single-inverter tests, the inverters were connected to a variety of island circuit topologies designed to represent the variety of multiple-inverter islands that may occur on real distribution circuits. The interconnecting circuits and the resonant island load itself were represented in the real-time PHIL model. PHIL techniques similar to those employed here have been previously used and validated for anti-islanding tests, and the PHIL resonant load model used in this test was successfully validated by comparing single-inverter PHIL tests to conventional tests using an RLC load bank.
Effects of magnetic islands on drift wave instability
Magnetic islands have been implemented in the gyrokinetic toroidal code to study the effects of the islands on microturbulence. The pressure profile flattening is verified in the simulation with the islands. Simulations of ion temperature gradient instability find that different toroidal modes are linearly coupled together and that toroidal spectra become broader when the island width increases. The real frequencies and growth rates of different toroidal modes approach each other with the averaged value independent of the island width. The linear mode structures are enhanced at the island separatrices and weakened at the island centers, consistent with the flattening of the pressure profile inside the islands.
Runaway electrons and magnetic island confinement
The breakup of magnetic surfaces is a central feature of ITER planning for the avoidance of damage due to runaway electrons. Rapid thermal quenches, which lead to large accelerating voltages, are thought to be due to magnetic surface breakup. Impurity injection to avoid and to mitigate both halo and runaway electron currents utilizes massive gas injection or shattered pellets. The actual deposition is away from the plasma center, and the breakup of magnetic surfaces is thought to spread the effects of the impurities across the plasma cross section. The breakup of magnetic surfaces would prevent runaway electrons from reaching relativistic energies were it not for the persistence of non-intercepting flux tubes. These are tubes of magnetic field lines that do not intercept the walls. In simulations and in magnetic field models, non-intercepting flux tubes are found to persist near the magnetic axis and in the cores of magnetic islands even when a large scale magnetic surface breakup occurs. As long as a few magnetic surfaces reform before all of the non-intercepting flux tubes dissipate, energetic electrons confined and accelerated in these flux tubes can serve as the seed electrons for a transfer of the overall plasma current from thermal to relativistic carriers. The acceleration of electrons is particularly strong because of the sudden changes in the poloidal flux that naturally occur in a rapid magnetic relaxation. Furthermore, the physics of magnetic islands as non-intercepting flux tubes is studied. Expressions are derived for (1) the size of islands required to confine energetic runaway electrons, (2) the accelerating electric field in an island, (3) the increase or reduction in the size of an island by the runaway electron current, (4) the approximate magnitude of the runaway current in an island, and (5) the time scale for the evolution of an island.
The effect of sheared toroidal rotation on pressure driven magnetic islands in toroidal plasmas
The impact of sheared toroidal rotation on the evolution of pressure driven magnetic islands in tokamak plasmas is investigated using a resistive magnetohydrodynamics model augmented by a neoclassical Ohm’s law. Particular attention is paid to the asymptotic matching data as the Mercier indices are altered in the presence of sheared flow. Analysis of the nonlinear island Grad- Shafranov equation shows that sheared flows tend to amplify the stabilizing pressure/curvature contribution to pressure driven islands in toroidal tokamaks relative to the island bootstrap current contribution. Finally, as such, sheared toroidal rotation tends to reduce saturated magnetic island widths.
Islander: A database of precisely mapped genomic islands in tRNA and tmRNA genes
Genomic islands are mobile DNAs that are major agents of bacterial and archaeal evolution. Integration into prokaryotic chromosomes usually occurs site-specifically at tRNA or tmRNA gene (together, tDNA) targets, catalyzed by tyrosine integrases. This splits the target gene, yet sequences within the island restore the disrupted gene; the regenerated target and its displaced fragment precisely mark the endpoints of the island. We applied this principle to search for islands in genomic DNA sequences. Our algorithm identifies tDNAs, finds fragments of those tDNAs in the same replicon and removes unlikely candidate islands through a series of filters. A search for islands in 2168 whole prokaryotic genomes produced 3919 candidates. The website Islander (recently moved to http://bioinformatics.sandia.gov/islander/) presents these precisely mapped candidate islands, the gene content and the island sequence. The algorithm further insists that each island encode an integrase, and attachment site sequence identity is carefully noted; therefore, the database also serves in the study of integrase site-specificity and its evolution.
High endemism of mosquitoes on São Tomé and Príncipe Islands: evaluating the general dynamic model in a worldwide island comparison
Abstract The archipelago of São Tomé and Príncipe is a major centre of endemism for vertebrates but, to date, arthropods remain poorly studied, with the exception of mosquitoes due to their medical and economic relevance. Previous studies, over 20 years old, recorded 26 species of mosquitoes for São Tomé and 14 for Príncipe, of which six and two were endemic to each island respectively. We collected mosquito larvae and adults on both islands and found three species as a first record, putting the number of mosquito species collected at least once on São Tomé at 29 and 15 for Príncipe. We compared the mosquito richness on São Tomé and Príncipe islands to 71 other oceanic islands represented within 20 archipelagoes worldwide. We used the general dynamic model of oceanic island biogeography and the associated ATT 2 (for Area+Time+Time 2 ) model to explain two different metrics: (i) the richness of the single‐island endemics and (ii) the richness of archipelago endemics. We also included other island characteristics in our model, such as isolation and latitude. Our data compilation and comparative analyses revealed an overall low richness of endemic mosquitoes on islands, with the highest number of single‐island endemics found on São Tomé Island. As predicted, we found a positive relationship between our two richness metrics and the general dynamic model ATT 2 , although the model with the area (A) alone also explained endemic richness. Isolation was also predictor of mosquito diversification.
Modern analogs for understanding pollen-vegetation dynamics in a Mediterranean mosaic landscape (Balearic Islands, Western Mediterranean)
This paper presents the study of modern pollen analogs from the Balearic Islands. While similar studies have been largely applied to mainland areas, research focused on modern vegetation dynamics on Mediterranean islands remains very rare. In this research, we combine vegetation surveys, pollen analysis and multivariate statistics to understand landscape composition. The main objectives of are: (1) to examine pollen-vegetation relationships in relation to environmental and land-use variables; (2) to understand modern pollen representation in a mosaic landscape structure; and (3) to propose pollen indicators that characterize the primary vegetation types from the Balearic Islands to better interpret past pollen records in Mediterranean island environments. Pollen results and Redundancy Analysis (RDA) distinguish three major groups: (a) Holm oak and box formations; (b) maquis and garrigues; and (c) anthropogenic and open habitats. Landscape form, mean decadal rainfall, mean decadal temperature, fire activity, trampling, slope percentage, wet/flooded soil, saline soil, distance to agropastoral cells, gHM index, domestic herbivory presence, agropastoral use, and soil type are the major variables explaining modern pollen assemblage variation in our research. Poaceae undiff., Plantago sp., Apiaceae undiff., Cerealia-t, and Cichorieae are highly correlated to human activities but should be interpreted cautiously when occurring in low values. Quercus ilex-t, Hypericum, and Buxus are correlated to humid locations while Pistacia, Pinus, Juniperus-t, and Olea to high mean decadal temperatures. Our study indicates how pollen analysis and multivariate analysis are powerful tools for characterizing the mosaic landscape, with special focus on the main vegetation types of the Balearic Islands.
An Islanding Detection Test Platform for Multi-Inverter Islands Using Power HIL
When an unintentional island is formed on the electric power system, distributed energy resources (DERs) are typically required to detect and de-energize the island. This requirement may become more challenging as the number of DERs in an island rises. Thus, it is of interest to experimentally verify whether DERs can successfully detect and de-energize islands containing many DERs connected at different points. This paper presents a power hardware-in-the-loop (PHIL) platform for testing the duration of islands containing multiple inverters connected at multiple points in a network. The PHIL platform uses real-time simulation to represent islanded distribution circuits, with DER inverters connected in hardware. This allows efficient testing of a large number of island configurations simply by changing the distribution circuit model in the real-time simulator. A method for calculating the quality factor of an arbitrary distribution circuit in real time, designed for anti-islanding tests, is also presented. Experimental results are included demonstrating the use of the PHIL method to test a variety of three-inverter islands.
Inverter Anti-Islanding with Advanced Grid Support in Single- and Multi-Inverter Islands
As PV and other DER systems are connected to the grid at increased penetration levels, island detection may become more challenging for two reasons: 1. In islands containing many DERs, active inverter-based anti-islanding methods may have more difficulty detecting islands because each individual inverter's efforts to detect the island may be interfered with by the other inverters in the island. 2. The increasing numbers of DERs are leading to new requirements that DERs ride through grid disturbances and even actively try to regulate grid voltage and frequency back towards nominal operating conditions. These new grid support requirements may directly or indirectly interfere with anti-islanding controls. This report describes a series of tests designed to examine the impacts of both grid support functions and multi-inverter islands on anti-islanding effectiveness.
Long Island Steatite Sourcing from Rhode Island Quarries During the Late Holocene Era
This study aimed to investigate the possible sourcing of Long Island steatite artifacts from quarries in Rhode Island. Seven steatite samples from archeological sites on Long Island including Nassau County, Suffolk County (North Fork and South Fork) and Shelter Island were compared to two Rhode Island quarry sample putative sources using X-ray fluorescence (XRF) microscopy at the Submicron Resolution X-ray spectroscopy (SRX) beamline of the National Synchrotron Light Source II at Brookhaven National Laboratory. The site-derived steatite samples displayed numerous similarities in elemental composition compared to the two Rhode Island quarries, Oaklawn and Ochee Springs. This study provides new evidence to support the possible sourcing of Long Island steatite from Rhode Island during the late Holocene era.
Nucleation, morphology, and structure of sub-nm thin ceria islands on Rh(111)
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Strategic Environmental Assessment practices in European small islands: Insights from Azores and Orkney islands
The literature concerning Strategic Environmental Assessment (SEA) often refers to the importance of context-specific approaches. However, there is a lack of systematised and consistent studies that enhance tailor-made SEA practices and procedures. Small islands are bounded units of study which may help explore SEA theory and practice in special territories. Small islands present particular features and unique values, such as, small size and population, geographic isolation, limited resources and vulnerable ecosystems. Hence, the main goal of this research was to profile SEA practices and procedures in European small islands and provide a background for future research aiming to improve context-specific SEA applications. To achieve this goal, an exploratory case study was developed using Azores (Portugal) and Orkney (Scotland) archipelagos. An analysis of the corresponding mainland was also carried out to contextualise both case studies. The data collection was achieved through a qualitative content analysis of 43 Environmental Reports. The research found that there is not an SEA context-specific approach used within these European small islands, including guidelines, assessment topics, assessment techniques, follow-up and stakeholders engagement. The debate concerning specific approaches to small islands must be re-focused on the enhancement of SEA capacity-building amongst different stakeholders (including decision-makers), on the development and implementation of collaborative approaches, and on the exchange of knowledge and experiences between small islands networks. - Highlights: • Reviewed the differences between the Portuguese and Scottish SEA system • Showed a low integration of SEA specific features in reports of European small islands • Provides background for future SEA research for small islands approaches.
The transport characteristics of passing fast ions produced by nonlocal overlapping of drift island surfaces and magnetic island surfaces
The structure of the drift-island surface of passing fast ions (PFIs) is investigated in the presence of the resonant interaction with a magnetic island. Two overlapping regions of the drift-island surface and the magnetic island surface are found, one corresponding to local overlapping region and the other to non-local one. Here, the word “nonlocal” denotes that the resonances in the core plasma can have effects on the PFIs near the plasma boundary, while the “local” represents that the PFIs just near the resonant location are influenced. The nonlocal overlapping constructs a transport path along which the PFIs can become losses. There are three kinds of drift-island surfaces to join in forming the transport paths. A pitch angle region, which is called pitch angle gap, is found near the plasma boundary, where the drift-island surface cannot be formed and few PFIs are lost. The pitch-angle selective features of PFI losses are obtained by analyzing the three kinds of drift-island surfaces. The coupling between the crowd drift island surfaces and the collision can induce the prompt losses of PFIs and rapidly slowing down of PFI energy. The time of the prompt losses and the slowing down rate are calculated. Qualitatively, the theoretical results are in well agreement with the experimental observations in ASDEX Upgrade [M. García-Muñoz et al., Nucl. Fusion 47, L10 (2007)].
Long Island Steatite Sourcing from Rhode Island Quarries During the Late Holocene Era
This study aimed to investigate the possible sourcing of Long Island steatite artifacts from quarries in Rhode Island. Seven steatite samples from archeological sites on Long Island including Nassau County, Suffolk County (North Fork and South Fork) and Shelter Island were compared to two Rhode Island quarry sample putative sources using X-ray fluorescence (XRF) microscopy at the Submicron Resolution X-ray spectroscopy (SRX) beamline of the National Synchrotron Light Source II at Brookhaven National Laboratory. The site-derived steatite samples displayed numerous similarities in elemental composition compared to the two Rhode Island quarries, Oaklawn and Ochee Springs. This study provides new evidence to support the possible sourcing of Long Island steatite from Rhode Island during the late Holocene era.
SMALL-SCALE MAGNETIC ISLANDS IN THE SOLAR WIND AND THEIR ROLE IN PARTICLE ACCELERATION. I. DYNAMICS OF MAGNETIC ISLANDS NEAR THE HELIOSPHERIC CURRENT SHEET
Increases of ion fluxes in the keV–MeV range are sometimes observed near the heliospheric current sheet (HCS) during periods when other sources are absent. These resemble solar energetic particle events, but the events are weaker and apparently local. Conventional explanations based on either shock acceleration of charged particles or particle acceleration due to magnetic reconnection at interplanetary current sheets (CSs) are not persuasive. We suggest instead that recurrent magnetic reconnection occurs at the HCS and smaller CSs in the solar wind, a consequence of which is particle energization by the dynamically evolving secondary CSs and magnetic islands. The effectiveness of the trapping and acceleration process associated with magnetic islands depends in part on the topology of the HCS. We show that the HCS possesses ripples superimposed on the large-scale flat or wavy structure. We conjecture that the ripples can efficiently confine plasma and provide tokamak-like conditions that are favorable for the appearance of small-scale magnetic islands that merge and/or contract. Particles trapped in the vicinity of merging islands and experiencing multiple small-scale reconnection events are accelerated by the induced electric field and experience first-order Fermi acceleration in contracting magnetic islands according to the transport theory of Zank et al. We present multi-spacecraft observations of magnetic island merging and particle energization in the absence of other sources, providing support for theory and simulations that show particle energization by reconnection related processes of magnetic island merging and contraction.
Modeling of early stages of island growth during pulsed deposition: Role of closed compact islands
After a brief review of recent modeling of growth during Pulsed Laser Deposition (PLD), we present the study of a role of adatom interactions on growth of surface islands during PLD in submonolayer regime. We employ kinetic Monte Carlo simulation with reversible growth. Attachment of monomers to islands is irreversible at low temperatures while it becomes reversible at higher temperatures, small islands become unstable with growing temperature. In the case of real system we have to take into account not only diffusion of monomers but also diffusivity of dimers and larger clusters and theirs stability. Our new code allows us to study processes which proceed on different time scales which are typical in PLD experiments: fast deposition (on scale order of 10{sup -5} s) during individual pulses, and relaxation of a system between pulses (on scale order of 0.1 s). We calculate and compare the temperature dependence of island density for two modes pulsed deposition and continuous Molecular Beam Epitaxy (MBE) growth. The island densities in PLD mode are substantially higher than in MBE mode, provided the temperature is sufficiently high. In the case of PLD, we observe anomalous temperature dependence of the island density in a certain temperature interval. It is due to the interplay between a cluster decay time and an interval between pulses. The cluster decay time depends not only on temperature but also on clusters size and shape. The anomalous behavior is caused by the temperature limited stability of the closed--compact clusters. This scenario was revealed for the simplified model with only nearest-neighbor interaction. Now, it is elucidated further and we also include interaction to second and third neighbors. We analyze role of the closed-compact surface island in kinetics of both growth modes. Furthermore, by varying interactions energies, diffusion barrier and parameters of deposition, we compare results of simulations with the PLD experiment for Fe/Mo system.