Micro instabilities and rotating spokes in the near-anode region of partially magnetized plasmas

International audienceElectron and ion transport in the near-anode region of a partially magnetized plasma under conditions typical of Hall thrusters or magnetron discharges is studied with fully kinetic, Particle-In-Cell Monte Carlo Collision (PIC-MCC) simulations assuming a uniform magnetic field and no ionization. We derive a simple relation that defines the magnetic field at the transition point between negative and positive sheath. For magnetic fields around or above this transition point, PIC-MCC simulations show the development of short wavelength azimuthal instabilities that cascade to longer wavelengths ("rotating spokes") as the magnetic field is increased. Both short-wavelength and large-wavelength fluctuations can coexist under some conditions. A detailed study of the fluid dispersion relation is used to analyze the PIC-MCC results. Small coherent structures can be associated with the destabilization of ion sound waves by density gradient and collisions. Longer wavelengths or rotating spokes are characteristic of the collisionless Simon-Hoh instability. The small structures are dominant for larger plasma density gradients while the larger structures correspond to smaller density gradients and larger magnetic fields. Anomalous transport associated with these instabilities can be significant, with effective collision frequencies larger than 2 × 10 7 s −1 in xenon for magnetic fields above the transition point

Micro instabilities and rotating spokes in the near-anode region of partially magnetized plasmas

International audienceElectron and ion transport in the near-anode region of a partially magnetized plasma under conditions typical of Hall thrusters or magnetron discharges is studied with fully kinetic, Particle-In-Cell Monte Carlo Collision (PIC-MCC) simulations assuming a uniform magnetic field and no ionization. We derive a simple relation that defines the magnetic field at the transition point between negative and positive sheath. For magnetic fields around or above this transition point, PIC-MCC simulations show the development of short wavelength azimuthal instabilities that cascade to longer wavelengths ("rotating spokes") as the magnetic field is increased. Both short-wavelength and large-wavelength fluctuations can coexist under some conditions. A detailed study of the fluid dispersion relation is used to analyze the PIC-MCC results. Small coherent structures can be associated with the destabilization of ion sound waves by density gradient and collisions. Longer wavelengths or rotating spokes are characteristic of the collisionless Simon-Hoh instability. The small structures are dominant for larger plasma density gradients while the larger structures correspond to smaller density gradients and larger magnetic fields. Anomalous transport associated with these instabilities can be significant, with effective collision frequencies larger than 2 × 10 7 s −1 in xenon for magnetic fields above the transition point

Hunting and migratory movements of white sharks in the eastern North Pacific

The aim of these studies was to determine local movements of hunting white sharks (Carcharodon carcharias) near a seal rookery and global movements during migration. Seven adults were monitored locally with attached ultrasonic tags that received and telemetered animal position and behavior via an array of three-acoustic-positioning (RAP) buoys moored off Ano Nuevo Island, California. Migratory movements of 6 adults departing this island and nearby Southeast Farallon Island were tracked for 2-6 months with attached pop-up satellite archival tags. Sharks began hunting seals at Ano Nuevo Island in October, spending 40% of the day patrolling the 1 km2 receptive field within 400 m of the island at a depth of 30 m or less. For six weeks, they did not stray far or long from the area, were equally active at night as by day, were non-territorial, and fed infrequently. This nearshore phase at both island rookeries ended abruptly in winter as the sharks moved offshore to a region of the subtropical eastern Pacific half way to Hawaii. An adult male went further, traveling to Hawaii where it remained until migrating back to California, only to repeat the journey the following year. Electronic tagging provides vital information on the hunting and migratory behavior of this apex predator

Ocean climate and seal condition

BACKGROUND: The condition of many marine mammals varies with fluctuations in productivity and food supply in the ocean basin where they forage. Prey is impacted by physical environmental variables such as cyclic warming trends. The weaning weight of northern elephant seal pups, Mirounga angustirostris, being closely linked to maternal condition, indirectly reflects prey availability and foraging success of pregnant females in deep waters of the northeastern Pacific. The aim of this study was to examine the effect of ocean climate on foraging success in this deep-diving marine mammal over the course of three decades, using cohort weaning weight as the principal metric of successful resource accrual. RESULTS: The mean annual weaning weight of pups declined from 1975 to the late 1990s, a period characterized by a large-scale, basin-wide warm decadal regime that included multiple strong or long-duration El Niños; and increased with a return to a cool decadal regime from about 1999 to 2004. Increased foraging effort and decreased mass gain of adult females, indicative of reduced foraging success and nutritional stress, were associated with high ocean temperatures. CONCLUSION: Despite ranging widely and foraging deeply in cold waters beyond coastal thermoclines in the northeastern Pacific, elephant seals are impacted significantly by ocean thermal dynamics. Ocean warming redistributes prey decreasing foraging success of females, which in turn leads to lower weaning mass of pups. Annual fluctuations in weaning mass, in turn, reflect the foraging success of females during the year prior to giving birth and signals changes in ocean temperature cycles

Respiration and Heart Rate at the Surface between Dives in Northern Elephant Seals

All underwater activities of diving mammals are constrained by the need for surface gas exchange. Our aim was to measure respiratory rate (fb) and heart rate (fh) at the surface between dives in free-ranging northern elephant seals Mirounga angustirostris. We recorded fb and fh acoustically in six translocated juveniles, 1.8-2. 4 years old, and three migrating adult males from the rookery at Ano Nuevo, California, USA. To each seal, we attached a diving instrument to record the diving pattern, a satellite tag to track movements and location, a digital audio tape recorder or acoustic datalogger with an external hydrophone to record the sounds of respiration and fh at the surface, and a VHF transmitter to facilitate recovery. During surface intervals averaging 2.2+/−0.4 min, adult males breathed a mean of 32.7+/−5.4 times at a rate of 15. 3+/−1.8 breaths min(−)(1) (means +/− s.d., N=57). Mean fh at the surface was 84+/−3 beats min(−)(1). The fb of juveniles was 26 % faster than that of adult males, averaging 19.2+/−2.2 breaths min(−)(1) for a mean total of 41.2+/−5.0 breaths during surface intervals lasting 2.6+/−0.31 min. Mean fh at the surface was 106+/−3 beats min(−)(1). fb and fh did not change significantly over the course of surface intervals. Surface fb and fh were not clearly associated with levels of exertion, such as rapid horizontal transit or apparent foraging, or with measures of immediately previous or subsequent diving performance, such as diving duration, diving depth or swimming speed. Together, surface respiration rate and the duration of the preceding dive were significant predictors of surface interval duration. This implies that elephant seals minimize surface time spent loading oxygen depending on rates of oxygen uptake and previous depletion of stores

Modeling of the plasma jet of a stationary plasma thruster

We have developed a two-dimensional hybrid fluid – particle-in-cell Monte Carlo collisions (PIC- MCC) model to study the plume of a stationary plasma thruster. The model is based on a fluid description of the electrons (the electron density follows a Boltzmann distribution) and a particle description of the ion and neutral transport. Collisions between heavy species are taken into account with a Monte Carlo method. The electric field is obtained from Poisson's equation or from the quasineutrality assumption. We first show that the results from the PIC-MCC model are close to the results of a more time-consuming direct simulation Monte Carlo approach. We then compare the model predictions of the plume density and ion energy distribution with experimental measurements. Finally, we present a brief discussion on the assumptions of the model and on its ability to give reliable predictions on important issues such as the flux of ions backscattered to the satellite. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70320/2/JAPIAU-91-12-9521-1.pd

Predicted Properties of Microhollow Cathode Discharges in Xenon

A fluid model has been developed and used to help clarify the physical mechanisms occurring in microhollow cathode discharges (MHCD). Calculated current-voltage (I-V) characteristics and gas temperatures in xenon at 100 Torr are presented. Consistent with previous experimental results in similar conditions, we find a voltage maximum in the I-V characteristic. We show that this structure reflects a transition between a low-current, abnormal discharge localized inside the cylindrical hollow cathode to a higher-current, normal glow discharge sustained by electron emission from the outer surface of the cathode. This transition, due to the geometry of the device, is a factor contributing to the well-known stability of MHCDs

Semimetalic antiferromagnetism in the half-Heusler compound CuMnSb

The half-Heusler compound CuMnSb, the first antiferromagnet (AFM) in the Mn-based class of Heuslers and half-Heuslers that contains several conventional and half metallic ferromagnets, shows a peculiar stability of its magnetic order in high magnetic fields. Density functional based studies reveal an unusual nature of its unstable (and therefore unseen) paramagnetic state, which for one electron less (CuMnSn, for example) would be a zero gap semiconductor (accidentally so) between two sets of very narrow, topologically separate bands of Mn 3d character. The extremely flat Mn 3d bands result from the environment: Mn has four tetrahedrally coordinated Cu atoms whose 3d states lie well below the Fermi level, and the other four tetrahedrally coordinated sites are empty, leaving chemically isolated Mn 3d states. The AFM phase can be pictured heuristically as a self-doped Cu$^{1+}$Mn$^{2+}$Sb$^{3-}$ compensated semimetal with heavy mass electrons and light mass holes, with magnetic coupling proceeding through Kondo and/or antiKondo coupling separately through the two carrier types. The ratio of the linear specific heat coefficient and the calculated Fermi level density of states indicates a large mass enhancement $m^*/m \sim 5$, or larger if a correlated band structure is taken as the reference

Sinking versus suspended particle size distributions in the North Pacific Subtropical Gyre

The particle size distribution (PSD) is a fundamental property that influences all aspects of phytoplankton ecology. In particular, the size (e.g., diameter d [μm]) and sinking speed w (m/day) of individual particles are inextricable, but much remains unknown about how d and w are related quantitatively for bulk particulate matter. There is significant interest in inferring sinking mass fluxes from PSDs, but doing so requires knowing how both mass and w scale with d . To this end, using both laser diffraction and imaging, we characterized for the first time both sinking and suspended PSDs in the oligotrophic North Pacific subtropical gyre. Comparing these PSDs via a power law parameterization indicates an approximately linear w ‐to‐d scaling, suggesting particles are more fractal‐like than sphere‐like in this respect. This result is robust across multiple instruments, depths, and sediment trap deployments and is made comparatively precise by a high degree of replication