Electron beam-plasma discharge in GDT mirror trap: Experiments on plasma start-up with electron gun

The paper describes experiments on the injection of an electron beam into a gas at the Gas Dynamic Trap (GDT) and develops a technique for creating a starting plasma with parameters sufficient for its subsequent heating by neutral beams. It is found that a relatively thin electron beam is capable of ionizing plasma in the entire volume of the trap, and the plasma turbulence it excites is capable of accelerating some of the electrons to energies tens of times higher than the initial energy of the beam. It is shown that, in contrast to early experiments on tabletop open traps, collective beam relaxation under GDT conditions occurs in the vicinity of the entrance magnetic mirror. Since the electron cyclotron frequency in this region significantly exceeds the plasma frequency, it is necessary to study the mechanism of a beam-plasma discharge under these conditions. As a first step along this path, we measure the radial diffusion coefficient of fast particles, as well as the rate at which they gain energy.

Ultrastructure of intracytoplasmic Rickettsia-like infection of the gills of the teleost Archosargus probatocephalus (Sparidae) in northeastern Brazil

A histopathological survey was conducted to investigate the presence of microparasites in fish Archosargus probatocephalus in a river near Maceió, Brazil. Light microscope observations of fragments of gill showed the presence of small cysts containing numerous myxospores that were morphologically identified as Henneguya. Transmission electron microscopy observations further revealed several gill cells containing groups of prokaryotic cells within large cytoplasmic vacuoles. Each infected host cell displayed a single vacuole containing a variable number of Rickettsia-like cells (up to 11), some of which presented the dumbbell shape characteristic of binary fission. The Rickettsia-like cells were pleomorphic, without a nucleus and with chromatin dispersed in the cytoplasm. They had a thin electron-dense wall of Gram-negative type. The morphology of these prokaryotic was similar to those of the order Rickettsiales and was described as a Rickettsia-like organism. Histopathological evaluation showed that several vacuole membranes had a lysed appearance. Some had ruptured, thus allowing direct contact between the Rickettsia-like organism and the cytoplasm of the host cell. The rupturing of the branchial epithelium may have contributed towards reduction of the surface area of the gills, but it is not possible to say that this was the cause of the host’s death.

Mind the gap: micro-expansion joints drastically decrease the bending of FIB-milled cryo-lamellae

Cryo-focussed ion beam (FIB)-milling of biological samples can be used to generate thin electron-transparent slices from cells grown or deposited on EM grids. These so called cryo-lamellae allow high-resolution structural studies of the natural cellular environment by in situ cryo-electron tomography. However, the cryo-lamella workflow is a low-throughput technique and can easily be obstructed by technical issues like the bending of the lamellae during the final cryo-FIB-milling steps. The severity of lamella bending seems to correlate with shrinkage of the EM grid support film at cryogenic temperatures, which could generate tensions that may be transferred onto the thin lamella, leading to its bending and breakage. To protect the lamellae from these forces, we milled “micro-expansion joints” alongside the lamellae, creating gaps in the support that can act as physical buffers to safely absorb material motion. We demonstrate that the presence of such micro-expansion joints drastically decreases lamella bending. Furthermore, we show that this adaptation does not create instabilities that could constrain subsequent parts of the cryo-lamella workflow, as we obtained high-quality Volta phase plate tomograms revealing macromolecules in their natural structural context. The minimal additional effort required to implement micro-expansion joints in the cryo-FIB-milling workflow makes them an easy solution against cryo-lamella bending in any biological sample milled on EM grids.

Ultrastructural characterization of Acarispora falculifera n.gen., n.sp., a new microsporidium (Opisthokonta: Chytridiopsida) from the feather mite Falculifer rostratus (Astigmata: Pterolichoidea)

Only about 20 species of microsporidia have been described from mites. All except one species produce typical spores with a long polar filament and a polaroplast. This paper is the first study of an atypical microsporidium infection in a feather mite (Falculifer rostratus). The infection of the pigeon feather mite is restricted to the colon epithelium where it leads to hypertrophy of the concerned cells. During sporogony, a multinucleate plasmodial aggregate is formed within a sporont (endogenous sporogony resulting in a polysporophorous vesicle). The cisterns delimiting the single sporoblasts later form the spore walls. Sporogonial stages are in direct contact to the host cell cytoplasm. Merogonial stages were not present. Spores are tiny (3.6 μm × 2.6 μm), broad oval in form and monokaryotic. The spore wall of mature spores consists of a three-layered endospore and a thin, electron-dense, wavy exospore. The polar filament is anisofilar and completely coiled in 3-4 turns. In cross-sections, it has a star-like appearance because the electron-dense core forms rounded compartments of lucent material at its surface. In superficial sections, this results in a honeycomb-like pattern. A polaroplast is missing. The polar filament arises subapically at a polar sac that lacks an internal anchoring disk. These atypical spore structures clearly classify the species from the feather mite as a member of the order Chytridiopsida. It could not be clearly affiliated to one of the known genera, so we created a new genus, Acarispora, with the species A. falculifera.

In-situ TEM studies of ion-irradiation induced bubble development and mechanical deformation in model nuclear materials

ABSTRACTThe MIAMI* facility at the University of Huddersfield is one of a number of facilities worldwide that permit the ion irradiation of thin foils in-situ in a transmission electron microscope. MIAMI has been developed with a particular focus on enabling the in-situ implantation of helium and hydrogen into thin electron transparent foils, necessitating ion energies in the range 1 – 10 keV. In addition, however, ions of a variety of species can be provided at energies of up to 100 keV (for singly charged ions), enabling studies to focus on the build up of radiation damage in the absence or presence of implanted gas.This paper reports on a number of ongoing studies being carried out at MIAMI, and also at JANNuS (Orsay, France) and the IVEM / Ion Accelerator Facility (Argonne National Lab, US). This includes recent work on He bubbles in SiC and Cu; the former work concerned with modification to bubble populations by ion and electron beams and the latter project concerned with the formation of bubble super-lattices in metals.A study is also presented consisting of experiments aimed at shedding light on the origins of the dimensional changes known to occur in nuclear graphite under irradiation with either neutrons or ions. Single crystal graphite foils have been irradiated with 60 keV Xe ions in order to create a non-uniform damage profile throughout the foil thickness. This gives rise to varying basal-plane contraction throughout the foil resulting in almost macroscopic (micron scale) deformation of the graphite. These observations are presented and discussed with a view to reconciling them with current understanding of point defect behavior in graphite.*Microscope and Ion Accelerator for Materials Investigations

Numerical investigation for shape controlling of ultrathin electron layer

To control the shape of the ultra-thin electron layer produced by directly interaction of ultrahigh contrast laser with ultra-thin foil target, we investigated the spacial distribution and temporal evolution of electron layers produced from single and double foil targets through two-dimensional particle-in-cell simulations. Results show that electron layers produced from double foil targets can fly with unperturbed velocity for a much longer time than in the single foil case, which can be explained by the integrated contribution of charge separation field from both the two foils. Further studies show that through adjusting the foil expansion, electron layers with different shapes can be obtained. Detailed studies on the forming process of layers show that electron momentum distribution evolves rapidly along with the pump laser and then the vanishing of electron transverse momentum induced by the reflected laser results in the forming of layer shape. So different foil expansion corresponds to different moments that reflected laser interact with electron layer, when the electron transverse momentum distribution is different. After the reflected laser interact with electron layer, the resultant longitude momentum distribution will finally lead to various electron layer shapes.