Experimental Observation of a Current-Driven Instability in a Neutral Electron-Positron Beam.

We report on the first experimental observation of a current-driven instability developing in a quasineutral matter-antimatter beam. Strong magnetic fields (≥1 T) are measured, via means of a proton radiography technique, after the propagation of a neutral electron-positron beam through a background electron-ion plasma. The experimentally determined equipartition parameter of ε_{B}≈10^{-3} is typical of values inferred from models of astrophysical gamma-ray bursts, in which the relativistic flows are also expected to be pair dominated. The data, supported by particle-in-cell simulations and simple analytical estimates, indicate that these magnetic fields persist in the background plasma for thousands of inverse plasma frequencies. The existence of such long-lived magnetic fields can be related to analog astrophysical systems, such as those prevalent in lepton-dominated jets.

Thin-shell instability in collisionless plasma.

Thin-shell instability is one process which can generate entangled structures in astrophysical plasma on collisional (fluid) scales. It is driven by a spatially varying imbalance between the ram pressure of the inflowing upstream plasma and the downstream's thermal pressure at a nonplanar shock. Here we show by means of a particle-in-cell simulation that an analog process can destabilize a thin shell formed by two interpenetrating, unmagnetized, and collisionless plasma clouds. The amplitude of the shell's spatial modulation grows and saturates after about ten inverse proton plasma frequencies, when the shell consists of connected piecewise linear patches.

Table-top laser-based source of femtosecond, collimated, ultrarelativistic positron beams.

The generation of ultrarelativistic positron beams with short duration (τ(e+) ≃ 30 fs), small divergence (θ(e+) ≃ 3 mrad), and high density (n(e+) ≃ 10(14)-10(15) cm(-3)) from a fully optical setup is reported. The detected positron beam propagates with a high-density electron beam and γ rays of similar spectral shape and peak energy, thus closely resembling the structure of an astrophysical leptonic jet. It is envisaged that this experimental evidence, besides the intrinsic relevance to laser-driven particle acceleration, may open the pathway for the small-scale study of astrophysical leptonic jets in the laboratory.

Time-resolved characterization of the formation of a collisionless shock.

We report on the temporally and spatially resolved detection of the precursory stages that lead to the formation of an unmagnetized, supercritical collisionless shock in a laser-driven laboratory experiment. The measured evolution of the electrostatic potential associated with the shock unveils the transition from a current free double layer into a symmetric shock structure, stabilized by ion reflection at the shock front. Supported by a matching particle-in-cell simulation and theoretical considerations, we suggest that this process is analogous to ion reflection at supercritical collisionless shocks in supernova remnants.

Dynamics of self-generated, large amplitude magnetic fields following high-intensity laser matter interaction.

The dynamics of magnetic fields with an amplitude of several tens of megagauss, generated at both sides of a solid target irradiated with a high-intensity (~10(19) W/cm(2)) picosecond laser pulse, has been spatially and temporally resolved using a proton imaging technique. The amplitude of the magnetic fields is sufficiently large to have a constraining effect on the radial expansion of the plasma sheath at the target surfaces. These results, supported by numerical simulations and simple analytical modeling, may have implications for ion acceleration driven by the plasma sheath at the rear side of the target as well as for the laboratory study of self-collimated high-energy plasma jets.

Weibel-induced filamentation during an ultrafast laser-driven plasma expansion.

The development of current instabilities behind the front of a cylindrically expanding plasma has been investigated experimentally via proton probing techniques. A multitude of tubelike filamentary structures is observed to form behind the front of a plasma created by irradiating solid-density wire targets with a high-intensity (I ~ 10(19) W/cm(2)), picosecond-duration laser pulse. These filaments exhibit a remarkable degree of stability, persisting for several tens of picoseconds, and appear to be magnetized over a filament length corresponding to several filament radii. Particle-in-cell simulations indicate that their formation can be attributed to a Weibel instability driven by a thermal anisotropy of the electron population. We suggest that these results may have implications in astrophysical scenarios, particularly concerning the problem of the generation of strong, spatially extended and sustained magnetic fields in astrophysical jets.

Generation of a purely electrostatic collisionless shock during the expansion of a dense plasma through a rarefied medium.

A two-dimensional numerical study of the expansion of a dense plasma through a more rarefied one is reported. The electrostatic ion-acoustic shock, which is generated during the expansion, accelerates the electrons of the rarefied plasma inducing a superthermal population which reduces electron thermal anisotropy. The Weibel instability is therefore not triggered and no self-generated magnetic fields are observed, in contrast with published theoretical results dealing with plasma expansion into vacuum. The shock front develops a filamentary structure which is interpreted as the consequence of the electrostatic ion-ion instability, consistently with published analytical models and experimental results.

Selective breeding for deficient sensorimotor gating is accompanied by increased perseveration in rats.

Prepulse inhibition (PPI) of the acoustic startle response is a measure of sensorimotor gating that is deficient in some neuropsychiatric disorders, such as schizophrenia and Tourette's syndrome. Experimentally induced PPI deficits in rats are regarded as endophenotype to study the biological mechanisms and therapeutic strategies of these disorders. We have recently shown that selectively breeding rats for high and low PPI levels, respectively, leads to groups with different PPI performance that remains stable from the second generation on. We here tested whether the low PPI is accompanied by other behavioral deficits. Different spatial and operant learning paradigms were used to assess rats' learning and memory abilities as well as their behavioral flexibility. In the delayed alternation T-maze task the two groups did not differ in task acquisition and working memory. Rats with low PPI showed enhanced perseveration during switching between an egocentric and allocentric radial maze task. Enhanced perseveration was also found in an operant behavioral task, where different demands, i.e. a different number of lever presses for a pellet-reward, were assigned to and switched between two levers of a Skinner box. Rats with low PPI stayed longer at the ineffective lever before switching, thus being less able to adjust their behavior to changing reward values. Additionally, PPI low rats had a higher breakpoint value during a progressive ratio-schedule of reinforcement. Rats selectively bred for low PPI showed some cognitive deficits that are apparent in a number of psychiatric disorders with deficient information processing. Specifically in both, spatial and operant behavioral paradigms, PPI low rats are deteriorated in their ability to modulate behavior based upon new changing information. They may thus provide a non-pharmacological model that can be used to evaluate new therapeutic strategies ranging from pharmacological treatment to functional neurosurgery.

Compact x-ray microtomography system for element mapping and absorption imaging.

We have designed and built a compact x-ray microtomography system to perform element mapping and absorption imaging by exploiting scanning fluorescence tomography and full-field transmission microtomography, respectively. It is based on a low power microfocus tube and is potentially appropriate for x-ray diagnostics in space. Full-field transmission tomography yields the three-dimensional inner structure of an object. Fluorescence microtomography provides the element distribution on a virtual section through the sample. Both techniques can be combined for appropriate samples. Microradiography as well as fluorescence mapping are also possible. For fluorescence microtomography a small and intensive microbeam is required. It is generated using a polycapillary optic. Operating the microfocus tube with a molybdenum target at 12 W, a microbeam with a full width at half maximum lateral extension of 16 microm and a flux of about 10(8) photonss is generated. As an example of application, this beam is used to determine the element distribution inside dried plant samples. For full-field scanning tomography, the x-ray optic is removed and the sample is imaged in magnifying projection onto a two-dimensional position sensitive detector. Depending on the sample size, a spatial resolution down to about 10 microm is possible in this mode. The method is demonstrated by three-dimensional imaging of a rat humerus.

Particle simulation of an ultrarelativistic two-stream instability.

A two-stream instability in an unmagnetized plasma is examined by a particle-in-cell simulation. Each beam initially consists of cold electrons and protons that stream at a relative Lorentz factor 100. This is representative for plasma close to the external shocks of gamma-ray bursts. An electrostatic wave develops which saturates by trapping electrons. This wave collapses and the resulting electrostatic turbulence gives an electron momentum distribution that resembles a power law with a spectral break. Some electrons reach Lorentz factors over 1000.

Electron acceleration to energies beyond GeV by a relativistic ion beam instability.

Synchrotron emission suggests the presence of TeV electrons at various astrophysical objects. We propose a mechanism for the acceleration of electrons to ultrahigh energies (UHE) by intense electrostatic waves (ESWs). The latter are driven by dense proton beams that move at relativistic speeds relative to a background plasma and the electrons are accelerated by their nonlinear interaction with the ESWs. We follow the evolution of the wave instability by means of particle-in-cell (PIC) simulations. After the instability has saturated, we obtain spatially confined electron voids in which secondary instabilities develop due to resonant interactions between the beams and the background protons, generating intense ESWs which accelerate electrons to ultrarelativistic speeds within times of a few hundred inverse plasma frequencies.

Connecting shock velocities to electron-injection mechanisms.

Electrons can be accelerated by their interaction with nonlinearly saturated electrostatic waves up to speeds with which they can undergo diffusive acceleration across supernova remnant shocks. Here, we model this wave-electron interaction by particle-in-cell and Vlasov simulations. We find that the lifetime of the saturated wave is considerably longer in the Vlasov simulation, due to differences in how these simulation methods approximate the plasma. Electron surfing acceleration which requires a stable saturated wave may thus be more important for electron acceleration at shocks than previously thought. For beam speeds above a critical value, which we estimate here, both simulation codes exclude surfing acceleration due to a rapid wave collapse.

Surfatron and stochastic acceleration of electrons at supernova remnant shocks.

The surfatron offers the possibility of particle acceleration to arbitrarily high energies, given a sufficiently large system. Surfatron acceleration of electrons by waves excited by ions reflected from supernova remnant (SNR) shocks is investigated using particle simulations. It is shown that surfatron and stochastic acceleration could provide a seed population for the acceleration of cosmic ray electrons at SNR shocks.

Protein size resolution in human eye lenses by dynamic light scattering after in vivo measurements.

Dynamic light scattering (DLS) measurements were conducted with lowest intensity levels on human eye lenses (3.2 mW/cm2) within measurement times of 3-5 s. A characterisation of the human eye under in-vivo conditions along the optical axis is given and a careful interpretation of the data is made, referring not only to in-vitro results of investigated solutions of lens chemistry and various crystallin fractions but also to measurements performed on intact human lenses under various scattering angles. The clinical study was expanded to 79 subjects with ages varying from 9 to 85 years with no serious diseases of the ocular lenses. A normalisation of Scheimpflug photography density data to the data obtained by DLS enables comparison of the two techniques and shows good agreement. The bimodal character of the viscoelastic properties of healthy eye lenses was confirmed; with an assumed viscosity of 2 cP, the mean size parameter of the smaller component is 5.13 +/- 1.6 nm and of the polymeric fraction 690 nm.

Hemiballismus due to contralateral sphenoid ridge meningioma.

Primary intracranial tumour is a very uncommon cause of hemiballismus. In our review of the literature only two verified cases were found: one meningioma and one meningoblastoma. We report a patient with right hemiballismus due to a contralateral meningioma of the sphenoid ridge. His symptoms disappeared completely after resection of the tumour. The patient had no signs of increased intracranial pressure. Possible pathogenic mechanisms are briefly discussed. A consequence of such an experience is that magnetic resonance imaging or computed tomography scans should be made of every patient with uncommon extrapyramidal disorders, particularly if they do not respond to drug therapy.

[The occurrence and significance of enterotoxin-producing Clostridium perfringens strains in the intestinal tract of horses]. / Untersuchungen zum Vorkommen und zur Bedeutung von enterotoxinbildenden Clostridium-perfringens-Stämmen im Darmkanal von Pferden.

100 faecal samples from clinically healthy horses of different age groups and feeding habits, 50 samples of faeces from horses suffering from enteropathy accompanied by diarrhoea and small and/or large intestine from 25 horses that had died after an intestinal disease were examined for the presence of Clostridium (Cl.) perfringens. The frequency with which Cl. perfringens was detected was 22% in clinically healthy horses, 32% in horses with diarrhoea and 52% in the dead horses. In two faecal samples from the horses with diarrhoea the microbial count of Cl. perfringens was ca. 10(6) cfu/g faeces. The occurrence of Cl. perfringens in clinically healthy horses was not influenced by age. In animals fed exclusively on grass silage Cl. perfringens was found more frequently. The enterotoxin of Cl. perfringens was identified in one of 36 faecal samples from horses with diarrhoea. 54 Cl. perfringens strains isolated from material examined were studied for their ability to sporulate and to produce enterotoxin. 98% of the strains formed spores. Enterotoxin production was negative in all cases. Experimental investigations with enterotoxin producing Cl. perfringens strains on four ponies did not show any signs of a possible durable colonization of the intestinal tract by this organism. The results of these investigations do not suggest any pathogenic relevance of enterotoxin producing Cl. perfringens strains to the development of enteropathy in horses.

[Stomach ulcers in the horse--clinical and gastroscopic findings in 12 horses (1989-1990)]. / Magenulzera beim Pferd--klinische und gastroskopische Befunde bei 12 Pferden (1989-1990).

Twelve horses with clinical symptoms of a gastric disorder were studied by gastroscopy. Symptoms of gastric disorders were periprandial colic, bruxism, ructus and reflux. Preliminary to gastroscopy the horses were fasted for 24 h. Access to water was not restricted. The gastroscopy could be conducted easily using a fiberscope 2.5 m in length and 11 mm in outer diameter. While ulcers were present in the squamous fundus of all horses only one horse showed ulceration of the glandular fundus. Solitary ulcers near the margo plicatus were found in horses with mild clinical symptoms. In contrast, diffuse gastroesophageal ulceration was accompanied by severe clinical symptoms. Four horses were affected by an acute gastroesophageal ulceration with gastric reflux and subsequent aspiration pneumonia. Two of those horses suffered from acute gastric ulceration 3-4 days following laparatomy. All horses were treated with cimetidine (5 mg/kg bwt/q.i.d.) until clinical symptoms ceased.

[Fallot's tetralogy in a calf--a case report]. / FALLOT'sche Tetralogie bei einem Kalb--ein Fallbericht.

A report is given on a calf with tetralogy of Fallot. This is the most common cyanotic congenital heart lesion in calves. Diagnosis was made by two-dimensional echocardiography and heart catheterisation. Tetralogy of Fallot was confirmed by necropsy.

Transduction of cellular neo mRNA by retrovirus-mediated recombination.

Transduction of cellular oncogenes by retroviruses is thought to be a multistep process, involving transcriptional activation of a cellular gene by upstream proviral integration and joining of cellular DNA to retroviral transcriptional signals, followed by copackaging and recombination with a helper virus genome during reverse transcription. To examine the molecular mechanism of the reverse transcriptase-mediated recombination, we introduced into mouse fibroblast cells a variety of constructs in which the neo selectable marker was joined to flanking retroviruslike or cell-like sequences. After superinfection and copackaging with a replication-competent Mo-MuLVsupF virus, the formation of recombinant neo transducing viruses was assessed in a second round of virus infection by the ability to confer G418 resistance to infected cells. Our results showed that recombinant neo proviruses were generated from neo RNA containing either a 5' or 3' retroviral end, implying that one recombination event with helper virus RNA was sufficient to incorporate the neo gene into proviral DNA. Recombination occurred with an apparent frequency of 10(-4) to 10(-5) per replication cycle in the absence of homology between the two recombining partners. This frequency, however, increased at least 100-fold if homology was provided at the site of recombination. Our results support the hypothesis that neo-transducing viruses arise via reverse transcriptase-mediated recombination of RNA rather than by recombination proceeding through DNA intermediates. Unexpectedly, removal of the retroviral packaging site psi reduced the number of neo recombinants only slightly. Our data indicated that although RNAs lacking the psi site are poorly packaged into virions, those RNAs that are included in the virions undergo frequent recombination, even if there is no selection for recombination. Many of the neo recombinants formed with the psi- constructs had undergone additional recombinations and often incorporated the psi site from the helper RNA.