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Shocks in supersonic flows offer both high density and sharp density gradients that are used, for instance, for gradient injection in laser-plasma accelerators. We report on a parametric study of oblique shocks created by inserting a straight axisymmetric section at the end of a supersonic "de Laval" nozzle. The effect of different parameters, such as the throat diameter and straight section length on the shock position and density, is studied through computational fluid dynamics (CFD) simulations. Experimental characterizations of a shocked nozzle are compared to CFD simulations and found to be in good agreement. We then introduce a newly designed asymmetric shocked gas jet, where the straight section is only present on one lateral side of the nozzle, thus providing a gas profile well adapted for density transition injection. In this case, full-3D fluid simulations and experimental measurements are compared and show excellent agreement. [ABSTRACT FROM AUTHOR]

The electric and magnetic properties of microbridges made from 440 nm thick Y–Ba–Cu–O films prepared on Al2O3 substrates with CeO2 sublayers by the pulsed injection metal organic chemical vapor deposition (MOCVD) method were studied in temperatures ranging from 20 K to 300 K in order to investigate the possibilities of using these microbridges as fast fault current limiters. The application of an external magnetic field () causes the flux flow induced magneto-resistive effect, which is proportional to and to at low and high temperatures, respectively. The experimentally obtained S-shaped I–V characteristic and the change of the microbridge resistance over time when affected by a step-like magnetic field can be well explained using a thermo-electrical model based on Joule heating and the flux flow nature of the resistive state (RS). The transition from the superconducting to the RS, studied using ns duration rectangular waveform electrical pulses able to create up to ≈2106 A cm−2 current densities without rises of microbridge temperatures, demonstrated that the I–V characteristics of the RSs consisting of an assembly of straight lines corresponding to the different pinning centers. At temperatures close to the critical temperature of superconductivity, the number of these lines increases, and the I–V characteristics can be described with high accuracy by a power law. It was concluded that microbridges made from these films can be used as protectors against complicated waveform electromagnetic pulses having short rise times, high voltage peak amplitudes and long low-value over-current 'tails'. [ABSTRACT FROM AUTHOR]

Laser–plasma acceleration at kilohertz repetition rates has recently been shown to work in two different regimes with pulse lengths of either 30 fs or 3.5 fs. We now report on a systematic study in which a large range of pulse durations and plasma densities were investigated through continuous tuning of the laser spectral bandwidth. Indeed, two laser–plasma accelerator (LPA) processes can be distinguished, where beams of the highest quality, with a charge of 5.4 pC and a spectrum peaked at 2–2.5 MeV, are obtained with short pulses propagating at moderate plasma densities. Through particle-in-cell (PIC) simulations, the two different acceleration processes are thoroughly explained. Finally, we proceed to show the results of a 5-h continuous and stable run of our LPA accelerator accumulating more than 18 × 10 6 consecutive shots, with a charge of 2.6 pC and a peaked 2.5 MeV spectrum. A parametric study of the influence of the laser driver energy through PIC simulations underlines that this unprecedented stability was obtained thanks to micro-scale density gradient injection. Together, these results represent an important step toward stable laser–plasma accelerated electron beams at kilohertz repetition rates. [ABSTRACT FROM AUTHOR]

The results of fabrication of nanostructured La1−xSrxMnO3 films grown by pulsed injection metalorganic chemical vapour deposition technique onto special disordered glass-ceramics substrate are presented. Two groups of films were produced in order to determine what would be their main physical properties when certain fabrication changes were introduced: (1) films with thicknesses in the range of 25-900 nm, which were grown at a temperature of 750 °C; and (2) films having thicknesses of 400 nm, which were grown at different deposition temperatures ranging from 600 °C up to 775 °C in steps of 25 °C. It was determined that the morphology and microstructure of the films depends on the thicknesses of these films and their deposition temperatures. The thinnest films (25 nm) grew mainly in amorphous phase, while the thicker films had well-pronounced structure made of column-shaped crystallites. These had average column widths of 40-65 nm, were spread throughout the whole thickness of the films, and were separated by 5-10-nm-thick grain boundaries. The influence of growth conditions on the colossal magnetoresistance effect in these films was studied in pulsed magnetic fields of up to 20 T. The dependences of the magnetoresistance on the magnetic flux density were analysed using modified Mott’s hopping model. It was demonstrated that these nanostructured films behave as superparamagnetic materials with reduced magnetic properties due to disordered grain boundaries. The obtained results allow the tuning of the resistivity, magnetoresistance and the anisotropy of La1−xSrxMnO3 manganite films, which were deposited onto glass-ceramic substrates, and thus to use them for the fabrication of high pulsed magnetic field sensors. [ABSTRACT FROM AUTHOR]

The electrical field-induced changes of the yeast Saccharomyces cerevisiae cells permeabilization to tetraphenylphosphonium (TPP+) ions were studied using square-shaped, nanosecond duration high power electrical pulses. It was obtained that pulses having durations ranging from 10 ns to 60 ns, and generating electric field strengths up to 190 kV/cm significantly (up to 65 times) increase the absorption rate of TPP+ ions without any detectible influence on the yeast cell viability. The modelling of the TPP+ absorption process using a second order rate equation demonstrates that depending on the duration of the pulses, yeast cell clusters of different sizes are homogeniously permeabilized. It was concluded, that nanosecond pulse-induced permeabilization can be applied to increase the operational speed of whole cell biosensors. [ABSTRACT FROM AUTHOR]

We report on the use of the colossal magnetoresistance (CMR) effect in manganites for the measurement of pulsed magnetic fields up to the megagauss limit. To increase the application range in a magnetic field, we fabricated nanostructured La-Sr-Mn-O films consisting of nanocrystallites cummulated into clusters separated by highly amorphous inter-cluster boundaries. We demonstrate that the CMR effect does not saturate in these films at 77 K up to 91.4 T. Moreover, the magnetoresistance behavior at 290 K shows that nanostructured manganite films are promising candidates for the development of magnetic field scalar sensors operating in wide field and temperature ranges. [ABSTRACT FROM AUTHOR]

A new fast high-power fault current limiter made from epitaxial and polycrystalline \La0.83\Sr0.17\MnO3 thin films and operating at 80 K has been demonstrated. An epitaxial film that exhibits a current-induced increase in its resistance is connected in series with a 50-\Omega-impedance transmission line, while a polycrystalline film whose resistance decreases with an increase in current is connected in parallel. It has been demonstrated that, when the fault current pulse has an amplitude of 200 V, a rise time of 0.5 ns, and a pulselength of 18 ns, the limitation of its amplitude is 3.5 dB after the rise time and 8 dB at the end of the pulse. [ABSTRACT FROM AUTHOR]

The following peculiarities of electron transport in In0:53Ga0:47As/In0:52Al0:48As quantum wells with δ-Si-doped In0:52Al0:48As barriers at high electric fields are discovered: (1) an enhancement of electron mobility by inserting the InAs phonon wall into the In0:53Ga0:47As/In0:52Al0:48As quantum well, as well as increasing the InAs content in the modulation-doped In0:8Ga0:2As/In0:7Al0:3As heterostructure; (2) a large decrease in electron mobility and a change of electron density with increasing electric field in the range of 1-4 kV/cm; (3) a magnetic field dependence of the threshold electric field for intervalley scattering of electrons; and (4) microwave current oscillations in high electric fields. [ABSTRACT FROM AUTHOR]