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The long-distance stable transport of relativistic electron beams (REBs) in plasmas is studied by full three-dimensional particle-in-cell simulations. Theoretical analysis shows that the beam transport is mainly influenced by three transverse instabilities, where the excitation of self-modulation instability, and the suppression of the filamentation instability and the hosing instability are important to realize the beam stable transport. By modulating the transport parameters such as the electron density ratio, the relativistic Lorentz factor, the beam envelopes and the density profiles, the relativistic bunches having a smooth density profile and a length of several plasma wave periods can suppress the beam-plasma instabilities and propagate in plasmas for long distances with small energy losses. The results provide a reference for the research of long-distance and stable transport of REBs, and would be helpful for new particle beam diagnosis technology and space active experiments.
Experimental analyses of synthetic jet control (SJC) effects on aerodynamic characteristics of rotor in steady state and in hover were conducted. To ensure the structural strength of rotor and enough interior space for holding the synthetic jet actuators (SJAs), a particular blade with a frame-covering structure was designed and processed, and the experiment was conducted with low free stream velocities and rotor rotation speeds. There were three test conditions. In steady state, there were three free stream velocities (10m/s, 15m/s and 20m/s). In hover state, the rotor was worked with two rotation speeds of 180RPM and 240RPM. In forward flight, the rotor was worked with a rotation speed of 180RPM and a free stream velocity of 7.5m/s. To measure the synthetic jet control effect on rotor in stall, the range of collective pitch was set from 10° to 28° in steady state. The aerodynamic forces and sectional velocity field were measured by using the six-component balance and the Particle Image Velocimetry (PIV) system in the wind tunnel. Flow control effects on the blade based on the synthetic jets (SJ) were experimentally investigated with different jet parameters, such as jet locations, jet angles, and jet velocities. In steady state, the jet closer to the leading edge, and the jet angle of 90° had more advantages in improving the aerodynamic characteristics. Furthermore, the aerodynamic forces and sectional velocity field measurement of rotor in hover were conducted, it showed that SJAs could increase flow velocity at the upper surface, which led to lower upper surface pressure. As a result, the normal forces of rotor with two rotation speeds were increased significantly. These results indicated that the synthetic jet has a capability of increasing the normal force and delaying or preventing the stall of rotor.
Theoretical investigations on ferroelectric tunnel junctions (FTJs) with asymmetric electrodes and a composite barrier are presented. A large tunneling electroresistance effect exists for the Pt/SrTiO3/BaTiO3/SrRuO3 junction; on the other hand, exchange of the dielectric and ferroelectric layer stacking sequence can seriously degrade the performance. These correlations are rationalized by the proposed concept of an asymmetry factor, defined as the ratio between the average barrier heights of FTJs for two opposite polarization orientations. We show that a large asymmetry factor is beneficial to FTJs. This work may provide a way to enhance the performance of FTJs by structure engineering.
In inertial confinement fusion the target implosion non-uniformity is introduced by a driver beams’ illumination non-uniformity, a fuel target alignment error in a fusion reactor, the target fabrication defect, etc. For a steady operation of a fusion power plant the target implosion should be robust against the implosion non-uniformities. In this paper the requirement for the implosion uniformity is first discussed. The implosion uniformity should be less than a few percent. A study on the fuel hotspot dynamics is also presented and shows that the stagnating plasma fluid provides a significant enhancement of vorticity at the final stage of the fuel stagnation. Then non-uniformity mitigation mechanisms of the heavy-ion beam (HIB) illumination are also briefly discussed in heavy ion inertial fusion (HIF). A density valley appears in the energy absorber, and the large-scale density valley also works as a radiation energy confinement layer, which contributes to a radiation energy smoothing. In HIF a wobbling HIB illumination was also introduced to realize a uniform implosion. In the wobbling HIBs illumination, the illumination non-uniformity oscillates in time and space on a HIF target. The oscillating-HIB energy deposition may contribute to the reduction of the HIBs’ illumination non-uniformity by its smoothing effect on the HIB illumination non-uniformity and also by a growth mitigation effect on the Rayleigh–Taylor instability.
Intense Compton-scattering γ-ray radiation driven by laser wakefield acceleration (LWFA) and generation of ultrashort positron beams are investigated by Monte Carlo simulation. Using an LWFA driven GeV electron bunch and a 45 femtosecond, 90 mJ/pulse, and 10 Hz Ti:Sapphire laser for driving the Compton scattering, fs γ-ray pulses were generated. The latter have a flux of ≥108/s, peak brightness of ≥1020 photons/(s mm2 mrad2 0.1% bandwidth), and photon energy of 5.9 to 23.2 MeV. The γ-ray pulses then impinge on a thin high-Z target. More than 107 positrons/s in the form of sub-100 fs pulses at several MeV can be produced. Such ultrashort positron pulses can be useful as the pump-probe type positron annihilation spectroscopy as well as in other applications.
Collimated proton beams from laser interaction with a slab having a hole on its backside are investigated using particle-in-cell simulation. The hot target electrons driven by the laser expand rapidly into the hole. However, at the hole's corners the electrons are strongly compressed and an intense electron jet is emitted from each corner, tightly followed by the ions. The plasma jets focus and collimate along the axis of the hole and can propagate without divergence within the hole. The effect of the hole diameter on the collimated proton beam is considered.
A total of 266 type III bursts observed with the 2.6 - 3.8 GHz high temporal resolution dynamic spectrometer of NAOC during the 23rd solar cycle (from April in 1998 to January in 2003) are statistically analyzed in this present paper. The frequency drift rates (normal and reverse slop), durations, polarizations, bandwidth, starting and ending frequencies are analyzed in detail. From the statistical results of starting and ending frequencies we show that the regions of starting frequencies are very large, which are from less than 2.6 GHz to greater than 3.8 GHz; but the ending frequencies regions are relative concentration, which are from 2.82 GHz to 3.76 GHz. These phenomena mean that the sites of electrons acceleration are quite scatter, while the cutoff regions of the radio type III bursts are in the limiting domain. The bursts number with positive and negative drift rates are nearly equal. This correlation may interpret the suggest that a proportional number of electron beams in the directions of upward and downward are accelerated in the range of 2.6 - 3.8 GHz. The other statistical results are similar to those of decimetric type III bursts as statistics in previous literature. The emission mechanisms of microwave type III bursts are mainly caused by the plasma radiation and electron gyro-maser radiation.
From the statistics of microwave type III bursts and associated coronal mass ejections (CMEs), it is found that the 36% of type III bursts (97) are corresponding to the CMEs for occurring time and site. The correlation between the type III bursts and CMEs is not close, and most type III bursts are occurred in the time regions of 26 – 30 minutes before CMEs. This means that the partial microwave type III bursts may be a precursor of the CMEs.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html
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