Evolution Of Electron Beam Research Articles

Nearly hyperuniform, nonhyperuniform, and antihyperuniform density fluctuations in two-dimensional transition metal dichalcogenides with defects

Hyperuniform many-body systems in $d$-dimensional Euclidean space are characterized by completely suppressed (normalized) infinite-wavelength density fluctuations, and appear to be endowed with novel exotic physical properties. In this work, we analyze the effects of localized defects on the density fluctuations across length scales and on the hyperuniformity property of experimental samples of two-dimensional transition metal dichalcogenides. In particular, we extract atomic coordinates from time series annular dark field-scanning transmission electron microscopy imaging data of 2D tungsten chalcogenides with the 2H structure showing continuous development and evolution of electron-beam induced defects, and construct the corresponding chemical-bonding informed coordination networks between the atoms. We then compute a variety of pair statistics and bond-orientational statistics to characterize the samples. At low defect concentrations, the corresponding materials are nearly hyperuniform. As more defects are introduced, the (approximate) hyperuniformity of the materials is gradually destroyed, and the system becomes non-hyperuniform even when the material still contains a significant amount of crystalline regions. At high defect concentrations, the structures become antihyperuniform with diverging (normalized) large-scale density fluctuations. Overall, the defected materials possess varying degrees of orientation order, and there is apparently no intermediate hexatic phase emerging. We also construct a minimalist structural model and demonstrate that the experimental samples can be essentially viewed as perturbed honeycomb crystals with small correlated displacements and double chalcogen vacancies.

Electron bunch evolution in laser-wakefield acceleration

We report on systematic and high-precision measurements of the evolution of electron beams in a laser-wakefield accelerator (LWFA). Utilizing shock-front injection, a technique providing stable, tunable and high-quality electron bunches, acceleration and deceleration of few-MeV quasimonoenergetic beams were measured with cutting-edge technology sub-5-fs and 8-fs laser pulses. We explain the observations with dephasing, an effect that fundamentally limits the performance of LWFAs. Typical density dependent electron energy evolution with $57--300\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ dephasing length and 6--20 MeV peak energy was observed and is well described by a parabolic fit. This is a promising electron source for time-resolved few-fs electron diffraction.

Drift frequency of kilometric radio burst type III with solar wind speed and density

Solar radio burst is a signature of solar flare generated by plasma emission mechanism. Radio burst is a diagnostic tool to study electron beam evolution and plasma background properties in corona and interplanetary medium. Sudden release magnetic energy up to 1032 ergs due to magnetic connection accelerates electron located at accelerated site to propagate near relativistic. This fast group of electron propagating through plasma background induced high level of Langmuir wave due to bump in tail instability. Non linear wave-wave causing radio burst near the local plasma frequency. In this paper, frequency drift of radio burst is calculated only from 1 MHz to 400 kHz and the data is obtained from Coordinated Data Analysis Web (CDAweb). The data is analysed by identifying time during maximum intensity of burst at the particular frequency range. Only 45 burst observed in September 2017 is considered in this paper. Meanwhile the data of solar wind speed and density is collected from Advanced Composition Explorer (ACE) satellite. Drift frequency of burst then compared to average of solar wind and density. From the analysis, drift frequency of burst decreasing against solar wind speed and solar wind density. But both relationship shows very weak correlation between drift frequency and solar wind properties.

Spatial Expansion and Speeds of Type III Electron Beam Sources in the Solar Corona

Abstract A component of space weather, electron beams are routinely accelerated in the solar atmosphere and propagate through interplanetary space. Electron beams interact with Langmuir waves resulting in type III radio bursts. They expand along the trajectory and, using kinetic simulations, we explore the expansion as the electrons propagate away from the Sun. Specifically, we investigate the front, peak, and back of the electron beam in space from derived radio brightness temperatures of fundamental type III emission. The front of the electron beam travels at speeds from 0.2c to 0.7c, significantly faster than the back of the beam, which travels at speeds between 0.12c and 0.35c. The difference in speed between the front and the back elongates the electron beam in time. The rate of beam elongation has a 0.98 correlation coefficient with the peak velocity, in line with predictions from type III observations. The inferred speeds of electron beams initially increase close to the acceleration region and then decrease through the solar corona. Larger starting densities and harder initial spectral indices result in longer and faster type III sources. Faster electron beams have higher beam energy densities, and produce type IIIs with higher peak brightness temperatures and shorter FWHM durations. Higher background plasma temperatures also increase speed, particularly at the back of the beam. We show how our predictions of electron beam evolution influences type III bandwidth and drift rates. Our radial predictions of electron beam speed and expansion can be tested by the upcoming in situ electron beam measurements made by Solar Orbiter and Parker Solar Probe.

Microstructure evolution of electron beam welded joints of Ti-43Al-9V-0.3Y and Ti-6Al-4V alloys

Ti-43Al-9V-0.3Y (at%) and Ti-6Al-4V (at%) alloys were successfully welded by electron beam to investigate the microstructure evolution of the joints. The results showed that the microstructure of the weld is comprised of α2-Ti3Al, γ-TiAl, together with a few residual α-TiAl solid solution and B2 phase (ordering by β-TiAl solid solution). The casting structure is formed during the solidification process of the weld, which inevitably causes component segregation in some parts of the joint. In high Al content regions, Al element drives α phase ordering into α2 phase, and eventually introduces the formation of α/γ or α2/γ lamellar structures. Conversely, in some regions with low Al content, α phases hardly experience microstructure evolution and thus remains to room temperature.

Electron beam melting of Inconel 718: effects of processing and post-processing

ABSTRACTThis study reports the effect of process temperature on microstructure evolution of electron beam melted Inconel 718. Samples fabricated at 915°C had fine grain boundary δ (∼200 nm) along with coarse intragranular δ spanning through the length of the grains. On the other hand, samples fabricated at 990°C, only had grain boundary δ along with secondary carbides. During hot isostatic pressing, the distribution of carbides governs the grain growth vs. lack of it. The samples fabricated at 990°C having grain boundary carbides had no grain growth owing to the pinning effect of carbides. In contrast, the sample processed at 915°C had significant grain growth owing to dissolution of grain boundary δ phase and absence of grain boundary carbides.

Evolution of the shape of a high-current and moderate energy electron beam at its propagation through D2 at low pressure

The paper presents the experimental data on the time-spatial evolution of electron beam that is formed and propagates in a dielectric tube of large volume filled with D2 at low pressure of 1-2 Torr. The e-beam is generated by gas discharge in a short gap powered by the stepwise high voltage with amplitude up to 25 kV. Initially, this discharge appears in a strongly over-voltage regime which is unstable and eventually (in several hundreds of ns) transits into abnormal glow discharge at the voltage about of 1.5 - 2.5 kV and the current amplitude of several hundreds of Amps. The every of the mentioned discharge regimes generates the e-beam with energy up to 25 and 2.5 keV respectively. The ability of the over-voltage gap to generate the high- and low-energy e-beams happening one by one can be of great interest for many applications. The features in spatial structures of high-energy and low-energy e-beams in the course of their propagation over long distance through deuterium at low pressure are revealed and discussed.

Temporal evolution of electron beam generated Argon plasma in pasotron device

The plasma- assisted slow wave oscillator (PASOTRON) is a high power microwave source in which the electron beam in the interaction region is confined by the background plasma. The plasma is generated by impact ionization of background gas with the electron beam. A model has been developed for temporal evolution of Argon plasma in pasotron device. In this model, we consider electron beam of energy E interacting with Argon gas. The resulting ionization creates quasi neutral argon plasma composed of argon Ar atoms, singly ionized ions Ar+1and electrons having energy from 0 to E. Electron impact excitation, ionization, radiative decay, radiative recombination and three body recombination processes are considered in this model. Population of ground and excited states of argon atom, ground state of argon ion as well as the population of electron energy groups is calculated by solving time dependent rate equations. Temporal evolution of electron beam generated plasma is given.

High-brightness electron beam evolution following laser-based cleaning of a photocathode

Laser-based techniques have been widely used for cleaning metal photocathodes to increase quantum efficiency (QE). However, the impact of laser cleaning on cathode uniformity and thereby on electron beam quality are less understood. We are evaluating whether this technique can be applied to revive photocathodes used for high-brightness electron sources in advanced x-ray free-electron laser (FEL) facilities, such as the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory. The laser-based cleaning was applied to two separate areas of the current LCLS photocathode on July 4 and July 26, 2011, respectively. The QE was increased by 8--10 times upon the laser cleaning. Since the cleaning, routine operation has exhibited a slow evolution of the QE improvement and comparatively rapid improvement of transverse emittance, with a factor of 3 QE enhancement over five months, and a significant emittance improvement over the initial 2--3 weeks following the cleaning. Currently, the QE of the LCLS photocathode is holding constant at about $1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, with a normalized injector emittance of about $0.3\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ for a 150-pC bunch charge. With the proper procedures, the laser-cleaning technique appears to be a viable tool to revive the LCLS photocathode. We present observations and analyses for the QE and emittance evolution in time following the laser-based cleaning of the LCLS photocathode, and comparison to the previous studies, the measured thermal emittance versus the QE and comparison to the theoretical model.

Evolution of self-injected quasi-monoenergetic electron beams in a plasma bubble

The evolution of electron beam injection and acceleration in the bubble regime of a laser wakefield is studied experimentally. When a 50 TW, 35 fs laser pulse is focused to a spot size of ∼22 μm (FWHM) in a tenuous 1-cm-long helium gas jet, well-collimated (a few milliradians) electron beams with energies in the sub-GeV level are produced. Multiple electron bunches are observed when the laser–plasma channel (i.e., interaction length) is long (<∼1 cm). However, only one bunch is generated when the channel is short (∼2 mm). These experimental results confirm the evolution scenarios of the electron bunches observed in 3D PIC simulations [refer to the publications: [6] N. Hafz et al., Nat. Photon. 2 (2008) 571 and [7] S. Kalmykov et al., Phys. Rev. Lett. 103 (2009) 135004], which suggested that multiple electron injections are a characteristic process for a long acceleration length and an evolving plasma bubble.

Evolution of a high-density electron beam in the field of a super-intense laser pulse

AbstractThe evolution of a high-density electron beam in the field of a super-intense laser pulse is considered. The one-dimensional (1D) theory for the description of interaction, taking into account the space-charge forces of the beam, is developed, and exact solutions for the equations of motion of the electrons are found. It was shown that the length of the high-density electron beam increases slowly in time after initial compression of the beam by the laser pulse as opposed to the low-density electron beam case, where the length is constant on average. Also, for the high-density electron beam, the sharp peak frozen into the density distribution can appear in addition to a microbunching, which is characteristic for a low-density electron beam in a super-intense laser field. Characteristic parameters for the evolution of the electron beam are calculated by an example of a step-like envelope of the laser pulse. Comparison with 1D particle-in-cell simulations shows adequacy of the derived theory. The considered issue is very important for a special two-pulse realization of a Thomson scattering scheme, where one high-intensity laser pulse is used for acceleration, compression and microbunching of the electron beam, and the other probe counter-streaming laser pulse is used for scattering with frequency up-shifting and amplitude enhancement.

Generation of short THz bunch trains in a RF photoinjector

A radiofrequency electron gun joined to a compressor can generate trains of THz sub-picosecond electron pulses by illuminating the photocathode with trains of laser pulses. Assuming a prompt electron emission, each laser pulse generates an electron disk at the cathode. The disk train evolves toward a slightly modulated slug having a peculiar sawtooth energy modulation. This kind of energy modulation is transformed into a density modulation within a velocity-bunching compressor recovering at a good extent the initial multi-disk profile. The electron beam evolution is analyzed through simulations looking at its features as function of pulse and frequency characteristics of the laser and of the accelerator parameters.

Numerical modeling of type III solar radio bursts in the inhomogeneous solar corona and interplanetary medium

The first numerical calculations are presented for type III solar radio bursts in the inhomogeneous solar corona and interplanetary medium that include microscale quasilinear and nonlinear processes, intermediate-scale driven ambient density fluctuations, and large-scale evolution of electron beams, Langmuir and ion-sound waves, and fundamental and harmonic electromagnetic emission. Bidirectional coronal radiation driven by oppositely directed beams is asymmetric between the upward and downward directions due to downward beam narrowing in velocity space, and harmonic emission dominates fundamental emission, consistent with observations and theoretical analysis. In the interplanetary medium, fundamental and/or harmonic emission can be important depending on beam parameters and plasma conditions. Furthermore, Langmuir waves are bursty, ion-sound waves also show some degree of irregularity, while electromagnetic radiations are relatively smooth, all qualitatively consistent with observations. Moreover, the statistics of Langmuir wave energy agree well with the predictions of stochastic growth theory, indicating that the beam-Langmuir wave system evolves to a stochastic growth state.

Self-organization of plasma due to electron beam instability

The evolution of electron beams in dense plasmas has been investigated by using two-dimensional particle-in-cell simulations with mobile and immobile ions, respectively. It is shown that electron beams are split into many filaments at the beginning due to the Weibel instability, and then different filamentation beams attract each other and coalesce. It is observed that there exist two different current density distributions, the Bennett-like equilibrium and the delta-like equilibrium, which are mainly determined by the total beam current. When the initial beam current exceeds Alfvén current, the system approaches a delta-like state. Otherwise, the system approaches a Bennett-like state. The physical mechanism for this phenomenon is pointed out; also the temporal evolution of particle distribution function is discussed.

Numerical Simulations of Type-III Solar Radio Bursts

The first numerical simulations are presented for type-III solar radio bursts in the inhomogeneous solar corona and interplanetary space, that include microscale quasilinear and nonlinear processes, intermediate-scale driven ambient density fluctuations, and large scale evolution of electron beams, Langmuir and ion sound waves, and fundamental and harmonic electromagnetic emission. Bidirectional coronal emission is asymmetric between the upward and downward directions, and harmonic emission dominates fundamental emission. In interplanetary space, fundamental and/or harmonic emission can be important. Langmuir and ion sound waves are bursty and the statistics of Langmuir wave energy agree well with the predictions of stochastic growth theory.

Microstructure evolution of electron beam welded Ti 3Al–Nb joint

The microstructure evolution characterization in high containing Nb, low Al titanium aluminide alloy of electron beam welded joints was investigated by means of OM, SEM, XRD, TEM and microhardness analysis. The results indicated that the microstructure of the weld metal made with electron beam under the welding conditions employed in this work was predominantly metastable, retaining ordered β phase (namely B2 phase), and was independent of the welding parameters but independent of the size and the orientation of the weld solidification structures. As the heat input is decreased, the cellular structure zone is significantly reduced, and then the crystallizing morphology of fusion zone presented dendritically columnar structure. There existed grain growth coarsening in heat affected zone (HAZ) for insufficient polygonization. Both fusion zone (FZ) and the HAZ had higher microhardness than the base metal.

Energetic electron beam generation and anisotropy of hard x-ray emission from 0.9 to 1.0 MA high-Z X pinches

An analysis of the energy, spatial character, and temporal evolution of electron beams and hard x rays from 0.9 to 1.0 MA high-Z X pinches is presented. Experimental results from Ti, Fe, Mo, and W X pinches show that X pinches are an effective source of hard x rays with energies greater than 30 keV. Electron beams with energies up to 2 MeV higher than the applied anode-cathode voltage are generated along the pinch axis before the maximum current is reached. The beams have diameters of about 3 mm and generate bursts of hard x rays with sizes between 1 and 2 mm and total time durations of up to 150 ns. The measured 100–500 keV x-ray distribution is spectrally anisotropic. Hard x-ray synchrotron radiation in the side-on direction is proposed as a possible explanation of this anisotropy.

Simulations of the 100 kW TJNAF FEL using a short Rayleigh length

The TJNAF FEL can be upgraded to operate at 100 kW average power and then explore the use of a short Rayleigh length in order to reduce the power density on the resonator mirrors. The short Rayleigh length can only work with a relatively short undulator. Multimode simulations are used to self-consistently model the optical mode interaction with the electron beam. The steady-state resonator mode is affected by the complex, non-linear electron beam evolution as well as the resonator design.

Evolution of electron beam generated waves resulting in transverse ion heating and filamentation of the plasma

We present here a systematic simulational study on electron beam driven waves and their consequences in terms of plasma electrodynamics. The study is performed by using three‐dimensional particle‐in‐cell code, parallelized to simulate a large volume of plasma. Our simulation shows that an initial electron beam of finite radius with beam velocity along the ambient magnetic field triggers a series of events in the evolution of the waves and the plasma. In the initial stage (t ≤ 200 ωpo−1, ωpo being the electron plasma frequency with the total electron density n0), high frequency waves near ω ∼ ωpo are driven. These waves progressively disappear giving way to the dominance of lower hybrid (LH) waves. The phase of the lower hybrid waves lasts over the time interval . In this time period the LH waves stochastically accelerate ions transverse to B0, and the beam electrons are scattered outside the initial beam volume to occupy the entire volume of the simulation plasma. The ion acceleration leads to the formation of elongated tail in the perpendicular velocity distribution. The large‐amplitude LH waves are seen to undergo a parametric decay into resonance cone waves at frequencies ω &lt; Ωi, the ion cyclotron frequency. Such extremely low‐frequency (ELF) waves are the electrostatic version of the inertial Alfven waves. The phase of the strong LH waves is followed by a stage in which HF waves with frequencies ω ∼ ωpo appear again, but in this stage they are strongly modulated by the already present ELF waves and other low‐frequency waves in the frequency range near the ion cyclotron frequency Ωi and its harmonics. Beginning with the LH wave stage and continuing into the late stages of ELF waves, the plasma density is highly filamented and the filaments oscillate with the ELF frequencies. The relevance of our results to the observations on plasma waves from satellites is brought out.

Fundamental and Harmonic Emission in Type III Solar Radio Bursts – II. Dominant Modes and Dynamic Spectra

Recent theoretical estimates of the emissivity of fundamental and harmonic radiation in type III solar radio bursts are combined with calculations of electron beam evolution, radiation scattering and propagation delays to estimate dynamic spectra at a remote observer. The burst intensity, brightness temperature, temporal evolution, and dominant mode of emission are then calculated. A simple explanation of the recently observed low-frequency cutoff to type III emission is found and it is noted that some type III beams may propagate without significant radio emission. Criteria for observation of harmonic structure in dynamic spectra are also obtained. The results are shown to be consistent with a wide range of observations.