Beam Front Research Articles

Unusual dynamics and nonlinear thermal self‐focusing of initially focused magnetoacoustic beams in a plasma

AbstractUnusual thermal self‐focusing of two‐dimensional beams in plasma which axis is parallel to the equilibrium straight magnetic field is considered. The equilibrium parameters of plasma determine scenario of a beam divergence (usual or unusual) which is stronger as compared with a flow without magnetic field. Nonlinear thermal self‐action of a magnetosonic beam behaves differently in the ordinary and unusual cases. Damping of wave perturbations and normal defocusing in gases leads to reduction of the magnitude of initially planar perturbations at the axis of a beam. Additional thermal self‐focusing nonspecific for gases occurs in plasma under some condition which counteracts this reduction. The theory and numerical examples concern thermal self‐action of initially focused (defocused) magnetosonic beam. Dynamics of perturbations in a beam is determined by dimensionless parameters responsible for diffraction, damping of the wave perturbations, initial radius of a beam's front curvature, and the ratio of viscous to thermal damping coefficients.

Tracking X-Ray Source Movement in a Retracting Flux Tube

Solar flares produce sources of localized, enhanced X-ray emission, thought to be due to the acceleration of nonthermal electrons and the transport of energy away from the reconnection site. The 2002 November 28 C1.6 limb flare showed clear X-ray source motion in the Reuven Ramaty High Energy Solar Spectroscopic Imager observations at 3–10 keV propagating from the apex of the flaring arcade, down toward the footpoints, and then rising back into the corona. Previous work attempted to model this motion using simulations driven by heating with an electron beam or thermal conduction front, finding reasonable agreement only if there were large initial densities. This work extends the previous model by considering a flux tube that retracts through a current sheet away from a magnetic reconnection site. The retraction model includes drag to slow motion in the current sheet, which allows us to vary the energy released by the retraction. This retraction causes a dense and superhot plug of material to form at the loop apex, naturally causing a thermal X-ray source to form in the corona. We find that the observed X-ray source motion, however, is most likely thermal and a signature of the evaporation fronts after initially filling the flux tube.

Intense laser-generated ion beams in plasmas: the rapid heating regime

Previous work (Robinson 2022 Plasma Phys. Control. Fusion 64 105014) has shown that there is a regime of ion beam transport in plasmas where the electron–ion drag becomes so intense that the electron temperature rapidly rises to temperatures at which the electron–ion drag is strongly reduced. This regime is mainly thought to pertain to short-pulse laser-generated ion beams, because of the ion energy fluences required to enter this regime. The regime for was thought to be determined by a dimensionless number, . Here it is shown that there is another dimensionless number that needs to be considered. Numerical simulations show that this dimensionless number, χ h , does not affect the half-energy range of the ion beam, but does determine how strong stopping is at the ion beam front, and thus how rapidly the beam energy falls past the half-energy range.

Multi-layer structure formation of relativistic electron beams in plasmas

A two-dimensional electromagnetic particle-in-cell simulation model is proposed to study the density evolution and collective stopping of electron beams in background plasmas. We show here the formation of the multi-layer structure of the relativistic electron beam in the plasma due to the different betatron frequency from the beam front to the beam tail. Meanwhile, the nonuniformity of the longitudinal wakefield is the essential reason for the multi-layer structure formation in beam phase space. The influences of beam parameters (beam radius and transverse density profile) on the formation of the multi-layer structure and collective stopping in background plasmas are also considered.

Methods of increasing the dielectric strength of the accelerating gap in an electron source with a plasma cathode

This work represents the investigations for decreasing acceleration gap breakdown probability of plasma source of electrons SOLO, with grid stabilization of the boundaries of the arc cathode plasma. We increased the distance to the treated target, bent the transportation channel of the electron beam, created additional plasma in the anode space, and increased the beam front. The effect of the above measures on the breakdown probability when the target is exposed of a low-energy electron beam with a power density of up to 0.5 MW/cm2 with a diameter of 2.5 cm was investigated separately. Beam deflection is most effective at relatively long pulse durations of 150 μs and accelerating voltage of 20 kV, rather than a lower one. It was possible to double the maximum power for the same beam transport length applied to a low-melting target. Preionization in the anode proved to be effective for relatively short beams of 15 μs duration.

Dynamic analysis and vibration reduction of mechanical-hydraulic coupled tunnel boring machine (TBM) main drive system

Tunnel Boring Machine always works in the changeable geologies with multiple drivers, which leads to severe vibration of the TBM main drive system and key component failures. The vibration characteristics of TBM under different working conditions and the vibration reduction analysis have important meanings. First of all, by considering the time-varying random loads of the cutters, the contact force of the gears, the stiffness of the main bearing, and the stiffness of the cylinders, a mechanical-hydraulic coupling nonlinear dynamic model of the TBM main drive system was built according to the assembly relationship and load transmission path of the main drive system. Secondly, the dynamic model of the TBM main drive system is verified by comparing the theoretical vibration with the real vibration of the TBM main drive system. The error of the vibration acceleration is 10% to 30%. Three typical loads are defined under typical working conditions, and the vibrations of the TBM main drive system under three typical loads were analyzed. Finally, the sensitivity analysis of the cylinder damping shows that the damping at the position of the propulsion cylinder has a great influence on the vibration of the TBM main drive system. The results show that when the damping coefficient is 2.5 × 106 N·s/m, the maximum reduction of axial acceleration of cutterhead is 0.64 g, and that of the main beam front section is 0.55 g. The variable damping coefficient vibration reduction strategies under three typical loads are verified.

Thermocapillary dewetting-based dynamic spatial light modulator.

Dynamic spatial light modulators (SLMs) are capable of precisely modulating a beam of light by tuning the phase or intensity of an array of pixels in parallel. They can be utilized in applications ranging from image projection to beam front aberration and microscopic particle manipulation with optical tweezers. However, conventional dynamic SLMs are typically incompatible with high-power sources, as they contain easily damaged optically absorbing components. To address this, we present an SLM that utilizes a viscous film with a local thickness controlled via thermocapillary dewetting. The film is reflowable and can cycle through different patterns, representing, to the best of our knowledge, the first steps towards a dynamic optical device based on the thermocapillary dewetting mechanism.

Dynamic interaction between a laser beam and keyhole front wall during fiber laser welding

A bright spot (laser beam spot-center-induced metal vapor) on the keyhole front wall (KFW) was observed during fiber laser keyhole welding of a ‘sandwich’ specimen to reveal the interaction between the laser beam and the KFW. The dynamic behavior of the bright spot shows that the keyhole formation is the result of periodical drilling of the laser beam spot center on the KFW during welding. Welding parameters have little effect on the drilling period. The drilling speed first increases and then decreases with the increase in the keyhole depth, and the next drilling process begins before the drilling speed reduces to 0 m s−1. The average drilling speed can be increased by increasing the line energy or power density of the laser.

Electron streams in air during magnetic-resonance image-guided radiation therapy.

To investigate the undesired irradiations outside of the treatment field by electron streams in air (air-electron-stream) during magnetic-resonance image-guided radiation therapy (MR-IGRT). A custom-made support phantom adjusting angles between the beam central axis (CAX) and the phantom surface (termed phantom-angles), were used. Using the ViewRay system, a rectangular parallelepiped phantom placed on the support phantom, was irradiated with field sizes of 6.3 cm × 6.3 cm (FS6.3) and 12.6 cm × 12.6 cm (FS12.6) at gantry angles of 0°, 30°, and 330°, and phantom-angles of 10°, 20°, and 30°. For each beam delivery, the isocenter was located at the center of mass of the phantom and 3 Gy was delivered to the isocenter (prescription dose = 3 Gy). The doses given by the air-electron-streams were measured using the EBT3 films on the panels placed orthogonal to the direction of the magnetic field at distances of 10 and 17 cm from CAX. Two dose distributions per irradiation were measured on the panel facing the phantom surface of the incident beam (front panel) and on the panel facing the phantom surface of the beam exit (end panel). We investigated the doses by the air-electron-streams by calculating the average doses inside the circles drawn around a point of the maximum dose with radii of x cm (DRx) from the dose distributions on the panels (x = 1–5 cm). The largest value of DRx was DR1 (1.64 Gy, 55% of the prescription dose) at 10 cm distance from CAX, with FS12.6, at 30° phantom-angle and 330° gantry angle. The average difference of the DR1 at the end panels (FS12.6) between the calculations and measurements was 1.36 Gy. The average global gamma passing rate with 3%/3 mm on the dose distributions at the end panels (FS12.6) was 40.3%. The calculated dose distributions on both panels were not coincident with the measured dose distributions. The Spearman’s rank correlation coefficients between the projected areas and the DRx values were always higher than 0.75 (all with p < 0.001). The doses by the air-electron-streams increased with the projected areas of the cross-sections of the treatment beams on the panels.

Phase-imposed regime of relativistic backward-wave oscillators

We investigate in detail the method of coherent summation of multiple sub-gigawatt Ka-band backward-wave oscillators (BWOs) based on their phase-imposed excitation by an incoming short seed electromagnetic pulse. For theoretical analysis, we use particle-in-cell simulations as well as a basic model that describes both spontaneous and stimulated Cherenkov emission of an electron beam moving in corrugated waveguides. In the scope of the model, the influence of the electron beam front edge length, the phase and the power of the seed pulse, and other conditions important for the performed experiments is studied. The predictions of the model are compared to the experiments in which both superradiant and longer-pulse BWOs are explored. Despite sub-nanosecond duration of the seed pulse, phase stability throughout the entire generated pulse with up to nanosecond duration is demonstrated.

Solar type III radio burst time characteristics at LOFAR frequencies and the implications for electron beam transport

Context. Solar type III radio bursts contain a wealth of information about the dynamics of electron beams in the solar corona and the inner heliosphere; this information is currently unobtainable through other means. However, the motion of different regions of an electron beam (front, middle, and back) have never been systematically analysed before. Aims. We characterise the type III burst frequency-time evolution using the enhanced resolution of LOFAR (LOw Frequency ARray) in the frequency range 30–70 MHz and use this to probe electron beam dynamics. Methods. The rise, peak, and decay times with a ~0.2 MHz spectral resolution were defined for a collection of 31 type III bursts. The frequency evolution was used to ascertain the apparent velocities of the front, middle, and back of the type III sources, and the trends were interpreted using theoretical and numerical treatments. Results. The type III time profile was better approximated by an asymmetric Gaussian profile and not an exponential, as was used previously. Rise and decay times increased with decreasing frequency and showed a strong correlation. Durations were shorter than previously observed. Drift rates from the rise times were faster than from the decay times, corresponding to inferred mean electron beam speeds for the front, middle, and back of 0.2, 0.17, 0.15 c, respectively. Faster beam speeds correlate with shorter type III durations. We also find that the type III frequency bandwidth decreases as frequency decreases. Conclusions. The different speeds naturally explain the elongation of an electron beam in space as it propagates through the heliosphere. The expansion rate is proportional to the mean speed of the exciter; faster beams expand faster. Beam speeds are attributed to varying ensembles of electron energies at the front, middle, and back of the beam.

A 6-Port Two-Dimensional $3\times 3$ Series-Fed Planar Array Antenna for Dual-Polarized X-Band Airborne Synthetic Aperture Radar Applications

A six-port 3×3 series-fed planar antenna array design for dual polarized X-band airborne synthetic aperture radar (SAR) applications is presented in this paper. The antenna array patches interconnected with series feeding one to another as the structure to realize directional radiation and vertical/horizontal polarization. The patch elements are excited through 50 Ω microstrip line feeding in series with quarter wave transformer for good impedance characteristics. A six-port feeding allows selecting the direction of the traveling waves, and consequently the sense of linear (vertical and horizontal) polarization. A prototype of the antenna is fabricated and validated the proposed method. The dimensions of the fabricated prototype 3×3 array antenna are 3.256λg×3.256λg×0.0645λg (λg is guided wavelength at center frequency 9.65 GHz). The measured S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> <; -10 dB reflection bandwidths are 1.4% at each port. Furthermore, an interport and intraport isolation S21<;-25 dB was also achieved across the operation band. The measured peak gains are higher than 11.5 dBi with 90% efficiency for all individual ports/polarizations V1, V2, V3 H1, H2, and H3). In order to satisfy the different performance requirement of series-fed array antenna, such as realized gain, radiation efficiency, side lobe level, half power beam width and front to back ratio are also examined. Measured results of an antenna prototype successfully validate the concept. The proposed antenna is found suitable for dual polarized airborne SAR applications.

Fast Rise Time High Current Electron Beam: Emission, Acceleration, and Drift Motion

Fast processes associated with emission, acceleration, and drift motion of a high current, moderately magnetized electron beam (particles energy ~300 keV; current ~3 kA) were studied experimentally when tubular explosive-emission cathode was supplied by subnanosecond rise time voltage pulse. Kinematic effect causes the sharpening of the observed beam front which was proven by particle-in-cell numerical modeling. The angular structure of tubular beam was studied through the current waveform records from collector probe after beam propagation through a radial-slit collimator. Current waveforms had time resolution no worse than 10 ps and provided the analysis of the beam temporal structure after its acceleration as well as in the process of further electrons drift motion in a finite guiding magnetic field.

Effects of width and density of supersonic molecule beam on penetration depth of tokamak

The penetration depth and the fueling efficiency of the supersonic molecular beam injection (SMBI) are affected by both the intrinsic parameters of the SMBI and the parameters of background plasma. The purpose of the present paper is to explore the possible methods of improving the fueling efficiency of SMBI by varying the beam parameters. The penetration depths and the transport processes of SMBI with different beam densities and different beam widths are studied using the trans-neut module of the three-dimensional (3D) edge turbulence simulation code BOUT++. In our present study, the number of the injected molecules per unit time the injection speed and the injected flux are kept constant throughout the SMB fueling process, but the beam density and beam width are adjusted. The simulation is based on the real magnetic configuration of the HL-2A tokamak. Our results indicate that the deeper injection depth can be obtained with a supersonic molecular beam (SMB) with smaller density and larger width. However, the injection depth decreases when the beam density or the beam width increases. The residence time of the beam front can be lengthened by increasing the beam density and widening the beam width. If the beam density increases or the beam width enlarges, not only the injection depth decreases, but also the residence time shortens. The front of the atom density exhibits the behaviors analogous to that of the SMB, namely, both its depth and its residence time decreases with beam density increasing and beam width decreasing. At the same time, the dissociation rate has a larger range in the spatiotemporal coordinate. The global growth of dissociation rate is inhibited by the molecular dissociation localization. However, the localization of the molecular dissociation accelerates the local growth of the dissociation rate, and the global growth of the molecular dissociation rate is promoted. When the promoting effect is dominant, under the condition of constant flux and fixed injection speed, the smaller molecular injection width will lead to the shallower molecular penetration depth. The simulation results suggest that if we attempt to promote the fueling efficiency and to increase the injection depth of SMBI, we should utilize the SMBI with a smaller density and larger beam width. Of course, the concrete influences of the SMBI on injection depth and fueling efficiency should be studied further by varying other relevant parameters of the SMB and the backgroud plasma.

Optimized stability of a modulated driver in a plasma wakefield accelerator

AbstractWe analyze the transverse stability for a configuration of multiple Gaussian bunches subject to the self-generated plasma wakefield. Through a semi-analytical approach we first study the equilibrium configuration for the modulated beam and then we investigate the evolution of the equilibrium configuration due to the emittance-driven expansion of the beam front that results in a rigid backward shift. The rear-directed shift brings the modulated beam out of the equilibrium, with the possibility for some of the bunch particles to be lost with a consequent deterioration of the driver. We look therefore for the proper position of the single bunches that maximize the stability without severely affecting the accelerating field behind the driver. We then compare the results with three-dimensional particle in cell simulations.

Investigation of the Squeeze Film Dynamics Underneath a Microstructure With Large Oscillation Amplitudes and Inertia Effects

This paper presents direct simulation Monte Carlo (DSMC) numerical investigation of the dynamic behavior of a gas film in a microbeam. The microbeam undergoes large amplitude harmonic motion between its equilibrium position and the fixed substrate underneath. Unlike previous work in literature, the beam undergoes large displacements throughout the film gap thickness and the behavior of the gas film along with its impact on the moving microstructure (force exerted by gas on the beam's front and back faces) is discussed. Since the gas film thickness is of the order of few microns (i.e., 0.01 &lt; Kn &lt; 1), the rarefied gas exists in the noncontinuum regime and, as such, the DSMC method is used to simulate the fluid behavior. The impact of the squeeze film on the beam is investigated over a range of frequencies and velocity amplitudes, corresponding to ranges of dimensionless flow parameters such as the Reynolds, Strouhal, and Mach numbers on the gas film behavior. Moreover, the behavior of compressibility pressure waves as a function of these dimensionless groups is discussed for different simulation case studies.

Electron Properties in Collisionless Mesothermal Plasma Expansion: Fully Kinetic Simulations

This paper presents a fully kinetic particle-in-cell simulation of collisionless mesothermal plasma expansion with a focus on the macroscopic electron properties. The results show that the electron thermal properties are anisotropic and nonuniform. In the beam core region, the electrons are thermalized due to interactions between the trapped electrons and the potential well between the beam exit and the beam front. The electron expansion inside the beam is equilibrium and significant cooling occurs associated with the expansion. Immediately out of the beam boundary, the electrons undergo an isothermal expansion. In the outer expansion region, the electrons transition into a nonequilibrium expansion and again exhibit significant cooling. We find that the commonly used assumption of Boltzmann electron simplification is not valid for modeling electrons in mesothermal plasma expansion. An analysis of the electron temperature and density correlation along each individual ion streamlines suggests that simplified models for electron dynamics may be constructed using multiple polytropic laws in different plume regions.

First boundary electrical feedthroughs for the heating neutral beams injectors of ITER

The first boundary feedthroughs of the ITER heating neutral beam (HNB) injectors are key components of the neutral beam system. They allow the penetration of signals and power from/to the beam line components (BLC) and front end components (FEC), through the confinement boundary of ITER.The design status of the two types of feedthroughs is presented in the first part. This includes the design requirements, cabling strategy, the analysis done and the maintenance strategies. The second part deals with the qualification of the feedthroughs. Being made of brittle material and not covered by mechanical standard a qualification by testing strategy is proposed based on existing rules and norms.The design of the feedthroughs is almost finished and satisfies all the requirements. The next step is the procurement for their integration in the MITICA [1] test bed and the detailed definition of the qualification tests to be carried out for the HNB feedthroughs.

Spatial-temporal evolution of the current filamentation instability

The spatial-temporal evolution of the purely transverse current filamentation instability is analyzed by deriving a single partial differential equation for the instability and obtaining the analytical solutions for the spatially and temporally growing current filament mode. When the beam front always encounters fresh plasma, our analysis shows that the instability grows spatially from the beam front to the back up to a certain critical beam length; then the instability acquires a purely temporal growth. This critical beam length increases linearly with time and in the non-relativistic regime it is proportional to the beam velocity. In the relativistic regime the critical length is inversely proportional to the cube of the beam Lorentz factor . Thus, in the ultra-relativistic regime the instability immediately acquires a purely temporal growth all over the beam. The analytical results are in good agreement with multidimensional particle-in-cell simulations performed with OSIRIS. Relevance of the current study to recent and future experiments on fireball beams is also addressed.

Amplification of Weibel instability in the relativistic beam–plasma interactions due to ion streaming

On the basis of a three-dimensional relativistic electromagnetic particle-in-cell (PIC) code, we have analyzed the Weibel instability driven by a relativistic electron–ion beam propagating into an unmagnetized ambient electron–ion plasma. The analysis is focused on the ion contribution in the instability, considering the earliest evolution in shock formation. Simulation results demonstrate that the Weibel instability is responsible for generating and amplifying the small-scale, fluctuating, and dominantly transversal magnetic fields. These magnetic fields deflect particles behind the beam front both perpendicular and parallel to the beam propagation direction. Initially, the incoming electrons respond to field fluctuations growing as the result of the Weibel instability. Therefore, the electron current filaments are generated and the total magnetic energy grows linearly due to the mutual attraction between the filaments, and downstream advection of the magnetic field perturbations. When the magnetic fields become strong enough to deflect the much heavier ions, the ions begin to get involved in the instability. Subsequently, the linear growth of total magnetic energy decreases because of opposite electron–ion currents and topological change in the structure of magnetic fields. The ion current filaments are then merged and magnetic field energy grows more slowly at the expense of the energy stored in ion stream. It has been clearly illustrated that the ion current filaments extend through a larger scale in the longitudinal direction, while extension of the electron filaments is limited. Hence, the ions form current filaments that are the sources of deeply penetrating magnetic fields. The results also reveal that the Weibel instability is further amplified due to the ions streaming, but on a longer time scale. Our simulation predictions are in valid agreement with those reported in the literature.