Generator of broadband noiselike microwave oscillations on turbulent electron beams

Generator of chaotic microwave oscillations on turbulent electron beams has been studied by experimental and theoretical methods. The presence of a magnetic field with sufficiently large amplitude and period leads to the formation of space charge bunches in the electron beam, which results in the generation of broadband noiselike oscillations.     

An experimental study of electron acceleration with detuning of the bunch repetition frequency from that of an excited wake field

We have experimentally studied the excitation of wake fields in a dielectric structure by a train of relativistic electron bunches and the acceleration of subsequent bunches of the same train due to detuning of the bunch repetition frequency relative to that of the wake field excited in the dielectric structure at the Cherenkov resonance. Electron bunches of the first (leading) part of the train excite the wake wave, and bunches of the second (trailing) part of this train are shifted to the accelerating phase of the wake wave so as to gain additional energy. The possibility of controlling the number (repetition frequency) of bunches exciting the wake field in the dielectric structure and the number of subsequently accelerated bunches has been investigated by changing the value of detuning.     

Charged particles accelerated by wake fields in a dielectric resonator with exciting electron bunch channel

We have studied the acceleration of electrons by wake fields excited in a resonator by a train of electron bunches. The resonator comprised a cylindrical metal waveguide section, containing a dielectric sleeve with a vacuum channel and ends closed by metal walls. Expressions describing the wake field excited by uniformly moving exciting electron bunches have been derived. The self-consistent process of resonator excitation by a train of charged bunches and the particle acceleration in the excited wake field has been numerically simulated.     

Criteria for the optimization of parameters of a multibunch scheme of wakefield acceleration

We propose criteria for the optimization of a wakefield acceleration scheme in a dielectric-lined waveguide structure, which is aimed at increasing the efficiency of energy transfer from the driving to witness (driven) electron bunches with simultaneous increase in the accelerating field gradient. It is suggested to use a multimode acceleration structure excited by a train of short electron bunches profiled in a special manner. The proposed scheme provides for a maximum energy transformer ratio simultaneously with an increase in the wakefield as compared to the values typical of the traditional linear acceleration scheme.     

Compensating for frequency detuning in multibunch wakefield acceleration schemes

The wakefield electron acceleration scheme provides effective means of creating high accelerating field gradients in both dielectric-lined waveguide structures and plasma. We have studied the influence of errors in the structure of this scheme on the efficiency of energy transfer from the driving to witness (driven) electron bunches. Deviation of the wakefield frequency from the calculated optimum value leads to a misfit in the mutual positioning of bunches in the driving train and the corresponding decrease in the energy transformer ratio. It is suggested to compensate for the arising misfit by correcting distances between bunches in the driving train, with a resulting increase in the efficiency of energy transfer. Results of calculations are presented for a dielectric-filled 13-GHz structure with parameters of the Argonne Wakefield Accelerator.     

The status and evolution of plasma Wakefield particle accelerators

The status and evolution of the electron beam-driven Plasma Wakefield Acceleration scheme is described. In particular, the effects of the radial electric field of the wake on the drive beam such as multiple envelope oscillations, hosing instability and emission of betatron radiation are described. Using ultra-short electron bunches, high-density plasmas can be produced by field ionization by the electric field of the bunch itself. Wakes excited in such plasmas have accelerated electrons in the back of the drive beam to greater that 4 G eV in just 10 cm in experiments carried out at the Stanford Linear Accelerator Centre.     

Second generation beatwave experiments at UCLA

The NEPTUNE Laboratory, under construction at UCLA, will be a user facility for exploring concepts useful for advanced accelerators. The primary programmatic goal for the laboratory is to inject extremely high-quality electron bunches into a laser-driven plasma beat wave accelerator and explore ideas for extracting a high-quality ΔE/E < 0.1, < 10 π mm mrad), high-energy (100 MeV) beam from a plasma structure operating at about 1 THz and about 3 GeV/m. The lab will combine an upgraded MARS CO₂ laser and the state-of-the-art SATURNUS RF gun and linac, also undergoing an upgrade. The new MARS laser will be about 1 TW (100 J, 100 ps), up from 0.2 TW (70 J, 350 ps). This allows for doubling the spot size of the laser beam and thereby quadrupling the interaction length while still driving gradients of 3 GeV/m. The large diameter of the accelerating structure relative to the injected electron bunches (10:1 ratio) will minimize the deleterious effects of the radial dependence of the accelerating field and soften the radial focusing thus permitting, in principle, the extraction of a high-quality accelerated beam.     

Limitation of runaway electron beam duration in air-filled gap with inhomogeneous field

Alternative factors that account for a limitation of the period of injection of picosecond runaway electron bunches in air-filled diode with inhomogeneous electric field are analyzed. Experimental data on the characteristics of such electron beams have been obtained under the conditions with variable emissive properties of the cathode, time of the voltage prepulse action, and electric field strength in the region of electron injection. Based on these data, a hypothesis is formulated and justified that the mechanism of limitation related to a transition from the field electron emission to the explosion of microinhomogeneities is less probable than the mechanism of current limitation by a screening plasma cloud formed over the point electron emitters.     

Field ionization effects on ion acceleration in laser-irradiated clusters

Field ionization is included in our two-dimensional particle-in-cell simulations of femtosecond laser pulse interactions with submicron water clusters. Though laser field does not penetrate into the interior of submicron clusters, field ionization is shown to be efficient there due to the electric fields induced by quasineutrality violation due to expelled electrons by laser wave together with hot electron bunches which cross the cluster in the next half-period of the laser. Thus, oxygen atoms are ionized up to Z≥6 in the whole submicron cluster by 40 fs laser pulses of amplitude a₀≥0.5. The impact of oxygen ion charge state on the spectrum of accelerated protons is shown and comparison of the spectrum with simulations with fixed ion charge states is presented.     

Dynamic observation of vortices in superconductors using a holography electron microscope

Individual vortices in superconductors are directly observed with our holography electron microscope. Vortices cannot be viewed as an electron micrograph, but only as a holographic interference micrograph or a defocused micrograph (Lorentz micrograph), since vortices are phase objects to the illumination electron beam. Since the flux itself, and not its replica, is detected, even the dynamic behavior can be observed. The dynamics of vortices in a Nb thin film can be observed in real time when the sample conditions, such as the temperature or the applied magnetic field, change.     

Edge effects in flow around a plasma actuator

We have studied the structure of flow formed in the boundary layer at the lateral edge of the discharge zone of a dielectric barrier discharge plasma actuator. It is established that a region with nonzero component of tangential velocity exists near the plasma layer boundary. At some distance downstream of the actuator, a concentrated vortex is formed with the axis aligned with the flow direction. In the presence of two closely spaced boundaries of the plasma region, a pair of counter-rotating vortices is formed. Separate microdischarges in the plasma layer also appear to be a source of similar longitudinal vortices with smaller amplitude.

     

Volume x-ray emission in gas diodes at atmospheric pressure

The generation of x-rays and high-energy electron beams in gas diodes filled with air and nitrogen at atmospheric pressure has been studied by experimental and theoretical methods. It is established that soft x-ray radiation is not only generated in the region of dense discharge, but is predominantly emitted from a weak-current discharge region. For a high-energy electron beam formation in the gap, the role of the voltage pulse front is not less important than that of the voltage amplitude; the electric field strength at the cathode has an optimum value for the electron beam formation.     

The effect of beam-cavity interaction on the closed orbit and the beam shape

Analytic formulas for the closed orbit distortion (COD) and the beam shape distortion (BSD) due to the beam-cavity interaction are obtained with the cumulative, i.e., the multi-turn effect taken into account. The formulas are extended so that they are applicable to storage rings with many cavities and several counter-rotating bunches. In particular, the COD and the BSD of the bunches at the collision point are estimated for the case of TRISTAN. The effect of the COD and the BSD on the luminosity is found to be small. The beam-cavity interaction causes a difference between the closed orbits of the electron bunches and the positron bunches of almost 10% of the closed orbit due to magnet imperfections.     

Analysis and elimination of the distortion in electro-optic measurement of low-energy subpicosecond electron bunches

Electro-optic (EO) technique is widely used to characterize longitudinal profile of electron bunches. However, electron bunches with a low energy (MeV) and a short time duration (subpicosecond) cannot be well resolved by EO measurement, which leads to distortion. The convolution theorem is proposed to analyze this distortion in EO measurement. And the factors leading to the distortion are discussed, among which the divergent Coulomb field of electrons is the main one. Distortion elimination and reconstruction of electron bunch profiles from detected EO signals is an ill-posed inverse problem. This paper proposes an iterative Tikhonov regularization method to solve this inverse problem and to reconstruct electron bunch profiles from the EO signals detected by the EO measurement. The feasibility of our proposal is tested and numerically verified based on subpicosecond electron bunches with several MeV energy.     

The flow structure of a puff

From time-resolved stereoscopic particle image velocimetry measurements over the entire circular cross section of a pipe, a first-of-its-kind quasi-instantaneous three-dimensional velocity field of a turbulent puff at a low Reynolds number is reconstructed. At the trailing edge of the puff, where the laminar flow undergoes transition to turbulence, pairs of counterrotating streamwise vortices are observed that form the legs of large hairpin vortices. At the upstream end of the puff, a quasi-periodic regeneration of streamwise vortices takes place. Initially, the vortex structure resembles a travelling wave solution, but as the vortices propagate into the turbulent region of the puff, they continue to develop into strong hairpin vortices. These hairpin vortices extract so much energy from the mean flow that they cannot be sustained. This structure provides a possible explanation for the intermittent character of the puffs in pipe flow at low Reynolds numbers.     

液舱晃荡的CIP法数值模拟

数值模拟是求解液舱晃荡问题的重要方法。基于CIP方法,该文建立了有限差分法求解二维不可压缩流体的Navier-Stokes方程的数值模型,使用对时间积分的分步算法求解流场,利用THINC格式捕捉自由液面,将液舱晃荡问题视为包括气体、液体和固体的多相流问题。分别模拟了无隔板和带纵隔板的液舱晃荡。该模型可以模拟晃荡过程中自由面破碎、翻卷和对舱壁的冲击压力以及隔板附近涡的生成与脱落,模型在不同网格的计算结果一致,并将计算结果与试验结果进行了对比。结果表明：该模型可以较好地模拟液舱晃荡问题并且纵隔板对液舱晃荡有明显的抑制作用。     

Carbon nanotube based photocathodes

This paper describes a novel photocathode which is an array of vertically aligned multi-walled carbon nanotubes (MWCNTs), each MWCNT being associated with one p-i-n photodiode. Unlike conventional photocathodes, the functions of photon-electron conversion and subsequent electron emission are physically separated. Photon-electron conversion is achieved with p-i-n photodiodes and the electron emission occurs from the MWCNTs. The current modulation is highly efficient as it uses an optically controlled reconfiguration of the electric field at the MWCNT locations. Such devices are compatible with high frequency and very large bandwidth operation and could lead to their application in compact, light and efficient microwave amplifiers for satellite telecommunication. To demonstrate this new photocathode concept, we have fabricated the first carbon nanotube based photocathode using silicon p-i-n photodiodes and MWCNT bunches. Using a green laser, this photocathode delivers 0.5?mA with an internal quantum efficiency of 10% and an I(ON)/I(OFF) ratio of 30.     

Time-of-Flight Experiments of Vortex Rings Propagating from Turbulent Region of Superfluid 4He at High Temperature

We report the time-of-flight of quantized vortex rings generated by a vibrating wire in superfluid ⁴He which contains normal fluid component. A cover box of vibrating wires and slow cooling of superfluid reduce the number of vortices attached to wire surfaces, enabling us to study vortex rings propagating from a turbulent region. Using two vibrating wires as a generator and a detector of vortices, the time-of-flight of vortices propagating a distance of 0.88 mm was measured at 1.25 K. We find that the time-of-flights distribute from 0.06 s to 27.4 s, much larger than the lifetimes of circular vortex rings limited in the size of a generator amplitude. These results imply that large vortex rings with non-circular shape or vortex tangles are created by the generator, propagating slowly and colliding with the detector before complete disappearance.     

Reconstruction of the Charge Density Wave State Under the Applied Electric Field

Charge density wave (CDW) under an applied electric field in constraint geometry experience stresses, which can easily exceed a plastic threshold. The stress is resolved by the ground state reconstruction which proceed via creation of topological defects like solitons and dislocations??the CDW vortices. These states can be observed experimentally either in average at macroscopic scales of X-ray and multijunction space resolved studies, at mesoscopic scales of coherent X-ray micro-diffraction and nano-junctions or individually as by the STM. Here, we report numerical modeling taking into account multiple fields in their mutual nonlinear interactions: the phase and the amplitude of the CDW order parameter, distributions of the electric field, of the density and the current of normal carriers. Following events of creation and the subsequent evolution of dislocations, we find that vortices are formed in the junction when the voltage across, or the current through, exceed a threshold. The number of vortices remnant in the reconstructed ground state increases stepwise??in agreement with experiments. The vortex core concentrates the voltage drop across the junction giving rise to observed peaks of the interlayer tunneling. The studied reconstruction in junctions of CDWs may be relevant to modern efforts of the field-effect transformations in other correlated electronic systems.