Velocity Spread Research Articles

Design and simulation of a W‐band gyrotron oscillator based on self‐consistent nonlinear theory

AbstractBased on the self‐consistent nonlinear theory, a W‐band continuous wave 30 kW fundamental harmonic gyrotron oscillator is designed and simulated. With a 30.0 kV, 3.0 A electron beam, the output power of more than 35 kW is obtained and the maximum electronic efficiency is equal to 40.3%. The effects of electron beam voltage, beam current, magnetic field intensity, guiding center radius, and velocity spread on electron efficiency and output power are analyzed in detail. In addition, the images of correlative electron clustering states in the process of beam‐wave interaction are presented intuitively.

Design and Simulation of Coaxial Magnetron Injection Gun for a 170-GHz MW-Class Gyrotron

This article presents a design of the triode coaxial magnetron injection gun (CMIG) for a 170-GHz MW-class gyrotron. The trade-off equations have been used to initialize the design of gun geometry and beam parameters. The operating mode of gyrotron is TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">34,11</sub> at fundamental harmonic. The 3-D electron tracing code CST Particle Studio has been employed to simulate and optimize the CMIG design. An optimum electron beam with transverse to longitudinal velocity ratio α = 1.3 and transverse velocity spread ββ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</sub> = 3.29% has been obtained. The sensitivity analysis of electron gun parameters are also investigated including geometric deviations and misalignments.

Directional spreading of a viscous droplet on a conical fibre

If a droplet is placed on a substrate with a conical shape it spontaneously starts to spread in the direction of a growing fibre radius. We describe this capillary spreading dynamics by developing a lubrication approximation on a cone and by the perturbation method of matched asymptotic expansions. Our results show that the droplet appears to adopt a quasi-static shape and the predictions of the droplet shape and spreading velocity from the two mathematical models are in excellent agreement for a wide range of slip lengths, cone angles and equilibrium contact angles. At the contact line regions, a large pressure gradient is generated by the mismatch between the equilibrium contact angle and the apparent contact angle that maintains the viscous flow. It is the conical shape of the substrate that breaks the front/rear droplet symmetry in terms of the apparent contact angle, which is larger at the thicker part of the cone than that at its thinner part. Consequently, the droplet is predicted to move from the cone tip to its base, consistent with experimental observations.

Design and simulation investigations of high-power millimeter-wave gyrotron

ABSTRACTIn this paper, a high power millimeter wave gyrotron cavity was modelled and studied for its beam-wave interaction, beam-misalignment, and thermal issues. The sub-assemblies including the magnetron injection gun (MIG), RF output window, and electron beam collector were investigated. The MIG has produced an annular electron beam with transverse velocity spread less than ∼4%. The Particle–In–Cell simulation predicted an RF output of 470 kW in TE10,4 mode at ∼139.52 GHz for a DC drive of 80 kV, 20 A. The temperature on the inner, outer surfaces of the cavity and the corresponding average radial expansion of the cavity were calculated. Due to the radial expansion of the cavity, the resonant frequency and RF output power has changed to ∼0.514 GHz and ∼50 kW, respectively. Finally, the overall efficiency of the present gyrotron was improved from ∼29% to ∼47% by using a single–stage depressed collector.

Comparative analysis of terahertz fourth-harmonic gyro-TWAs and multiplying gyro-TWAs with axis-encircling electron beams

The harmonic operation in a terahertz gyrotron traveling-wave amplifier (gyro-TWA) permits a reduced magnetic field, whereas a harmonic multiplying gyro-TWA enables magnetic field reduction and frequency multiplication. This study comparatively analyzes the 400-GHz fourth-harmonic gyro-TWAs and fourth-harmonic multiplying gyro-TWAs with axis-encircling electron beams. This property of the gyro-TWAs with axis-encircling electron beams enables us avoid the appearance of the majority of the competing modes. According to the simulation results obtained using multi-mode codes, the attenuating severs suppress the remaining competing modes. In the case of a harmonic gyro-TWA containing a sever section, the copper-section start-oscillation length is significantly influenced by a decrease in the sever-section radius when compared with an increase in the sever-section length. Furthermore, a stable fourth-harmonic TE41-mode gyro-TWA containing sever sections is proposed. We propose fourth-harmonic TE41-mode multiplying gyro-TWAs, where the drive stages operate in the fundamental-harmonic TE11 mode to reduce the frequency of the drive wave. We subsequently develop a stable fourth-harmonic multiplying gyro-TWA with sever sections in the drive and amplified stages to avoid the competing modes and enhance the output power. The stable high-gain fourth-harmonic gyro-TWA can yield a peak output power of 2.7 kW at 400.6 GHz with a saturated gain of 75 dB and a bandwidth of 0.7 GHz for a 75-kV and 2-A electron beam with an axial velocity spread of 3%. Furthermore, a peak output power of 1.7 kW can be obtained by the fourth-harmonic multiplying gyro-TWA at 400.4 GHz with a saturated gain of 57 dB and a bandwidth of 0.4 GHz. Subsequently, we calculate the power and gain scaling for the harmonic gyro-TWA and harmonic multiplying gyro-TWA.

Dissipation-Induced Luttinger Liquid Correlations in a One-Dimensional Fermi Gas.

We study a one-dimensional Fermi gas in the presence of dissipative coupling to environment through the Lindblad equation. The dissipation involves energy exchange with the environment and favours the relaxation of electrons to excitations. After switching on the dissipation, the system approaches a steady state, which is described by a generalized Gibbs ensemble. The fermionic single particle density matrix resembles deceivingly to that in a hermitian interaction quench. It decays inversely with the distance for short times due to the fermionic correlations in the initial state, which changes into a noninteger power law decay for late times, representing dissipation-induced Luttinger liquid behavior. However, the crossover between the two regions occurs due to dissipation-induced damping, and is unrelated to the propagation of excitations. The velocity of information spreading is set by the dissipative coupling, and differs significantly from the original sound velocity. The thermodynamic entropy grows as -t ln t initially, and saturates to an extensive value. Our results can be tested experimentally in one-dimensional Dirac systems.

Splat formation mechanism of droplet-filled cold-textured groove during plasma spraying

In this study, a numerical simulation and experiments were performed to analyze the formation process of splat in the laser texture groove of filling molybdenum droplets during atmospheric plasma spraying. Texture grooves were extracted using a 3D surface topography instrument, and the heat transfer and phase transition processes of droplet hydrodynamics were analyzed. A 3D numerical model of the droplet-filling texture groove was developed using a commercial software. The droplet diameter, impact velocity, and temperature were set to 50 μm, 110 m/s, and 2917 °C, respectively, to fill the cold-textured grooves. The cross-sectional morphology and element distribution of the filled coating were characterized using scanning electron microscopy and energy dispersive spectroscopy, respectively. Numerical results are consistent with those of the experiment. This study reveals the splat formation mechanism of droplet impact, spreading, and solidification in textured grooves and the grooves’ wall temperature. Results show that the fastest spreading velocity of the droplet contact groove is 4.63 times that of the initial impact velocity. The groove wall has a maximum melting depth of 2.6 μm. In the process of droplet spreading, the fluid layer gradually solidifies, and the transient spreading is accompanied by solidification.

Asynchronism of the spreading dynamics underlying the bursty pattern**Project supported by the National Natural Science Foundation of China (Grant Nos. 61703281, 11547040, 61803266, 61503140, and 61873171), the PhD Start- Up Fund of Natural Science Foundation of Guangdong Province, China (Grant Nos. 2017A030310374 and 2016A030313036), the Science and Technology Innovation Commission of Shenzhen, China (Grant No. JCYJ20180305124628810), and the China Scholarship

The potential mechanisms of the spreading phenomena uncover the organizations and functions of various systems. However, due to the lack of valid data, most of early works are limited to the simulated process on model networks. In this paper, we track and analyze the propagation paths of real spreading events on two social networks: Twitter and Brightkite. The empirical analysis reveals that the spreading probability and the spreading velocity present the explosive growth within a short period, where the spreading probability measures the transferring likelihood between two neighboring nodes, and the spreading velocity is the growth rate of the information in the whole network. Besides, we observe the asynchronism between the spreading probability and the spreading velocity. To explain the interesting and abnormal issue, we introduce the time-varying spreading probability into the susceptible-infected (SI) and linear threshold (LT) models. Both the analytic and experimental results reproduce the spreading phenomenon in real networks, which deepens our understandings of spreading problems.

Spreading speed in a farmers and hunter-gatherers model arising from Neolithic transition in Europe

We study spreading phenomena in a two-component reaction-diffusion system, modeling the Neolithic transition from hunter-gatherer societies to agricultural societies in Europe. When farmer populations were introduced into the region inhabited by hunter-gatherer populations, farmer populations spread as an expanding wave through Europe. The speed of the wave is observed to tend to be a constant. We analytically compute the spreading velocity of the model, and characterize the longtime behavior of solutions of the model. Without comparison principle, we introduce a new technique that can locate the spreading front in reaction-diffusion systems.

High-resolution imaging of molecular collisions using a Zeeman decelerator.

We present the first crossed beam scattering experiment using a Zeeman decelerated molecular beam. The narrow velocity spreads of Zeeman decelerated NO (X2Π3/2, j = 3/2) radicals result in high-resolution scattering images, thereby fully resolving quantum diffraction oscillations in the angular scattering distribution for inelastic NO-Ne collisions and product-pair correlations in the radial scattering distribution for inelastic NO-O2 collisions. These measurements demonstrate similar resolution and sensitivity as in experiments using Stark decelerators, opening up possibilities for controlled and low-energy scattering experiments using chemically relevant species such as H and O atoms, O2 molecules, or NH radicals.

Current and Future Spatiotemporal Patterns of Lyme Disease Reporting in the Northeastern United States

The incidence and geographic range of Lyme disease continues to increase in the United States because of the expansion of Ixodes scapularis, the species of tick that is the main Lyme disease vector. Currently, no dynamic model for the disease spread exists that integrates information of both acarological and human case surveillance data. To characterize the spatiotemporal spread of Lyme disease in humans among counties in US endemic regions. This cross-sectional study modeled the spread of Lyme disease county-level case reporting, accounting for county-level demographic factors, environmental factors associated with tick presence and human exposure, and the spatiotemporal association between counties. The analyses were conducted between January and August 2019. The setting was 1405 counties in the following regions of the United States: West North Central, East North Central, New England, Middle Atlantic, and the South. Assessments were based on publicly available Lyme disease case data reported to the US Centers for Disease Control and Prevention (CDC) between January 2000 and December 2017. Probability of reporting the first case of Lyme disease by county by year. Between 2000 and 2017, a total of 497 569 Lyme disease cases were reported to the CDC in the study area. Reporting a first case of Lyme disease was associated with a county's and county's neighbors' forest coverage, elevation, percentage of population living in the wildland-urban interface, tick presence, county's population size, proportion of neighbors reporting cases, and neighbors' years since first reporting. The model that included these variables showed high predictive power, with a mean area under the receiver operating characteristic curve of 81.1 (95% CI, 68.5-86.2). The model predicted the first reported Lyme disease case a mean (SD) of 5.5 (3.5) years earlier than was reported to the CDC, with a mean spread velocity estimated at 27.4 (95% CI, 13.6-54.4) km per year. Among 162 counties without reported cases, 47 (29.0%) had a high probability of reporting Lyme disease by 2018. The estimated mean time lag between the first reported case in a neighboring county and any county was 7 (95% CI, 3-8) years. This study's findings suggest that, if updated regularly and expanded geographically, this predictive model could enable states and counties to develop more specific Lyme disease prevention and control plans, including improved sensitization of the general population and medical community.

Kinematic unrest of low mass galaxy groups

In an effort to better understand the formation of galaxy groups, we examine the kinematics of a large sample of spectroscopically confirmed X-ray galaxy groups in the Cosmic Evolution Survey with a high sampling of galaxy group members up to z = 1. We compare our results with predictions from the cosmological hydrodynamical simulation of HORIZON-AGN. Using a phase-space analysis of dynamics of groups with halo masses of M200c ∼ 1012.6 − 1014.50 M⊙, we show that the brightest group galaxies (BGG) in low mass galaxy groups (M200c &lt; 2 × 1013 M⊙) have larger proper motions relative to the group velocity dispersion than high mass groups. The dispersion in the ratio of the BGG proper velocity to the velocity dispersion of the group, σBGG/σgroup, is on average 1.48 ± 0.13 for low mass groups and 1.01 ± 0.09 for high mass groups. A comparative analysis of the HORIZON-AGN simulation reveals a similar increase in the spread of peculiar velocities of BGGs with decreasing group mass, though consistency in the amplitude, shape, and mode of the BGG peculiar velocity distribution is only achieved for high mass groups. The groups hosting a BGG with a large peculiar velocity are more likely to be offset from the Lx − σv relation; this is probably because the peculiar motion of the BGG is influenced by the accretion of new members.

Theory Analysis of Nonlinear Excitation of Second Harmonic THz Gyrotron for DNP-NMR

As an important parameter, start current reveals the excitation condition of cavity modes. Normally, the method of successive iterations with one mode associated in the calculation is used to get the start current, which just reflects the self-excitation conditions in the presence of single mode. Nevertheless, in many cases, the prior-excited mode may affect the start current of the subsequent ones. In this article, we re-derived electron motion equations with two modes involved in the calculation and calculated the start current numerically based on those equations. We analyzed start currents of operation mode TE26+ and parasitic modes of a 394-GHz gyrotron with the two methods. In the calculation using the conventional method, TE26+ satisfies self-excitation condition while TE26− does not. In the calculation using the latter method, the start current of TE26− decreases under the influence of TE26+. This nonlinear oscillation of TE26− is further observed in time domain multimode simulation. In practical application, the beam velocity spread, ohmic loss, and high-frequency loss are inevitable. We studied their effects on mode interaction and then optimized the operation parameters. When the equivalent conductivity is $1.7e^{7}~\text {s/m}$ and beam velocity spread is 0.03, the optimized output power is 149.2 W.

Effects of electric current and sample orientation on flame spread over electrical wires

Electrical wire is one of the primary causes of electrical fires. Experiments were conducted to study the effects of electric current and sample orientation on flame spread over polyethylene (PE) insulated copper wires. The flame spread velocity showed a non-monotonic trend with the inclination angle, i.e., the flame spread velocity firstly decreased and then increased with the increasing inclination angle. In the upward flame spread configuration, the molten polymer flew down along the wire if the wire was inclined, which resulted in a periodic flame spread phenomenon, and a sub-flame was observed behind the primary flame. The periodic time was shorter for the smaller inclination angle or the lower electric current. The downward spread seemed to be more susceptible to the electric current. A flame spread model considering the effects of inclination angle and electric current was developed. The heat transfer analyses in the preheating region were conducted to study the controlling mechanism of flame spread.

Design and analysis on a 0.1‐THz second harmonic low‐voltage, low‐current gyrotron with complex cavity

AbstractThis article presents the design and analysis of a complex cavity that will be used in a 10 kW‐level 94 GHz second harmonic low‐current, low‐voltage gyrotron. The mode pair of TE+7.2/TE+7.3 is selected as the operating modes of the complex cavity. In cold‐cavity design, a throat is introduced in the complex cavity to increase the diffractive quality factor, which can make the gyrotron operate at low‐current and low‐voltage conditions. In hot‐cavity analysis, the beam‐wave interaction efficiency affected by different factors has been studied in detail. At the operating point with beam voltage of 32 kV and beam current of 1.5 A, the mode competition has been studied based on the analysis of starting current and a time‐dependent multifrequency code; the transverse velocity spread influence on the efficiency has also been investigated. The results indicate that the operating mode eventually dominates the mode selection, while the competing modes are all at noise level. Meanwhile, the interaction efficiency can keep more than 30% with maintaining the spread below 8%, and the output power can hold at 10 kW‐level when the spread is below 12%.

Experimental Investigation of Thermophysical Properties and Combustion Characteristics of Thickened Jet Fuel

ABSTRACT The thermophysical properties and combustion characteristics of PIB thickened jet fuel are investigated based upon both aims of expanding basic combustion theory (i.e., the coupled gas phase and liquid phase flows and heat transfer involving flame spread over liquid fuel) and potential practical applications (i.e., the oil additive in two-stroke gasoline engine and in-situ burning of leaked oils). The effects of PIB on the thermophysical parameters of jet fuel including density, flashpoint, viscosity and surface tension are experimentally measured. The logarithm of the dynamic viscosity varies linearly with the proportion of PIB, which is in good agreement with predictions. Then, the impact of PIB on fire safety of jet fuel is examined through flame spread and ignition experiments. The temperature evolution, flame spread rate, length and velocity of subsurface flow are quantitated and analyzed. The addition of PIB into jet fuel reduces the flame spread rate, but promotes the ignitability. The viscosity augment plays the principal role in hindering flame spreading over thickened jet fuel.

Nonadiabatic Effects on Beam-Quality Parameters for Frequency-Tunable Gyrotrons

We propose an unconventional electron gun structure in which the emitter is located on a concave cathode surface with a non-uniform electric field. Such a design violates the intuition that an emitter should place close to a uniform electric field to reduce the velocity spread. The commonly employed design guide based on the adiabatic condition predicts a huge velocity spread of 24%, but the simulation using EGUN code and verified with CST particle studio shows a very low spread of 2.8%. Examining the magnetic moment and the kinetic energy of the beam reveals that the electrons experience a relatively long acceleration process due to the much weak electric field. That's why the non-adiabatic effect matters. In addition to the cyclotron compression and the E$\times$B drift, the "resonant" polarization drift plays a crucial role in reducing the overall velocity spread.Simulations show a decent beam quality with the pitch factor of 1.5 and the transverse velocity spread of 2.8% over a wide range of the magnetic field (7.4-8.0 T) and the beam voltage (12-22 kV) with a high structural tolerance. The promising results with the wide working range enable the development of continuous frequency-tunable gyrotrons.

Conditions for the Stable Acceleration of Ion Rings by Collapsed Liners

The conditions of suppressing the collective slowing-down of an ionic beam on electrons, which is caused by the excitation of electron plasma oscillations (beam instability) by ions, are found. In contrast to available explicit and implicit indications, a large spread of the energies only in an ionic beam is insufficient for this suppression. The acceleration of ions is shown to become stable when a sufficiently large spread of the electron velocities is simultaneously present. The beam instability of ions is suppressed by the Landau damping of the ion-excited plasma waves at electrons. The results obtained are used to analyze the possibility of ion acceleration by collapsed cylindrical plasma liners.

Chapter 7 Quick clay behaviour in sensitive Quaternary marine clays – a UK perspective

Abstract The term quick clay has been used to denote the behaviour of highly sensitive Quaternary marine clays that, due to post depositional processes, have the tendency to change from a relatively stiff condition to a liquid mass when disturbed. On failure these marine clays can rapidly mobilise into high velocity flow slides and spreads often completely liquefying in the process. For a clay to be defined as potentially behaving as a quick clay in terms of its geotechnical parameters it must have a sensitivity (the ratio of undisturbed to remoulded shear strength) of greater than 30 together with a remoulded shear strength of less than 0.5 kPa. The presence of quick clays in the UK is unclear, but the Quaternary history of the British islands suggests that the precursor conditions for their formation could be present and should be considered when undertaking construction in the coastal zone.

Study of the beam-wave interaction of a 0.34-THz confocal waveguide gyro-TWT with confocal beam

In this paper, in order to enhance the efficiency of the confocal gyrotron travelling wave tube (confocal gyro-TWT), the confocal beam is first introduced based on the field distribution of the operating mode in the confocal waveguide. Besides, a confocal gyro-TWT with a confocal beam operating at 0.34 THz is simulated and designed based on linear and nonlinear theories. It is verified that the confocal beam can enhance the interaction efficiency and output power significantly through simulation. In addition, the effects of electron beam parameters on output power are also analyzed in this paper. The simulation results indicate that the confocal gyro-TWT can operate at TE06 stably with an output power of 17.3 kW when the beam voltage is 60 kV, beam current is 2 A, input power is 40 mW, and transverse velocity spread is 5%. The corresponding frequency and interaction efficiency are 0.34 THz and 14.4%, respectively.