Vacuum Gap Research Articles

Anodic Glow and Conductive Channel Formation in Single and Double Long Vacuum Gaps

The evolution processes of vacuum breakdowns in single- and double-break vacuum interrupters are reported in this paper, in order to reveal the formation mechanisms for the conductive channels in vacuum arcs. Breakdown experiments in two vacuum interrupters containing 10-30 mm gaps with single-break and double-break are studied. The light emission during breakdowns is observed by an intensified charge coupled device camera, with the current and voltage waveforms being recorded simultaneously. According to the results, there is no difference between the evolution processes of breakdowns in small gaps and large gaps. The establishment of the conductive channel and sustaining of the vacuum arc can be achieved by the effect of the cathode without any contribution from the anodic glow. In the double-break interrupter, the transition from insulation to conduction for the two serial connected gaps starts simultaneously but evolves independently.

Electron-beam interaction with emission-line clouds in blazars

Context. An electron-positron beam escaping from the magnetospheric vacuum gap of an accreting black hole interacts with recombination-line photons from surrounding gas clouds. Inverse-Compton scattering and subsequent pair production initiate unsaturated electromagnetic cascades exhibiting a characteristic spectral energy distribution. Aims. By modelling the interactions of beam electrons (positrons) with hydrogen and helium recombination-line photons, we seek to describe the spectral signature of beam-driven cascades in the broad emission-line region of blazar jets. Methods. Employing coupled kinetic equations for electrons (positrons) and photons including an escape term, we numerically obtained their steady-state distributions and the escaping photon spectrum. Results. We find that cascade emission resulting from beam interactions can produce a narrow spectral feature at TeV energies. Indications of such an intermittent feature, which defies an explanation in the standard shock-in-jet scenario, have been found at ≈ 4σ confidence level at an energy of ≈ 3 TeV in the spectrum of the blazar Mrk 501. Conclusions. The energetic requirements for explaining the intermittent 3 TeV bump with the beam-interaction model are plausible: Gap discharges that lead to multi-TeV beam electrons (positrons) carrying ≈ 0.1% of the Blandford-Znajek luminosity, which interact with recombination-line photons from gas clouds that reprocess ≈ 1% of the similar accretion luminosity are required.

Effect of CO Molecule Orientation on the Reduction of Cu-Based Nanoparticles.

The adsorption of CO on the surface of Cu-based nanoparticles was studied in the presence of an external electric field by means of scanning tunneling microscopy (STM) and spectroscopy (STS). Nanoparticles were synthesized on the surface of a graphite support by the impregnation–precipitation method. The chemical composition of the surface of the nanoparticles was determined as a mixture of Cu2O, Cu4O3 and CuO oxides. CO was adsorbed from the gas phase onto the surface of the nanoparticles. During the adsorption process, the potential differences ΔV = +1 or −1 V were applied to the vacuum gap between the sample and the grounded tip. Thus, the system of the STM tip and sample surface formed an asymmetric capacitor, inside which an inhomogeneous electric field existed. The CO adsorption process is accompanied by the partial reduction of nanoparticles. Due to the orientation of the CO molecule in the electric field, the reduction was weak in the case of a positive potential difference, while in the case of a negative potential difference, the reduction rate increased significantly. The ability to control the adsorption process of CO by means of an external electric field was demonstrated. The size of the nanoparticle was shown to be the key factor affecting the adsorption process, and particularly, the strength of the local electric field close to the nanoparticle surface.

Analysis of the Optimal Thickness and the Heat Transmission for the Triple Glazing System with Vacuum and Carbon Dioxide Gaps

Analysis of the Optimal Thickness and the Heat Transmission for the Triple Glazing System with Vacuum and Carbon Dioxide Gaps

Performance optimization of graphene thermionicdevices based on charge and heat transport

In recent years, researchers have proposed a model of graphene thermionic energy converter (GTEC) for the utilization of high-grade thermal energy, which is used to extensively study the physical mechanism and parametric optimization. However, the influences of space charge accumulation and near-field radiative effects on the GTEC’s energy conversion performance are rarely reported. In the present work, the theories of thermionic emission, Langmuir space charge, non-equilibrium thermodynamics, and fluctuating electrodynamics are used to construct an improved model, in which the coupling effects of thermionic transport, near-field radiative heat transfer, and Newton heat transfer are considered. Firstly, the dependence of additional potential barrier, current density, power density, efficiency, and heat flows on the voltage and the vacuum gap are analyzed by neglecting the Newton heat transfer. The results show that the vacuum gap has a significant influence on the power density, while it has a negligible effect on the efficiency, the optimal power density and efficiency can be obtained at two different voltages. Secondly, the variations of power density and efficiency with voltage are analyzed on condition that the electrodes’ temperatures are restricted by the energy balance equation. It is found that Newton heat transfer has a significant influence on the power density, while it has a negligible effect on the conversion efficiency; the anode’s temperature at the optimal power density is higher than the ambient temperature, and the temperature at the optimal efficiency is close to the ambient temperature; the optimal regions of voltage, vacuum gap, and anode’s temperature are determined by considering the trade-off between power density and efficiency. The results obtained in this work can provide a theoretical basis for the development of practical devices.

Some features of discharge initiation in vacuum gap by optical range radiation

The hypothesis of discharge initiation in vacuum gap by optical range radiation based on pre-viously obtained experimental data. During the laser pulse interaction withelectrode erosionproductsthe glow discharge has ignited. In result of ionization-overheating instability the dis-charge has had current channel contraction and has transferred to arc. Thedependences ofmaterial of target thermo dynamical parameters on theminimal and thresholdlaser pulse en-ergyhave demonstrated. The threshold laser pulse energy –the energy which enough to effec-tive impact on the laser plasma.

Effect of Magnetic Field on Different Opening Processes of Multi-Break Vacuum Gaps

Vacuum gap saturation of single break can be solved by multiple breaks in series. In different breaking processes, the unbalanced energy and charges injected into the breaks lead to unbalanced voltage distribution. The purpose of this paper was to compare the voltage distribution under the regulation of magnetic field in different opening process. The magnetic fields with 3 ms width were applied at the point 3 ms before current through zero. Comparing with no axial magnetic field, the voltage distribution unbalances deceased both in synchronous and asynchronous opening processes. Under the condition that the arcing time of high-voltage break remained at 6.6 ms and the magnitude of axial magnetic fields was 60 mT, voltage distributions of high-voltage break were approximately proportional to the arcing times of the low-voltage break from 3.4 ms to 6.6 ms. It demonstrated that axial magnetic fields could effectively improve the voltage distribution of multi-break vacuum circuit breakers in synchronous and asynchronous opening processes.

Thermodynamic performance of near-field electroluminescence and negative electroluminescent refrigeration systems

Electroluminescent (EL) and negative electroluminescent (NEL) devices are radiative thermoelectric energy converters that use electric power for refrigeration. For the EL system, we apply a forward bias to the emitter that we want to cool, whereas a reverse bias voltage is applied to the hot absorber for the NEL system. In this work, we derive the thermodynamic limits of the cooling power density and coefficient of performance (COP) of near-field EL and NEL refrigeration systems based on entropy analysis that considers near-field effects. We show numerically that operating the EL and NEL systems in the near-field regime could increase the cooling power density and the COP bounds to a certain extent. As the vacuum gap decrease from 1000 to 10 nm, the near-field effects improve the performance of the NEL system all the time, but the performance of the EL system increases to the optimal value and then decreases. In addition, the increase in temperature difference weakens the performance of both refrigeration systems greatly. Moreover, we also investigate the effects of the absence of sub-bandgap thermal radiation on the performance of the EL and NEL systems. Our work indicates significant opportunities for evaluating the performance of near-field radiative thermoelectric energy converters from the perspective of thermodynamic limits. Meanwhile, these results establish the targets for cooling power density and COP of the near-field EL and NEL systems.

Approximate theoretical analysis of gas penetration in metal micro spallation

After high pressure shock, the shock wave in the metal is unloaded at the metal-gas interface, and micro spallation occurs when the metal melts. When the micro spallation develops to a certain extent, the high pressure gas penetrates the zero pressure vacuum gap between the metal melt droplets. In this paper, the phenomenon of gas penetrating metal micro spallation zone is analyzed theoretically. Based on the regular hexahedron periodic arrangement of metal droplets, the calculation formulas of the maximum penetration depth, the sealing time of the penetration channel and the maximum mass of the gas penetrating the metal micro spallation zone are given through theoretical analysis under the quasi-static and semi-dynamic conditions. The quasi-static process is considered to be the gas penetration process that can be approximated as the escape process of gas into the vacuum, and the gap in the metal micro spallation zone will be filled with gas. The semi-dynamic analysis is based on two basic assumptions: one is the equal droplet size and spacing in the micro spallation zone and the other is the critical sealing condition of gas penetration. In the process of semi-dynamic analysis it is demonstrated that the initial critical sealing distance is independent of the shape factor of the droplet single control volume. The semi-dynamic analysis can give various critical sealing information when the gas stops penetrating the metal micro spallation zone. The results of quasi-static analysis can be used as the upper limit of gas penetration, and the semi-dynamic analysis results can be used as the lower limit of gas penetration. From the sensitivity analysis, it can be seen that the change law of physical phenomena given by theoretical analysis accords with the basic physical understanding of the problem. Through this study, the upper and lower limit of the mixed state of gas penetrating the metal micro spallation zone can be estimated, which can provide more accurate initial metal-gas mixed state for subsequent research of the evolution of mixed state. The theoretical analyses given in this paper are based on a lot of uncertain assumptions, and the in-depth study of this phenomenon is still needed based on the law summary and mutual confirmation of experiment and simulation.

Electronically tunable near-field radiative heat transfer between doped silicon and graphene-covered silicon dioxide

The key to the micro/nanoscale thermal management of precision instruments is flexibly controlling the heat flux in the near-field following the actual demand. In this paper, a near-field radiative thermal switch (NFRTS) made of the n-type doped silicon (D-Si) and graphene-covered silicon dioxide (SiO2) plates is proposed to achieve the active near-field radiative heat transfer modulation. The radiative heat flux, thermal switching factor, and thermal modulation factor are calculated for different graphene chemical potential values from 0 to 1 eV and D-Si doping concentrations at different vacuum gaps. This is achieved by considering the breakdown voltage of the SiO2, and with the fluctuational electrodynamics and fluctuation-dissipation theorem. The SiO2 is also replaced by the silicon carbide (SiC) in the thermal switch to clarify further the effect of the breakdown voltage on the performance of the NFRTS. In combining the graphene chemical potential value and D-Si doping concentration, it is obtained that the optimal thermal switching factor is 93.5% for a doping concentration of 1018 cm−3 at a 10 nm vacuum gap for SiO2 system when the heat source and heat sink temperatures are 400 K and 300 K, respectively. This is mainly due to the combination of the major angular frequency band of the transmission coefficients for bulk D-Si determined by the doping concentration and the surface mode of the graphene-covered SiO2 modulated by altering the graphene chemical potential value via applying the external voltage bias. Results also reveal that the performance of the NFRTS with SiC substrate is significantly affected and weakened by the limited breakdown voltage of SiC, and almost always worse than that of the NFRTS with SiO2 substrate. This work paves a way for designing the active near-field thermal management devices for simple structures.

Enhancement and active mediation of near-field radiative heat transfer through multiple nonreciprocal graphene surface plasmons

Metasurfaces, the two-dimensional counterparts of three-dimensional metamaterials, have recently attracted much attention due to their interesting properties, such as negative refraction, hyperbolic dispersion, and the ability to manipulate the evanescent spectrum. In this work, we propose a theoretical model for near-field radiative heat transfer between two multilayered systems consisting of anisotropic metasurfaces. The choice of metasurface is graphene, with an adjustable drift current, since this provides an ideal platform to support a high density of modes around the plasmon frequency. In this configuration, multiple nonreciprocal surface plasmon polaritons are excited, providing a strong way for near-field energy transport. The resulting heat transfer, assisted by the graphene's multilayered structure and with a high drift-current velocity, is more than 36 times stronger than that of a graphene monolayer structure without a drift current for the same vacuum gap. By adjusting the vacuum gap and the thickness of a dielectric spacer, this enhanced effect can be modulated over a large range, and can even turn into a suppression. Our findings provide a powerful way to enhance and regulate energy transport, and in turn, open up a way to enrich the moir\'e physics inherent to the anisotropic optical properties of a metasurface.

Exterior tuning and switching of non-equilibrium Casimir force

We consider a non-equilibrium system consisting of two slabs having the same temperature separated by a vacuum gap, and surrounded by vacuum with a temperature that is different from that of the slabs. We show that the force on one of the slabs can be tuned or switched by placing an additional thin layer on its exterior surface, and by changing the dielectric permittivity of this layer. Moreover, here the tuning or switching is entirely a non-equilibrium effect, and the same change does not affect the force if the system is in equilibrium. Based on this effect, we consider a switch for non-equilibrium Casimir force where a V O 2 thin film is placed on the exterior surface. This switch exhibits a sharp transition in its Casimir force as a function of temperature, only when the system is out of equilibrium.

Molecular dynamic simulation for studying transport characteristics of confined water between sandwiched albite-quartz systems under cyclic thermal–mechanical couplings

In this work, we make a first attempt at performing a molecular dynamic simulation to gain insights into the transport characteristics of H2O molecules confined in sandwiched albite-quartz system, where a mechanical loading couples a thermal cycling was carried out·H2O molecules were placed within the vacuum gap of the albite-quartz slab to accomplish the sandwich model. Based on the viewpoint of energetics aspect, the coulombic interactions are the dominant contributors to the total potential energy. The diffusion coefficient, orientation parameter and hydrogen bonding were examined for analysis of the transport characteristics of confined H2O molecules. Not only the water–mineral interaction but also the substrate structure reflects the dynamics of confined H2O molecules, as evident by qualitatively analyzing the trajectories of some selected H2O molecules located in different regions. The cyclic thermal–mechanical (T-M) coupling contribute a wide band distribution after 1000th cycles instead of two distinct peaks after the 100th cycles. The decrease in amplitude occurring primarily around 1.7 Å is indicative of a weaken H-Bond and a greater rotational freedom of those molecules, leading to the gathering of H2O molecules near quartz and albite slabs and a dense arrangement of the H2O molecules in the inter-spacing region, which could be ascribed to the interconnected aspect of diffusion process.

Reshaping of Micro-morphology Feature in Electrode Surface under Conditioning by Lightning Impulse Voltage in Vacuum Gaps

The objective of this paper is to determine the reshaping of the micro-morphology feature in both the cathode and anode electrode surfaces after conditioning by lightning impulse voltage in vacuum gaps. Materials of the experimental rod-plane electrodes are Cu and stainless steel. The micro-morphology feature in the electrode surface is recorded by a 3D laser microscopy. Experimental results indicate that conditioning by the lightning impulse voltage reshaped the micro-morphology feature in both the cathode and anode electrode surfaces. The BD characteristics are closely related to the micro-morphology feature and roughness in the cathode surface after conditioning. Moreover, the conditioning process takes different effects in the cathode and anode surfaces. For cathode electrode, the main difference of the micro-morphology feature is in the distributing characteristics of the erosion regions. For anode electrode, there aree different patterns of the anodic conditioning mechanism of the micro-morphology feature. In the molten pattern, conditioning melted the anode surface and removed the micro-protrusions effectively. In the crater pattern, with the increasing BD voltage and energy, the material below the anode surface is heating, which is the cause of formation of the many craters in anode surface.

Hadronic High-energy Emission from Magnetically Arrested Disks in Radio Galaxies

Abstract We propose a novel interpretation that gamma rays from nearby radio galaxies are hadronic emission from magnetically arrested disks (MADs) around central black holes (BHs). The magnetic energy in MADs is higher than the thermal energy of the accreting plasma, where the magnetic reconnection or turbulence may efficiently accelerate nonthermal protons. They emit gamma rays via hadronic processes, which can account for the observed gamma rays for M87 and NGC 315. Nonthermal electrons are also accelerated with protons and produce MeV gamma rays, which is useful to test our model by proposed MeV satellites. The hadronic emission from the MADs may significantly contribute to the GeV gamma-ray background and produce the multi-PeV neutrino background detectable by IceCube-Gen2. In addition, gamma rays from MADs provide electron–positron pairs through two-photon pair production at the BH magnetosphere. These pairs can screen the vacuum gap, which affects high-energy emission and jet-launching mechanisms in radio galaxies.

Coupling of light and mechanics in a photonic crystal waveguide

Observations of thermally driven transverse vibration of a photonic crystal waveguide (PCW) are reported. The PCW consists of two parallel nanobeams whose width is modulated symmetrically with a spatial period of 370 nm about a 240-nm vacuum gap between the beams. The resulting dielectric structure has a band gap (i.e., a photonic crystal stop band) with band edges in the near infrared that provide a regime for transduction of nanobeam motion to phase and amplitude modulation of an optical guided mode. This regime is in contrast to more conventional optomechanical coupling by way of moving end mirrors in resonant optical cavities. Models are developed and validated for this optomechanical mechanism in a PCW for probe frequencies far from and near to the dielectric band edge (i.e., stop band edge). The large optomechanical coupling strength predicted should make possible measurements with an imprecision below that at the standard quantum limit and well into the backaction-dominated regime. Since our PCW has been designed for near-field atom trapping, this research provides a foundation for evaluating possible deleterious effects of thermal motion on optical atomic traps near the surfaces of PCWs. Longer-term goals are to achieve strong atom-mediated links between individual phonons of vibration and single photons propagating in the guided modes (GMs) of the PCW, thereby enabling optomechanics at the quantum level with atoms, photons, and phonons. The experiments and models reported here provide a basis for assessing such goals.

Optimal design of a solar cell-driven electroluminescent refrigerator

Near-field electroluminescent refrigerators (NFERs) have potential applications in thermal management. The input electricity can manipulate the photons’ electrochemical potential to realize heat transfer from cold side to hot side. However, the coupled properties and the overall optimum performances of the NFER driven by actual power sources have not been studied. A model of solar cell powered NFER is conceptually proposed and optimally designed. The electrical coupling characteristics between the solar cell and the NFER are discussed. The operating region of the NFER’s input voltage is provided. The structure and the electrical parameters are optimized to obtain the maximum efficiency. The effects of the vacuum gap, the emitter’s thickness, and the solar irradiance on the optimum performances are analyzed. Making trade-offs between the efficiency and cooling heat flow rate, the optimum region of solar irradiance is determined. The proposed model and the parametric optimal analysis can provide a route for solar-driven refrigerators.

Effect of plasma hydrodynamics on laser-produced bremsstrahlung MeV photon dose

We detail a laser plasma experiment aimed at enhancing laser to MeV electron energy coupling and then the x-ray dose produced when a short pulse laser propagates through a long preformed plasma. This study can be of interest not only for radiography of high areal mass objects requiring large doses but also for radiation safety of large scale laser facilities such as LMJ or NIF able to produce long preformed plasmas through which a short pulse laser can propagate. A low-intensity (∼1014 W/cm2) ns beam explodes a thin foil deposited on a high-Z solid target to generate an underdense plasma. An intense (>1018 W/cm2) and short (<1 ps) laser pulse then (with an adjustable delay δt) interacts with this plasma and produces multi-MeV electrons. These high-energy electrons are converted into a bremsstrahlung emission of MeV x-ray photons in the high-Z target. In a second target design, a vacuum gap between the foil and the conversion target is also tested to let the plasma expand on both sides of the foil, increasing the interaction length even more. Results show how the vaporization of the foil produces an underdense plasma over several hundreds of micrometers which significantly enhances x-ray doses, with harder x-ray spectra obtained at an optimum delay, δt, until the short pulse laser is affected by refraction. Increasing the interaction length with gap targets is at the origin of a much more complex plasma hydrodynamics involving on-axis plasma stagnation which delays the optimum time for the maximum x-ray dose production.

Beating absorption in solid-state high harmonics

Since the new millennium coherent extreme ultra-violet and soft x-ray radiation has revolutionized the understanding of dynamical physical, chemical and biological systems at the electron’s natural timescale. Unfortunately, coherent laser-based upconversion of infrared photons to vacuum-ultraviolet and soft x-ray high-order harmonics in gaseous, liquid and solid targets is notoriously inefficient. In dense nonlinear media, the limiting factor is strong re-absorption of the generated high-energy photons. Here we overcome this limitation by generating high-order harmonics from a periodic array of thin one-dimensional crystalline silicon ridge waveguides. Adding vacuum gaps between the ridges avoids the high absorption loss of the bulk and results in a ~ 100-fold increase of the extraction depth. As the grating period is varied, each high harmonic shows a different and marked modulation, indicating their waveguiding in the vacuum slots with reduced absorption. Looking ahead, our results enable bright on-chip coherent short-wavelength sources and may extend the usable spectral range of traditional nonlinear crystals to their absorption windows. Potential applications include on-chip chemically-sensitive spectro-nanoscopy.

Enhancement of spectrally controlled near-field radiation transfer by magnetic polariton generated by metal–insulator–metal structures

Near-field radiation transfer between a metal–insulator–metal (MIM)-structured emitter and receiver was investigated through numerical simulation using a finite difference time domain (FDTD) method. Both the emitter and receiver consist of a squared-island-type metal array composed of nickel (Ni), an insulator layer composed of silicon dioxide (SiO2), and a metal substrate composed of nickel (Ni). The emitter and receiver were set up with a vacuum gap of a few hundred nanometers. The results showed that the near-field radiation flux was enhanced by a factor of approximately four, when compared to that between blackbody surfaces. Simultaneously, the enhancement was spectrally controlled over frequencies ranging from 9 × 1014 to 20 × 1014 rad/s (approximating to a wavelength range of 0.9 to 2.1 μm) depending on the size of the squared island. Images of the magnetic field in both the emitter and receiver clarified that the spectrally enhanced radiation flux was caused by the magnetic polariton (MP)) generated in the insulator between the squared-island metal and the substrate metal when the frequency of the MP coincided with that of the near-field radiation. Moreover, the resonance mode between the frequencies of the MP and near-field radiation was predicted by an impedance model that considered the capacitance between the islands in the emitter and receiver, in addition to the conventional circuit of a MIM structure.