Vacuum Gap Research Articles

Electroacoustic shear waves in the hollow structure of two piezoelectric

Dispersion properties of electroacoustic waves in the gap structure of two different piezoelectrics are considered. It is shown that in the considered structure PbTiO3 – vacuum gap – BaTiO3 in the presence of a difference of shear wave velocities in piezoelectrics there are no purely symmetric and antisymmetric modes, and the coefficients of the boundary localization of the shear wave will be significantly different.

Design and test of a module of a breathable Electrostatic Shield for the MITICA 1 MV negative ion Beam Source

The electrical insulation of the MITICA Beam Source at 1 MV is a challenging issue, which has not been fully addressed so far on the basis of experimental results and of theoretical models available in literature. Being MITICA the full-size prototype of the Heating Neutral Beam Injector for the ITER fusion experiment, its electrical insulation is constituted just by vacuum gaps and alumina insulators, since other insulating materials such as SF6 gas or fibreglass-reinforced plastic (FRP) would be quickly degraded by the expected neutron flux produced by fusion reaction. Extrapolations based on HV tests on reduced-scale models have recently indicated the risk of electrical breakdowns in the vacuum gap between electrodes nominally operating at -1 MV and the vacuum vessel (at ground potential). The risk of electrical breakdown can be mitigated by introducing an intermediate Electrostatic Shield (ES), which essentially is an equipotential (metallic) enclosure surrounding the HV electrode, so as to divide the vacuum gap in two independent insulating gaps of 400 kV and 600 kV respectively. However, for optimal negative ion production, the ion source shall operate in H2 or D2 at a pressure of ∼ 0.3 Pa and unavoidably produces a flow of gas leaking out in the surrounding vacuum. Thus, the presence of an intermediate shield can substantially increase the background gas pressure in the vacuum gaps, and, due to the large gap length (0.6 m), exacerbate the risk of breakdown when the pressure approaches the conditions of Paschen-type discharges. In addition to this, RF-induced breakdowns were found on the rear side of the ion source during the operation of the prototype source SPIDER, which were somewhat correlated to a relatively high hydrogen pressure in that area. For these reasons, a structure capable of constituting a full equipotential barrier all around the BS and, at the same time, having sufficient gas conductivity (breathability) to allow efficient pumping of background gas, has been designed. In the first part of the paper, the requirements and design optimization of a breathable module of the intermediate ES are described. Then, an experimental campaign for the validation of the electrode implementation the test configurations and the experimental procedure is discussed.

Research on Conduction Delay Time Characteristics of Double-Gap Surface Flashover Triggered Multistage Vacuum Switch

Due to the insulation saturation effect of a vacuum gap, the series connection is the main method to improve the capability of the triggered vacuum switch (STVS) to withstand voltage. In this paper, a double-gap surface flashover triggered multistage vacuum switch (DMVS) is constructed based on a removable vacuum chamber. The DMVS consists of a surface flashover triggered vacuum gap (STG) and a self-breakdown vacuum gap (SBG) in series. The trend in DMVS conduction delay time under different operating parameters and gap distances is firstly measured through experiments, and then the conduction delay time characteristics of DMVS and single-gap STVS compared. The test results show that increasing the trigger current and operating voltage and decreasing the gap distance of each vacuum gap can optimize the conduction delay time of the DMVS, but the voltage division ratio between the STG and the SBG has no obvious influence on the conduction delay time. The comparison result between the DMVS and the STVS shows that increasing the number of series gaps increases the conduction delay time but, for a certain vacuum gap distance, DMVS exhibits superior conduction delay time characteristics compared to STVS.

Enhancement and modulation of near-field radiative heat transfer in Bi2Se3/graphene-based three-body configurations

The manipulation of near-field radiative heat transfer (NFRHT) through complex photon tunneling in many-body systems has garnered significant attention, particularly for advancing thermal management in micro- and nanodevices. In this work, we theoretically investigate the enhancement and modulation of NFRHT within a graphene/Bi2Se3-based three-body configuration. Compared to a two-body configuration, the three-body modulator enhances the near-field radiative heat flux (NFRHF) by 2.77, primarily due to the enhancement of surface plasmon polaritons facilitated by graphene. Moreover, we analyze how NFRHF is modulated by varying the vacuum gap between the emitter and the intermediate, adjusting the thickness of Bi2Se3 films in emitter and intermediate, as well as manipulating the chemical potentials of graphene. Results indicate that by tuning the chemical potentials of graphene, thermal regulation can be enhanced up to a ratio of 2.55. This enhancement is attributed to the excitation of SPPs, which are concentrated in a narrower wave vector space and higher angular frequency range as the chemical potential of graphene increases. These findings have implications for controlling thermal radiation applications utilizing graphene and Bi2Se3 heterostructures. This work demonstrates the superiority of the three-body system in enhancing NFRHT and contributes new insights into the potential applications of Bi2Se3 in NFRHT.

The Kunming–Montreal Global Biodiversity Framework and the Paris Agreement need a joint work programme for climate, nature and people

Abstract The global climate and biodiversity crises are closely intertwined, yet no policy instrument currently tackles both jointly. This is problematic, as the lack of international platforms for designing coordinated and coherent policy frameworks that align targets across the nexus of biodiversity and climate change poses risks to nature and people. Here, we argue that the successful implementation of both the Paris Agreement and the Global Biodiversity Framework (GBF) requires a joint work programme between the United Nations Framework Convention on Climate Change (UNFCCC) and the Convention on Biological Diversity (CBD). Such a joint work programme could fill the current global governance vacuum and address critical implementation gaps, while fostering innovation and synergies in climate and biodiversity action. Based on a review of existing work programmes under the UNFCCC and interagency and joint work programmes established with the CBD, we provide a concept of how this joint programme could be set up, ideas on the type of issues it should tackle and recommendations for successful implementation. Policy implications: The UNFCCC and CBD are unique platforms for providing the evidence to, and guiding the way through the changes we need, but higher levels of integration between the biodiversity and climate change agendas are needed to plug the implementation gap between the GBF and the Paris Agreement. The upcoming Conference of the Parties (COP) for the United Nations Framework Convention on Climate Change (COP29) and the Convention on Biological Diversity (COP16) present a clear policy window for the two conventions to introduce a formal governance structure, such as the one proposed in this contribution, that brings together ideas, people, organizations and processes necessary for joining the dots on how to both stabilize our climate and recover our nature.

Dirac Theory in Noncommutative Phase Spaces

Based on the position and momentum of noncommutative relations with a noncanonical map, we study the Dirac equation and analyze its parity and time reversal symmetries in a noncommutative phase space. Noncommutative parameters can be endowed with the Planck length and cosmological constant such that the noncommutative effect can be interpreted as an effective gauge potential or a (0,2)-type curvature associated with the Plank constant and cosmological constant. This provides a natural coupling between dynamics and spacetime geometry. We find that a free Dirac particle carries an intrinsic velocity and force induced by the noncommutative algebra. These properties provide a novel insight into the Zitterbewegung oscillation and the physical scenario of dark energy. Using perturbation theory, we derive the perturbed and nonrelativistic solutions of the Dirac equation. The asymmetric vacuum gaps of particles and antiparticles reveal the particle–antiparticle symmetry breaking in the noncommutative phase space, which provides a clue to understanding the physical mechanisms of particle–antiparticle asymmetry and quantum decoherence through quantum spacetime fluctuation.

Near-field radiative thermal rectification assisted by Bi2Se3 sheet

The ability to control heat flux at the nanoscale opens up numerous exciting possibilities in modern electronics and the field of information processing. In this research, we propose a design with the focus on achieving efficient thermal rectification at moderate gap and relatively low temperature. This study centers on near-field thermal radiation between temperature dependent indium antimonide (InSb) and silicon carbide (3C-SiC) coated with bismuth selenide (Bi2Se3). Our investigation sheds light on the critical role played by the Bi2Se3 layer in enhancing various key parameters, including the net radiative flux, and thermal rectification efficiency (η). We achieved a substantial improvement in the η of a near-field radiative thermal rectifier (NFRTR) due to the presence of the Bi2Se3 sheet. This enhancement is contingent on factors such as the Fermi energy (Ef) of Bi2Se3, emitter temperature, and the vacuum gap (d). Our study culminated in the identification of an optimal design, achieving an impressive η of 75% at an emitter temperature (TH) of 350 K, with vacuum gap (d) set to 20 nm. Furthermore, increasing TH to 500 K resulted in even more promising outcomes, with the highest η reaching 93%. The need for operating the optimized device at moderate temperatures is to strike a balance between efficiency, safety, cost-effectiveness, and material compatibility. These findings represent a significant step forward in the development of efficient Bi2Se3-based NFRTRs, paving the way for future applications in thermal management, energy conversion systems, and thermal logic gates.

Influence of the shape of a conducting chamber on the stability of rigid ballooning modes in a mirror trap

MHD stabilization of flute and ballooning modes in an axisymmetric mirror trap is studied under the assumption of strong finite Larmor radius effect that suppresses all perturbations with azimuthal numbers m⩾2 and makes the m = 1 mode ‘rigid’. The rigid mode can be effectively suppressed using perfectly conducting lateral wall without any additional means of stabilization or in combination with end MHD anchors. Numerical calculations were carried out for an anisotropic plasma produced in the course of neutral beam injection into the minimum of the magnetic field at the right angle to the trap axis. The stabilizing effect of the conducting shell made of a straightened cylinder is compared with a proportional chamber, which, on an enlarged scale, repeats the shape of the plasma column. It is confirmed that for convincing wall stabilization of the rigid modes, the plasma beta (β, the ratio of the plasma pressure to the magnetic field pressure) must exceed some critical value βcr2 . When conducting lateral wall is combined with conducting end plates imitating MHD end anchors, there are two critical betas and, respectively, two stability zones β<βcr1 and β>βcr2 that can merge, making the entire range 0<β<1 of betas allowable for stable plasma confinement. The dependence of the critical betas on the plasma anisotropy, mirror ratio, width of the vacuum gap between the plasma column and the lateral wall, radial pressure profile and the axial magnetic field profile is examined.

Active control of nonreciprocal near-field radiative heat transfer with a drift-current biased graphene/α-MoO3 heterostructure.

The interaction between the drift-current biased graphene plasmonics and the hyperbolic phonon polaritons of α-MoO3 provides a promising way to manipulate near-field radiation heat transfer (NFRHT). Through examination of the drift biased graphene/α-MoO3 heterostructure, it has been discovered that drift-current applied to the graphene effectively enhances photon tunneling. Consequently, they dynamically modulate the coupling effect of the two excitations, thereby offering a reliable pathway for the modulation of NFRHT. Furthermore, the influencing mechanism of vacuum gaps on nonreciprocal NFRHT with different drift-current rates is revealed, and it is discovered that the vacuum gaps can filter the nonreciprocal surface plasmon polaritons with high nonreciprocity. Our findings make it possible to manipulate nanoscale thermal rectification and noncontact thermal modulation.

InSitu Observation of Field-Induced Nanoprotrusion Growth on a Carbon-Coated Tungsten Nanotip.

Nanoprotrusion (NP) on metal surface and its inevitable contamination layer under high electric field is often considered as the primary precursor that leads to vacuum breakdown, which plays an extremely detrimental effect for high energy physics equipment and many other devices. Yet, the NP growth has never been experimentally observed. Here, we conduct field emission (FE) measurements along with insitu transmission electron microscopy (TEM) imaging of an amorphous-carbon (a-C) coated tungsten nanotip at various nanoscale vacuum gap distances. We find that under certain conditions, the FE current-voltage (I-V) curves switch abruptly into an enhanced-current state, implying the growth of an NP. We then run field emission simulations, demonstrating that the temporary enhanced-current I-V is perfectly consistent with the hypothesis that a NP has grown at the apex of the tip. This hypothesis is also confirmed by the repeatable insitu observation of such a nanoprotrusion and its continued growth during successive FE measurements in TEM. We tentatively attribute this phenomenon to field-induced biased diffusion of surface a-C atoms, after performing a finite element analysis that excludes the alternative possibility of field-induced plastic deformation.

Corner- and edge-mode enhancement of near-field radiative heat transfer.

It is well established that near-field radiative heat transfer (NFRHT) can exceed Planck's blackbody limit1 by orders of magnitude owing to the tunnelling of evanescent electromagnetic frustrated and surface modes2-4, as has been demonstrated experimentally for NFRHT between two large parallel surfaces5-7 and between two subwavelength membranes8,9. However, although nanostructures can also sustain a much richer variety of localized electromagnetic modes at their corners and edges10,11, the contributions of such additional modes to further enhancing NFRHT remain unexplored. Here we demonstrate both theoretically and experimentally a physical mechanism of NFRHT mediated by the corner and edge modes, and show that it can dominate the NFRHT in the 'dual nanoscale regime' in which both the thickness of the emitter and receiver, and their gap spacing, are much smaller than the thermal photon wavelengths. For two coplanar 20-nm-thick silicon carbide membranes separated by a 100-nm vacuum gap, the NFRHT coefficient at room temperature is both predicted and measured to be 830W m-2 K-1, which is 5.5 times larger than that for two infinite silicon carbide surfaces separated by the same gap, and 1,400 times larger than the corresponding blackbody limit accounting for the geometric view factor between two coplanar membranes. This enhancement is dominated by the electromagnetic corner and edge modes, which account for 81% of the NFRHT between the silicon carbide membranes. These findings are important for future NFRHT applications in thermal management and energy conversion.

Polarized QED cascades over pulsar polar caps

ABSTRACT The formation of e± plasmas within pulsar magnetospheres through quantum electrodynamics (QED) cascades in vacuum gaps is widely acknowledged. This paper aims to investigate the effect of photon polarization during the QED cascade occurring over the polar cap of a pulsar. We employ a Monte Carlo-based QED algorithm that accurately accounts for both spin and polarization effects during photon emission and pair production in both single-particle and particle-in-cell (PIC) simulations. Our findings reveal distinctive properties in the photon polarization of curvature radiation (CR) and synchrotron radiation (SR). CR photons exhibit high linear polarization parallel to the plane of the curved magnetic field lines, whereas SR photons, on average, demonstrate weak polarization. As the QED cascade progresses, SR photons gradually dominate over CR photons, thus reducing the average degree of photon polarization. Additionally, our study highlights an intriguing observation: the polarization of CR photons enhances e± pair production by approximately 5 per cent, in contrast to the inhibition observed in laser–plasma interactions. Our self-consistent QED-PIC simulations in the corotating frame reproduce the essential results obtained from single-particle simulations.

Insight on relationship between crystallinity and band gap energies of polyhydroxyalkanoates polymers

This study explores the electronic properties of polyhydroxyalkanoates (PHAs), a class of biodegradable polymers. The focus lies in examining the relationship between crystallinity and band gap energies in PHAs. A notable discovery is the identification of a band gap plateau, suggesting a threshold in electronic properties with increasing polymer chain length. DFT calculations were employed to model the complexities of PHA polymers, with the conductor-like polarizable continuum model (CPCM) offering insights into solvent effects. The results demonstrate a strong negative correlation between crystallinity and band gap in both vacuum and CPCM environments. This study underscores the significance of structural order in shaping the electronic properties of PHAs, paving the way for the development of sustainable polymeric materials for various applications.

Performance analysis of a novel façade-based building integrated photovoltaic-thermal system with phase change material for domestic hot water

Since the current contribution of solar thermal energy to the amount of energy consumed for domestic hot water in buildings worldwide is very low (8.4 %, approximately), the potential of this renewable energy to meet the demand for domestic hot water is quite significant. In this context, this paper presents a new façade-based building integrated photovoltaic-thermal system with phase change material which has been patented recently. It is a modular device for installation on façades and roofs, enabling the production of domestic hot water and electricity using solar energy. The main novelty of the system lies in the use of a photovoltaic glass sheet, which improves the building’s aesthetics and facilitates the architectural integration of this type of devices. Thus, a photovoltaic glass is used for the production of electricity, a phase change material for heat storage and a water-phase change material exchanger for the production of domestic hot water. The design and verification stages of the system were developed for a case study under typical usage conditions. Two dimensional numerical models of five different configurations (of the energy harvesting and storage module and two dimensional and three dimensional numerical models of the finned exchanger were carried out to simulate the charge and discharge processes. The most suitable configuration of the system, characterised by a 12 mm vacuum gap and a low emissivity of the absorber, was able to meet the requirements in terms of water flow rate and supply temperature (45 °C) for a shower time of 5 min.

Active stabilization of an open-access optical microcavity for low-noise operation in a standard closed-cycle cryostat

Open-access optical microcavities are Fabry–Perot type cavities consisting of two micrometer-size mirrors, separated by an air (or vacuum) gap typically of a few micrometers. Compared to integrated microcavities, this configuration is more flexible as the relative position of the two mirrors can be tuned, allowing for easy changes in parameters such as cavity length and mode volume and the selection of specific transverse cavity modes. These advantages come at the expense of the mechanical stability of the cavity itself, which is particularly relevant in noisy closed-cycle cryostats. Here, we show an open-access optical microcavity based on scanning-probe microscope design principles. When operated at 4 K in a tabletop optical closed-cycle cryostat without any dedicated mechanical low-pass filter, we obtain stabilities of 5.7 and 10.6 pm rms in the quiet and full periods of the cryocooler cycle, respectively. Our device has free-space optical access, essential, for instance, for full polarization control.

Radio Pulsar B0950+08: Radiation in the Magnetosphere and Sparks above the Surface

We observed the nearby radio pulsar B0950+08, which has a 100% duty cycle, using the Five-hundred-meter Aperture Spherical Radio Telescope. We obtained the polarization profile for its entire rotation, which enabled us to investigate its magnetospheric radiation geometry and the sparking pattern of the polar cap. After we excluded part of the profile in which the linear polarization factor is low (≲30%) and potentially contaminated by jumps in position angle, the rest of the swing in polarization position angle fits a classical rotating vector model (RVM) well. The best-fit RVM indicates that the inclination angle, α, and the impact angle, β, of this pulsar, are 100.°5 and −33.°2, respectively, suggesting that the radio emission comes from two poles. We find that, in such RVM geometry, either the annular vacuum gap model or the core vacuum gap model would require that the radio emissions come from a high-altitude magnetosphere with heights from ∼0.25 R LC to ∼0.56 R LC, with R LC being the light cylinder radius. Both the main and interpulses’ sparking points are located away from the magnetic pole, which could relate to the physical conditions on the pulsar surface.

Novel correction procedure for compensating thermal contraction errors in the measurement of the magnetic field of superconducting undulator coils in a liquid helium cryostat.

Superconducting undulators (SCUs) can offer a much higher on-axis undulator field than state-of-the-art cryogenic permanent-magnet undulators with the same period and vacuum gap. The development of shorter-period and high-field SCUs would allow the free-electron laser and synchrotron radiation source community to reduce both the length of undulators and the dimensions of the accelerator. Magnetic measurements are essential for characterizing the magnetic field quality of undulators for operation in a modern light source. Hall probe scanning is so far the most mature technique for local field characterization of undulators. This article focuses on the systematic error caused by thermal contraction that influences Hall probe measurements carried out in a liquid helium cryostat. A novel procedure, based on the redundant measurement of the magnetic field using multiple Hall probes at known relative distance, is introduced for the correction of such systematic error.

Investigation on Ion extraction processes in M-Type electrode configuration and the influence of top electrode

In the realm of ion extraction from photoplasma, the design of electrode configuration exerts a significant influence over-extraction efficiency by tailoring the vacuum gap and electric potential distribution. Over time, various electrode configurations have been explored, including parallel-plate, wire-type, M-type, and Π-type designs. Notably, the M-type configuration has shorter ion extraction times. In the present study, a comprehensive investigation of the ion extraction process with the M-Type setup is performed using a 2D2v-electrostatic Particle-in-Cell simulation. The study focuses on the spatiotemporal evolution of photoplasma, the formation of the plasma sheath, and the spatial distribution of electric potential. Subsequently, a series of computational experiments have been conducted by systematically altering the size of the top electrode in the M-type configuration, effectively transforming it into wire-type and Π-type. The objective of these experiments is to explore the impact of the top electrode on ion extraction. The obtained simulation results report a significant finding: the formation of a virtual anode between the cathodes. This distinct phenomenon substantially contributes to an exceptional level of efficiency, exceeding 90%, among different simulated configurations. Furthermore, it also reports an approximate 95% collection efficiency with minimum collection time with an M-type configuration over the Π-type electrode configuration.

Development of X-Ray Collimators to Identify Sources of Radiation in Devices Insulated by Large Vacuum Gaps

Development of X-Ray Collimators to Identify Sources of Radiation in Devices Insulated by Large Vacuum Gaps

Thermal Performance of a Flat-Plate Solar Collector for Drying Agricultural Crops

In this study, the finite volume method was used to evaluate the thermal performance of a flat-plate solar collector used to dry agricultural crops. A 3D numerical model was created and used to predict the outlet air velocities and temperatures for three inlet air velocities. When compared with experimental measurements, the numerical predictions showed good agreement under all testing conditions. Then, the numerical model was used to predict the internal airflow and heat transfer characteristics of the collector. The internal baffles were found to increase the dwell time and efficiency but also promote flow separation, which resulted in flow loss. In addition, the collector has a transparent cover that results in a substantial heat loss, which can be mitigated by adding a vacuum gap between the flow inside the collector and the cover. Increasing the flow rate increased the heat loss and decreased the heat uptake, which decreased the temperature difference between the inlet and outlet of the collector. Because the heat was lost through long-wavelength radiation via the transparent cover and sidewalls, coating the absorber plate with black matte paint to increase the solar radiation absorption coefficient did not improve the drying performance.