Vacuum Breakdown Research Articles

Cathode-initiated breakdown in vacuum

New methods and criteria are proposed for evaluating the cathode-initiated breakdown in vacuum. Using the optimum regimes of pulse conditioning, it is possible to significantly improve the state of the cathode surface and substantially increase the electric strength with respect to cathode initiation. The proposed methods have been experimentally verified in the range of pulse durations from 10 to 800 ns.

New formulation of the synthesis problem in electron optics

We describe a new approach to solving the problem of harmonic field synthesis for electron-optical systems. Numerical solutions obtained in practice with the classical approach often lead to operational difficulties, such as numerical instability and vacuum breakdown. Our approach differs from the classical one in that it considers a mixed formulation where one part of the information is responsible for representing objective functionals—Cauchy data, while the other part of the information represents the boundary conditions defined on boundaries of specified geometry. This approach takes into account the a priori structural constraints of the synthesis problem, analytically extending the solution to the subdomain of synthesis as well as finding the tolerances for small perturbations of the objective functional. Numerical algorithms proposed in this work are based on the integral representation of a field by a potential of a single and double layer as well as by a volume potential for charged particle beams. A classification of the basic types of electronic lenses with minimal aberrations is given. Numerical examples of test and practical problems are presented. This approach is implemented in the code “Synthesis-2” of Computer Aided Design system “TOPAZ”.

Vacuum breakdown of carbon-nanotube field emitters on a silicon tip

Findings are given from the experimental observation of the vacuum breakdown of carbon-nanotube (CNT) field emitters on a Si tip. The CNTs were grown on the apex of a Si microtip by microwave plasma-enhanced chemical vapor deposition. The electrical contact of the CNT-Si junction was shown to be of ohmic type. A fine tungsten microprobe in combination with a scanning electron microscopy (SEM) system was employed for both the field emission and the contact conductivity measurements. This arrangement allows to precisely measure the characteristics of individual CNT and to in situ inspect the morphology of the CNT emitters on Si tips before and after vacuum breakdown events. An upper limit in emission current density of ∼103 A/m2 from the CNT emitters was recorded before a vacuum breakdown event is initiated. Clear evidence was found to show that the vacuum breakdown of the CNTs results in melting of the Si tip. These findings enhance the understanding of the failure mechanism of CNT emitters. It also has important technical implication to the development of ultrabright electron source.

Electron work function estimated from the delay time of breakdown in vacuum

The delay time of an interelectrode gap breakdown in vacuum was studied. A method for estimating the electron work function of a cathode with an extended working surface is proposed based on the breakdown delay measurements using applied voltage in the form of rectangular pulses or a sawtooth wave.

Electrical breakdown in atmospheric air between closely spaced (0.2 μm-40 μm) electrical contacts

The increasing importance of electrical contacts in air with micrometer spacing prompted recent experiments on the electrical breakdown behavior of these gaps. The electrical field between the contacts used in one of the experiments was analyzed using finite element analysis to model the electric field. The experimental data on the electrical breakdown voltage could be divided into three regions as a function of the gap spacing. First, at close gap spacing (/spl les/4 /spl mu/m) both the breakdown voltages as well as the electrical fields at the cathode were similar to values measured during the breakdown of vacuum gaps of less than 200 /spl mu/m. Second, at larger gaps (>6 /spl mu/m) the breakdown voltages followed Paschen's curve for the Townsend electron avalanche process in air. Finally, in between these two regions the breakdown values were below the expected values for purely vacuum breakdown or purely Townsend breakdown. The breakdown phenomena have been discussed in terms of field emission of electrons from the cathode and their effect on initiating the observed breakdown regimes.

Conformal ion implantation

Using a 40–50 kV system we have made measurements of how small the sheath can be in plasma implantation with the objective of finding the limits of the process. We have made measurements of sheath size in plasma implantation using the probe technique developed at the University of Wisconsin [M.M. Shamim et al., J. Appl. Phys. 70 (1991)]. We also developed a tritium tracer technique for rapidly measuring the relative implanted dose. The tritium tracer technique is particularly attractive because the amount of tritium required is extremely small (below the activity level which requires regulatory approval) and it can be obtained from a source such as a display tube. In the measurements, we find that implantation with sheaths of distance 1.5–3 cm at voltages of 40–50 kV is similar to operation in other parameter regimes. The only limits occur in the range (100 kV/cm) where vacuum breakdown of the substrate is anticipated in any case.

Visualization of late breakdown in vacuum by ICCD camera

Experimental observations of late breakdowns in a vacuum circuit breaker were made using an intensified charge coupled device camera (ICCD). These late restrikes can cause arc reignition and interruption failure. This type of failure is rare and can be initiated by a number of mechanisms. This paper illustrates the method used to observe this phenomenon. Some key image examples of the phenomenon occurring during breakdown initiation are shown.

Global static electrospheres of charged pulsars

In this paper we present self-consistent models of the magnetosphere of inactive, charged, aligned rotator pulsars. We have devised an efficient semi-analytical and numerical algorithm to construct such models. The only free parameter is the total charge of the system. These charge-separated electrospheres consist of an equatorial belt carrying charge of one sign, partially in differential rotation, and of two oppositely charged domes located over the poles which corotate with the neutron star. The dependence of the shape of these plasma-filled regions surrounding the star on the total charge of the system and of their differential rotation is investigated. It is shown that our solutions are stable to vacuum breakdown by electron-positron pair production in most of the light-cylinder volume, except perhaps in the case of millisecond pulsars. The small regions where vacuum breakdown occurs are shown to behave merely as an effective extension of the star's volume. We have also found that no permanent null-charged wind emanating from the polar caps can exist in a stationary state. Indeed, for a given total charge of the system determined by the net outgoing charged flux, the potential configuration becomes unfavorable to particles escaping to infinity. Finally, we have shown that the geometric and kinematic structure of the electrosphere is uniquely determined by the total charge of the system.

Development of a large volume negative-ion source for ITER neutral beam injector

Development of the negative-ion sources has been conducted to realize a high power neutral beam injector for International Thermonuclear Experimental Reactor (ITER). A high negative-ion current density of 31 mA/cm2 (H−) at a very low pressure of 0.1 Pa has been produced in a cesium seeded multicusp plasma generator which has the same concept of the ITER source. For a vacuum insulated accelerator, a voltage holding experiment of long distance vacuum gaps up to ∼1.8 m has been performed. It was clarified that the transition region of product pressure distance (pd) from the vacuum breakdown to the gas discharge is about 0.2 Pa m which is high enough from the operating region of the ITER source. A prototype vacuum insulated accelerator was fabricated based on the experiment and tested. A high-energy H− beam acceleration up to 970 keV, 37 mA, and 1 s has been successfully demonstrated.

精密機械加工を施した無酸素銅電極の真空中における絶縁破壊特性

Vacuum breakdown characteristics of oxygen-free copper electrodes processed by precision machining were investigated by applying impulse voltage. Before breakdown measurements, He ion-beam sputtering was performed to the electrode surface. Chemical compositions on the electrode surface before and after the ion-beam sputtering and after the 500 times repetitive breakdowns were analyzed by X-ray Photoelectron Spectroscopy (XPS). In addition we investigated the vacuum breakdown characteristics of oxygen-free copper electrodes that ware processed by precision machining and heated in the hydrogen firing. As a result, processed electrodes by precision machining showed higher breakdown fields than those for electrodes without the process. And their conditioning saturated rapidly. The results of the XPS spectra confirmed that organic contaminants due to C were removed by repetitive breakdowns. The hydrogen firing improved breakdown fields of the early stage of conditioning process.

Low-macroscopic-field electron emission from carbon films and other electrically nanostructured heterogeneous materials: hypotheses about emission mechanism

Thin flat dielectric films can be low-macroscopic-field (LMF) electron emitters, able to generate electrons when subject to a macroscopic electric field in the range 1–50 V μm −1. This phenomenon is a known cause of pre-breakdown currents in high-voltage vacuum breakdown, and is now the basis of a broad-area electron-source technology, using carbon-based thin films and other materials. The phenomenon occurs because the dielectric film is, or becomes, an electrically nanostructured heterogeneous (ENH) material, with quasi-filamentary conducting channels between its surfaces. These channels connect to emitting features near or on the film/vacuum surface, or act as electron emitters themselves. The film may contain conducting or semiconducting particles that assist with conductivity and/or act as emitting features. Several forms of thin-film LMF emitter exist: in each case the situation geometry ensures that sufficient field enhancement occurs at the ‘tip’ of the emitting feature for the emission process to be some form of tunnelling field electron emission (probably ‘cold’ in some cases, ‘hot’ in others). This paper explores aspects of the theory of thin-film LMF emission, starting from the need to understand the behaviour of emitters based on amorphous carbon films. A summary review, with extensive references, is given of relevant past work outside the immediate ‘carbon field emission’ context. Relevant aspects of semiconductor field emission theory are noted. Comment is made on the original experiments on diamond field emission, and on theoretical misconceptions in the carbon field emission literature. Analysis of carbon-film emitter behaviour suggests that emission must primarily be due to geometrical field enhancement, that in at least some cases arises from conducting nanostructure inside the film. In one case, published film characteristics can be used to show that sufficient field enhancement should be available. Some problems with an ‘internal field enhancement’ hypothesis are considered and disposed of. Difficulties with Latham’s theory of field-induced emission from ENH materials are pointed out: a new theory, largely qualitative at this stage, can explain longstanding problems: this assumes that dielectric films must be treated as ‘hopping conductors’ not semiconductors. Electron emission takes place via localised surface states: transition to a channel-limited current regime takes place when the surface states no longer have high enough occupation probability to screen the external field, and is accompanied by anomalous band bending at the channel tip. Mathematical theories of band bending and field emission for hopping conductors are required. Some consequences for the design of LMF emitters are noted.

Particle acceleration and curvature TeV emission by rotating, supermassive black holes

It is shown that particles accelerating near the event horizon of a spinning supermassive black hole that is threaded by externally supported magnetic field lines suffer severe curvature losses that limit the maximum energy they can attain to values well below that imposed by the maximum voltage drop induced by the black hole dynamo. It is further shown that the dominant fraction of the rotational energy extracted from the black hole is radiated in the TeV band. The implications for vacuum breakdown and the observational consequences are discussed.

Microwave magnetic field effects on high-power microwave window breakdown

Microwave window breakdown in vacuum is investigated for an idealized geometry, where a dielectric slab is located in the center of a rectangular waveguide with its normal parallel to the microwave direction of propagation. An S-band resonant ring with a frequency of 2.85 GHz and a power of 60 MW is used. With field enhancement tips at the edges of the dielectric slab, the threshold power for breakdown is observed to be dependent on the direction of the microwaves; i.e., it is approximately 20% higher for the downstream side of the slab than it is for the upstream side. Simple trajectory calculations of secondary electrons in an RF field show a significant forward motion of electrons parallel to the direction of microwave propagation. Electrons participating in a saturated secondary avalanche on the upstream side are driven into the surface, and electrons on the downstream side are driven off the surface, because of the influence of the microwave magnetic field. In agreement with the standard model of dielectric surface flashover for dc conditions (saturated avalanche and electron-induced outgassing), the corresponding change in the surface charge density is expected to be proportional to the applied breakdown threshold electric field parallel to the surface.

Prebreakdown and breakdown investigation of needle-plane vacuum gaps in the micron/submicron regime

This article reports investigations of insulation failure of needle-plane gaps of micrometric dimensions. Using a piezoelectric translational stage it is possible to set the gap spacing in the range between 0.1 and 40 μm with a resolution less than 0.04 μm. The prebreakdown and breakdown characteristics of the needle-plane gap were investigated as a function of the gap spacing, emitter radius, ambient pressure, and conditioning effects. The experimental results showed that the breakdown voltage of a microtip-plane gap is relatively high. For a 2 μm radius tip set at a 1 μm gap distance from a planar electrode, the breakdown voltage was about 500 V. It was found that an increase of pressure from 10−6 to 10−2 Torr did not influence the value of the threshold voltage for breakdown. dc glow discharge conditioning in air at a pressure of ∼1 Torr increased the value of threshold voltage for breakdown in vacuum by about 20%–30%. The obtained prebreakdown characteristics of microtip type gaps is very important for the development of vacuum microelectronic devices; specifically, these data provide valuable insights to the practical limits to which microtip-gate gaps can be stressed in practical field emitter arrays in field emission displays.

Vacuum emission and breakdown characteristics of a planar He–Xe microdischarge

Vacuum emission, breakdown, and current–voltage characteristics of a large surface area-to-volume ratio planar dc microdischarge have been measured for different He–Xe mixtures in the range of 10%–100% Xe and pressures in the range of 60–500 Torr. The electrical measurements show that the Paschen breakdown curves and the current–voltage characteristics are primarily controlled by the xenon concentration in the mixture. A study of the vacuum ultraviolet emission of xenon atoms at 147 nm and of xenon dimers at 150 nm normalized to the discharge power reveals the presence of local maxima in the emitted intensity as a function of discharge pressure and xenon concentration, indicative of complex excited-state atomic and excimer kinetic processes.

Breakdown of the high-voltage sheath in metal plasma immersion ion implantation

It is suggested that breakdown of a space-charge sheath obeys similar breakdown laws as known for vacuum breakdown. When metal plasmas of vacuum arcs are used, the sheath between a biased substrate and plasma is very thin and the electric-field strength is very high. Field enhancement (e.g., at sharp edges of the substrate) leads to thermal instability of electron emission centers, followed by dense plasma formation which, in turn, electrically short circuits the sheath (breakdown). Theoretical and experimental evidence for this mechanism is presented.

Changes of Cu electrode surface conditions and prebreakdown current characteristics caused by repetitive breakdowns in a vacuum

Changes of Cu electrode surface conditions and prebreakdown current characteristics caused by repetitive breakdowns in a vacuum

An initiation mechanism of thermal instability of a metal-diamond-vacuum field emission regime

An analysis is carried out of the physical origin of thermal instability that can trigger a catastrophic vacuum breakdown event in vacuum microelectronic devices based on flat diamond emitters. The temperature rise in a diamond film will enhance internal field emission across metal–diamond interface. This effect can lead to a regenerative process that can initiate a breakdown event at temperature lower than the melting point of an emitter. A set of equations has been developed. These theoretical findings are successfully applied to explain the instability of field emission from the nondoped diamond films.

Stability consideration of metal–diamond–vacuum micro-emission regimes

Details are given of both experimental and theoretical studies of the vacuum breakdown phenomena associated with field emission from a metal–diamond–vacuum micro-emission regime. Real-time recording in conjunction with the transparent anode imaging technique was employed to study the process of the breakdown event. It was found that such an event can be cathode initiated due to the thermal heating in diamond film emitter. The theoretical study shows that a new mechanism is responsible for thermal instability of a channel: the effect of temperature rise of the channel on field emission from substrates. The results calculated based on the new theory are qualitatively consistent with the experimental findings.

Thermal instability of molybdenum and silicon tips

Thermal effects in molybdenum and silicon tips are analyzed with the aim of understanding the initiating mechanism of thermal instability leading to vacuum breakdown. A new theoretical treatment is employed to study the thermal response of tip emitters, which takes into account the correlative effect between field emission and its resultant temperature rise at the apex of tips. The results show that for a typical Si tip, thermal instability would occur before the tip temperature reaches its melting point, while thermal effect is not significant in the case of the Mo tip with the same geometry and dimension of a silicon tip. This remarkable difference between two types of tip is mainly due to their large discrepancies in electrical resistivity, but the Nottingham effect contributes significantly as well.