Vacuum Breakdown Research Articles

Extreme field science

We discuss the progress in the development of extreme light sources that will open new horizons for studying a wide range of fundamental science and astrophysics problems. The regimes of dominant radiation reaction, which completely change the electromagnetic wave–matter interaction, will be revealed, resulting in a new extremely powerful source of ultrashort high-brightness gamma-ray pulses. The possibility of abundant electron–positron pair creation via multi-photon processes and the possibility of reaching the critical field of quantum electrodynamics, which would lead to vacuum polarization and breakdown, are attracting considerable attention.

Slowly balding black holes

The "no hair" theorem, a key result in General Relativity, states that an isolated black hole is defined by only three parameters: mass, angular momentum, and electric charge; this asymptotic state is reached on a light-crossing time scale. We find that the "no hair" theorem is not formally applicable for black holes formed from collapse of a rotating neutron star. Rotating neutron stars can self-produce particles via vacuum breakdown forming a highly conducting plasma magnetosphere such that magnetic field lines are effectively "frozen-in" the star both before and during collapse. In the limit of no resistivity, this introduces a topological constraint which prohibits the magnetic field from sliding off the newly-formed event horizon. As a result, during collapse of a neutron star into a black hole, the latter conserves the number of magnetic flux tubes $N_B = e \Phi_\infty /(\pi c \hbar)$, where $\Phi_\infty \approx 2 \pi^2 B_{NS} R_{NS}^3 /(P_{\rm NS} c)$ is the initial magnetic flux through the hemispheres of the progenitor and out to infinity. We test this theoretical result via three-dimensional general relativistic plasma simulations of rotating black holes that start with a neutron star dipole magnetic field with no currents initially present outside the event horizon. The black hole's magnetosphere subsequently relaxes to the split monopole magnetic field geometry with self-generated currents outside the event horizon. The dissipation of the resulting equatorial current sheet leads to a slow loss of the anchored flux tubes, a process that balds the black hole on long resistive time scales rather than the short light-crossing time scales expected from the vacuum "no-hair" theorem.

Effect of Internal Electrical Field on the Electrical Breakdown Properties of Contact Materials

In order to get a deep understanding of the effect of internal electrical field on the electrical breakdown properties of contact materials, two kinds of W-Cu and Cr-Cu joints were prepared in a vacuum sintering furnace. The vacuum breakdown tests were respectively performed at the interface of two joints and pure metal ends in an arc extinguishing chamber, and the surface morphologies after electrical breakdown 50 times were characterized by a scanning electron microscopy equipped with an energy dispersive spectroscopy. The results show that the breakdown strength at the interface is much larger than that of pure metal ends, and the breakdown site deviates from the interface. It is suggested that the existence of internal electrical field at the interface of two contact metals changes the electronic structure, and, thus, the electrical breakdown behavior is influenced.

Ageing effect on the field emission reproducibility of multiwalled carbon nanotubes

The ageing effect on the field emission (FE) reproducibility of multiwalleded carbon nanotubes (MWCNTs) synthesised by plasma enhanced chemical vapour deposition was investigated by scanning electron microscopy. To investigate the ageing effect on the physical state of the MWCNTs, a scanning electron microscope (SEM), a Raman spectrometer, an X-ray photoelectron spectroscope and an X-ray diffractometer were used to characterise the MWCNTs comprehensively before and after the ageing. Through ageing, the FE reproducibility became better because the larger number and more evenly distributed shorter MWCNTs – (resulted from the local vacuum breakdown of higher MWCNTs due to Joule heating and oxidisation) – became the dominating emitters.

Effect of Fe on vacuum breakdown properties of CuW alloys

CuW alloys with different Fe additions were prepared by the infiltration method. The microstructures of the CuW alloys prepared were characterized by a transmission electron microscope (TEM). The effect of Fe content on the surface erosion morphology was observed by a scanning electron microscope (SEM). The distribution of cathode spots was analyzed by a digital high-speed video camera. The results show that Fe addition can improve the relative density and thermal conductivity of CuW alloy with less than 0.6 wt.% Fe. However, excessive Fe addition can deteriorate the relative density and thermal conductivity. TEM results show that Fe addition can promote the formation of CuW solution. The area and depth of the erosion pits on the surface of CuW alloy depend on the breakdown position. Fe addition can strengthen the interface bond of Cu and W, improve the arc stability and the division of the cathode spots and, thus, avoid the concentration erosion.

Liquid phase separation of Cu–Cr alloys during the vacuum breakdown

The microstructures of the surfaces and cross sections of Cu–Cr contact alloys were analyzed after the vacuum breakdown was repeated for 100 times. The results reveal that the liquid phase separation was involved in the microstructure evolution of the alloys during the breakdown. The Cr-rich phase from the liquid separation featured the morphology of spherical- and sheeted-shaped particles. The sizes of the Cr spheres ranged from several nanometers to hundreds of nanometers, and were not strongly dependent on the compositions of the alloys. The large Cr-rich droplets could be elongated and swirled due to the liquid flow, resulting in the formation of the Cr phase with sheeted-shaped particles. The fractions of Cr sheets increased with the increasing of the Cr contents; however, their sizes decreased significantly, compared with those of their parent alloys.

Stripper foil failure modes and cures at the Oak Ridge Spallation Neutron Source

The Oak Ridge Spallation Neutron Source comprises a 1 GeV, 1.5 MW linear accelerator followed by an accumulator ring and a liquid mercury target. To manage the beam loss caused by the ${H}^{0}$ excited states created during the ${H}^{\ensuremath{-}}$ charge-exchange injection into the accumulator ring, the stripper foil is located inside one of the chicane dipoles. This has some interesting consequences that were not fully appreciated until the beam power reached about 840 kW. One consequence was sudden failure of the stripper foil system due to convoy electrons stripped from the incoming ${H}^{\ensuremath{-}}$ beam, which circled around to strike the foil bracket and cause bracket failure. Another consequence is that convoy electrons can reflect back up from the electron catcher and strike the foil and bracket. An additional contributor to foil system failure is vacuum breakdown due to the charge developed on the foil by secondary electron emission. In this paper we detail these and other interesting failure mechanisms and describe the improvements we have made to mitigate them.

Study of large gap 200 kV breakdowns in vacuum with high stored energy for ITER neutral beam accelerator

In the context of the ITER contract “ITER/CT/07/219–200 kV Stored Energy Tests”, electrical breakdown tests have been performed in vacuum with a stored energy of up to 425 J. The experiments have been conceived and performed with the collaboration of Consorzio RFX. The tests are being performed in the 1 MV test facility at IRFM, CEA-Cadarache. They should simulate the conditions that will be found in the ITER Neutral Beam accelerator, at 200 kV. This paper presents the set-up of the test bed, the choice of critical components, the diagnostic equipments and the results obtained with 200 kV applied on the anode electrode.

Avalanche mechanism of vacuum breakdown

Justification of the hypothesis of the avalanche mechanism of vacuum breakdown is examined. For this purpose, values of dc and impulse breakdown voltage have been measured under well-defined laboratory conditions, in the pressure range from 10−6 mbar to 1 bar, and for inter-electrode distances from 0.1 to 1 mm. Variable parameters of the experiment included the type of residual gas and the material of the electrodes in the system used for forming a homogeneous electric field. Statistical analysis of experimental results and their comparison with the predictions of a theoretical model attest that the avalanche mechanism of breakdown does come about in low vacuums.

Schwinger Limit Attainability with Extreme Power Lasers

High intensity colliding laser pulses can create abundant electron-positron pair plasma [A. R. Bell and J. G. Kirk, Phys. Rev. Lett. 101, 200403 (2008)], which can scatter the incoming electromagnetic waves. This process can prevent one from reaching the critical field of quantum electrodynamics at which vacuum breakdown and polarization occur. Considering the pairs are seeded by the Schwinger mechanism, it is shown that the effects of radiation friction and the electron-positron avalanche development in vacuum depend on the electromagnetic wave polarization. For circularly polarized colliding pulses, these effects dominate not only the particle motion but also the evolution of the pulses. For linearly polarized pulses, these effects are not as strong. There is an apparent analogy of these cases with circular and linear electron accelerators to the corresponding constraining and reduced roles of synchrotron radiation losses.

Vacuum breakdown limit and quantum efficiency obtained for various technical metals using dc and pulsed voltage sources

For the SwissFEL project, an advanced high gradient low emittance gun is under development. Reliable operation with an electric field, preferably above 125 MV/m at a 4 mm gap, in the presence of an ultraviolet laser beam, has to be achieved in a diode configuration in order to minimize the emittance dilution due to space charge effects. In the first phase, a dc breakdown test stand was used to test different metals with different preparation methods at voltages up to 100 kV. The authors show that gradient achieved for rough machined (Ra<200 nm) metal electrodes followed by an argon glow plasma are similar to the one obtained using a mirrorlike electrode (Ra<40 nm). In addition, high gradient stability tests were also carried out over several days in order to prove reliable spark-free operation with a minimum dark current. In the second phase, electrodes with selected materials were installed in the 250 ns full width at half maximum, 500 kV electron gun and tested for high gradient breakdown and for quantum efficiency using an ultraviolet laser. Routine electron beam operation, breakdown-free, at 50 MV/m (6 mm gap, 10 Hz repetition rate) at various charges is now achieved using different metal electrodes.

Effect of Contact Mode on the Electrical Transport and Field‐Emission Performance of Individual Boron Nanowires

AbstractVapor–liquid–solid processing of boron nanowires (BNWs) can be carried out either using a bottom‐up or top‐down growth mode, which results in different contact modes between the nanowire and the substrate. The contact mode may strongly affect the electrical transport and field‐emission performance of the individual boron nanowires grown on a Si substrate. The electrical transport and field‐emission characteristics of individual boron nanowires of different contact modes are investigated in situ using a scanning electron microscope. The contact barriers are very distinct for the different contact modes. Moreover, the transition from a “contact‐limited” to a “bulk‐limited” field‐emission (FE) process is demonstrated in nanoemitters for the first time, and the proposed improved metal–insulator–vacuum (MIV) model may better illustrate the nonlinear behavior of the Fowler‐Nordheim (FN) plots in these nanoscale systems. Individual BNWs with different contact modes have a discrepancy in their emission stability and vacuum breakdown characteristics though they have similar aspect ratios, which suggests that their electrical transport and field‐emission performance are closely related to their contact mode. Boron nanowires grown in the base‐up mode have better field‐emission performances and are more beneficial than those grown in the top‐down mode for various device applications.

Validity of the Space-Time Enlargement Law for vacuum breakdown

This paper investigates, theoretically and experimentally, the applicability of the Space-Time Enlargement Law to vacuum-insulated systems. A discussion on how characteristics of possible vacuum breakdown mechanisms determine the distribution function of the breakdown voltage random variable is presented. By superimposing effects of electrode surface enlargement and inter-electrode gap enlargement, expressions for the mean value and standard deviation of the breakdown voltage random variable are obtained. In the case of time extensions, the assumption of complete independence of consecutive discharge processes is discussed. Experimental testing of the Enlargement Law was performed on Rogowski type two-electrode systems, with different electrode surface areas, inter-electrode gaps and vacuum pressures. Measurements were conducted using industrial ac voltage, dc voltage with 50 V/s rate of rise, standard atmospheric pulse voltage (1,2/50 μs), and commutational pulse voltage (250/2500 μs). The final conclusion, based on the comparison of theoretical considerations and the experimental results, is that the Space-Time Enlargement Law can be applied in the design phase during the development of vacuum devices, with certain limitations, regardless of the type of the applied voltage.

Avoiding Current Loss in an X-Pinch by Shortening the Gap between Anode and Cathode

A current loss happens in the load section of an X-pinch, and it most possibly results from a vacuum breakdown somewhere from the side top of the cathode outward to the anode. The current loss was avoided by shortening the a-c gap, which is the gap between the anode and the cathode. It was found that the shortening of the a-c gap has reduced the impedance of the a-c gap, leading to a reduced impedance of the load section. The smaller impedance has two effects. If the incident voltage wave to the load section is kept unchanged, the voltage applied to the load section is lowered due to the lesser reflection as a result of the smaller impedance of the load section. If the voltage applied to the load section is kept unchanged, the voltage applied to the a-c gap is reduced due to a decreasing percentage of the a-c gap impedance in the load impedance. Both effects work together to reduce, in turn, the voltage applied to a-c gap and the electric field in the region close to the side top of the cathode, which avoids the breakdown as well as the current loss.

Effect of Al2O3 Content on Electrical Breakdown Properties of Al2O3/Cu Composite

Al2O3/Cu composites were prepared by external addition of Al2O3, and the effect of Al2O3 content on microstructure, density, hardness, electrical conductivity and vacuum electrical breakdown properties was studied. The results show that with increasing Al2O3 addition, the density of Al2O3/Cu composite significantly decreases, the hardness sharply increases and then slowly decreases, but the electrical conductivity invariably decreases. The vacuum breakdown test shows that with increasing Al2O3 addition, the breakdown strength first sharply increases and then decreases when the Al2O3 content exceeds 1.2 wt.%; the chopping current always exhibits a decreasing trend and the arc life first increases and then decreases. According to the morphology of arc erosion and analysis, the arc erosion resistance increases and then decreases sharply. In the range of experiments, the optimal arc erosion resistance of Al2O3/Cu composite can be obtained with the addition of 1.2 wt.% Al2O3.

Electrical breakdown mechanisms in vacuum diodes

The aim of this paper is to determine a statistical model which provides a satisfactory description of the “vacuum breakdown voltage” random variable and its relation to the vacuum breakdown mechanism. For that purpose, theoretical and experimental investigations of pulse and dc breakdown voltage statistical properties in two-electrode vacuum-insulated diodes were conducted. During the experiment vacuum pressures were 10 −9, 10 −6, 10 −4, 10 −3 bar and inter-electrode gaps were 0.1, 0.3 and 0.5 mm. Cylindrical copper electrodes with polished surfaces to a high glam were used. The field in the inter-electrode region was homogeneous with slight inhomogeneity at the edges of the electrode system (edge effects). According to results obtained it should be pointed out that, under conditions of the experiment, at the pressure value of 10 −9 bar breakdown mechanism is emission mechanism of cathode type, while at the pressure of 10 −6 and 10 −4 bar breakdown mechanism is combination of emission mechanism of cathode type and vacuum avalanche mechanism, while at the pressure of 10 −3 bar breakdown mechanism is Townsend gas mechanism with occurrence of anomal Paschen effect.

Investigation of the dc vacuum breakdown mechanism

Breakdowns occurring in rf accelerating structures will limit the ultimate performance of future linear colliders such as the Compact Linear Collider (CLIC). Because of the similarity of many aspects of dc and rf breakdown, a dc breakdown study is underway at CERN to better understand the vacuum breakdown mechanism in a simple setup. Measurements of the field enhancement factor $\ensuremath{\beta}$ show that the local breakdown field is constant and depends only on the electrode material. With copper electrodes, the local breakdown field is around $10.8\text{ }\text{ }\mathrm{GV}/\mathrm{m}$, independent of the gap distance. The $\ensuremath{\beta}$ value characterizes the electrode surface state, and the next macroscopic breakdown field can be well predicted. In breakdown rate experiments, where a constant field is applied to the electrodes, clusters of consecutive breakdowns alternate with quiet periods. The occurrence and lengths of these clusters and quiet periods depend on the evolution of $\ensuremath{\beta}$. The application of a high field can even modify the electrode surface in the absence of breakdown. Measurements of time delays to breakdown show two distinct populations, immediate and delayed breakdowns, indicating that two different mechanisms could exist. The ratio of these two populations depends on the conditioning state of the electrodes and on material. Gas release during breakdown is dominated by ${\mathrm{H}}_{2}$ and CO. This degassing is mainly due to electron-stimulated desorption. During the quiet periods without breakdown, gases are also released but the quantities are much smaller. All the measurements presented here emphasize the crucial role of field emission in the breakdown triggering.

Effect of Al 2O 3 particle size on vacuum breakdown behavior of Al 2O 3/Cu composite

In order to clarify the effect of Al 2O 3 particle size on the arc erosion behavior of the ceramic-reinforced Al 2O 3/Cu composite, Al 2O 3/Cu composites with different sizes of Al 2O 3 particles were prepared by powder metallurgy, the effect of Al 2O 3 particle size on the characteristics of arc motion was studied, and the mechanism of arc erosion of Al 2O 3/Cu composites was discussed as well. The results show that with decrease in the size of Al 2O 3 particles, the erosion area increases significantly and the erosion pits become shallower. The vacuum breakdown is preferred to appear in the area between Al 2O 3 particle and the copper matrix. Based on the experimental results and theoretical analysis, a particle partition arc model is proposed.

A new approach for explanation of specimen rupture under high electric field

Failure of needle-shaped specimens under the action of high electric fields is one of the most serious problems in many field-ionization devices and methods. It was shown that the length of what was removed during discharge part of specimens in most of the cases was several orders greater than that of an initial radius of the specimen apex. Experimental results obtained in this work indicate that electrical breakdown in field-ion microscopes is initiated by the impact of nanoparticles detached from a point anode in a high electric field. A new approach for explanation of an observed specimen failure in a high electric field was proposed, where the formation and expansion of the cathode plasma play the key role in the mechanical rupture and melting of field-ion tips. The expanding plasma behaves as a dynamic capacitor plate characterized by a high velocity and increasing field strength at the specimen tip. The field rise rate at a constant voltage in a dynamic plasma-to-tip capacitor is of the order of >10 20 V/m/s. The present experimental data along with the ecton model of the vacuum breakdown allow the results to be discussed as a mechanical sample failure under the action of the superhigh positive field pulse induced by the cathode plasma expansion.

Optimization technique for electrical insulation design of vacuum interrupters

Because of the excellent high insulation performance, vacuum is expected to be one of the alternatives of SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas. In order to reduce the use of SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas in substation equipment, the applicability of vacuum circuit breakers (VCB) and vacuum interrupters (VI) should be extended to higher (typically 145-300 kV) voltage. For the high voltage VI, the role of electrical insulation design becomes more important. For accurate insulation design of the VI, we developed an optimization technique to improve the electrical insulation performance. We evaluated several design variables necessary for optimizing the electrode contour of the main contactor and center shield in VI. The electrode area effect on vacuum breakdown characteristics as well as the electric field distribution was considered in the optimization process. In order to verify the accuracy and the efficiency of the proposed technique, we applied it to a practical model of VI. In addition, we examined the influence of the parameters of the area effect, arrangement of VI in VCB on the optimization result. From the calculation results, we confirmed that the optimization based on the area effect had higher availability and higher accuracy than the electric field optimization from the viewpoint of the improvement of the electrical insulation performance of vacuum insulated equipment.