Vacuum Breakdown Research Articles

A contribution to consideration of stochastic properties of mechanisms of electrical breakdown in vacuum

A contribution to consideration of stochastic properties of mechanisms of electrical breakdown in vacuum

Efficient generation of multigigawatt rf power by a klystronlike amplifier

This article addresses the new development of high-power rf klystronlike amplifiers using modulated intense relativistic electron beams. Development of these amplifiers follows earlier research in which the interaction between a high-impedance (120-Ω) intense relativistic electron beam and a low-power rf pulse resulted in the generation of coherent bunches of electrons with excellent amplitude and phase stabilities. In the present experiment a low-impedance (30-Ω) large-diameter (13.2-cm) annular electron beam of power ∼8 GW was modulated using an external rf source (magnetron at 1.3 GHz) of 0.5 MW power. The interaction of the modulated electron beam with a structure generated a 3-GW rf pulse that was radiated into the atmosphere. The self-fields of the intense beam provided significant electrostatic insulation against vacuum breakdown at the modulating gaps and at the rf extraction gap.

真空絶縁破壊電圧に対するスパッタリング薄膜の効果

真空絶縁破壊電圧に対するスパッタリング薄膜の効果

A new protection system against high voltage vacuum breakdowns

A passive, electrical fast-switch-off/on system for protecting electrodes subject to fortuitous high voltage vacuum breakdown has been developed. This system is based on the principle of oscillatory discharges in an RLC network coupled with the use of a diode. When a breakdown occurs, the current is automatically and rapidly switched off, preventing the damaging of electrodes and power supply by the electrical arc. The interruption lasts as long as required to stop the short-circuit metal vapor arc: a couple of milliseconds are generally sufficient. Then the current is automatically restored, allowing a rapid recovery of the normal electrical operating conditions. Besides the economical advantages of this circuit, one should note that its operating characteristics are very useful for electrode conditioning. The study of this protection circuit has been made for protecting some elements of the neutral beam injectors in the Tore Supra Tokamak experiment, but its field of application should be quite large.

Use of a helical resonator as a capacitive transducer in vibrating reed measurements

A new design of the vibrating reed apparatus for studying the high-resonant overtones of very small samples is described. Using the high Q (∼103) helical rf resonator as a capacitive transducer, great sensitivity has been obtained. The theoretical sensitivity of the detector is investigated with transmission-line theory, and the result shows that the Johnson-noise-limited sensitivity is proportional to the Q of the resonator instead of Q1/2, as is the case of lumped LC-tuned circuit analyses. For a nonsuperconducting resonator, the Johnson-noise-limited minimum detectable displacement (Δdmin) at room temperature, with bandwidth B, is about 10−7 Å (B/Hz)1/2, if the rf electrical field level is only limited by vacuum breakdown. An application of the apparatus in studying the temperature-dependent changes in the Young’s modulus of single crystals of TaS3 is shown.

Synchrotron radiation lithography system in an atmospheric environment (invited)

The atmospheric environmental exposure system for synchrotron radiation (SR) lithography has been integrated using the Photon Factory storage ring (2.5 GeV). The system, composed of a highly reliable beamline, an SR extracting chamber and a prototype SR stepper, aims at attaining higher accuracy and throughput. Based on a fail-safe mechanism notion, a double-vacuum protection system, in which two sets of a fast closing valve and acoustic delay line are installed in the main beamline and branch beamline, respectively, has been organized. Vacuum breakdown tests indicated that any vacuum breakdown, a beryllium (Be) window rupture in the worst case, exerts little influence on the storage ring ultrahigh vacuum. The SR extracting chamber, equipped with a Be window and an extraction window, is filled with helium at atmospheric pressure. Particularly, the 50-μm-thick, 35-mm-diam Be window, vacuum-sealed by a Viton O-ring, was preliminarily employed and, so far, has operated successfully, giving a 25-mm square exposure area. In terms of practical availability and simplicity, the SR stepper in an atmospheric environment has been constructed. A novel differential mode linear Fresnel zone plate alignment method, which can detect an alignment error between a mask and a wafer during exposure, was developed.

Pulsed electrical discharges in vacuum at cryogenic electrode temperatures

A review is presented of recent investigations concerning the effect of cathodes at cryogenic temperatures on the initiation of a pulsed vacuum breakdown. It has been observed that cooling Nb and W cathodes from 300 K to 4.2 K results in a monotonic increase of the critical emission current and the pulsed electric strength of the vacuum gap. This appears to be due to variations of the thermophysical characteristics (resistivity, thermal conductivity and heat capacity) of the cathode on cooling. The superconducting state of an Nb cathode essentially does not affect the vacuum breakdown delay time. Also given are the results of a study of the initiation of a vacuum breakdown between electrodes made of high-temperature superconductors.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Methods of increasing the surface flashover potential in vacuum

The effects of surface coating and magnetic fields on dielectric surface breakdown in vacuum are described. Coatings of metals and meta oxides increase the pulsed surface hold-off voltage by factors of up to three, depending on the material. Measurements of the secondary emission coefficient show substantial differences between coated and uncoated samples at high energies. The influence of magnetic fields on dielectric surface breakdown for uncoated samples is investigated in the pressure range of 10/sup -5/ to 1 Pa, for both DC and pulsed electric and magnetic fields. Insulation effects (increase in breakdown voltage of up to 2* at magnetic field amplitudes of 0.5 T) are observed when the magnetic field is parallel to the surface of the dielectric and perpendicular to the applied electric field. Magnetic insulation shows a strong material dependence and decreases with increasing pressure and surface roughness.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Statistical properties of electrical breakdown in vacuum

The breakdown probability distribution function was determined for DC and impulse voltages, for conditions of gap pressure of 10, 0.1, and 10/sup -4/ Pa and gap length of 0.1 mm. It was found that DC and impulse voltages are associated with different types of probability distribution function due to different initiation mechanisms. The statistical influence of the number of previous breakdowns on the probability distribution of the breakdown voltage was investigated. By applying the U test for analysis of measured data, it was found that at lower gap pressure, the breakdown voltage probability changes after a smaller number of breakdowns than it does at higher gap pressure. >

Insulation of a hollow beam gun

The authors are investigating the possibility of accelerating particles with a high gradient in a wake-field transformer. They focus on the short-pulse, high-current hollow-beam gun being developed for this experiment and especially on the insulation problems. A laser-driven gun that works in a solenoid field of 0.2 T was chosen. At a cathode voltage of 100 kV, the gun produces a hollow beam of 109-cm diameter with a space-charge-limited current of 100 A over a pulse length of a few nanoseconds. The cathode voltage is limited by vacuum breakdown, caused by electrons, emitted from the cathode and from the surfaces of optical elements between the electrodes. >

The effect of low temperature on the role played by electrodes in vacuum breakdown

Electrical discharge in a vacuum is discussed, showing that the electrical breakdown requires the creation of plasma in the interelectrode space. Both electrodes are a source of plasma, but the more important dense plasma is created on the anode. In a system with an insulating spacer, the insulator materials is the source of plasma. Plasma creation from a cooled anode needs more energy. Similarly, cooling of the parts of the insulator where the plasma is created increases the electrical strength. >

Applicability of simple expressions for electrical breakdown probability in vacuum

Simple analytical expressions for the law of breakdown probability are suggested. They are tested by comparison with experimental data obtained by impulse breakdown in 10-Pa vacuum for very small gaps (d >

Breakdown phenomena in high-power klystrons

In the course of developing new high-peak-power klystrons, high electric fields in several regions of these devices have become an important source of vacuum breakdown. In addition, a renewed interest in breakdown phenomena for nanosecond-pulse, megavolt-per-centimeter fields has been sparked by recent work in the area of gigawatt RF sources. The most important regions of electrical breakdown are in the output cavity gap area, the RF ceramic windows, and the gun ceramic insulator. The experiments and results on the breakdown in these regions are discussed, as well as the solutions to alleviate this breakdown problem. >

Vacuum breakdown

A short review is given of physical processes which limit high voltage insulation in vacuum. Prebreakdown field emission, ignition events, current flow during full breakdown and conditioning procedures are described and discussed. It is argued that cathodic phenomena are dominating for well conditioned electrodes.

Fast cathode processes in conditioning of vacuum electrodes

High-speed streak photographs of vacuum breakdown show oscillations in light emission at the cathode. Experimental results that show the change in frequency of that oscillation as conditioning progresses are presented. The link between the oscillations and conditioning supports the hypothesis that adsorbed gas removal is the cause of conditioning in vacuum breakdown.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Area effect on electric breakdown of copper and stainless steel electrodes in vacuum

The breakdown field strength E/sub b/(= alpha Vb/d) of gaps made with pure copper and stainless steel depends on the effective area S/sub eff/. It can be expressed by the single experimental equation E/sub b/=K/sub 1/S/sub eff//sup -n/, where K/sub 1/ is a constant, n is about 0.24, S/sub eff/ is the electrode area subjected to more than 90% of maximum field strength, alpha is the shape factor, d is the gap length, and V/sub b/ is the breakdown voltage. The field intensification factor beta /sub l/ obtained through the measurement of prebreakdown current can be expressed by a single experimental equation beta /sub l/=K/sub 2/S/sub eff/n, where K/sub 2/ is a constant. This means that the area of effect of E/sub b/ is represented by the area effect of beta /sub l/ and that the vacuum breakdown may be initiated by field emission.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Changes in components of residual gases and conditions of electron bombarded surface during prebreakdown situation in a vacuum gap.

Gas liberation from electron-bombarded electrode surface during prebreakdown situation is one of important factors that cause electrical breakdown in a vacuum gap. Changes in residual gas components were monitored with a quadrupole mass specrometer, and electron-bombarded electrode surfaces were analyzed with an X-ray photoelectron spectrometer. The ions formed in the prebreakdown gap were also analyzed by a time of flight mass spectrometer installed in a prebreakdown measurement apparatus. It was found that the residual gas components in the prebreakdown situation are H, H2, H2O, CO, and CO2, and the mass numbers of the carriers formed in the vacuum gap were the same as those of the residual gas components. The reaction of the electron bombarded surface and the effect of the reaction on vacuum breakdown phenomena are also discussed.

Development of highly reliable synchrotron radiation lithography beamline

The reliable beamline structure for synchrotron radiation lithography has been investigated using the Photon Factory storage ring (2.5 GeV). The recently built beamline aims at attaining system reliability and safety. This beamline, one of three branch lines split from a basic beamline, is a 10−7 Pa ultrahigh‐vacuum system with an oscillating mirror. In addition to a 40 ms fast closing valve (FCV) and an acoustic delay line (ADL), installed in the basic beamline, a <15 ms FCV and 40 ms ADL were set up to protect the storage ring from accidental breakdown. The FCV and ADL were placed far upstream of the oscillating mirror, to cope with accidental gas leakage caused by the oscillating mechanism. A vacuum breakdown test demonstrated that the FCV and ADL are greatly effective in vacuum protection. In order to protect operators from x‐ray exposure, two auxiliary shutters made of tantalum were placed upstream of the oscillating mirror. The oscillating mirror, driven through bellows by a combination of a direct current servomotor and a cam mechanism, enabled a highly reliable oscillation. A double‐structured bellows was adopted to provide against gas leakage. In addition, a silicon carbide plane mirror (40×17×4 cm) was employed because of its high‐heat‐resistance capability.

An Energy Explanation of the Area Effect in Electrical Breakdown in Vacuum

The mechanism of the so called "area effect" in vacuum dis charge studies was investigated. Point-plane aluminum electrodes were used with point radius 1.5 mm and gaps between 0.5 and 10 mm. It was found that a capacitor connected in parallel with an electrode gap reduces the electrical strength in a manner similar to that observed with increasing electrode area. The capacitance was found to play two major roles. On one hand, the larger stored energy associated with a larger capacitance promoted greater cathode damage. As a result, subsequent voltage breakdowns occurred at lower voltage levels. On the other hand, when addi tional stored energy was placed within 15 cm of a discharge site, the initial voltage application resulted in a breakdown voltage less than that observed in the absence of the capacitor during a comparable test. The results presented here advocate an energy based explanation of the area effect rather than a probabilistic weak link theory.

真空中の破壊電圧に及ぼす電界分布の影響

真空中の破壊電圧に及ぼす電界分布の影響