Vacuum Breakdown Research Articles

Compression of powerful x-ray pulses to attosecond durations by stimulated Raman backscattering in plasmas

Backward Raman amplification (BRA) in plasmas holds the potential for longitudinal compression and focusing of powerful x-ray pulses. In principle, this method is capable of producing pulse intensities close to the vacuum breakdown threshold by manipulating the output of planned x-ray sources. The minimum wavelength limit of BRA applicability to compression of laser pulses in plasmas is found.

Scaling laws for dielectric window breakdown in vacuum and collisional regimes

The scaling laws for the initiation time of radio frequency (rf) window breakdown are constructed for three gases: Ar, Xe, and Ne. They apply to the vacuum, to the multipactor-triggered regime, and to the collisional rf plasma regime, and they are corroborated by computer simulations of these three gases over a wide range of pressures. This work elucidates the key factors that are needed for the prediction of rf window breakdown in complex gases, such as air, at various pressures.

Electrical breakdown of short multiwalled carbon nanotubes

Electrical breakdown of short (∼50nm) multiwalled carbon nanotubes (MWNTs) is studied utilizing mechanically controllable break junction technique with gold electrodes. Measurements of the conductance-bias characteristics near the breakdown point revealed two different types of breakdown behavior for the short MWNTs. In one type designated as type I, the conductance increases nearly linearly with the bias and, over the breakdown point, decreases stepwise. On the other hand, in the type-II breakdown, the conductance shows a transient and irreversible increase right before the breakdown and subsequently jumps to zero. We consider that the type-II breakdown in vacuum is contact related, whereas the type-I breakdown occurs through the usual MWNT heating.

Addressing the lower limit of breakdown delay time in vacuum

Based on the Joule mechanism of vacuum breakdown initiation, the dependence of a pulsed electric strength on the high-voltage pulse duration at a finite rate of electric field buildup in the vacuum gap is calculated. The results of calculations are confirmed by experimental data available for subnanosecond pulses. It is shown that the breakdown delay time has no lower limit at an electric strength of up to ∼3 × 1010 V/m observed in experiment. The independence of the breakdown delay time of the electric field strength for pulse durations comparable with the pulse front width is explained by the finite rate of applied voltage buildup.

Limits for light intensification by reflection from relativistic plasma mirrors

Ultraintense laser pulses might be capable of producing plasma mirrors through ultrarelativistic oscillating electrons. Two kinds of such mirrors with relativistic factors γ∼100–300 were contemplated recently as a tool for producing huge frequency upshifts 4γ2 of reflected laser pulses as well as the compressing and focusing of those pulses. The combination of these effects would result in dramatic light intensification toward the vacuum breakdown intensities (Schwinger limit) [S. V. Bulanov et al., Phys. Rev. Lett. 91, 085001 (2003) and S. Gordienko et al., Phys. Rev. Lett. 94, 103903 (2005)]. The analysis performed in these publications was limited, however, to idealized situations of cold uniform plasmas and uniform laser intensities. The analysis here of effects of electron thermal motion and random inhomogeneities in the plasma density or laser intensity indicates that the largest relativistic factors allowed within these schemes are much smaller than those assumed in the idealized models, unless essentially new physical mechanisms are adduced in addition to those already considered.

Field emission current and vacuum breakdown by a pointed cathode

Vacuum breakdown of very small gap in the range of 30 nm to 2 μm, between a pointed cathode of thin tungsten wire and a plane anode of stainless steel, was experimentally investigated. The experimental setup, which consists of both electrodes and precise positioning mechanisms actuated by piezoelectric devices, was installed in scanning electron microscope (SEM). Breakdown voltage decreases with decrease of gap length and/or radius of curvature at the tip of cathode. Theoretical consideration shows that the evaporation from the anode surface heated by field emission current is a dominant factor of triggering vacuum breakdown.

On achieving better uniform carbon nanotube field emission by electrical treatment and the underlying mechanism

The uniformity in field emission from a carbon nanotube film is very important to many applications. A study has been carried out to obtain uniform emission from multiwall carbon nanotube (CNT) films by electrical treatment. The electrical treatment is based on local vacuum breakdown of CNTs under direct-current conditions. The optimal current density was found, under which the best effect of the treatment may be observed. The physical mechanism responsible for the effect of local vacuum breakdown of CNTs on emission uniformity was investigated.

Priorities of cathode and anode contaminations in triggering the short-pulsed voltage breakdown in vacuum

Modern theoretical notations on electrical breakdown in vacuum consider cathode triggering mechanisms to be most responsible on short-pulsed (<1 /spl mu/s) breakdowns while anode mechanisms to be responsible in a part on DC and long-pulsed breakdowns. Following those notations, we tried to reveal conditions at which either mechanism steps aside to another one. The study involved several experimental techniques including the anode-probe surface scanning, pulsed electron-beam surface melting in vacuum for surface cleaning, and intentional dust particle contamination of electrode surfaces. Breakdown tests were performed using a pulser capable of producing 220 kV quasi-square pulses that were adjustable to /spl sim/30 to 80 ns pulse length. Our experiments showed that cathode emission sites are responsible for breakdowns at relatively low hold-off fields. At higher electric fields of up to 1 MV/cm, the anode share in the mechanism of triggering breakdown becomes probably more significant than the cathode mechanism.

Estimation of cathode-initiated breakdown in vacuum

Cathode initiation of vacuum breakdown in DC and pulse modes is considered. New criteria and methods for evaluating the cathode initiation mechanism are proposed. They are based on consideration of relative changes in cathode surface state and electric strength. The elaborated criteria and methods have been verified with experiments results using DC and pulse modes of high-voltage treatment of electrodes. Accuracy of the proposed methods exceeds known methods precision. It is shown that optimum modes of pulsed conditioning can be used for definition of top boundary of cathode initiation mechanism applicability in DC mode.

Vacuum electrical breakdown characteristics and surface condition of ti electrodes with oxidation conditions

Outgassing from an electrode surface is regarded as a major factor leading to electrical breakdowns in vacuum. Recently oxidation treatment at 200/spl deg/C was reported as an effective means of reducing Ti outgassing. In this paper, we report our measurement and comparison of the electrical breakdown characteristics of Ti electrodes with different oxidation conditions (without oxidation, oxidation at 200/spl deg/C, oxidation at 450/spl deg/C). In addition, we analyzed electrode surfaces before and after breakdown experiments in situ with X-ray photoelectron spectroscopy (XPS). Before oxidation, we machined the electrode's surfaces to the roughness of 0.8 /spl mu/m Rmax with diamond turning. Breakdown experiments demonstrated that the breakdown field is highest at the first application of voltage to electrodes with oxidized at 200/spl deg/C. Before breakdown experiment, surface analysis revealed that all the sample electrodes had a large amount of carbon originating from the hydrocarbons of contaminants, and after the experiments, they revealed that the carbons had disappeared. To obtain breakdown characteristics of electrodes with smoother surfaces, we conducted experiments on electrodes with a surface roughness of 0.05 /spl mu/m Ra. For these electrodes, the breakdown field was higher at first breakdown; the repetitions required to achieve saturated breakdown fields were significantly fewer, and the amount of carbon on electrode surfaces before breakdown was less.

Pulsed HV vacuum breakdown of polished, powder coated, and E-beam treated large area stainless steel electrodes with 0.5 to 7 mm Gaps

An investigation of the HV vacuum breakdown between polished, powder coated, and e-beam treated 304L and 316L stainless steel electrodes is described. Tests were performed with 160 ns, 1-cos(/spl omega/t), and 260 ns flat-top voltage pulses of up to 500 kV. The high voltage hold-off for the 160 ns pulse was /spl sim/130 kV/mm for 2 mm gaps for 80-mm diameter polished stainless steel electrodes, and 15% lower for 120-mm polished and e-beam treated electrodes. The longer 260 ns pulse gave 15% lower hold-off for 80-mm electrodes. These electrodes showed voltage hold-off that scaled as the square root of the gap between 0.5 and 7 mm. This total voltage effect has been interpreted in the past as due to accelerated particles. We analyze our data in terms of this mechanism and show that only nanoparticles of molecular size could be responsible. We also discuss how ions or background gas could affect the breakdown thresholds but existing models do not predict square root dependence. We test how extremely fine powers affect hold-off and show that contaminated surfaces have relatively constant reduced breakdown E-fields that intersect the clean-electrode voltage-dependent breakdown at critical gaps defined by the type and quantity of contamination. The hold-off was /spl sim/55 and 65 kV/mm with copper powder on the cathode and anode for 2 to 6.5 mm gaps, respectively, and /spl sim/95 and 75 kV/mm for talc powder on the cathode and anode for gaps <3.5 and 6.5 mm. Optical diagnostics show no difference in the light emission from clean and contaminated electrode breakdown arcs.

スパークコンディショニング処理された陰極と陽極の真空中絶縁破壊特性

スパークコンディショニング処理された陰極と陽極の真空中絶縁破壊特性

The influence of the electric field shape on the gas breakdown under low pressure and small inter-electrode gap conditions

The connection between nonhomogeneities of the electric field and the value of the breakdown voltage [direct current (dc), pulls] in gases (N/sub 2/, SF/sub 6/) under low pressure and small inter-electrode gap was investigated in this paper. The obtained results show that the values of the breakdown voltages for nonhomogenous fields are considerably higher than the values for homogenous fields. It was also established that there is some disagreement between experimentally and theoretically (numerically) obtained values of the dc breakdown voltages for nonhomogenous fields. These results could be explained by the small distance effect and by the combination of the gas and vacuum breakdown mechanisms.

Theoretical analysis of the field enhancement in a two-dimensional triple junction

The field enhancement at a triple junction is theoretically investigated. For a long metal–dielectric–vacuum junction, the field behavior in the three media is described as a function of the geometry and the dielectric constant. It is found that the key factor determining the field enhancement is the ratio of the angles subtended by the dielectric and vacuum portions. This phenomenon can account for the field emission enhancement and the vacuum breakdown observed in experiments. The current result implies that the triple junction can be a new type of field emission source.

Electrode phenomena before long delayed breakdowns in vacuum after switching capacitive currents

Due to breakdown of vacuum interrupters used for switching of capacitor banks, high overvoltages are produced, which could result in damages in capacitor banks. Besides breakdowns of the vacuum gap during voltage rise, some breakdowns with much larger delays have been observed. The mechanism of such breakdowns is studied in this paper experimentally by considering the light intensity emitted from the vacuum gap during voltage application. The emitted light intensity is recorded using a charge coupled device (CCD) camera. The recorded light intensities show that an emission center building at the anode surface precedes the breakdown of the vacuum gap.

Manipulating ultraintense laser pulses in plasmas

An efficient way for manipulating ultraintense laser pulses in plasmas using resonant three-wave interactions is proposed. Poor quality ultraintense laser pulses can be efficiently transformed into high quality focused laser pulses, while the entropy is taken up by resonant plasma waves. This can be accomplished within plasma layers thin enough that parasitic scatterings and instabilities of laser pulses do not have enough time to develop. Combined with laser pulse compression in plasmas, this scheme, using, for example, the lasers that power the National Ignition Facility (see, for instance, the NIF project website: http://www.llnl.gov/nif/project), is potentially capable of producing vacuum breakdown intensities.

Influence of surface properties of an oxygen-free copper electrode on electrical breakdown in vacuum

Influence of surface properties of an oxygen-free copper electrode on electrical breakdown in vacuum

Cathode-initiated vacuum breakdown in a pulse regime

Based on an analysis of the Joule heating of emitters by thermionic current, new methods and criteria are proposed for evaluating vacuum breakdown cathode-initiated in a pulse regime for an arbitrary cathode shape. The applicability of the proposed methods and validity of the new criteria are confirmed by the results of experiments using nanosecond pulses applied to coaxial copper electrodes with a gap width of d=0.2 mm and an area of S=2500 mm2.

Triggers for RF breakdown

We outline a model of breakdown in RF cavities. In this model, the breakdown trigger is the injection of ions, atoms and clusters into cavities by either of two mechanisms. One mechanism is some combination of fracture and field evaporation of ions from solid surfaces caused by locally high electric fields. The second mechanism, driven by high local current densities, is localized Ohmic heating at grain boundaries and defects. Field evaporation and fracturing are similar processes, both driven by the high tensile stresses in the electric field that occur at local electric fields of ∼10 10 V/m, which have been measured in a number of experimental environments. We also outline how ions can be injected into cavities in the presence of large RF electric fields. The model can explain most of the behavior seen in a variety of cavities at different frequencies without assuming that melting or gas emission occurs at breakdown sites. This model may also be relevant to DC vacuum breakdown.

Mechanism responsible for initiating carbon nanotube vacuum breakdown.

We report a physical mechanism responsible for initiating a vacuum breakdown process of a single carbon nanotube (CNT) during field emission. A quasidynamic method has been developed to simulate the breakdown process and calculate the critical field, critical emission current density and critical temperature beyond which thermal runaway occurs before the CNT temperature reaches its melting point. This model is in good agreement with experiments carried out with a single CNT on a silicon microtip.