Coaxial Gap Research Articles

Volt-Time Curves for a Coaxial Cylindrical Gap in SF6-N2 Mixtures

The impulse breakdown voltage-time characteristics of a coaxial cylindrical gap in 10% SF6-90% N2 mixtures are presented. The theoretical models for the determination of the envelopes of the v-t characteristics are examined. A method has been suggested for the determination of the characteristic parameters used in the theoretical models.

Impulse Voltage Breakdown Characteristics of Teranry Gas Mixtures in A Coaxial Cylinder Gap

Impulse breakdown characteristics for ternary gas mixtures were studied using a coaxial cylinder geometry. The experimental mental pressure ranged from 100 to 600 kPa. The mixtures comprised SF6 with an additive C-C4F8 and various proportions of buffer gases including N2, C02, CO, and dry air. For 50% flashover voltages, mixtures containing CO exhibited highest dielectric strength for pressures up to 500 kPa. The studies also showed that for a coaxial cylinder geometry the breakdown strength is not affected significantly by increasing the amount of the additive gas C-C4F8 but is determined by the total percentage of the electronegative gas component.

Effect of Wavefront on Impulse Breakdown Voltages of Slightly Nonuniform Field Gaps in Nitrogen, Air and SF6

The influence of the wavefront on impulse breakdown voltages for enclosed slightly-nonuniform-field gaps is reported. A 25 mm sphere-plane gap and a 20/50 mm diameter coaxial electrode system were used to obtain different fielt nonuniformity factors and different critical volumes. The wavefronts used in this work were 0.45, 1.35, 20, 50 and 90 ?s while the wavetail was around 450 , ?s for all cases. A 30 W UV lamp and 2 mCi caesium isotope were employed as irradiation sources. Experimental results show that strongly wavefront-dependent breakdown characteristics for small enclosed gaps reported by some other investigators were probably due to insufficient irradiation conditions. On the other hand, for the coaxial cylinder gap having a large critical volume, artificial irradiation was not necessary.

New insulating materials and their use to achieve high operating stresses in electrostatic machines

Compressed gas insulation has provided the main insulation for sustaining terminal voltages of electrostatic accelerators. Essentially coaxial geometry is used with mechanical support of the terminal achieved by long columns which also support the acceleration tubes. Because of the vacuum insulation in the acceleration tubes, the electric gradient along the columns is typically 10–20 kV/cm, whereas the radial gas gap can operate at stresses about ten times larger. Until now, the terminal support has always been located in the low stress axial direction along the column and not in the radial high stress region. This paper is concerned with support insulation to be used in the radial direction. Advantages of radial supports include: simpler, more compact column structures, higher total voltages, and support of discrete stress redistribution electrodes such as vivitron. Important factors to the design of radial support insulators include the insulation constraints imposed by the gas gap, mechanical contact to the solid insulator, and basic limits of gas-solid dielectric interfaces. The gas gap insulation strength is shown to be limited by surface microirregularities and this accounts for electrode area and pressure effects. Based on the gas gap requirements, a design strategy for the insulators is developed. Epoxy is employed as the dielectric to allow the use of cast-in metal inserts at the ends. The inserts provide mechanical contact, shielding of the triple junction, and redistribution of the interface electric stresses. By careful design, the electric stress on the interface is made lower than that in the plain coaxial electrode gap. Practical experience shows that voltage increases linearly with insulator length and that designs achieve more than 10 MV/m into the multimegavolt region.

Non-stationary helium heat transfer

This Paper presents the results of an experimental investigation of heat transfer to helium in stationary conditions, in conditions of increase and decrease of heat power at various rates and with pulse heating. Heat transfer both from a free surface and in a coaxial gap of 0.5 mm was studied. The experiments were carried out within the range of liquid saturation temperatures from 1.94 to 4.23 K. The values of the heat loads in a pulse were ≈1.5 – 15 times the steady-state value of the critical heat flux for helium nucleate boiling. It has been shown that transient thermal processes may have higher heat transfer coefficients compared with steady-state conditions. The most pronounced effect is observed under pulse heating conditions.

Rise Time of a Vacuum Gap Triggered by an Exploding Wire

Detailed investigations on the rise time (tr) of a coaxial vacuum gap, triggered by exploding wire (EW) plasma have been reported for the first time. The tr behavior, studied as a function of energy (Ee) put into EW, exhibits ripples at low gap voltages. At large gap voltages, the ripples disappear resulting in a flat characteristic making tr independent of Ee. This complex tr behavior has been qualitatively explained on the basis of varying proportions between metallic clumps and metallic vapor, and change in the degree of ionization in the EW plasma, as Ee is varied.

Adjustable long duration high-intensity point light source

A new long duration high-intensity point light source with adjustable light duration and a small light spot locally stable in time has been developed. The principle involved is a stationary high-temperature plasma flow inside a partly constrained capillary of a coaxial spark gap which is viewed end on through a terminating Plexiglas window. The point light spark gap is operated via a resistor by an artificial transmission line. Using two exchangeable inductance sets in the line, two ranges of photoduration 10–130 μs and 100–600 μs can be covered. For a light spot size of 1.5 mm diameter the corresponding peak light output amounts to 5×106 and 1.6×106 candelas, respectively. Within these ranges the duration is controlled by an ignitron crowbar to extinguish the plasma. The adjustable photoduration is very useful for the application of continuous writing rotating mirror cameras, thus preventing multiple exposures. The essentially uniform exposure within the visible spectral range makes the new light source suitable for color cinematography.

Some Computations and Observations on Corona-Stabilized Breakdown in SF6

Loeb retracted his support for Hermstein's negative ions cloud theory and proposed that the reduction of electric field at the anode, due to the presence of positive ions space charge, could be responsible for the corona-stabilization in electronegative gases. Based on this comment, Abdel-Salam et al. proposed a semi-empirical criterion for the calculation of the breakdown voltage during corona-stabilization. It is shown in this paper that this criterion, which is based on the avalanche growth in the presence of space charge field, would not work. We computed several discharge parameters such as the total charge, the current flowing and the power dissipated in the gap at experimental breakdown voltages in coaxial cylinders and concentric spheres gaps. The corona onset voltages were also calculated based on the streamer theory. The purpose was to explore if a new, simple and reasonably accurate semi-empirical criterion for the calculation of the breakdown voltage could be established. Results indicate that the calculated values of the corona onset voltage agreed well with the experimental results but no simple equation can correlate the discharge parameters, the field non-uniformity, the gas pressure and the breakdown voltage.

Beam stability and current loss in magnetic insulation

A two-dimensional electromagnetic particle code has been used to investigate the effect on magnetic insulation of a beam of electrons injected longitudinally into the coaxial vacuum gap. It is found that a parapotential beam, though force balanced at the input, does not propagate more than a short distance before being totally disrupted, apparently by beam instability. Even a very hollow beam is lost to the anode by a diffusion process which may explain the anomalous current leakage observed in some experiments.

Particle-Initiated Breakdown in SF6 Insulated Systems Under High Direct Voltage

Conducting particle charging and detachment into a compressed SF6 coaxial electrode gap energized with high direct voltage have been investigated using the charge simulation technique and dimensional analysis.

Influence of particles on AC and DC electrical performance of gas insulated systems at extra-high-voltage

High-voltage breakdown measurements were made in two similar particle contaminated coaxial test systems, one with AC and the other with DC voltages. Information is presented on the effects of particle size, shape, and material for both SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> and N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> gases at pressures up to 15 atm in a plain coaxial gap and a coaxial gap including a post-type support spacer. Particle motion and location were found to strongly influence insulation performance. Measured values of electric fields which lifted and drove the particles, so that they bounced vertically and laterally, compare favorably with calculated levels. Movement into the the higher stress region at the center conductor was correlated with the initiation of sparkover. These breakdowns could be at levels more than a factor of five lower than those obtained when contamination was not introduced. Large variations in breakdown voltage of as much as 3 to 1 encountered under DC correspond to conditions where particle motion could be restricted, presumably by corona discharge, to motion near the outer electrode. AC sparkover levels were typically at the lower limits of the DC range. Both free and attached particles on the dielectric spacer surface would trigger flashover at the same low levels as were measured in the gas gap.

Field attenuation as the underlying principle of cryo-current comparators 2. Ring cavity elements

In Part 1 the connexion between the error of a cryo-current comparator and the magnetic field attenuation in the gap region of its superconducting shield has been treated for the special but very common case of co-axial gap regions. This paper extends the error analysis on gaps forming a ring cavity. Such gap elements occur in toroidal cryo-current comparators as a link between two adjacent co-axial cavities. The results for these comparators are: the only field component which may penetrate the gap without attenuation is the one containing the useful signal about the current linkage of the comparator. All other field components contribute to the error of the comparator. They are attenuated according to their degree of symmetry. As the exact way the error functions is known it will be possible to construct cryo-current comparators with a predictable upper limit to their dc error.

Dielectric Breakdown of High Pressure SF6 in Sphere and Coaxial Cylinder Gaps

This paper describes the breakdown characteristics of SF6 at pressures up to 15 kg/cm2 absolute at 20°C against power frequency, switching impulse and lightning impulse voltages in sphere and coaxial cylinder gaps.

Theoretical investigation of a low-pressure arc discharge in a cylindrical diode

Using systematic numerical calculations of an arc discharge in a coaxial interelectrode gap, the potential drops and the electron density and temperature are obtained as a function of the current density, the plasma pressure, the azimuthal magnetic field, and the interelectrode distance. General conclusions are drawn from the relations obtained.

The effect of ionization and flow velocity upon spark gap recovery

Breakdown measurements for a small cylindrical cathode in an atmospheric-pressure flowing plasma of variable ionization (between 1015 and 1018 m−3) are reported for both DC voltages and fast-rise pulses (rise time 0·16 μs) applied to the cathode. For these measurements the breakdown voltages are found to be reduced to as little as fraction one-sixth of the breakdown voltage with no ionization. The measurements agree reasonably well with a theoretical model in which the anode is not the positive electrode in the plasma, but rather the edge of the coaxial sheath which surrounds the cathode, with breakdown voltages being given by the well-known breakdown relation for coaxial gaps. (For cylindrical geometry and a thick sheath, the ion space charge in the sheath is shown not to change materially the electric field from its value calculated for a conventional coaxial gap of the same dimensions.) In this experiment the impedance of the plasma between the sheath edge and the actual positive electrode is low enough for breakdown not to occur over this region. It is expected that this model will be used to predict reignition voltages at short recovery times in circuit breakers.

A CO2-laser-triggered spark gap

A coaxial spark gap triggered by a high-pressure CO 2 laser has been constructed. High-voltage pulses with subnanosecond rise time and very low jitter have been obtained. The observed low-laser energy threshold required to initiate switching suggests an application of the spark gap in controlling and manipulating high-intensity infrared radiation by electrooptic shutters.

Impulse breakdown between co-axial electrodes at low pressure

A capacitor bank of 9-20 μF, charged to 5-7 kV was suddenly connected to a short coaxial gap, having an electrode separation of 4 cm, filled with N2 at pressures between 4 and 4 × 10-2 torr. It was observed that in spite of gap and external electric circuit symmetry, the discharge with a peak current of 10,000 A to 60,000 A always developed initially in one or more radial columns and never in a continuous luminous ring, and that the development of each radial column occurred in three distinct stages.

Cylindrical Spark Chamber

A spark chamber consisting of eight coaxial cylindrical gaps around a liquid hydrogen target has been built and used. The sparks are viewed by a single camera looking axially up into the chamber. Stereo information is provided in the same view by reflections of the sparks from tilted mirrors at the top of the chamber.

Self-Excited 150-Kilovolt Resonant Cavity for Operation at 87 Megacycles

A high-voltage, fixed frequency re-entrant cavity of moderate power requirements designed for use in the Cornell 1.5-Bev synchrotron is described. Design considerations and details of the cavity gap structure are presented. The problem of multipactoring is described. Advantages of coaxial gap construction using an internal insulator are given. In particular a dc bias to prevent multipactoring can easily be applied. A simplified analysis of the operation of the cavity in terms of equivalent circuits is also given.