Sessile water droplets on insulating surfaces subject to high AC stress effect of contact angle

Surface pollution of outdoor high-voltage insulators is an important cause of flashover. We have undertaken an experimental study of electrical breakdown at the edges of a sessile water droplet on a planar, polymeric, insulating surface when subject to AC stress, parallel to the insulator surface, up to 2MV/m. The static contact angle between droplet and surface was varied by controlling the physical properties of the droplet and by inclining the insulator plane from the horizontal. The partial discharge activity from the water droplet was investigated using a combination of high-speed video camera, operated at up to 3,000 frames per second, and an electrical partial discharge detection system. We have used this to examine the location of partial discharge at the edges of the water droplet

Analysis of the transient process in underwater spark discharges

lf water is stressed with a voltage pulse having a rise time of tens of nanoseconds which creates a sufficiently high electric field, streamers develop and a highly conductive channel forms between the electrodes. The intense Joule heating of the plasma in the channel results in the collapse of its electrical resistance from a few Ohms to a few tens of milliOhms with the behavior of the collapse depending on the parameters of the discharge circuit. The rapid decrease of the resistance occurs during the first quarter of the current oscillation in the circuit. During this time, the pressure inside the channel rises to several GPa, causing the channel to expand in water with a velocity of 100 to 1000 m/s driving a high power ultrasound pulse. In the present paper, a phenomenological model is discussed which describes the dynamics of the resistance of underwater spark discharges during its initial stage and allows the pressure in the acoustic pulse to be obtained. The model is based on the plasma channel energy balance equation used by Braginskii and links the hydrodynamic characteristics of the channel and the parameters of the electric driving circuit. The dynamics of the transient cavity during the dissipation of the electrical energy in the plasma channel is described and the analytical results are compared with experimental measurements of the current in the electrical circuit and the acoustic pulse profiles radiated by the transient cavities

Factors affecting the operation of laser-triggered gas switch (LTGS) with multi-electrode spark gap

Multi-electrode spark switches can be used for switching applications at elevated voltages or for command triggering. Symmetrical field graded electrodes allow the electrical stress across individual gaps to be controlled, thus maximising the hold off voltage and reducing switch pre-fire. The paper considers some aspects of multielectrode switch design and their influence on switching behavior. Non-symmetrical, uni-directional electrode topologies can be employed with advantages over traditional symmetrical design. The choice of working gas and gas pressure can influence switching performance in terms of delay-time and jitter. Transient analysis of switch characteristics has been undertaken in order to understand multi-electrode switching

Fundamental Science Investigations to Develop a 6-MV Laser Triggered Gas Switch for ZR: First Annual Report.

In October 2005, an intensive three-year Laser Triggered Gas Switch (LTGS) development program was initiated to investigate and solve observed performance and reliability issues with the LTGS for ZR. The approach taken has been one of mission-focused research: to revisit and reassess the design, to establish a fundamental understanding of LTGS operation and failure modes, and to test evolving operational hypotheses. This effort is aimed toward deploying an initial switch for ZR in 2007, on supporting rolling upgrades to ZR as the technology can be developed, and to prepare with scientific understanding for the even higher voltage switches anticipated needed for future high-yield accelerators. The ZR LTGS was identified as a potential area of concern quite early, but since initial assessments performed on a simplified Switch Test Bed (STB) at 5 MV showed 300-shot lifetimes on multiple switch builds, this component was judged acceptable. When the Z{sub 20} engineering module was brought online in October 2003 frequent flashovers of the plastic switch envelope were observed at the increased stresses required to compensate for the programmatically increased ZR load inductance. As of October 2006, there have been 1423 Z{sub 20} shots assessing a variety of LTGS designs. Numerous incremental and fundamental switch design modifications have been investigated. As we continue to investigate the LTGS, the basic science of plastic surface tracking, laser triggering, cascade breakdown, and optics degradation remain high-priority mission-focused research topics. Significant progress has been made and, while the switch does not yet achieve design requirements, we are on the path to develop successively better switches for rolling upgrade improvements to ZR. This report summarizes the work performed in FY 2006 by the large team. A high-level summary is followed by detailed individual topical reports

Influence of gamma radiation on the properties of polyethylenectapthalate (PEN)

Foil samples of polyethylenenapthalate, 25 mum thick have been subjected to radiation doses of 26.5 and 158.4 kGy from a Co-60 source to promote ageing. The current transients produced by the application and removal of a DC voltage to the samples were measured over an extended period of time. Peaks in both the charging and discharge currents were observed whose positions were dependent on both temperature and radiation dose. Activation energies were obtained from isochronal charging currents and from the frequency domain. These showed a dose dependence. Curve fitting to the initial discharge transient and to the dielectric loss peak in the frequency domain indicated that the ageing had not produced changes in the type of loss mechanisms. A linear relationship was found between the received dose and the dieelctric loss peak position

Investigation of the mechanisms leading to the electrical breakdown of a triggered water gap

The breakdown of a triggered, plane-parallel electrode system with water dielectric has been investigated. The gap was triggered by a discharge initiated at an electrically isolated trigger pin, positioned in the center of one of the electrodes, using a 500 ns voltage pulse. A 5000 fps CCD-camera monitored events occurring in the gap during such a discharge and the intensity variations of a laser beam transmitted through the electrode gap was also monitored. The results indicate the initiation, expansion and collapse of a gas bubble generated at the trigger electrode. The subsequent application of a voltage between the plane electrodes results in the complete breakdown of the gap due to the trigger discharge. The effect of a delay time between the trigger pulse and the application of the main gap voltage was consistent with the growth and collapse of the trigger-initiated bubble

Structure and evolution of pulsed surface discharges in noble gases

This paper looks at the structure and evolution of pulsed surface discharges in noble gase

Discharge velocity effects across charged insulator surfaces

Surface discharge velocity measurements are reported. An aluminium strip fixed on the underside of a polyester film trapped corona charge on the surface and guided the discharge initiated by a 100 ns voltage pulse of between 10 to 30 kV. The surface charge was of opposite polarity to the voltage pulse. Results for the discharge velocity under various conditions are presented and indicate that both surface charge and the voltage pulse are responsible for driving the discharge across the length of the polyeste

The application of principal component analysis to study TSDC spectra

The technique of principal component analysis has been applied to spectra obtained by thermally stimulated discharge current measurements on polyimide films. The films had been aged by gamma irradiation followed by long term exposure to elevated temperatures. The relationship between the aging history and the changes observed in the principal component values derived from the spectra are discussed

Development of pulsed surface discharges for pumping dye lasers

This paper looks at the development of pulsed surface discharges for pumping dye laser