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

Optimisation of the spark gap parameters for high powered ultrasound applications

There is considerable interest in the industrial and commercial applications of high power ultrasound (HPU) generated using pulsed power techniques. These applications include metal peening, the treatment of ores and minerals before extraction, drilling technologies and the comminution and recovery of waste materials. In all of these applications, it is important to optimise the parameters of the discharge causing the shock wave in the working medium to maximise the efficiency of the treatment. In a research project at the University of Strathclyde, some applications of HPU to the treatment of waste to assist in recycling have been investigated. Two systems have been considered, slag from the manufacture of stainless steel and bottle glass. With the slag material, it is intended to separate stainless steel from the silicate matrix to permit its recovery. With the bottle glass, the intention is comminution of the material to allow it to be recycled in a more valuable form. Measurements of the efficiency of these processes have been made in terms of the mass of material processed versus the energy input as the parameters of the discharge gap have been varied. In parallel with this work, measurements have been made using pinducer sensors to determine the energy in HPU pulses generated by discharges under identical conditions. Correlations are made between the efficiency of material treatment and the intensity of the HPU pulse measured in the far field. It is hoped that this approach will allow the optimal gap parameters to be determined using pinducer measurements rather than time consuming trials based around materials processing

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

Short range nucleon correlations studied with electron and photon probes

This paper reviews experimental research into two- and three-body nucleon-nucleon interactions, carried out by the University of Glasgow Nuclear Physics research group. A key aim of these studies has been to elucidate the role of short-range nucleon-nucleon correlations (SRC). Studies of photon-induced two- and three-nucleon emission reactions from a range of light nuclei, carried out at the 840 MeV Mainz electron microtron MAMI-B, have provided detailed information on the contributing mechanisms. More recent electron scattering studies at the 6.0 GeV Jefferson Laboratory have probed SRC at high values of Q^2 and x_B and suggested a connection between SRC and the EMC effect

Surface flashover of oil-immersed dielectric materials in uniform and non-uniform fields

The applied electrical fields required to initiate surface flashover of different types of dielectric material immersed in insulating oil have been investigated, by applying impulses of increasing peak voltage until surface flashover occurred. The behavior of the materials in repeatedly over-volted gaps was also analyzed in terms of breakdown mode (some bulk sample breakdown behaviour was witnessed in this regime), time to breakdown, and breakdown voltage. Cylindrical samples of polypropylene, low-density polyethylene, ultra-high molecular weight polyethylene, and Rexolite, were held between two electrodes immersed in insulating oil, and subjected to average applied electrical fields up to 870 kV/cm. Tests were performed in both uniform- and non-uniform-fields, and with different sample topologies. In applied field measurements, polypropylene required the highest levels of average applied field to initiate flashover in all electrode configurations tested, settling at similar to 600 kV/cm in uniform fields, and similar to 325 kV/cm in non-uniform fields. In over-volted point-plane gaps, ultra-high molecular weight polyethylene exhibited the longest pre-breakdown delay times. The results will provide comparative data for system designers for the appropriate choice of dielectric materials to act as insulators for high-voltage, pulsed-power machines

The suitability of N2 to replace SF6 in a triggered spark-gap switch for pulsed power applications

The high dielectric strength of sulphur hexafluoride (SF6) when compared with other gases, coupled with safety benefits such as non-flammability and non-toxicity, has seen the widespread use of SF6 for the insulation of switching components. However, SF6 is now widely recognised as a highly damaging greenhouse gas, and investigations of the switching properties of alternative gases to replace SF6 within the bounds of existing system topologies are required. In the present paper, a comparative study has been carried out on a triggered spark-gap of type presently deployed in industrial pulsed-power machines, to determine the suitability of nitrogen (N2) to replace SF6 as the switching medium, without compromising on functionality. Experiments were performed with fast-rising trigger pulses to minimise the delay time to breakdown and jitter, and three distinct operational regimes have been identified for both gases as the pressure inside the switch is increased. The static breakdown characteristics and upper pressure boundaries of operation have been determined for both gases at a range of dc charging voltages. Measurements of the time to breakdown have shown jitters as low as 1.3 ns when operating in N2, highlighting the potential of N2 to replace SF6 without the need for re-design or replacement of the presently used switch

Avian community responses to the establisment of small garden allotments within a Mediterranean habitat mosaic

Estudios previos de ecología enfocados a los efectos que tienen las alteraciones de los hábitats a pequeña escala han hallado efectos positivos, nulos y negativos sobre la biodiversidad. En este trabajo describimos los efectos que tiene el establecimiento de un pequeño huerto sobre la comunidad de aves. Para ello, analizamos los valores de diversidad (i.e., riqueza de especies y abundancia) y el comportamiento (i.e., forrajeo, uso de perchas) de las comunidades de aves. Los resultados de este trabajo muestran que, aunque el cambio de uso de suelo es reciente y a pequeña escala, las comunidades de aves observadas en el huerto están dominadas por unas pocas especies, mientras que mostraron ser mayormente equitativas en las plantaciones naturalizadas de almendros (hábitat previo al establecimiento de los huertos). Cuando comparamos el comportamiento de las aves en ambos hábitats, encontramos una mayor proporción de aves en búsqueda activa de alimento en los huertos, mientras que el número de aves desarrollando otras actividades (descanso) fue mayor en las plantaciones naturalizadas. Aunque la presencia de nuevos hábitats puede elevar la riqueza regional de la avifauna en paisajes mediterráneos, nuestros resultados no muestran evidencia de un efecto positivo significativo en el aumento de la riqueza regional de aves debido al establecimiento de pequeños huertos. Proponemos que las futuras actividades agrícolas deban tener en cuenta la escala, intensidad y frecuencia de las perturbaciones generadas con la finalidad de lograr un efecto positivo sobre la biodiversidad. Palabras clave: Ecología aviar, Biodiversidad, Comunidades de aves, Transformación del uso del suelo.Estudios previos de ecología enfocados a los efectos que tienen las alteraciones de los hábitats a pequeña escala han hallado efectos positivos, nulos y negativos sobre la biodiversidad. En este trabajo describimos los efectos que tiene el establecimiento de un pequeño huerto sobre la comunidad de aves. Para ello, analizamos los valores de diversidad (i.e., riqueza de especies y abundancia) y el comportamiento (i.e., forrajeo, uso de perchas) de las comunidades de aves. Los resultados de este trabajo muestran que, aunque el cambio de uso de suelo es reciente y a pequeña escala, las comunidades de aves observadas en el huerto están dominadas por unas pocas especies, mientras que mostraron ser mayormente equitativas en las plantaciones naturalizadas de almendros (hábitat previo al establecimiento de los huertos). Cuando comparamos el comportamiento de las aves en ambos hábitats, encontramos una mayor proporción de aves en búsqueda activa de alimento en los huertos, mientras que el número de aves desarrollando otras actividades (descanso) fue mayor en las plantaciones naturalizadas. Aunque la presencia de nuevos hábitats puede elevar la riqueza regional de la avifauna en paisajes mediterráneos, nuestros resultados no muestran evidencia de un efecto positivo significativo en el aumento de la riqueza regional de aves debido al establecimiento de pequeños huertos. Proponemos que las futuras actividades agrícolas deban tener en cuenta la escala, intensidad y frecuencia de las perturbaciones generadas con la finalidad de lograr un efecto positivo sobre la biodiversidad. Palabras clave: Ecología aviar, Biodiversidad, Comunidades de aves, Transformación del uso del suelo.Ecological studies focused on small–scale habitat alterations have found positive, null, and negative effects on biodiversity. In this study, we describe the effects that establishing a relatively small area of garden allotments had on bird communities. To assess such effects, we analyzed avian community diversity (i.e., species richness and abundance) and behavioral traits (i.e., foraging, perching). Although land transformation was recent and on a small geographic–scale, our results showed that bird communities in the allotments were dominated by a few species, while in the almond plantation (former habitat) evenness was higher. When perching and foraging behavior was compared in the two study areas, we found a significantly higher proportion of foraging in the garden allotments, and a higher proportion of birds perching in the naturalized plantation. Although new habitats often enhance regional bird species richness in Mediterranean landscapes, we found no evidence of an increase in regional avian diversity related to the establishment of small garden allotments. We propose that future harvesting activities should consider the scale, intensity, and frequency of the generated perturbation in order to promote biodiversity. Key words: Avian ecology, Biodiversity, Bird communities, Land–use transformation