Search

It is impractical to achieve the desired combination of power and bandwidth from conventional electromechanical acoustic sources. However, these characteristics can be achieved by the use of pulsed power technology to generate high-power ultrasound (HPU). High-voltage pulses induce the electrical breakdown of water and the resulting bubble formation and collapse produce acoustic waves of high power and frequency. The dynamics of spark generated bubbles are formulated to predict the development of the bubble radius with time and an experimental system to produce a consistent source of spherically symmetric HPU acoustic waves is described. Pressure pulses due to both bubble formation and collapse were detected and, although their relative amplitudes varied, their frequency spectra did not differ significantly. The amplitude of the acoustic output rises sharply for applied pulse energies up to ~ 25 J but the effect saturates indicating little gain and poor efficiency by using high-energy pulses. Variation of the source topology in the form of the electrode separation was found to be the most important factor in the acoustic output. The detected HPU increased as the source became larger but as the two-thirds power of the electrode separation, thereby showing progressively diminishing enhancement. The frequency content of the acoustic signal did not appear to vary with either applied pulse energy or the electrode separation. Index Terms--Electrical breakdown of water, high-power ultrasound (HPU), pulsed power, spark discharges.

In recent years, a number of new applications have emerged where pulsed power is being used in the treatment of waste and effluent, foodstuffs, and beverages. One of these emerging applications is pulsed electric field (PEF) inactivation of microorganisms in liquid media. There are several ways in which PEF can be applied, including oscillatory, double exponential, and square wave pulses. Of these, the square-wave pulse is considered to be the most energy-efficient form of PEF delivery. It has been reported that the bipolar square-wave pulse, involving polarity reversal half way through the pulse, provides superior inactivation when compared to the monopolar pulse. However, results from the authors have shown that this is not always the case and that monopolar PEF is at least as effective for bacterial inactivation under the conditions investigated (Beveridge, et. al., 2004). Further results are presented here on the effect of changing the pulse duration of the monopolar and bipolar pulse, using total pulse durations of 1, 2, 3, and 4 [micro]s. These results, obtained from an improved system, demonstrate how the relative effectiveness of inactivation is a function of the pulse duration. When inactivation is plotted against energy delivered, they show the superiority of monopolar over bipolar pulses for 1- and 2-[micro]s pulse profiles. For a pulse duration of around 3 [micro]s, there is no significant difference, and with still longer pulses, bipolar pulses appear to be superior to monopolar. It is postulated that electric-field-induced orientation may, in part, be responsible for this effect. These comparative experiments are conducted with complete control of the fluid temperature, which is maintained below 30 [degrees]C. Index Terms--Electronic sterilization, microbial inactivation, monopolar/bipolar, pulse profile, pulsed electric field.

With growing economic and environmental pressures, oil companies seek ways to increase the oil recovery from every well. Re-entry drilling of horizontal sidetracks from existing wells and multilateral drilling for new wells are techniques which help to achieve that increase. However, new approaches are needed to reduce the cost and to improve efficiency of rotary drilling methods used for these purposes. A new nonrotary plasma channel drilling (PCD) method for drilling of small diameter holes (3.5-15 cm) in oilfields for sidetrack creation and multilateral drilling is described in this paper. The method relies upon the use of the energy of impulse spark discharges which are formed inside the rock formation ahead of the drill position. Repeated formation of the plasma channel results in an effective and controlled drilling action. Plasma channel drills developed at Strathclyde University are able to cut clearly defined circular holes in medium hard sandstone with speeds of up to 16 cm/min using specific energies as low as 400 J/[cm.sup.3]. Plasma channel drilling has the potential to increase the lifetime of oil wells and would significantly reduce the cost of exploration drilling and subsurface data acquisition. PCD technology can also be a cost-effective and practical solution for other applications such as mineral mining, water boring and scale removal where micro-hole drilling is essential. Index Terms--Plasma channel drilling, pulsed-power, selective breakdown of solids, spark discharges.

This paper reports on the work undertaken to determine the effect of the gas type on the high repetition rate performance of a triggered corona stabilised (TCS) closing switch at a charging voltage of 23 kV. The voltage/pressure (V/p) characteristics as well as the ability of the switch to operate at high repetition rates were measured with S[F.sub.6], air and six different S[F.sub.6]/air mixtures having an S[F.sub.6] concentration ranging from 75 to 2% by pressure. The high repetition rate tests were conducted with a high-power facility at a pulse repetition frequency (PRF) ranging from 500 Hz up to a maximum of 3 kHz. During the investigation, it was found that for the given non-uniform switch geometry the high repetition rate performance of the device was closely associated with the efficiency of corona stabilisation, as measured from the V/p characteristics. This was clearly manifested by the superior performance of most of the S[F.sub.6]/air mixtures tested, which displayed a more efficient corona stabilisation compared to pure S[F.sub.6]. In contrast, air displayed the worst performance of all the gases tested, due to the weak V/p characteristic and inefficient corona stabilisation. Index Terms--Pulsed power, plasma closing switches, corona stabilised breakdown, non-uniform electric fields, gas discharges, gas mixtures, voltage recovery, electronegative gases.

An investigation has been undertaken into the influence of parameters such as the nature of the cover gas, the dielectric substrate, and the circuit parameters on the performance of surface discharges used as optical sources. The effects of the magnitude and duration of the driving voltage pulse on the radiated optical output from a pulsed surface discharge in an S[F.sub.6]-Ar gas mixture over a sapphire substrate have been measured within six spectral bands. It has been shown that the parameters studied can significantly affect the spectral distribution of the output of optical energy within these bands. The results show that the efficiency of optical emission within the 200-300 nm spectral band increases as the driving pulse duration increases from 250 to 1100 ns. This is in contrast to that found within the other spectral bands. The efficiency of optical emission in the 200-300 nm band has also increased with increasing discharge current, in contrast to the decrease observed for wavelengths greater than 700 nm. Index Terms--Energy, light sources, spectral emission, surface discharges.

The use of pulsed electric fields (PEF) for the inactivation of micro-organisms is currently of considerable interest as an alternative method to the conventional thermal methods of food pasteurization presently in widespread use. Many parameters of PEF influence the effectiveness of inactivation, one of which is the pulse profile, which is the subject of the present study. In this study, a pulse-forming network was designed and constructed to allow direct comparison of the effectiveness of a rectangular 'monophase' pulse (single polarity) with a rectangular 'biphase' pulse (incorporating a rapid reversal of polarity for the second half of the pulse). The pulses of 30 kV/cm in electric field intensity, 2/[micro]s in duration for monophase, and 1 [micro]s for each phase for biphase, were applied to parallel-plate test cells. Three different species of bacteria were tested including Gram negative and positive cell types. The bacteria were suspended in peptone water solutions of varied peptone concentration and hence, conductivity. The results show that, contrary to previous findings, biphase pulsing did not provide superior inactivation levels compared with monophase pulsing, for all three bacteria tested. Increasing the conductivity of the suspension fluid produced a general trend toward similar levels of inactivation of biphase compared with monophase PEF treatment, but in no case was it found that biphase PEF treatment was superior to monophase PEF treatment. Index Terms--Microbial inactivation, PEF, waveform comparison.

Treating milk samples with 2,500 pulses of a 30 kV electric field per centimeter at 50 degrees C. can inactivate Mycobacterium paratuberculosis. This bacterium causes Johne's disease in animals and may be linked to Crohn's disease in humans.

The susceptibility of a variety of medically important bacteria to inactivation by 405-nm light from an array of light-emitting diodes (LEDs), without the application of exogenous photosensitizer molecules is demonstrated. The results demonstrated a wide range of potential application for narrow-spectrum 405-nm visible-light illumination from an LED source as a novel decontamination method.