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Vacuum circuit breakers are widely used in medium-voltage class. In vacuum circuit breakers, restrikes and non-sustained disruptive discharges sometimes occur after current interruptions. It is said that these late breakdowns are triggered by the microparticles. This paper aims to investigate the relationship between the motion of microparticles and subsequent late breakdown phenomena. It does by artificially injecting microparticles into the vacuum gap and observing their motion using a highspeed video camera. We focused on the sputtering of the electrode material during the current interruption and the mechanical stress of the switching operation as being the origin of these microparticles. We found that the motion of microparticles can be grouped into two distinct patterns: bouncing and attaching. Bouncing microparticles seem to be made of metal, while most of the attaching microparticles attached themselves to the cathode, and their charge was about five times greater than that of the bouncing microparticles. The attaching microparticles seem to be positively charged by secondary electron emissions from the surface under electron bombardment due to the field emission current from the cathode. From these results, we believe that the surfaces of the attaching particles are covered with an insulating layer. From our observations of the production of microparticles by mechanical stress, we have found that the breakdown voltage of the gap drastically drops after a mechanical switching operation where microparticles are produced. The breakdown voltage is restored with repetitive mechanical switching operations and subsequent voltage application. However, the breakdown voltage drops again when the operating speed of the electrode is increased. We also found that microparticles are scattered from the cathode spots on electrodes that have not been current-conditioned, and they are hardly produced from electrodes that are current-conditioned. From these results, we believe that controlling the speed of the mechanical switching operation and the current conditioning are effective ways to prevent the microparticles from being produced. [ABSTRACT FROM AUTHOR]

Intracellular DNA damages caused by intense burst sinusoidal electric fields (IBSEFs) were investigated by means of an alkaline comet assay method. Non-thermal, 200 μs-long IBSEF with various frequency values (300 kHz-100MHz) and strengths (up to 200 kV/m) was applied to Chinese hamster ovary (CHO) cells in a suspending medium between 1 mm gap parallel electrodes. The comet assay suggests that 100 kV/m IBSEF with frequencies exceeding 1 MHz or 100 MHz IBSEFs with field strengths exceeding 3 kV/m induces significant DNA damage. According to the numerical calculation of the electric field over a simplified cell model under an alternating electric field, the intracellular field strength increases with increasing alternating frequency. The minimum level of the field strength that induces the DNA damage is in the range of 10-30 kV/m. This intracellular strong field might trigger biological processes leading to the DNA damage. [ABSTRACT FROM AUTHOR]

The authors would like to make the following corrections to the above paper. [ABSTRACT FROM AUTHOR]

This paper describes the biological response of cultured HeLa S3 cells to non-thermal, intense burst sinusoidal electric fields (IBSEF). The proliferation activity of the cells subjected to 300 kV/m, 200 μs-long IBSEF with a frequency between 0.3 MHz and 100 MHz was evaluated by using an automatic cell event monitor, xCELLigence, for 48 hours. The experiment showed that the cell proliferation depended on the pulse number and the frequency band of the IBSEF. The application of the smaller number of the pulses enhances the proliferation, whereas the large number of pulses tends to kill the cells. The dependence on the pulse number indicates the effect of the pulse is accumulated. The cells seem to be more sensitive to the pulses at the lower frequencies less than 1 MHz. The frequency of 100 MHz, which is sufficiently large for the field to penetrate into the cell, did not appear to influence the proliferation and the immediate death process. When the cells were subjected to 30 pulses of 3 MHz IBSEF, the proliferation was promoted 1.6 times as fast as that of the sham exposure. The frequency dependence of the proliferation might be associated with the electrical stress at the plasma membrane where several proteins related to the cell cycling are located. [ABSTRACT FROM PUBLISHER]

This study elucidates the influence of the pulsing sequence of repetitive nanosecond pulsed electric fields on the viability of human cancer (HeLa S3) cells. A magnetic pulse compressor was used to repetitively deliver 70 ns-long high voltage pulses to cultured HeLa cells in a 4 mm-gap cuvette electrode. The repetition frequency was varied between 0.01 and 250 pulses per second (pps), corresponding to interval from 100 to 0.004 s, respectively. The electric field and the number of pulses were fixed at 25 kV/cm and 25, respectively, which is a sub-lethal condition for the cells. Propidium iodide was used to identify dead cells in the population. Statistical analysis of a cell death ratio in approximately 5000 cells was performed using a flow cytometer. This experiment shows repetition frequency affects cell viability even with identical intra-medium electrical energy expenditure. The dead cell ratio was increased with decreasing pulse repetition frequency down to 0.33 pps, whereas the ratio is decreased with decreasing frequency below 0.1 pps. Since the resealing mechanism of nano-pores caused by the application of ns pulse lasts for a minute, the dependence of the cell viability on the pulsing sequence might be associated with time constants of biological processes related to recovery from field-induced damage. [ABSTRACT FROM PUBLISHER]