S. Kohno

Cable guns as a plasma source in a plasma opening switch

The characteristics of a plasma generated by cable plasma guns have been studied by a laser interferometer. Cable plasma guns are frequently used as a plasma source in plasma opening switches. In our experiments, the plasma source consists of eight coaxial cable guns mounted on the outer electrode of concentric coaxial electrodes. The reproducibility of the gun in subsequent shots is found to be better than 10%, and the gun-to-gun difference is less than 15%. Assuming a symmetry of eight guns, the contour maps of the electron plasma density are plotted as functions of time. The plasma density becomes maximum near the gun nozzle and near the inner coaxial electrode. The plasma density is low in the area between the coaxial electrodes during the early time of the discharge. At a later time, the plasma fills the space between the two guns more uniformly. Still photographs of the plasma luminosity show a good correspondence with the plasma density plots which were taken 10 /spl mu/s after the discharge initiation. The plasma gun system is designed for use in a 400-kA inductive voltage adder with the inductive energy storage system.

Effect of laser beam deflection on the accuracy of interferometer measurements

The laser interferometry is a frequently used diagnostics for plasma density measurements. The measurements in plasmas with no density gradients allow direct correspondence of the fringe shift and plasma density along the laser line of sight. The change of the interferometer contrast due to scene beam deflection in plasmas with density gradients reduces the fidelity of interferometer measurements due to the change in the contrast of the fringe. The analytic expression for the maximum acceptable deflection angle is derived, analyzed, and compared to the experiment. Ways to improve the accuracy of interferometer measurements are proposed.

Interferometer Measurements in Pulsed Plasma Experiments

The interferometer measurements are extremely informative in plasma experiments allowing direct evaluations of the electron density. The primary goal of the work presented, is to build a laser interferometer which meets the requirements of the highest possible simplicity, economy, convenience and ease of construction. These requirements are successfully satisfied while maintaining high sensitivity (±0.5°–of phase shift) and a wide density range (1014 and 1019 cm-2–line-integrated) of the interferometer. In our experiments we used a low average power (5 mW) He–Ne laser without complicated and costly stabilization or detection environments. The He–Ne laser interferometer with the Michelson arrangement was used to measure the line-integrated plasma densities in various plasma experiments. Time- and spatially-resolved density measurements were performed for a plasma opening switch, a laser produced plasma, an electrothermal launcher and railgun plasmas.

Repetitively operated plasma opening switch using laser produced plasma

A plasma opening switch using laser produced plasma is successfully operated at repetition rates up to 3.3 pulses per second. It is shown that the switch performance has good reproducibility without any maintenance. Reliability tests indicate that the switch lifetime is limited by the volume of the target material and the lifetime of the energy storage capacitor.

Improved operation of microsecond plasma opening switch by plasma source modification

The characteristics of a microsecond plasma opening switch have been improved by the modification of the plasma source. Conventional cable plasma guns have been used in the experiments, producing different directions of the plasma flow by changing the guns’ nozzles. The nozzles were arranged to provide the plasma flow, which is not exactly radial, but has an angle to the gun axis. The experiments with the modified plasma guns were carried out with the upstream, downstream, and azimuthally slanted plasma flow directions. The results are compared to a conventional radial flow plasma source. The plasma flow slanted in an angle of 30° downstream resulted in 50% improvement of the load current rise time and switch impedance. The obtained dependencies are explained in the frame of the snowplow model. Radial component of the plasma velocity at the end of the conduction phase is higher for downstream plasma flow. This results in a faster opening of the switch and is in good agreement with experimental data.

A novel scheme of laser interferometer–refractometer with high spatial and temporal resolutions

A novel scheme combining interferometer and refractometer for simultaneous plasma density and plasma density gradient measurements is described. The scheme employs a ribbon laser beam obtained from one-dimensional laser beam expansion by a pair of cylindrical lenses. This beam shape allows measurements at several spatial locations simultaneously. The beam deflection measurements give additional important information on the spatial distribution of plasma density gradient in the direction perpendicular to the major axis of the expanded beam. Fast pin photodiodes are used as light detectors for temporal resolution better than 1 ns. The high power (700 mW) of the argon ion laser allows 0.1° phase resolution and simultaneously 0.1 mrad refractional deflection angle resolution.

Variations in vacuum gap location in a microsecond plasma opening switch

The process of vacuum gap formation in a microsecond plasma opening switch has been studied using a He–Ne laser interferometer. Time and spatially resolved density measurements are performed in the plasma opening switch, showing a density decrease during switch opening in almost the whole interelectrode gap. Despite fine triggering accuracy and generally good shot-to-shot reproducibility of voltage and current waveforms, measurements made with the same laser beam location indicated remarkable shot-to-shot variation. The unstable formation of the vacuum gap in the microsecond plasma opening switch may be a major limiting factor for efficient switch operation.

Plasma opening switch using laser-produced plasma

In order to construct a practical inductive energy storage pulsed power generator, the development of the opening switch which can repeatedly conduct a large current and then rapidly interrupt this current is necessary. Though the plasma opening switch (POS) can interrupt a large current rapidly, the effective number of switch operations is limited because of the decrease of the carbon sprayed on the insulator with each shot. We proposed a plasma opening switch using laserproduced plasma as one of opening switches which can be repeatedly used. The another advantage of the POS with the laser produced plasma is that it becomes possible to select more suitable plasma species for the operation of POS by changing the target materials. Here, the target materials are changed.

Response of Biological Cell Exposed on Burst RF Fields

Burst RF fields (BRFFs) were applied to Chinese Hamster Ovary (CHO) cells, and the cell response to the BRFF of two different frequencies, 20 kHz and 50 MHz, was investigated by means of the fluorescent microscopy. The field strength and duration of the BRFF were fixed at 1 kV/cm, and 100 mus, respectively. Cells were placed in a 100 mum-gap electrode plated on the slide glass for a microscopy. Acridine orange (AO), which mainly reacts to DNA, was used as the fluorescent dye. From the experiment, the BRFF of 20 kHz initiates the increase in the permeability of cell membrane. In contrast, the BRFF of 50 MHz does not act on cell membrane but causes degeneration of DNA or RNA. We have experimentally demonstrated that the burst RF fields with only a small electric field of 1 kV/cm cause not only membrane reaction but also intracellular effects.

Inhomogeneity of currents in a wire array during its exploding phase

The current in a wire array during its exploding phase has been studied. The wire array consisting of many thin wires is used as a plasma source in a z-pinch X-ray radiation system. The current distribution in the exploding wire array plays an important role in producing an initial symmetrical plasma. In the present experiment, four tungsten wires of 0.1 mm in diameter were exploded in a vacuum. The experiment was carried out with an inductive voltage adder pulsed power generator providing a current of 160 kA at quarter period of 1.8 /spl mu/s The currents through and the voltage across the wires were measured simultaneously. It was found that the currents in the wires were inhomogeneous, especially after the wires became plasma. Once the currents became inhomogeneous, the inhomogeneity remained throughout the discharge. It was also studied how the dimensions of the wires affected the homogeneity of the currents. Wires of different lengths or different cross sectional areas resulted in strong inhomogeneities of the currents. The wires with shorter length or smaller cross-sectional area became plasmas earlier than other wires. Thus, the resistances of the wires were not equal. These differences in the resistance caused the inhomogeneity of the currents.