Summary form only given. Salinity changes in seawater are important for understanding global atmospheric and oceanic dynamics and their associated climate changes. Measurements of the electrical conductivity allow us to determine the salinity. Unfortunately, the electrical sensors, which are used for the measurement,are subject to biological growth (biofouling). Accurate measurements require frequent and careful cleaning of the sensors. In order to suppress biological growth at the sensors, an electrical sterilization method is being studied. It is based on the application of pulsed electric fields to the water at the intake of the sensor. This method has been successfully tested in the laboratory as well as in field experiments. The optimum pulse duration for biofouling prevention was found to be approximately 15 s at electric fields of 6 kV/cm. For an electrode gap of 2.5 cm this required voltages of 15 kV, generated by means of a Blumlein pulser with a thyratron as switch. Since the salinometer requires only a low flow rate, the intake in this system can be reduced from centimeter to millimeter size, and consequently allowed us to reduce the required voltage for biofouling prevention considerably. This in turn allowed us to replace the gas switch in the pulse gene tor by semiconductor, switches (MOSFETs). The voltage pulse has a duration of 700 ns (rise time: 20 ns), and an amplitude of 1000 V, generating an electric field of 6.7 kV/cm in the 1.5 mm electrode gap. The impedance of the load, the seawater between the electrodes, is approximately 5 Ohms. The repetition rate was set at 400 Hz. This allows us to apply at least two voltage pulses to every volume element of water entering the salinometer. The system is being tested in the Virginia Marine Science Museum in Virginia Beach, Va.

Summary form only given, as follows. Research on atmospheric pressure glow discharges in air is motivated by applications such as instantly activated reflectors and absorbers for electromagnetic radiation, remediation and detoxification of gaseous pollution, and surface treatment. One of the major obstacles in obtaining stable glow discharges at high electron densities (> 10/sup 11/ cm/sup -3/) is the glow-to-arc-transition, which develops generally in the cathode fall region. The use of a microhollow cathode discharge as independent electron emitter, has been shown to reduce the cathode fall drastically, and has therefore allowed us to generate direct current, atmospheric-pressure air glow discharges. The millimeter size plasmas, studied in earlier experiments, have been scaled to centimeter dimensions by operating MHCD sustained air glows in parallel and extending the electrode distance up to 2 cm. The electric field in these discharge plasmas, varies between 1.2 kV/cm, for a discharge current of 13 mA, to 2 kV/cm at 5 mA. The current density varies between 50 mA/cm/sup 2/ for high currents (13 mA) and 500 mA/cm/sup 2/ for low currents (5 mA), corresponding to electron densities between 10/sup 11/ cm/sup -3/ and 10/sup 12/ cm/sup -3/. The MHCD supported air glow discharge has a negative differential resistance. Parallel operation of the individual discharges therefore requires individual ballast. Two parallel discharges were generated, with their axes 0.4 cm apart. It was found that for this configuration the positive columns merged for currents exceeding 10 mA, creating a homogeneous plasma. The required electrical power density for electron densities of 1.5 /spl times/ 10/sup 11/ cm/sup -3/ was measured as 100 W/cm/sup 3/. By pulsing the plasma this power dissipation can be reduced considerably without sacrificing on the time-average value of the electron density.

Summary form only given, as follows. A 110 GHz interferometer is used to measure electron densities in a highly collisional plasma. The plasma is formed and maintained with a helical coil driven at 13.56 MHz. In this regime, the background pressure approaches atmospheric pressure, and the collisionality is of the order /spl nu//sub e-n//spl ap/10/sub 9/-10/sup 13/. Probe measurements in this pressure range are inaccurate and unreliable due to complicated effects that electron-neutral collisions have in the sheath. It is necessary to operate an interferometer at a frequency high enough that the dominant characteristic frequency is the operating frequency (/omega>/spl nu//sub e-n/) yet low enough that small phase changes can be detected as a wave passes through the plasma. In this experiment, plasma dimensions are of the order 5-10 cm. In addition, the inelastic nature of collisions in this experiment make it necessary to interpret small phase changes with great care. A model has been developed help in that interpretation. Typical operating parameters are pressures of 1-700 Torr, applied magnetic field of 0-1000 Gauss, operating gasses of nitrogen, oxygen, argon, and air, and input RF power levels of 100-3000 Watts.

Summary form only given, as follows. Glow-to-arc transitions in filamentary glow discharges in atmospheric air can be largely avoided by use of a plasma cathode, as has been demonstrated in short filamentary discharges in air. In these experiments a dc-driven microhollow cathode discharge (MHCD) was used as a plasma cathode to sustain a stable, direct current discharge between the plasma cathode and a third positively biased electrode. We have, using the same concept, extended the gap distance (distance between plasma cathode and third electrode) from previously 2 mm to the range from 6 mm to 20 mm and have studied the electrical, optical and plasma properties of such long filamentary glow discharges in atmospheric air. The MHCD is ignited between closely spaced molybdenum electrodes, separated by a 130 /spl mu/m thick alumina layer, with a 130 /spl mu/m hole through the sample. The filamentary discharge was ignited at small gap distances, in order to keep the ignition voltage at a low level, and then the gap was extended to the desired distance. In a certain range of current the filamentary glow discharge (FGD) current was found to be identical to the microhollow cathode discharge current. In this range control of the FGD by the MHCD is possible. From previous measurements of short gap filamentary discharge the gas temperature was found to be approximately 2000 K], the electron density was estimated as close to 10/sup 13/ cm/sup -3/. We will report on the results of measurements of these plasma parameters in long filamentary air discharges, and the electrical parameters, which determine the current range of MHCD control of the FGD. Parallel operation of these controlled filamentary glow discharges by using individual or distributed ballast might allow the generation of large volume, high pressure glows in air.

Summary form only given. High pressure nonthermal plasmas are gaining increasing importance because of their wide range of applications, e.g. in air plasma ramparts, gas processing, surface treatment, thin film deposition, and chemical and biological decontamination. In order to compare various methods of plasma generation with respect to efficiency, development of instabilities, homogeneity, lifetime etc., a central test facility for high pressure plasmas is being established. The facility will allow us to study large volume (>100 cm/sup 3/), nonthermal (gas temperature: <2000 K) plasmas over a large pressure range (10/sup -6/ Torr up to more than 1 atmosphere) in a standardized discharge cell. The setup was designed to generate plasmas in air as well as in gas mixtures. The available voltage range extends to 25 kV DC (10 kW power). The electrodes can be water cooled.

The electric field dependence of the absorption coefficient in semi-insulating GaAs at the absorption edge was measured in a high-voltage pulsed experiment. Pulse duration was kept below 50 ns in order to avoid thermal effects. A GaAs laser diode was used as a probe light source with wavelength varied from 902 to 911 nm. For fields up to 40 kV/cm the absorption coefficient increased from 3 to 17 cm−1 at 902 nm, with smaller absolute increases evident at the longer wavelengths. Calculation from theory was consistent with this behavior. The spatial variation of the electric field was also recorded with a CCD camera. This method was used as a diagnostic technique to study the field distribution during the switching cycle of a high-power photoconductive switch. The described system could be used as a simple electric field probe with temporal resolution of 100 ps, or as a field mapping system with spatial resolution approaching 1 μm.

Photoconductive GaAs-switches are used to switch electrical power in excess of one Megawatt and on a time scale of nanoseconds and less. When operated at very high applied fields and triggered with low energy lasers, the electrical breakdown leads to the development of current filaments with a forward voltage which corresponds to an average electric field of several kV/cm. The temporal development of the electrical breakdown in semi-insulating GaAs has been studied by means of electrical and optical diagnostics, particularly with a recently developed electro-absorption technique. The observed phenomena are analogous to glow-to-arc transitions in electrical discharges in electronegative gases, where the attachment coefficient is a rapidly increasing function of the electric field. Semi-insulating GaAs is characterized by the presence of a large concentration of electron and hole traps, with trapping cross-sections which in certain cases increase with the electric field intensity. As field enhanced electron attachment in gas discharges, field enhanced trapping in semi-insulating semiconductors can lead to the formation of high field domains. Impact ionization of deep centers in these domains is assumed to cause the generation of an electron-hole plasma. This plasma serves as the nucleus for streamers which develop into a current channel and bridge themore » gap between the contacts. The continuity of the current flow at forward electric fields of kV/cm is assumed to be due to double injection.« less