N. Bonifaci

Streamers in liquid nitrogen: characterization and spectroscopic determination of gaseous filament temperature and electron density

Streamers initiated in point-plane electrode geometry under impulse voltage are studied. Characterization up to 30?kV in both polarities shows two streamer types. In negative polarity, only slow `bush-like streamers with a velocity of about 100?m?s-1 are observed. In positive polarity, both `bush-like and filamentary streamers appear. Positive `bush-like streamers have similar velocity as in negative polarity, whereas filamentary streamers have a propagation velocity up to 30?km?s-1. This high velocity is correlated to large transient current and light emission. Spectroscopic study of the light emitted by filaments shows mainly the second positive system of nitrogen. This system allows us to estimate the gas temperature inside a filament by making a comparison with a numerically simulated spectrum. A temperature close to 500?K is found. Dilution of hydrogen gas in liquid nitrogen allows us to determine also the electron density by Stark broadening of the Balmer H? line. An electron density of about 1.1018?cm-3 is found.

Prebreakdown and breakdown phenomena under uniform field in liquid nitrogen and comparison with mineral oil

This paper presents a study of breakdown and prebreakdown phenomena (streamers) in liquid nitrogen and mineral oil under quasi uniform electric field, under ac and impulse voltage. Streamers preceding breakdown are studied up to 0.5 MPa by high-speed visualization and recording of emitted light. In these conditions, breakdown in LN/sub 2/ is mainly due to negative streamers, initiated at lower voltage than the corresponding positive voltage. Hydrostatic pressure has a limited effect on breakdown voltage, such as in mineral oil. It is shown that the ratio of impulse to ac breakdown voltage in LN/sub 2/ is surprisingly low (close to 1), whereas in the same conditions ac breakdown voltage in mineral oil is lower than impulse breakdown voltage. Practical consequences for the design of HV insulation in superconducting systems are discussed.

Characterization and spectroscopic study of positive streamers in water

This paper first presents a characterization of positive streamers in pure water (conductivity /spl sim/ 0.05 /spl mu/S/cm), initiated by a fast rectangular impulse voltage in a point-plane electrode geometry. According to the conditions, two main streamers types can be observed, with average propagation velocities from 0.5 to 30 km/s. Then, a spectroscopic investigation of positive streamer propagation is carried out. According to the conditions, spectra of the light emitted close to the point, show the OH band, atomic lines (OI, H/sub /spl alpha//) and a strong continuum from 350 to 500 nm. From an analysis of the OH band and from the broadening of the atomic lines (H/sub /spl alpha// and OI at 777 nm), the kinetic temperature and the electron density within filaments of both streamer types can be evaluated.

Determination of charge mobility in He gas from current-voltage measurements in point-plane geometry

In order to clarify whether or not is it possible to extract reliable values of positive and negative mobility for ions and even for electrons in gas when using sharp point-plane geometry, the authors have undertaken a systematic study using He gas. Monitoring the purity level of the same gas filling two different cells (point-plane and plane-parallel), they have compared mobility values deduced from current-voltage curves (mobility K) with time of flight measurements (actual mobility /spl mu/) as a function of gas pressure (0.1-15 MPa) and purity. For negative charge carriers (ions or electrons), they have directly measured K/sup -/ and /spl mu//sup -/ and then compared the results. They observed, as expected, that the measured negative charge carriers mobility was strongly dependent on gas purification. For commercial gas (O/sub 2/ of the order of 100 ppm), K/sup -/ was very close to the data obtained for negative O/sub 2//sup -/ ions (i.e., K/sup -/ /spl cong/ /spl mu/(O/sub 2//sup -/)) but, for better purification level, K/sup -/ increases quickly and can reach electronic values /spl mu//sub e/ (K/sup -/ /spl rarr/ /spl mu//sub e/). For positive ions, in all the range of studied pressure, it was shown that one can extract easily from I(V) measurements, mobility values K/sup +/ in accordance with the values of /spl mu//sup +/ of He/sup +/ previously given in the literature.

Negative corona discharge in liquid helium

Conductivity and ionization mechanisms in liquid helium have been investigated by using high field conduction phenomena occurring near a sharp needle electrode. When the voltage V applied between the needle and the plane is greater than some threshold voltage Vs, a corona-like discharge regime was observed. Charge carriers are generated in the high field region close to the point where an intense light emission is detected. Above Vs the mean current is space charge limited and the mobility of charge carriers has been deduced from analysis of current-voltage characteristics. Some parameters of the corona-like region have been deduced from a spectral analysis of the light emitted during the discharge

Prebreakdown and breakdown phenomena under uniform field in liquid nitrogen

We present a study of breakdown and prebreakdown phenomena (streamers) in liquid nitrogen under quasi uniform electric field, under ac and impulse voltage. Streamers preceding breakdown are studied by high speed visualization and recording of emitted light, with hydrostatic pressure up to 0.5 MPa. In these conditions, breakdown is mainly due to negative streamers initiated at lower voltage than positives. This result is in contrast with previous measurements carried out in point plane gaps: in this geometry breakdown is due to positive streamers since their propagation is much easier. It is also shown that the ratio of impulse to ac breakdown voltage in LN2 is surprisingly low, close to 1. Practical consequences for the design of high voltage insulation in superconducting systems are then discussed.

Impulsive Discharges in Water: Acoustic and Hydrodynamic Parameters

Underwater spark discharges are used in multiple practical applications including plasma closing switches, water treatment, plasma channel drilling and mineral processing, waste recycling, treatment of metals, and medical lithotripsy. Spark discharges in water have been studied for several decades; however, despite significant research efforts and progress in this area, further investigation into the efficiency of plasma acoustic sources and their optimization is required in order to expand their practical application. This paper is aimed at investigation of the electrical and hydrodynamic parameters of underwater plasma-generated cavities, including plasma resistance, energy delivered into the plasma cavity, period of cavity oscillations, and characteristics of pressure impulses. Different energy levels, breakdown voltages, and gap distances were used in the present study to allow systematic analysis of these electrical and hydrodynamic parameters. Empirical scaling relations that link the maximum acoustic pressure and the period of cavity oscillations with the energization parameters and the resistance of plasma have been obtained. These empirical functions can be used for optimization of the plasma acoustic sources and for tailoring their parameters for specific practical applications.

Direct measurements of ions mobilities in gaseous helium for validation of their indirect determinations using corona discharges

An experiment carried out with the same He gas shows the distinction between negative ions mobility data obtained both by time-of-flight and corona current methods. The negative charge carriers mobility magnitude extracted from corona discharge experiments could be either very close to the one obtained by time of flight (for the more dirty gas) or greater by several fold than that from the time-of-flight experiments (for the smallest impurity level). This is interpreted as the difference in production and capture of electrons in the 2 methods: for plane-plane geometry the number of electrons produced by the photo-cathode is low and they are immediately captured by impurities near the electrodes. In corona the electrons are multiplied by avalanche process before being captured, the lifetime of electrons depending strongly on impurity level. The mixture of both electrons and negative ions in current transport is revealed in the obtained indirect mobility which conversely could be used to appreciate the purity level.

Spectroscopic Study of the Negative Corona Discharge in the Liquid 4He

The corona discharge plasma in liquid 4He has been studied by spectroscopy as a function of the pressure between 0.1MPa and 10.4MPa. The rotational temperature was approximately equal to 350K at the pressure of 0.1MPa. With increasing pressure the blue shift and the broadening of the atomic helium line at 706.5nm (2p3P‐3s3S) and 728.1nm (2p1P‐3s1S) and the bands of the singlet d1Σu+→b1Πg transition at 639nm and triplet d3Σu+→b3Πg transition at 659nm have been observed.

Hydrodynamic parameters of air-bubble stimulated underwater spark discharges

Considerable amount of research work has been focused on investigation and optimization of strong acoustic waves generated by spark discharges in water. There are several methods to achieve and to stimulate underwater spark breakdowns, including free-discharges, wire-guided and gas-bubble stimulated discharges. In the present work, air bubbles are injected into water in order to achieve spark discharges in relatively long inter-electrode gaps. This paper reports on hydrodynamic and acoustic characteristics of spark discharges stimulated by air bubbles and presents the functional relationships between the hydrodynamic and electrical parameters of such discharges, including breakdown voltage, spark plasma resistance and energy available in the discharge. A hydrodynamic analytical model has been developed and used to calculate the acoustic efficiency of the underwater spark discharges.