Search

Liquid helium (LHe) at 4.2 K was electronically excited using a corona discharge for both negative and positive high voltages. The experiments were carried out for different pressures in the range from 0.1 to 10 MPa at constant temperature. The light emitted from the zone close to the tip was spectroscopically analyzed showing features from atoms and excimer helium. The shifts and widths of the observed lines and bands were found to depend on the applied hydrostatic pressure and on the tip polarity. Our analysis showed that classic pressure broadening theory cannot account for the observed widths and shifts rather than the presence of bubbles which surround single excited atoms and molecules. For positive tip polarities red shifted features distinct from pure He and [He.sub.2]* were observed and tentatively assigned to 'red satellites'. Index Terms--Helium spectroscopy, corona discharge, excimer 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 [V.sub.s], 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 [V.sub.s] 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. Index Terms--liquid helium, corona, space charge, charge carrier mobility, optical spectroscopy, spectrum analysis.

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 L[N.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 L[N.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. Index Terms--Insulating liquids, liquid nitrogen, cryogenic insulation, breakdown, prebreakdown phenomena, streamers.

This paper presents a study of prebreakdown phenomena (usually called streamers) occurring at high voltage in liquid nitrogen. A special cryostat has been constructed to allow dielectric measurements in liquid nitrogen up to 200 kV, and 0.5 MPa hydrostatic pressure. Streamers are detected and studied with various techniques: high speed visualization, streak camera, and recording of transient currents and emitted light. The propagation of positive and negative streamers is investigated at gap distances up to 80 mm. Streamers are then compared to similar phenomena observed in mineral oil. In liquid nitrogen, streamers are different compared to mineral oil. A transition from slow to fast positive streamers (velocity about 10 km/s) is observed at a threshold voltage below the breakdown voltage. The hydrostatic pressure has almost no influence on this phenomenon. Some experiments carried out with a solid barrier perpendicular to the gap axis are also presented. The practical consequences for the design of HV insulation in superconducting systems are then discussed.

An experimental study of high field conduction phenomena in liquid argon and liquid nitrogen in a point-plane electrode geometry is reported. These liquids behave qualitatively like saturated hydrocarbons except when they are studied close to their normal boiling point. The generation and dynamics of microscopic bubbles (thermally and electrically induced) as a function of various parameters (nature of the liquid, injected energy, hydrostatic pressure) are presented and discussed. Possible influences of these phenomena on the electrical insulation at cryogenic temperatures is also discussed.

The insulating properties of a liquid are commonly studied before a breakdown occurs. In HV industrial apparatus such as high energy plastic metallized and liquid impregnated films capacitors, some local breakdowns, named self-healing, occur and the ability of the liquid to remain insulating after electrical degradation is of interest. To understand the influence of the liquid on the behavior of such capacitors, the post-arc behavior of the liquid alone has to be studied in a range of energy (~ mJ) close to the self-healing process. The present work deals with the study of the ability of different liquids to restore in the millisecond range their insulating property after an electrical arc. Using an experimental apparatus based on the repetitive charging with a dc HV generator and discharging through an arc of a resistor-capacity system, different kind of liquids were tested. The liquids we used were alcane, esters, alkyl benzene and polyaromatic. We showed as expected that liquids with the smallest ratio of carbon versus hydrogen in their molecular formula present the best restoration property after an electrical arc, property that is crucial for liquids impregnating metallized capacitors.

When negative HV is applied to a needle of small radius of curvature ([r.sub.p] < 0.2 [[micro]meter]) in nonpolar liquids, a detectable field emission current (I > x[10.sup.-12] A) is observed above a voltage [U.sub.FN], the Fowler-Nordheim law. For larger point radii, only a regular current pulse regim is detected above a threshold voltage [U.sub.S] (Trichel regime). Analysis of these voltages [U.sub.s] and [U.sub.FN] as a function of conduction band energy [V.sub.0] of different liquids, shows that the apparent work function for high electric field strengths in liquids is lower than the one without applied field (2.1 compared to 4.5 eV for a tungsten cathode). This difference can be explained if one assumes the existence of a plasma-like layer around the HV cathode.

A comparative study of electrical conduction and emitted light in an atomic (Ar) and a molecular liquid ([N.sub.2]) is presented as a function of various parameters. From the spectroscopic analysis of emitted light (in the range 200 to 850 nm), the energy of the hot electrons in these liquids may be measured. A simultaneous study was carried out on dense gases as a function of the applied pressure. We discuss the electron swarm parameters deduced from these results, in an attempt to explain the behavior of electrons in the gas-liquid phase transition.