O. Lesaint

On the relationship between streamer branching and propagation in liquids: influence of pyrene in cyclohexane

This paper presents an experimental study on the influence of an aromatic additive (pyrene) on the propagation of positive filamentary streamers and breakdown in liquid cyclohexane. Experiments are carried out in point-plane gaps up to 5 cm over a wide voltage range. With pyrene, the propagation is facilitated, and thus the breakdown voltage is lowered in point-plane geometry. Streamers also become very branched, and a much larger number of filaments propagate than in pure cyclohexane. Correlated to this, a large increase of the inception voltage of fast streamers (acceleration voltage) is observed at high voltage. This fact can be explained by macroscopic electrostatic properties of streamers, which depend on their geometrical structure. When more and more filaments propagate at high voltage they shield each other, which in turn limits their tip field (and thus velocity) at a nearly constant value over a large voltage range. The propagation velocity is thus not only determined by microscopic processes at the filament heads. Consequently, it does not always constitute an adequate parameter to directly appreciate and compare microscopic propagation processes. A discussion about streamer mechanisms is also presented.

On the gaseous nature of positive filamentary streamers in hydrocarbon liquids. I: Influence of the hydrostatic pressure on the propagation

In this first part, the gaseous nature of filamentary positive streamers is demonstrated by the influence of hydrostatic pressure on their propagation in cyclohexane and pentane up to 7 MPa. At a fixed voltage, when the pressure is increased, the propagation velocity is found to be constant whereas the stopping length of streamers is greatly reduced. Correlated to this effect, the duration of transient currents and light emission signals is reduced. These effects are discussed in terms of the hypothesis that the conductivity of the streamer results from a gas discharge mechanism occurring within the filaments. These results constitute the experimental basis on which a study of the filament dynamics has been developed (part II).

On the gaseous nature of positive filamentary streamers in hydrocarbon liquids. II: Propagation, growth and collapse of gaseous filaments in pentane

This paper presents a study of the dynamics of positive streamer filaments in pentane using an optical method described in the text for pressures up to 9 MPa. It is observed that filaments expand and collapse, in a similar way to cavitation bubbles. Good agreement with the Rayleigh model shows that the dynamics of filaments is determined by liquid inertia. The influence of electric forces (electrostatic pressure) on the dynamics is found to be negligible. An evaluation of the energies involved shows that filaments are mainly composed of vapour, whose pressure varies with time and space and is on average higher than the hydrostatic pressure. Vaporization results mainly from energy dissipated at the filament extremity, which may be compared to a propagating point heat source of about 10 W power. At low pressure, a significant influence of energy dissipation within the filament, attributed to the transient current flowing during propagation, is also observed. At high pressure and/or high voltage, the stopping of the streamer is due to collapse of the gaseous filaments.

A comparison of negative and positive streamers in mineral oil at large gaps

The propagation of negative streamers in mineral transformer oil is investigated in large point-plane gaps up to 35 cm, under impulse voltage. Basic features of streamers are obtained: velocity, stopping length, propagation voltage, electrical charge, photographs of the emitted light. Many features of negative streamers resemble those already observed in positive polarity under similar conditions. The voltage required to initiate fast streamers (velocity higher than 50 km s-1) is identical in both polarities. The variation of propagation voltage versus distance and streak photographs both suggest that conduction mechanisms within long gaseous streamer channels are identical in positive and negative polarities. Some characteristics are however different: in negative polarity a higher voltage is required for the onset of propagation, propagating streamer heads emit more light and their velocity is lower. These differences are due to different propagation mechanisms occurring at the streamer heads. These mechanisms as well as the implications for practical high voltage insulation are discussed.

Ion implantation based on the uniform distributed plasma

Abstract For over a decade, our group has been working on the development of distributed electron cyclotron resonance (DECR) plasma sources. The uniform distributed plasma (UDP) is the latest outgrowth of DECR, which has proved to be a flexible concept, leading to task-adapted plasma sources. Our plasma-based ion implantation (PBII) reactor is a 60 cm diameter, 70 cm high cylinder. The inside of the cylinder wall is covered with an array of 24 tubular magnets, 2.45 GHz microwave power feeds, and wave propagators. This peripheral plasma source of a good square meter produces a uniform distributed plasma (UDP), suitable for the treatment of wafers, pipes or objects of arbitrary form. Initial PBII plans concern the nitridation of silicon wafers. At 1 mTorr pressure and 1.3 kW input power, the N 2 plasma has a density of 2 × 10 10 ions cm −3 and an electron temperature of 1.2 eV. The N + /N 2 + ratio of 7/3 in the N 2 UDP plasma, determined by quadrupole mass spectrometry, is favorable for PBII applications. A 45 kV pulsed power supply is available for initial tests, but should be supplanted by a more powerful source for meaningful experiments.

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.

New line of high voltage high current pulse generators for plasma-based ion implantation

The two general specifications required for plasma-based ion implantation are low pressure, large size plasmas and high voltage high current pulse generators. In addition, pulses with rise and fall times of the order of the inverse ion plasma frequency and with much longer durations than those of the inverse ion plasma frequency are most often required. To fulfill these requirements, a new type of high voltage generator using a pulse transformer has been developed. A “mettglass”® magnetic core is used as step-up pulse transformer. Voltage at the primary is provided by transistor switches which can achieve rise and fall times of less than 1 μs and maximum pulse currents of 100 A. The primary of the transformer consists of 96 turns wired in parallel and the secondary of 96 turns wired in series. The performances reported with this pulse generator were obtained on a test resistor and then on a substrate immersed in a plasma.

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.

Integrated low power and high bandwidth optical isolator for monolithic power MOSFETs driver

An integrated solution for the galvanic isolation between power transistors and their control unit is presented in this paper. This solution is based on a monolithic integration of a photodetector within a power MOSFET without any modification of its fabrication process. This photoreceiver can be associated with a monolithic driver to drive high side switches. Exhaustive characteristics for several integrated photodetectors are presented and discussed: quantum efficiency, step response, small signal analysis and sensitivity to the High Voltage MOSFETs Drain. The results of this analysis are photoreceivers with a Full Width at Half Maximum above 300MHz and a responsivity above 0.15A/W at a wavelength of 500nm. This leads to an integrated low power and high bandwidth optical isolation.

Breakdown of liquids in long gaps: influence of distance, impulse shape, liquid nature, and interpretation of measurements

Breakdown measurements are carried out in liquids under point-sphere and pointplane electrode geometries, over a large range of gap distances up to 35 cm. Non-polar hydrocarbon liquids with different chemical structures are studied: saturated, aromatic, polyaromatic, mineral oils, and ester. Two high voltage impulse shapes are used: the standard lightning impulse (LI), and a specific “step” impulse (ST). Step impulses are favorable to interpret breakdown measurements since the applied voltage remains constant while prebreakdown streamers propagate. Conversely, with lightning impulse and long gaps the propagation of streamers with a low propagation velocity (a few km/s) is quenched due to the rapidly decaying voltage. In this case, breakdown can result only from the propagation of faster streamers, which appear in very different conditions according to the liquid nature. The results obtained allow us to interpret the large variations of breakdown voltage observed in large gaps, when either the high voltage impulse shape or the liquid nature is changed. These results also help to define proper conditions for testing and comparing liquids. Testing with lightning impulse in short gaps, such as in standard tests, does not provides data relevant for very high voltage applications.