Thermal-buble initiated breakdown characteristics of liquid helium and nitrogen at atmospheric pressure
Experiments show that electric field promotes to form a vapor locking in a gap with a hot spot and in turn the formation of locked vapor produces the serious reduction of breakdown voltage. Especially it is argued that the dielectric strength of liquid helium under serious hot spot conditions decreases below that for saturated gas.
AC insulation property of fluorocarbon in thermal induced two-phase flow by self-heating electrode
Although it has been found that the insulating characteristics of thermal induced two-phase flow changes with pressure and heat power, its breakdown mechanism has not been clarified so far. In order to obtain actual working conditions of two-phase flow state of fluorocarbon, a special apparatus with self-heating electrodes has been manufactured to study AC breakdown voltage (BDV) under different pressure, electrode gap distance and heating density. The experimental data outline that the variation of breakdown voltage vs. heat density is an elongated "Z" shaped curve: Breakdown voltage maintains the liquid value in the Invariant Interval, and drops sharply in the Converting Interval, finally reduces to gas value and maintains in the Maintaining Interval. It is pointed out that the steep converting of BDV is closely related to the change of flow pattern and bubble generation on heating electrode surface. And the threshold of converting point is highly consistent with the peak of electrode surface overheating AT, therefore it can be used as an excellent signal to detect the BDV converting. An electric field model is established based on bubble thermodynamics to explain the discharge mechanism of "vapor lock" effect, offering a theoretical minimum limit of BDV value in the thermal induced two-phase flow, which provides a useful guide to design the insulation subjected to various high voltages.
Dielectric strength of liquid helium in millimeter gaps
In a previous paper,1 a program underway at Oak Ridge National Laboratory to investigate the high voltage electrical properties of fluids and solids for cryogenic systems was described. As the first result of these investigations, we report here on measurements of the breakdown strength of liquid helium boiling at atmospheric pressure, under voltages up to ± 130 kV de, and in gaps of up to 2. 5 mm. Preliminary results with ac voltage have also been obtained.