Many experiments on the dielectric behaviour of gaseous hefium have been carried out in the temperature range of 4"2 to 50 K. These experiments have been carried out in uniform fields as well as in nonuniform fields. In uniform fields, special attention has been paid to the breakdown values near the boiling point, and, in this zone, a strong variation has been pointed out. Moreover, the influence of some parameters such as external ionization has been studied.
In non-uniform field, with point-to-plane electrodes, an important polarity effect has been observed. From these experiments, it is possible to obtain some information on an electrode effect.
Dielectric breakdown of gaseous helium at very low temperatures B. Fallou, J. Galand, and B. Bouvier
In many cases, especially in the design of cryoelectrotechnical machines, knowledge of the dielectric behaviour of gaseous helium at very low temperatures is of prime importance. As a matter of fact, it may happen, in practical conceptions, that high voltage conductors must pass gaseous helium, from superconductors at 4.2 K to outside the cryogenic vessel at room temperature. Among the various dielectric properties, the most important .and the most critical is the breakdown strength.
In the following pages, this particular point is studied, in uniform as well as in non-uniform field. The tests which were performed were laboratory tests, with gaps ranging from 0.1 to 1 mm in uniform field and to 10 mm in non-uniform field. As the aim was to give development engineers practical data from which to build industrial machines, most of the experiments were made at 50 Hz.
Moreover, the behaviour of gaseous helium being well known above a certain temperature, the experiments were made only in the range from 4.2 to 50 K. From these values, it is easy to foresee the breakdown voltages for higher temperatures. All these experiments were made at atmospheric pressure.