Characterisation of a plasma closing switch filled with environmentally friendly gases

Sulphur hexafluoride (SF₆)´s advanced dielectric properties have ensured that this gas is the most common choice of dielectric media to be employed in gas-filled plasma closing switches (PCS). In recent years, however, environmental concerns over SF₆ usage adding to the greenhouse effect and economical reasons such as the continual increase in cost of SF₆ gas has lead the pulsed power research community to minimise the use of SF₆ in pulsed power systems with a view to removing it completely. In order to be able to remove SF₆ from these systems a suitable alternative must be found. Research has been carried out into minimizing SF₆ consumption by making use of mixtures of SF₆ with other gases as well as investigations into replacing SF₆ with other gases such as nitrogen (N₂) [1][2]. It is important to achieve similar performance of plasma closing switches filled with any alternative fluid as compared with SF₆-filled switches. The main parameters which define the switch performance include DC and triggered breakdown voltage, jitter, switch inductance and energy losses. The focus of the present paper is to study the characteristic parameters of typical plasma switch topologies used in industrial pulsed power systems such as Marx generators including DC charged PCS´s filled with environmentally friendly gases (gas mixtures). Experiments have been carried out using atmospheric air, nitrogen, and a 60% nitrogen/40% oxygen mixture at different pressures ranging from atmospheric pressure to 1 bar gauge pressure in a sphere-sphere topology. The paper presents breakdown characteristics of this switch including its self-breakdown voltage in the DC charged regime for various inter-electrode distances and gas pressures.