Breakdown characteristics and size effect in sub-cooled liquid nitrogen

We systematically investigated and discussed the ac breakdown (BD) characteristics and mechanisms of liquid nitrogen (LN₂) for different pressures (0.1-0.3 MPa) and temperatures (65-77 K) under quasi-uniform electric field (field utilization factor ζ = 0.986-0.674). The BD strength (E_B) in sub-cooled LN₂ (SLN₂) was analyzed and discussed in terms of size effect, taking account of the stressed liquid volume (SLV) with the electric field distribution in SLN₂. We also included the other reseachers' data on BD characteristics of LN₂, and extended the evaluation by applying the concept of α % SLV (SLV with the electric field strength higher than the decisive electric field factor (α) of the highest electric field strength). As the result, we found that E_B with the 50 % probability decreased linearly with the increase in α % SLV on a universal line, i.e. E_B = 78.4 × (α % SLV) ^-1/8.15, irrespective of the pressure and temperature of LN₂.