Prebreakdown and breakdown phenomena under uniform field in liquid nitrogen

We present a study of breakdown and prebreakdown phenomena (streamers) in liquid nitrogen under quasi uniform electric field, under ac and impulse voltage. Streamers preceding breakdown are studied by high speed visualization and recording of emitted light, with hydrostatic pressure up to 0.5 MPa. In these conditions, breakdown is mainly due to negative streamers initiated at lower voltage than positives. This result is in contrast with previous measurements carried out in point plane gaps: in this geometry breakdown is due to positive streamers since their propagation is much easier. It is also shown that the ratio of impulse to ac breakdown voltage in LN2 is surprisingly low, close to 1. Practical consequences for the design of high voltage insulation in superconducting systems are then discussed.