Instabilities in superconducting tunnel junctions in different thermal environments

Instabilities in superconducting tunnel junctions have been studied in three environments: low pressure He gas, normal liquid He I, and superfluid He II. It is demonstrated that the effect arises at a bias voltage V0 where junction dissipation is sufficient to drive a region of the electrode(s) normal. The transition is accompanied by a small excess current, which is due to the appearance of electrode resistance and junction heating. For Pb electrode junctions immersed in liquid He the critical junction dissipation marks the outset of film boiling, which shows up as excessive noise in the tunneling curves. This critical heat flux, approx. =1 W/cm2 at 4.2 K, limits the useful energy range for tunneling spectroscopy and leads to catastrophic burnout of high density Josephson arrays