Evaluation of bubble breakdown limit in LHe below 4.2 K

A simple model is presented to explain the high, almost intrinsic breakdown strength of LHe at low temperatures, despite the weakness of the vapor. A kind of intrinsic barrier against bubble growth is identified in the model. The first step of breakdown may be predischarges in microcavities. Extrinsic effects are consistent with the model and include the area effect, cathode surface conditions, and dynamical enhanced electron emission boosted by foreign particles. Whether these adverse effects may lower the liquid strength down to that of the vapor at temperatures much below 4.2 K is not only of theoretical significance but is also important for the design of advanced high field superconducting magnets