Saito, K. Konomi, T. JACoW Publishing Geneva, Switzerland 2673-5504 978-3-95450-234-9 10.18429/JACoW-SRF2023-MOPMB027 English 151-154 niobium cavity experiment SRF superconductivity Contribution to a conference proceedings 2023 2023-09 https://doi.org/10.18429/JACoW-SRF2023-MOPMB027 https://jacow.org/srf2023/papers/mopmb027.pdf To date, many theoretical formulas for superheating field on SRF cavity are already proposed based rather complicated calculations. This paper proposes the formulas by a very intuitive simple model: energy balance between RF magnetic energy and superconducting condensed one, and a condition of vanishing the mirror vortex line image. The penetration of a single vortex determines the superheating field for a type II superconductor. On the other hand, for type I superconductors, the surface flux penetration determines it. The formula fits very well quantitatively the results of niobium cavity and Nb₃Sn one. In addition, it gives a nice guideline for new material beyond niobium.