One of the dominant causes of the large gradient spread and gradient limits in superconducting cavities is the abrupt quench of superconductivity due to “defects” on the surface. Many types of defects have been identified ([1], pp. 203-204). Some involve the presence of impurities. But there are also pure Nb defects such as pits and protrusions. We present results of calculations to show that the field enhancement factor for pits can reach the value of 4. We also calculate the magnetic field enhancement at the surface of ellipsoidal protrusions. This enhancement has a value of about 1.5 for a semispherical projection
