摘要
SRF cavities are operated at Meissner state,which al-lows the electro-magnetic wave only interacts with the very surface of the cavity.The penetration depth of Nb at 4.2 is^100 nm,which means more than 99%of the expensive SRF grade Nb used on the SRF cavity is not involved in the interaction with the E-M wave.Fig.1 shows the schematics diagram of the interac-tion between magnetic field and a Nb coated Cu cavity.With a Nb film thicker than 3λ,the copper hide behind wouldn't be able to see the E-M wave nor generate RF dissipation.
