Investigation of Superconducting Niobium Cavities at S-Band

This paper describes the investigation of superconducting Nb cavities at S-band frequencies. The TE
₀₁₁
and TM
₀₁₁
cavities were machined by Varian Associates from solid billets of electron-beam-melted niobium and chemically polished to remove all work-damaged material. At low power levels, the TE
₀₁₁
cavity had a measured Q
_o
(4.2°K) 324 × 10
⁶
and a residual Qres ≈ 6.97 × 10
⁹
, whereas the TM
₀₁₁
cavity had a Q
_o
(4.2°K) ≈ 89 × 10
⁶
and a residual
^Q
res ≃ 425 × 10
⁶
. These results correspond to a surface resistance of R(4.2°K)/f
²
= 0.29 μΩ/GHz
²
. High level measurements were limited at a critical RF field H
^ac
_c
= 15.5 G in the TM
₀₁₁
mode and H
^ac
_c
= 290 G in the TE
₀₁₁
mode.