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Superconducting (SC) TEM-class spoke cavities have been an area of active research during the past decade with application to cw and pulsed ion linacs required for proposed facilities world-wide. Single- and multi-spoke geometries have been developed for use with ions over the full mass range and with velocities 0.2 < v/c < 0.8. Spoke cavities for this range, generally designed for 4 K operation, have several advantages over 2 K elliptical-cell cavities stemming mostly from the lower operating frequency. However, recent spoke-cavity results in 2 K operation, based on new and evolving cavity processing techniques such as clean assembly and hydrogen degassing, show very low rf losses even for high surface fields (EPEAK ~30 MV/m) required in operations. 2K results indicate even higher voltage gains per cavity with reduced heat loads are possible. Other implications of 2 Kelvin spoke cavity operation for ion linacs are discussed.
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