World Record Accelerating Gradient Achieved in a Superconducting Niobium RF Cavity

On November 16, 2004, a CW accelerating gradient of 46 MV/m was achieved in a superconducting niobium cavity with an unloaded quality factor (Q0) over 1 × 10¹⁰at a temperature of 1.9 °K. In pulsed mode, 47 MV/m was achieved. This represents a world record gradient in a niobium RF resonator. At a reduced temperature of 1.5-1.6 °K, an enhanced Q0was measured, ranging from 7 × 10¹⁰at 5 MV/m to 2 × 10¹⁰at 45 MV/m. The 1.3 GHz single-cell cavity has a reduced ratio of Hpk/Eacc, ensured by a reentrant geometry. The maximum peak surface electric and magnetic field exceeded 100 MV/m and 1750 Oe respectively. A soft multipacting barrier (predicted by calculations) was observed near 25 MV/m gradient and was easily processed through. Field emission in the cavity was negligibly small, and the highest field was limited by thermal breakdown. The cavity was built, processed, and tested with LEPP facilities at Cornell University. New techniques included half-cell heat treatment with yttrium for post-purification to RRR = 500, and vertical electropolishing the finished cavity.