Highest performance of TESLA 9-cell superconducting RF cavities by overcoming quenches in superfluid LHe

Accelerating gradients Eacc=26 MV/m with a world record Q of 3/spl times/10/sup 10/ in 9-cell TESLA superconducting RF (SRF) cavities have been reached in CW and pulsed RF modes. On the way to reaching the excellent performance, we experimentally characterized quench natures (most serious obstacle to higher gradients), investigated the Kapitza conduction and boiling heat transfer between cavities and LHe II, and studied the quench behaviors both in He II and normal LHe. Finally, we discuss the possibilities of further greatly raising the accelerating gradients to 50-100 MV/m by use of Nb/sub 3/Sn and some HTc superconducting materials.