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 x 10( 10) in 9-cell TESLA superconducting RF (SRF) cavities have been reache d in CW and pulsed RF modes. On the way to reaching the excellent perf ormance, 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 qu ench behaviors both in He II and normal LHe. Finally, we discuss the p ossibilities of further greatly raising the accelerating gradients to 50-100 MV/m by use of Nb3Sn and some HTc superconducting materials.