High-Power Tests And Solution To Overheating At Cavity-Coupler Interface Of The 166.6-Mhz Beta=1 Superconducting Quarter-Wave Resonator For Heps

166.6-MHz superconducting cavities have been chosen for the High Energy Photon Source (HEPS) as the main accelerating structures to provide 900 kW of beam power and 5.4 MV of accelerating voltage. A proof-of-principle cavity adopting the quarter-wave beta = 1 geometry was previously developed. Excellent performance was achieved in vertical tests at cryogenic temperatures. The cavity was later welded with a helium jacket, dressed with a power coupler and other ancillaries, and high-power tested in a test cryomodule. Performance degradation was observed and analyzed. Evidence from temperature sensor readout and heat loss measurement results suggested an overheating in the cavity-coupler interface region causing a "thermal runaway" and eventually quenching the cavity at its design voltage. Electromagnetic-fluid-thermal coupled simulation has thus been conducted, and the hypothesis was nicely validated. Finally, solutions were proposed including an elongated niobium extension tube at the coupler port and an optimized helium gas cooling of the power coupler's outer conductor. These modifications have been subsequently applied on the 166.6-MHz higher-order-mode damped superconducting cavities for the HEPS. Heat loss at 4.2 K contributed by the power coupler can be largely reduced with a modest gas cooling scheme. Similar design approaches can also be applied to other non-elliptical superconducting structures with on-cavity high-power coupler mountings.