Development and high-current cw operation of 166.6 MHz high-power, higher-order-modes-damped <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>β</mml:mi> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> </mml:math> quarter-wave

The High Energy Photon Source (HEPS) is a recently commissioned 6 GeV fourth-generation synchrotron light source that utilizes a novel low-frequency superconducting radio-frequency (SRF) system. This system was designed to handle high beam power and achieve heavy damping of higher-order modes (HOMs) within strict spatial constraints. To meet these stringent requirements, we in-house developed and subsequently installed five <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>β</a:mi> <a:mo>=</a:mo> <a:mn>1</a:mn> </a:math> quarter-wave SRF cavities operating at 166.6 MHz and 4.2 K with a compact envelope. The design incorporates an enlarged beampipe (505 mm in diameter) with a beamline absorber for effective HOM damping and adopts a single coaxial power coupler with on-cell coupling to handle a high continuous-wave power of 200 kW. Following rigorous cleaning and assembly protocols, the cryomodules demonstrated a peak electric field of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:mn>42</c:mn> <c:mtext> </c:mtext> <c:mtext> </c:mtext> <c:mi>MV</c:mi> <c:mo>/</c:mo> <c:mi mathvariant="normal">m</c:mi> </c:mrow> </c:math> with negligible radiation and maintained a low dynamic heat load of 5 to 7 W at the operating voltage of 1.2 MV, results that significantly surpassed the design goals. Installed in Summer 2025, the cavities were instrumental in the successful acceleration of a 100 mA electron beam at HEPS in September 2025. This work represents the first successful implementation of superconducting quarter-wave cavities as the main accelerating system for a high-current storage ring, thereby extending the application limits of quarter-wave cavities from their conventional use in low- <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mi>β</f:mi> </f:math> accelerators to high-current electron storage rings and establishing a proven technology for future high-performance light sources.