The Shanghai high repetition rate XFEL and extreme light facility linear accelerator employs two 3.9 GHz third harmonic cryomodules. The 3.9 GHz fundamental power coupler is one of the key components of the cryomodule and is designed to handle 2 kW of continuous-wave rf power, meeting operational requirements with a beam current of 0.3 mA and cavity gradient of $13.1\mathrm{MV}/\mathrm{m}$. A key innovation of the 3.9 GHz coupler is the incorporation of an adjustable antenna insertion depth, which enables online external quality factor ($Q_{\mathrm{ext}}$) tuning and eliminates the need for a three-stub tuner, reducing system complexity and cost. Two 3.9 GHz coupler prototypes were fabricated and underwent comprehensive rf testing. The couplers achieved excellent performance with an insertion loss of $-0.16\mathrm{dB}$ and return loss of $-43.22\mathrm{dB}$ at 3.9 GHz after structural optimization. High-power rf conditioning demonstrated stable operation at 2 kW in traveling wave mode and 1 kW in standing wave mode. Sixteen production 3.9 GHz couplers were manufactured and integrated into two third harmonic cryomodules, which achieved maximum total cavity voltages of 63.6 and 59.8 MV, respectively, with average usable accelerating gradients of $23.0$ and $21.6\mathrm{MV}/\mathrm{m}$. The cryomodules have been installed in the tunnel and initial beam commissioning has been completed successfully.

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• Radio frequency techniques
• Superconducting RF