Laser-driven high-order harmonic generation^1,2 provides spatially³ and temporally⁴ coherent tabletop sources of broadband extreme-ultraviolet (XUV) light. These sources typically operate at low repetition rates, f_rep ≲ 100 kHz, where phase-matched HHG is readily achieved^5,6. However, many applications demand the improved counting statistics or frequency-comb precision afforded by high repetition rates, f_rep > 10 MHz. Unfortunately, at such high f_rep, phase matching is prevented by steady-state plasma accumulated in the generation volume^7,8,9,10,11, strongly limiting the XUV average power. Here, we use high-temperature gas mixtures as the generation medium to increase the gas translational velocity, thereby reducing the steady-state plasma in the laser focus. This allows phase-matched XUV emission inside a femtosecond enhancement cavity at f_rep = 77 MHz, enabling a record generated power of ~ 2 mW in a single harmonic order. This power scaling opens up many demanding applications, including XUV frequency-comb spectroscopy^12,13 of few-electron atoms and ions for precision tests of fundamental physical laws and constants^{14,15,16,17,18,19,20}.

激光驱动的高次谐波^1,2产生了宽带极紫外（XUV）光的空间^3个和时间^4个相干的桌面光源。这些源通常以低重复率操作，˚F_代表 ≲  100千赫，在相位匹配的高次谐波被容易地实现^5,6。但是，许多应用要求通过提高重复频率f _rep  > 10 MHz来提高计数统计或频率梳精度。不幸的是，在如此高的f _rep处，由于在生成体积^{7,8,9,10,11中}积累的稳态等离子体阻止了相位匹配，强烈限制了XUV的平均功率。在这里，我们使用高温气体混合物作为生成介质，以提高气体平移速度，从而减少激光聚焦中的稳态等离子体。这允许在飞秒增强腔内相位匹配XUV发射˚F_代表 = 77兆赫，可实现〜2毫瓦的生成的记录功率在一个单一的谐波阶数。这个功率调整开辟了许多要求苛刻的应用，包括XUV频率梳光谱^12,13少数电子的原子和基本物理定律和常数精密测试离子^{14,15,16,17,18,19,20}。