Xu, Y.L. Doolittle, L.R. Du, Q. Galvanauskas, A. Huang, G. Huang, W.-H. Leemans, W. Li, D. Tang, C.-X. Wilcox, R.B. Yan, L.X. Yang, Y. Zhou, T. JACoW Publishing Geneva, Switzerland 978-3-95450-184-7 10.18429/JACoW-IPAC2018-THPML099 English 4895-4897 THPML099 Contribution to a conference proceedings 2018 2018-06 https://doi.org/10.18429/JACoW-IPAC2018-THPML099 http://jacow.org/ipac2018/papers/thpml099.pdf Coherent pulse stacking (CPS) is a new time-domain coherent addition technique that stacks several optical pulses into a single output pulse, enabling high pulse energy and high average power. We model the CPS as a digital filter in the Z domain, and implement two deterministic algorithms extracting the cavity phase from limited data where only the pulse intensity is available. In a 2-stage 15-pulse CPS system, each optical cavity is stabilized at an individually-prescribed round-trip phase with 0.7 deg and 2.1 deg RMS phase errors for Stage 1 and Stage 2 respectively. Optical cavity phase control with nm accuracy ensures 1.2% intensity stability of the stacked pulse over 12 hours.