Huang, N.Y. Chiu, M.-S. Chou, P.J. Luo, G.-H. Luo, H.W. Tsai, H.-J. Tseng, F.H. JACoW Publishing Geneva, Switzerland 2673-7035 978-3-95450-224-0 10.18429/JACoW-FLS2023-WE4P18 English 184-187 lattice emittance dipole storage-ring radiation Contribution to a conference proceedings 2024 2024-01 https://doi.org/10.18429/JACoW-FLS2023-WE4P18 http://jacow.org/fls2023/papers/we4p18.pdf We study the upgrade of Taiwan Photon Source (TPS) with energy saving as the prime objective. The upgrade design is dubbed TPS-II. To accommodate the constraints imposed by the existing TPS tunnel, we choose the higher-order achromat (HOA) lattice configuration which is composed of the 5BA and 4BA cells. This HOA lattice produces a natural beam emittance about 131 pm-rad for a 3 GeV, 518.4 m storage ring. The on-momentum dynamic aperture is about 8 mm and the estimated Touschek life time reaches around 5.7 hours at total beam current of 500 mA. As a result of the ultralow beam emittance, the brightness and coherence fraction (CF) of the photon beam are improved with a factor of several tens especially in the photon wavelength around 0.1 nm. The challenges and preliminary results of this HOA lattice design will be presented.