Persichelli, S. Byrd, J.M. De Santis, S. Li, D. Luo, T.H. Osborn, J.R. Swenson, C.A. Venturini, M. Yang, Y. JACoW Geneva, Switzerland 978-3-95450-180-9 10.18429/JACoW-NAPAC2016-THPOA33 English 1174-1176 THPOA33 ion impedance cavity vacuum simulation Contribution to a conference proceedings 2017 2017-01 http://dx.doi.org/10.18429/JACoW-NAPAC2016-THPOA33 https://jacow.org/napac2016/papers/thpoa33.pdf The proposed upgrade of the Advanced Light Source (ALS-U) consists of a multi-bend achromat ultralow emittance lattice optimized for the production of diffraction-limited soft x-rays. A narrow-aperture vacuum chamber is a key feature of the new generation of light sources, and can result in a significant increase in the beam impedance, potentially limiting the maximum achievable beam current. While the conceptual design of the vacuum system is still in a very early development stage, this paper presents a preliminary estimate of the beam impedance using a combination of electromagnetic simulations and analytical calculations. We include the impedance of cavities, select vacuum-chamber components and resistive wall in a multi-layered beam chamber with NEG coating.