Hwang, K. Qiang, J. JACoW Geneva, Switzerland 978-3-95450-180-9 10.18429/JACoW-NAPAC2016-WEPOA28 English 752-754 WEPOA28 ion linac cavity proton space-charge Contribution to a conference proceedings 2017 2017-01 http://dx.doi.org/10.18429/JACoW-NAPAC2016-WEPOA28 https://jacow.org/napac2016/papers/wepoa28.pdf The acceleration efficiency of the recirculating RF linac was demonstrated by operating electron machines. The acceleration concept of recirculating proton beam was recently proposed and is currently under study. In this paper, we present a 6D lattice design and beam dynamics tracking for a two-pass recirculating proton linac from 150 MeV to 500 MeV, which is the first section of the three acceleration steps proposed earlier. Issues covered are optimization of simultaneous focusing of two beams passing the same structure and achromatic condition under space-charge potential.