Lagniel, J.-M. JACoW Publishing Geneva, Switzerland 2673-5571 978-3-95450-253-0 10.18429/JACoW-HB2023-WEA3I1 English 241-244 cavity linac accelerating-gradient focusing acceleration Contribution to a conference proceedings 2024 2024-04 https://doi.org/10.18429/JACoW-HB2023-WEA3I1 https://jacow.org/hb2023/papers/wea3i1.pdf Synchronous phases (¿s) and transit time factors (T) are THE key parameters for linac designs and operations. While the couple (¿s, T) is still our way of thinking the longitudinal beam dynamics, it is important to have in mind that the original ¿Panofsky¿ definition of these parameters is no longer valid in the case of high accelerating gradients leading to high particle velocity changes and in the case of multi-gap cavities. In this case, a new (¿s, T) definition allowing to keep both acceleration and longitudinal focusing properties is proposed. Examples are given in the SPIRAL2 linac case.