Volkov, V. Petrov, V.M. JACoW Publishing Geneva, Switzerland 2226-0366 978-3-95450-194-6 10.18429/JACoW-LINAC2018-TUPO091 English 537-539 TUPO091 dipole cavity linac polarization focusing Contribution to a conference proceedings 2019 2019-01 https://doi.org/10.18429/JACoW-LINAC2018-TUPO091 http://jacow.org/linac2018/papers/tupo091.pdf This analyze argue that BBU instability both in separate cavities and in Linacs or ERLs is going due to the consequence of fundamental property of dipole modes. ’Head-tail’ bunch instability has also the same nature. New BBU instability testing methods are described and analytically proved in the article.