A clean way to measure nonlinear momentum compaction factor α1

α1 is an important lattice parameter for transition crossing. There exist several ways to measure _α1, such as debunching near transition. The extraction of _α1 from debunching rate depends on the momentum spread of beam, which is hard to measure accurately. Here we report, another way to bypass this difficulty. Instead of debunching, the beam is stored in a stationary bucket near transition. Since the bucket near transition is very small, the particles inside the bucket will fill the bucket and those outside will be lost if parameters are chosen properly. So the measured bunch length is equal to bucket length, which can be used to extract _α1. The nice thing about this method is that the measurement does not depend on initial distribution of bunch as long as its initial emittance is big enough to fill the stationary bucket near transition. A test has been carried out in the Fermilab Main Ring (MR).