Another Method to Measure the Low-Frequency Machine Impedance

The spectrum of long bunches samples the low-frequency part of the machine impedance which is mostly reactive. The voltage induced by the bunch produces the well known "potential well distortion" when RF is on, but also affects the debunching when RF is off. In this paper we present a method of estimating the reactive impedance by measuring the evolution of bunch parameters, such as the peak line density, during debunching. This method was used to find the inductive impedance of the CERN SPS with a single proton bunch injected above transition at 26GeV.