On the Measurement and Interpretation of Coupling Impedance Data in the Frequency and Time Domain

In order to prevent beam instabilities due to the coupling impedance of vacuum tanks containing septum magnets, electrostatic inflectors, etc. in a proton synchrotron we make routine measurements with portable instruments before installation. For frequencies >20 MHz the physical size of the devices is comparable to ¿/4 - the typical problem of microwave measurements - and impedance transformations by Carter- or Smith-Charts or computers are needed to obtain the true impedances. Therefore the measurement of reflection- and transmission-coefficients (S-parameters) is preferable because their magnitude is invariant along a transmission line. A center conductor of 1 cm diameter placed in our standard vacuum pipe forms a 125 ¿ coaxial line. If a nonstandard vacuum tank is inserted one observes reflection and transmission coefficients. To save the laborious impedance transformations in the frequency domain the reflected and transmitted waveforms are photographed in the time domain. Their interpretation in terms of frequency dependant impedances is discussed.