Impedance measurements on button electrodes

In the Advanced Light Source, there are about 400 capacitive button electrodes in the beam position monitor (BPM) system; hence the contribution of each button to the machine beam impedance must be very small. The impedance of a single button as sensed by a coax directly connected to the button surface was measured. This method is very sensitive and does not require a model of the total beam chamber as would the usual wire method. The measurements covered the range 0.1 to 20 GHz. The proportionality factor between the button impedance and the beam impedance depends upon geometry and frequency and was obtained from the measured sensitivity of a developmental BPM at low frequency. Discontinuities in the connection of the coax to the face of the button introduce parasitic effects that must be accounted for in interpreting the data. Below 5 GHz, the results compare very well with responses computed from mechanical dimensions of the electrode