MEASUREMENT OF HERA’S CENTRAL RF FREQUENCY

For the HERA luminosity upgrade the horizontal emit-tance of the electron beam was reduced from 41 nm to 22 nm firstly by an increased quadrupole focusing in the arcs’ FODO structures and secondly by a change of the damp-ing partition numbers by means of an rf frequency shift. The stronger focusing alone, which increases the phase ad-vance per FODO cell from 60 to 72 degrees, would only reduce the emittance to 35 nm. It was therefore critical for the luminosity upgrade to make sure that there is sufficient room in the damping window for increasing the frequency of HERA’s 500 MHz system by the required 300 Hz. For this purpose HERA’s rf frequency had to be compared to the central frequency of the ring, for which the design or-bit goes through the center of the quadrupoles. Since the accuracy of most methods of finding the central frequency is quite limited, we have used 7 different methods. Their results and their accuracies are compared in this paper.