GSI, Darmstadt
TEMF, TU Darmstadt, Darmstadt
This contribution focuses on extensive simulations based on Finite Element Methods (FEM) which were successfully used for the design of several Beam Position Monitor (BPM) types. These simulations allow not only to reduce the time required for BPM prototyping but open up new possibilities for the determination of characteristic BPM features like signal strength, position sensitivity etc. Since a precise visualization of the signal propagation along the BPM structure is possible, effects like resonances, field inhomogeneties or complex cross talks between adjacent electrodes can be controlled. Moreover, modern simulation programs enable to define a charge distribution that is moving also at non relativistic velocities, which has an impact on the electromagnetic field propagation. It is shown that for slow ion beams the frequency spectrum of the BPM signal depends on the beam position. A variety of simulation methods are discussed in the context of different BPM realizations applied in hadron accelerators.
GSI, Darmstadt
A precise tune determination is crucial for stable operation of GSI SIS-18 synchrotron especially for intense beam conditions. In order to avoid nearby resonances in the tune diagram the fractional part of coherent betatron motion needs to be measured with a resolution of 10-3 also during ramping mode. This is achieved using a fast digital readout system for Beam Position Monitors (BPM). The broadband BPM signal is sampled with a rate of 125 MSa/s which corresponds to an average of about 50 Sa per bunch for SIS-18 machine parameters. The signal is integrated bunch-by-bunch which minimizes thermal and digitization noise and the beam position is calculated. The tune is then determined in baseband directly by Fourier-transformation of the positions of a certain bunch typically over 2048 turns. This algorithm does not require any additional input parameter. Since particle losses due to significant emittance blow-up have to be avoided, excitation power has to be kept as low as possible. This was achieved using a digital pseudo random noise (PRN) generator for beam excitation, which produces white noise on a carrier frequency with adjustable bandwidth.