Capacitive beam position monitors and automatic beam centering in the transfer lines of GANIL

A non-interceptive beam position monitor, made of four capacitive electrodes, has been designed at GANIL in order to allow a permanent measurement of the ion beam position over a large intensity range (50 enA to 10 eμA). Signal processing is based on a 10 kHz heterodyne and on an amplitude to phase conversion in order to measure the beam position. The accelerator is equipped with ten of these probes, six of them on the first beam line. Seven other probes are planned. An immediate application of these monitors is the automatic beam centering. For this, two algorithms have been developed using the information on the center of gravity given by the beam position monitors which is then fed back to the steerers. The first is an iterative method based on an optimization algorithm (Pattern Search, Hooke and Jeeves). It consists in improving the criterion defined as the mean quadratic deviation of beam positions on the monitors. The second is a variational method which consists in determining for each steerers and each position monitor, the variation coefficient (mm/A) and solves a linear system. Both of these methods have been used on a section of beam line and have given similar and encouraging results. The next step is to center the beam on the completely equipped line