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 to allow a permanent measurement of the ion beam position over a large intensity range (50 enA to 10 e mu A). An immediate application of these monitors is 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 given by R. Hooke and T.A. Jeeves (1961). The second is a variational method which consists in determining for each steerer 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.<>