Towards the limit of frame transfer CCDs in beam instrumentation

CCDs are used for beam position and beam size measurements with screens and synchrotron radiation monitors. They have a large spatial resolution and a high dynamic range which make them interesting for high resolution two-dimensional measurements. In most applications, the video signal is digitised with a frame grabber and the beam parameters then computed. The results are limited in resolution, usually to 8 bits, and in rate. These restraints were seen as major limitations for fast and precise beam observations with synchrotron light monitors in LEP, where the revolution frequency is 11 kHz. They have been overcome by using the characteristics of Frame Transfer CCDs. Advantage is taken of the separate Image and Memory areas on the CCD chip to control independently the integration and digitising periods to achieve a 12 bit amplitude resolution at the individual pixel level. Using the CCD as a buffer memory, together with a pulsed intensifier, it is possible to make 9 to 18 two dimensional beam spot acquisitions up to a rate of 11 kHz. The most recent development uses the CCD as a summing device to perform on chip horizontal or vertical beam projections and several hundred successive profiles can be acquired in this mode.