A Single Pulse Beam Emittance Measurement for the CERN Heavy Ion Linac

A new device for transverse emittance measurement has been installed in the 4.2 MeV/u filter region of the CERN Heavy Ion Linac (Linac 3). It allows to obtain pulse-to-pulse (every 1.2 sec) visualisation of the Linac 3 beam parameters in order to tune the machine and to match the beam for injection into the first circular accelerator, the PS Booster. The system is based on the "multi-slit" technique similar to the well-known "pepper pot" method. A plate with a series of horizontal or vertical slits is placed in the beam, defining positions in the phase plane. Particles pass through the slits and drift to a scintillator screen where they produce light. The screen is looked at by an externally triggered high resolution CCD camera. For each slit position the light intensity distribution, in the limit of infinitesimal slit aperture, is proportional to the angle distribution of the particles and therefore, provides the angular distribution in the phase plane. The video signal from the camera is digitised and the result stored in a 512 x 512 x 8 frame store. An application program visualising slit images (raw data) as well as the calculated emittance ellipses has been developed. This program also delivers numerical values for the rms-emittance and the Twiss parameters. The paper discusses the method, the prototype realisation, the data treatment and first experimental results obtained during the running-in of Linac 3.