Beam size measurements with noninterceptive off-axis screens

At the end of the Stanford Linear Accelerator the transverse distributions of small electron and positron beams (σ≈100 μm) are measured by profile screens. To avoid constant interception and emittance blow-up of the production beams, the beams are deflected with fast magnets on to off-axis screens. One in a 1000 pulses is deflected. The required and achievable resolutions are described in another paper. Here we concentrate more on the magnets, the screen set-up, the readout, and signal processing of the video data. Together with the kicker magnet, two PCs, one for electrons, one for positrons, are triggered recording the next frame of the camera. Hardware and software process this information quickly producing a color-enhanced picture which is displayed in the control-room. With the resulting visual and digital information, the wakefield tails of the beam can be compensated using orbit oscillations. Minute by minute variations and slow drifts of the beam are recognizable