Diagnostic beam pulses for monitoring the SLC linac

The Stanford Linear Collider is a pulsed machine with a repetition rate of 120 Hz. By using fast devices such as kickers and triggers, individual pulses can be modified, measured and diagnosed, and then dumped to avoid any background in the experiment. For more than five years, a diagnostic pulse has been used to kick the beams onto off-axis screens at the end of the linac every 6 seconds. This provides a visual monitor of the beam size and loses about 0.14% of the rate or two minutes a day. The sensitivity of the linac optics to temperature and phase variations makes it desirable to monitor the phase advance between different locations in order to make local corrections. In principle, the feedback systems can measure the phase advance using the natural jitter of the beam. In practice, the phase jitter of the beam with respect to the rf may dominate the betatron jitter and distort the measurement. By using a large induced betatron oscillation, the two effects can be separated. To improve the monitoring of phase advance, a small kicker at the beginning of the linac is fired every few seconds and the orbit of this particular beam pulse measured and analyzed. The sensitivity, the measured variation and the correction scheme will be discussed