Experiments with Beam Position Sensors

Previous work on obtaining beam centroid position information using a beam excited resonant cavity with two orthogonal TM110 modes was extended to a higher harmonic cavity. The cavity operated at a frequency of 3220 MHz, the fourth harmonic of the accelerator frequency, and was fine tuned by using the frequency shift caused by varying the size of the beam hole. As predicted by beam-cavity interaction theory, the cavity output signal was shown to be proportional both to the square of the beam centroid displacement and to the square of beam current, and to be well correlated with position information obtained by self-powered γ-ray detectors. A crystal and a singly balanced mixer were used to process this signal to give information of both magnitude and sign of the beam position. The cavity was used to observe the effects on beam position when slow (0.01 Hz) sinusoidal disturbances were applied to three accelerator parameters: forward power, frequency and phase. Corresponding changes in the beam position have been found. The preliminary results of an automatic beam centering control show that slow disturbances on beam position can be corrected by driving a steering magnet with the cavity signals.