Beamscope - A Novel Device for Measuring Emittances and Betatron Amplitude Distributions

BEAMSCOPE (BEtatron AMplitude Scraping by Closed Orbit PErturbation) is a device developed at the CERN Proton Synchrotron Booster (PSB) for fast emittance measurements as well as the display of betatron amplitude distributions at any moment in the acceleration cycle. A local closed-orbit bump, produced by three synchronously pulsed dipoles, deflects the circulating beam in a controlled way into a fixed precision aperture restriction, where it is gradually lost within about 1 ms. During the whole loss period, five ADCs simultaneously track the three dipole shunt voltages, and the beam current as provided by a beam current transformer and its derivative. In the NORD-10 Beam Measurement Computer the bump amplitude at the position of the precision scraper is synthesized from the dipole shunt signals after correction for non-linearities due to saturation in the dipole yokes and to eddy currents. The beam diameter and emittance can then be derived from this synthesized orbit bump amplitude and from the properly normalized beam current signal, and like-wise the betatron amplitude distribution is computed from the derivative of the beam current and displayed. The computer also controls, via Serial CAMAC, both the multiplexing (4 rings, 2 planes) of power supplies and acquisition electronics, and the setting of process timing and sensitivities of the acquisition channels appropriate for the particular case out of the wide spectrum of beam properties encountered in the PSB. The accuracy of emittance measurements is comparable to that with the present mechanical targets.