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An Inductive Pick-Up (IPU) senses the azimuthal distribution of the beam
image current. Its construction is similar to a wall current monitor, but
the pick-up inner wall is divided into electrodes and each of which forms
the primary winding of a toroidal transformer. The beam image current
component flowing along each electrode is transformed to a secondary
winding, connected to a pick-up output. Four pick-up output signals drive
an active hybrid circuit, producing two difference signals proportional
to the horizontal and vertical beam positions, and one sum signal,
proportional to the beam current. The bandwidth of these signals, ranging
from below 1 kHz to beyond 150 MHz, exceeds five decades. Each electrode
transformer has an additional turn to which a pulse from a precise
current source is applied to calibrate the sensor for accurate beam
position and current measurements. The IPU has been developed for the
drive beam linac of the CLIC Test Facility 3. For that purpose it had to
be optimized for low longitudinal coupling impedance in the GHz range.
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