A General Analysis of Thin Wire Pickups for High Frequency Beam Position Monitors Page: 4 of 17
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matched stripline and shorted-end or "loop" pickups which are presently in use in the
CEBAF linacs.1J In addition, the results of longitudinal and transverse bench tests on an
open-ended pickup BPM which confirm the theory are presented.
Response Functions for Wireline Pickups
The BPM to be considered is illustrated schematically in figure 1. The monitor consists
of four thin wire antennas (x plane shown in figure 1) located orthogonally to each other
inside a circular beampipe of radius a. Each wire has length P, diameter d and is located
a distance b from the centerline of the monitor. As with conventional stripline pickups,
each wire forms a transmission line with characteristic impedance Ze determined by the
dimensions a, b, and d. For the general wireline pickup, the output is terminated by a load
impedance Z1, and the downstream end is terminated with an impedance Z2.
circular beampipe z
Figure 1 General wireline pickup BPM.
The goal of the analysis is to derive the frequency and impulse responses of the wireline
pickup for arbitrary real impedances Z1, Z2, and Ze. Subsequently, the special cases of
the conventional stripline (Z1 = Z2 = Zr), open-ended wireline (Z2 = oo) and shorted-end
wireline or "loop pickup" (Z2 = 0) can be examined in greater detail. In addition, the
longitudinal and transverse pickup impedances for the open-ended monitor are derived.
It is well established through the application of the Lorentz reciprocity and Panofsky-
Wenzel theorems that beam currents couple to pickups only in regions where electric fields
parallel to the current exist when the pickup is driven as a kicker.i12 For the pickups in
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Barry, Walter. A General Analysis of Thin Wire Pickups for High Frequency Beam Position Monitors, article, October 1, 1990; [Newport News, Virginia]. (https://digital.library.unt.edu/ark:/67531/metadc933929/m1/4/: accessed May 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.