A General Analysis of Thin Wire Pickups for High Frequency Beam Position Monitors Page: 3 of 17
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A GENERAL ANALYSIS OF THIN WIRE PICKUPS FOR
HIGH FREQUENCY BEAM POSITION MONITORS*
Continuous Electron Beam Accelerator Facility
12000 Jefferson Avenue, Newport News, VA 23606
In many particle accelerators, a large number of high frequency beam position mon-
itors (BPMs) are required to track and correct the orbit of the beam. Therefore, simple,
sensitive, low cost pickup designs for such BPMs are of widespread interest. In this pa-
per, a general analysis of arbitrarily terminated thin wire stripline or "wireline" pickups
for BPMs is presented. Subsequently, three specific cases of interest, the open-circuited
end, short-circuited end, and matched end wirelines are compared and contrasted in de-
tail. Of these three cases, the open-ended wireline pickup BPM is found to be particularly
well-suited for application in the arc regions of the Continuous Electron Beam Accelerator
Facility (CEBAF). Measurements of a 1.5 GHz prototype of this type of BPM yielded a
longitudinal (sum) pickup impedance of 72 Q, a transverse pickup impedance of 3.8 fl/mm,
and a bandwidth of 470 MHz, all of which are in excellent agreement with the theory.
It is presently estimated that 400-600 microwave BPMs operating at the machine
frequency (1.497 GHz) are required for the CEBAF accelerator. Because of the large
number of monitors involved, a simple low cost pickup design is of primary concern. In
addition, the BPMs must be capable of accurately measuring the position of beams with
average currents as low as 1 MA. For the extremely short bunch length at CEBAF, this
results in a 2 pA AC component of beam current at 1.497 GHz. Consequently, the BPMs
must be extremely sensitive as well as inexpensive.
A BPM pickup design well-suited to this application consists of a thin quarter wave-
length antenna or "wireline" inside the beampipe and is similar to the standard stripline
pickup design. However, in the present design, only the upstream end of the line is termi-
nated, and the line impedance is not necessarily matched to the termination impedance.
This results in a non-directional broadband pickup that, apart from a geometric factor,
depends solely on the wireline impedance. This latter feature is most important because in
practice, the line impedance can be made as high as 200 [2 which is a factor of four greater
than the standard termination impedance of 50 2 available at microwave frequencies.
The unterminated downstream or open-ended wireline pickup is a specific case of the
more general wireline pickup with arbitrary upstream and downstream terminations. In
this paper, a general analysis of the ideal wireline pickup is given. The characteristics of the
open-ended wireline pickup are then discussed and compared to those of the conventional
* This work was supported by the U.S. Department of Energy under contract DE-AC05-
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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/m1/3/: accessed May 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.