Instrumentation & Measurement Magazine 24-9 - 22

Fig. 1. A BPM system.
Some key aspects of any BPM system are:
◗ Resolution: The smallest change of the transverse beam
position detectable by the BPM. The resolution of the
BPM depends on several factors, the BPM pickup position
sensitivity and the measurement or integration time
of the read-out electronics are the most dominant ones.
The later is related to the insertion losses due to cables
and passive components between the BPM pickup and
the input of the read-out electronics, and the noise contribution
of the input gain stage, which together with the
quantization effects of the analog-to-digital converter
(ADC) defines the achievable signal-to-noise ratio (SNR).
For a short measurement time, e.g., single bunch or single
turn beam position acquisition in the ns or μs range the
resolution is typically in the 10 - 100 μm regime, for beam
orbit measurement in a ring accelerator, by averaging
the bunch signals over many 1000 turns, the resolution is
typically <1 μm.
◗ Precision: The spread of the beam position values reported
by the BPM instrument taking many measurements at the
same, constant beam position, which is basically the same
as the BPM resolution. Sometimes the digitally displayed
values of the beam position with many digits mislead to a
higher precision than the BPM instrument actually offers.
◗ Accuracy: This scaling error of the reported beam position
is typically related to non-linearities of the BPM pickup
and/or the read-out electronics. An accurate characterization
of the non-linearities and correction with look-up
tables or polynomial functions can improve the accuracy.
◗ Reproducibility / Repeatability: The stability of reported
beam position over long periods of time, e.g., within
12 hours of operation, day-by-day, week-by-week, etc.
Many factors play a role for this BPM characteristic, like
aging of electronics components and environmental
aspects like temperature and humidity. The extensive use
of digital signal processing and online calibration signals
in the acquisition system improves the long-term stability
of a BPM.
◗ Offset: The BPM offset, i.e., the zero position in the transverse
plane is of relevance to measure and keep the beam
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orbit in the magnetic center of the guide field magnets,
typically in the center of the vacuum chamber. Mechanical
tolerances of the BPM pickup and " electronic " offsets
need to be minimized. Still, a beam-based alignment procedure
is often required to measure and correct for the
residual offset and tilt of individual BPMs.
This introduction on BPMs is based on a tutorial given
in frame of the 2018 CERN Accelerator School, special topics
course on beam instrumentation for particle accelerators
[1]. The next section explains the principle of operation of a
beam position monitor, followed by the main section of this
paper, which is focused on the BPM pickup and includes an
excursion to image charges, beam signals, and explains the
" button-style " BPM pickup as example. The last section discusses
the processing of the BPM signals, including a note
on the BPM resolution; however, the topic on read-out electronics
is kept brief as it may quickly become outdated [2].
The closing point briefly mentions the performance of BPM
systems with an experimental observation on the LHC BPM
system.
Principle of Operation of a
Beam Position Monitor
This introduction on beam position monitoring covers BPM
systems based on non-invasive, electro-magnetic-type beam
pickups, however it should be noticed, there are other methods
and ways to detect the position of the beam. Fig. 2a
illustrates a typical pickup, in this example the so-called
" button-style " BPM, which consists of four round, metallic,
coin-like electrodes (the " buttons " ) which often are arranged
symmetrically along the horizontal and vertical axes, here
in a vacuum chamber of circular cross-section. The horizontal
(x) and vertical (y) offset of the beam trajectory with
reference to the center of the vacuum chamber is the beam
position or beam displacement, and has to be monitored with
high resolution, accuracy and repeatability, this is the goal of
the beam position measurement. In practice, the BPM button
also serves as vacuum feedthrough and is manufactured
as a compact unit with a coaxial RF connector as output signal
port, see Fig. 2b.
Broadband BPM pickups operate on the principle of image
currents (or image charges), each charged particle of the
beam is " compensated " by an image charge of opposite sign
in the metallic vacuum chamber. Fig. 2c shows the induced
image charges on a pair of electrostatic BPM electrodes and
the resulting bunch response voltage signal for the upper
electrode on a load resistor. The time-domain response of a
BPM electrode to a bunched beam appears as a differentiated
pulse as the image charges are induced as a displacement current
through the capacitive button electrode. Fig. 2d shows
the simplified equivalent circuit and the frequency-domain
response of this capacitive coupling BPM electrode, which
is equivalent to that of a simple, 1st
-order high-pass filter.
There is no " DC-coupling " , and therefore broadband BPM
pickups cannot operate with direct-current (DC), so-called
" debunched " beams.
IEEE Instrumentation & Measurement Magazine
December 2021
Instrumentation & Measurement Magazine 24-9

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