Instrumentation & Measurement Magazine 24-9 - 21

A Brief Introduction to
Beam Position Monitors for
Charged Particle Accelerators
Manfred Wendt
W
hile charged particle accelerators have their origin
in the research related to fundamental and
high-energy physics, they expanded their applications
to many other fields of applied research in physics,
material science, biology, chemistry, and medical science, including
the treatment of patients, to name some. However, in
all types of accelerators assembles of charged particles, e.g.,
electrons, protons, ions, sometimes their antimatter partners
positrons or p-bars (anti-protons) are accelerated to a desired
energy, typically in an evacuated, metallic tube, called beam
pipe. The acceleration is performed by providing an electric
field of high gradient to the charged particle assemble in the direction
of motion. In most cases this is performed by resonant
radio-frequency (RF) cavities, which are fed by a high-power
RF source. However, new acceleration schemes using high
power lasers, plasma wakefields, etc. are also studied these
days. A guide-field along the accelerator, typically provided
by sets of different types of magnets, ensures the charged particles
stay within the transverse boundaries of the beam pipe,
near its center, travelling on the wanted trajectory. Accelerating
elementary particles, like electrons, or quasi-elementary
particles, like protons, to very high energies (GeV range) results
their velocity v to be close to speed-of-light c, typically
expressed as relative velocity β = v / c ≈ 1, and any further acceleration
manifests in a gain of momentum p = γm0
2 being the Lorentz factor and m0
v, with
 
11
being the mass
of the particle at rest, 0.511 MeV and 938.26 MeV for electron
and proton, respectively. The use of resonant RF cavities for
the acceleration, causes the particles to form bunches, which
fill a longitudinal range of typically a few millimeters up
to some meters, depending on fRF
, γ and other factors. This
means, the large number of charged particles in the accelerator
are not uniformly distributed along the beam-line but
appear in bunches of typical N = 108
−1011
particles per bunch,
and the minimum space between the center of those bunches
is defined by the RF bucket length, given by the wavelength
λRF
= v / fRF
of the RF system.
There exist different types of particle accelerators, most popular
are the linear accelerators (linac), and the circular accelerators,
December 2021
e.g., betatrons and synchrotrons. Regardless of the application
of the machine, it has to deliver a charged particle beam with
well-controlled parameters, e.g., beam energy and energy
spread, beam or bunch intensity, transverse beam size or beam
emittance, etc. This requires beam diagnostic and monitoring instruments
to detect and monitor those parameters, obviously the
beam is invisible for the human eye. Among the various types
of beam instruments, the beam position monitors (BPM) are an
important and essential instrument in any particle accelerator
to detect and continuously measure the beam trajectory along
the accelerator, among many other applications. Therefore, the
BPMs are distributed along the accelerator beam-line, forming
a BPM system (see Fig. 1 for a ring accelerator), monitoring the
passing beam. Each beam position monitor consists out of:
◗ a BPM pickup, which is part of the vacuum chamber and
consists out of two or four symmetrically arranged electrodes,
which couple to the electromagnetic fields of the
beam and generate electrical signals at their output ports.
Typically, a BPM pickup is located near each quadrupole
magnet along the beam-line.
◗ the read-out electronics, which is used to condition and
process the signals from the pickup electrodes to provide
the beam position information in a digital data format,
such that it can be acquired and further processed by the
accelerator control system.
Popular BPM pickup styles are of button or stripline
type, both have broadband characteristics, and generate a
pulse-like output signal for each passing bunch. This enables
synchronous bunch-by-bunch (and turn-by-turn) BPM signal
processing possibilities which requires the synchronization
of the BPM data of all monitors in the system. In a ring accelerator
a synchronized turn-by-turn monitoring of the beam
position is of interest to follow the response of the beam to a
kick or chirp excitation, e.g., for beam optic studies and analysis.
Averaging the BPM data over many turns (ring accelerator)
or over many beam pulses (linac) from each BPM provides a
high-resolution measurement of the beam orbit or beam trajectory,
used for the alignment of the beam, for orbit feedback
purposes, etc.
IEEE Instrumentation & Measurement Magazine
1094-6969/21/$25.00©2021IEEE
21

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