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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - M O P 0 3 2ONE DIMENSIONAL BEAM POSITION MONITOR PROTOTYPE USINGINCOHERENT CHERENKOV DIFFRACTION RADIATION
A. Clapp∗ , Royal Holloway University of London, Egham, UKalso at Diamond Light Source, Didcot, UK
P. Karataev, Royal Holloway University of London, Egham, UK
L. Bobb, G. Cook, Diamond Light Source, Didcot, UK
Abstract
This paper proposes a novel advancement in both the studies of Cherenkov diffraction radiation (ChDR) and beam instrumentation. The proposed beam position monitor (BPM)
consists of two identical fused Silica prism radiators, witha fibre collimator attached to each one, which in turn areconnected to a photodetector via a series of optical fibres.
The setup will be implemented into the booster to storage
ring transfer line at Diamond Light Source - an electron lightsource with 3 GeV beam energy. The prototype proposed
aims to test the feasibility of a full BPM utilising ChDR. If
proven to be fully realisable, optical rather than capacitive
BPM pickups could be more widely distributed. The paper
will include the complete design and preliminary results ofa one-dimensional BPM, utilising the ChDR effect.

INTRODUCTION
Cherenkov radiation (CR) is the emission of electromagnetic radiation when a charged particle travels through adielectric medium with a speed greater than that of light inthat medium. Observed in 1934 by Cherenkov [1], it has
since been expanded upon both theoretically and experimentally. Cherenkov radiation is emitted at a well defined angle
denoted the Cherenkov angle 𝜃, given by1(1)𝛽𝑛where 𝛽 = 𝑣/𝑐 is the ratio of the charged particle velocity 𝑣 to the speed of light 𝑐, and 𝑛 is the refractive index
of the medium the charged particle particle is travellingthrough [1]. The emission and directionality of Cherenkov
radiation enables this phenomena to be a useful tool in many
fields ranging from astrophysics [2] to radiotherapy [3].
The theory of CR has been expanded to account for thesituation where the incident relativistic particle is instead
travelling close by a medium acting in a manner similar toclassical diffraction radiation [4] called Cherenkov Diffraction Radiation [5]. In addition to particle energy and therefractive index of the medium, ChDR is dependent on theimpact parameter 𝑏 which represents the distance between
the incident charged particle and the medium. ChDR isproduced by a hyper-relativistic particle travelling close toa medium, rather than through it, and given that ChDR isemitted at the well defined Cherenkov angle it is well suited
for accelerator diagnostic applications where a non-invasive
technique is of paramount importance.

The intensity of light emitted increases the closer theparticle is to the medium, as such it is necessary to ensurethat the incident particle is within the condition𝑏 ≤ 𝛾𝜆(2)where 𝛾 refers to the Lorentz factor of the incident particleand 𝜆 refers to the wavelength of emitted radiation. Thequantity 𝛾𝜆 refers to the effective electron field radius [6].
For optical wavelengths and an electron beam energy of3 GeV 𝑏 is ≤ 3 mm. ChDR has been investigated experimentally and had its applicability to diagnostics examined [7, 8].

BTS TEST STAND
The experiment is located on the Booster To Storage (BTS)transfer line at Diamond Light Source (DLS). The test standconsists of a six-way cross vacuum chamber with a fourdimensional manipulator system which allows for the insertion and removal of experiments. Vertical translation androtation of the manipulator are automated. It was previously
used for another ChDR experiment [9], which tested feasibility of beam position measurements and fundamental ChDR
properties.

cos 𝜃 =∗ alec.clapp.2021@live.rhul.ac.uk

Figure 1: Schematic of BTS test stand.
Figure 1 shows a schematic of the vacuum vessel stringon the BTS test stand consisting of three combined Optical Transition Radiation (OTR) and Yttrium Aluminium
Garnet (YAG) monitors, and one Inductive Beam Position
Monitor (IBPM) developed at CERN [10]. These diagnostic
tools are used as references for the beam profile and trajectory through the BTS test stand when taking data with theChDR BPM. The IBPM provides a reference beam position
measurement, which the results from the ChDR BPM arecompared against. The first OTR screen is used to measure

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the input beam profile and from this the input beam size
into the BTS test stand. OTR2 (see Fig. 1) is used to checkalignment of the ChDR BPM assembly with respect to thebeam path. Shadowing of the beam profile on this screen
indicates that the beam is in contact with the assembly andthat correct horizontal/rotational alignment is not achieved.
YAG screens have a higher photon yield allowing for beam
shadowing to be easier to identify, whereas OTR screensprovide better spatial resolution for beam size measurement.
The DLS storage ring has a vacuum rating of 10−10 mbar,
the BTS transfer line is targeted at Ultra High Vacuum(UHV), therefore the ChDR BPM assembly must also be
UHV compatible.

CHDR BPM DESIGN
The 1D ChDR BPM design utilises two pick-ups consisting of identical prisms positioned either side of the electron
beam to produce horizontal beam position measurements
using ChDR. The intensity of the emitted signal relies on
the impact parameter, which from theory displays an exponential decay with increasing impact parameter. For thecase of DLS using ChDR emitted at visible wavelengths,
the impact parameter should be less than 3 mm. This meansthat if the beam moves out of this range, the detected signal
will be within the noise, providing no useful information on
current beam position. Therefore the ChDR BPM pick-ups
should have a separation ≤ 3 mm to ensure that simultaneous
signals from the two pick-ups can be detected.

Figure 2: Intensity curve for the fused silica prism, showing
the diffraction radiation (DR) peak centered at zero and theChDR peak centered at the Cherenkov angle.
The ChDR BPM has an adjustable separation to allowfor flexibility in measurements. The ChDR output signal
intensity was calculated using the single target intensity
equation from [11] and is shown in Fig 2. The emitted
ChDR signal from one of the pick-ups will be at a maximum
when the impact parameter between the electron beam andthat pick-up is zero. The minimum separation between two
pick-ups should be such that when the electron beam is in thecentre of both pick-ups and has an equal impact parameterwith both, the emitted ChDR signal is at 1% of the maximum
value. This minimum separation was found by creating anFigure 3: Graph showing the decay of two identical pick-ups
either side of a beam with a separation of 1.1 mm.
intensity decay curve, shown in Fig. 3, for the two pick-ups
using the results from Fig. 2.
The decay curve in Fig. 3 found that the desired minimum
pick-up separation is at 1.1 mm. The separation between
the two pick-ups can be increased up to 6.1 mm through theuse of spacers. This maximum separation corresponds toan impact parameter of 2𝛾𝜆 for 500 nm emission or 𝛾𝜆 for1000 nm emission.
To couple the ChDR light out from the prisms to opticalfibres, collimators are used. To maximise the collected signal, the collimator lenses have been angled such that they areparallel to the output face of the prisms and are positionedby the assembly in close proximity. Figure 4 shows a 3D
rendering of the BPM assembly.

Figure 4: Autodesk inventor 3D rendering of the BPM assembly. Colours are used to differentiate the components:
silver is used to denote the prisms; the two collimators arein gold; the C-clamps are in dark orange; collimator bridges
are in red; the adjustable connection is in dark grey; the main
connecting structure in the light grey and; the spacers in thegreen and blue.
The assembly itself is made of stainless steel with stainlesssteel screws. All screw holes are either through or have had asmall hole drilled to ensure no cavities are formed to ensure
the assembly is UHV compatible. The C-clamps feature a
step angled at the Cherenkov angle, this aligns the target toMOP032

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ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

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ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - M O P 0 3 2ensure the generation face of the prism is flush with clamp
and is not angled with respect to the electron beam trajectory.
The main structure is connected to the central actuator,
which allows for the assembly to be lowered into the beam
path when conducting ChDR experiments. For user beam
operation of the storage ring the assembly is retracted beyond
the nominal beam pipe diameter to ensure the presence ofthe assembly does not impact beam injection to the storage
ring (see Fig. 5).

vertical size is required to be an order of magnitude larger
than the electric field radius 𝛾𝜆 to ensure that the target appears to be infinitely tall compared to the electric field. Thispredicts that the intensity decay due to increase in impact
parameter, will match that seen in Fig. 3.

Figure 6: Prismatic Radiator.

CHARACTERISATION OF
THE CHDR PICK-UPFigure 5: 3D rendering of the assembly in its retracted position inside the vacuum chamber from an upstream view.
The generated ChDR light is coupled from UHV compatible fibres inside the vacuum chamber to an external detector
via a UHV-to-air fibre feedthrough. The detector is an amplified photodiode with a direct fibre input. The detector
has a short rise time of 1 ns and a wide spectral range of
400 −1000 nm which is ideal for ChDR experiments as itallows for a wide range of experiments to be explored.

To allow for an an accurate measurement of ChDR emission power, all components have been bench tested with a638 nm laser and a power meter. Starting from the prisms
the laser was directed through the the component and then focused into the power meter to accurately measure the power
loss through that particular component. Each component
was tested individually and the assembly was gradually built
up such that the power loss through each section was accurately determined. The two most significant points of power
loss were the 0.5 m long UHV fibre and the 2 m long in-airfibre.

PRISM DESIGN
Each ChDR pick-up utilises a prismatic radiator as shownin Fig. 6. The prism is made of UV grade fused Silica.
Prisms have been optimised for 500 nm ChDR emission by
setting the angle of the output face at 43.2∘ with respect tothe electron beam path, to ensure that light emitted at thiswavelength has an outgoing wavefront parallel to this interface. The upstream face is angled at the Cherenkov angle for
500 nm at 46.8∘ such that all light at this wavelength travels
to the output face without encountering any reflections. To
reduce signal interference from synchrotron radiation inside
the beam pipe, the upstream face and the two triangular faces
are coated in a 0.5 µm layer of aluminium.
The vertical size of the target with respect to the electron beam path is 32 mm. To match current theory [11], theFigure 7: Power loss results of the components inserted into
the BTS chamber.
As Fig. 7 shows the overall signal loss through the system
is 1.6 dB, which corresponds to a percentage decrease of∼ 30.8%.

PRELIMINARY RESULTS
USING ONE CHDR PICK-UP
For initial testing of the assembly, only one pick-up is
installed (see Fig. 8). This allows for fundamental measure-MOP032
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ments to be undertaken which will then better optimise theinsertion of the full two-pick-up BPM. A one pick-up systemis far easier to properly align with respect to the electronbeam. The initial shifts consisted of finding the correct vertical and rotational positions for the ChDR BPM assemblywith respect to the electron beam path.

Figure 9: YAG image showing clear shadowing of the beamprofile due to ChDR assembly.

Figure 10: Oscilloscope signal showing the three IBPM
signals and the ChDR signal. Voltage divisions of: 50 mV
and 0.5 V for the IBPM and ChDR signals respectively.

CONCLUSION

Figure 8: Image of one pick-up ChDR BPM assembly froman angle perpendicular to expected beam path, with a £1
coin for scale.
Initial commissioning of the one pick-up ChDR BPM
assembly is underway. Figure 9 shows the beam profileon the YAG screen located at OTR2 (see Fig. 1) shadowed
from the ChDR pick-up. This confirms that the beam canbe moved sufficiently such that an impact parameter of zero
between the electron beam and the ChDR pick-up can beachieved.
Figure 10 shows the output signal given by the interaction
between the electron beam and the ChDR pick-up in Fig 9.
The shadowing of the electron beam means that the signal
from the ChDR detector will be a combination of CR andChDR. This confirms that radiation emitted at the Cherenkov
angle is successfully captured and transmitted through theBPM assembly.

A 1D BPM utilising incoherent ChDR has been designed
and characterised in the lab. The assembly has been installedwith one pick-up in place on the BTS transfer line at DLS.
Initial commissioning tests have been performed and firstsignals have been detected.

ACKNOWLEDGMENTS
The authors would like to thank the efforts of the engineersfrom Royal Holloway University of London for creating thestainless-steel assembly and the help of the DLS vacuum
group with installing the ChDR assembly into the BTS transfer line.

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ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

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JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0
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