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APPLICATION OF A CAMERA ARRAY FOR THE UPGRADE
OF THE AWAKE SPECTROMETER
Eugenio Senes∗ 1 , F. Pannell 2 , D.A. Cooke 2 , S. Mazzoni 1 ,M. Turner 1 , M. Wing 2 , G. Zevi Della Porta 1,3
1 CERN, Geneva, Switzerland
2 University College London, London, United Kingdom
3 Max Planck Institute for Physics, Munich, Germany
Abstract
The first run of the AWAKE experiment successfully
demonstrated the acceleration of an electron beam in theplasma wakefields of a relativistic proton beam. The planned
second run will focus on the control of the emittance of accelerated electrons, requiring an upgrade of the existing
spectrometer. Preliminary measurements showed that thismight be achieved by improving the resolution of the scintillator and with a new design of the optical system. This
contribution discusses the application of a digital camera
array in close proximity of the spectrometer scintillator, toenable the accelerated electron beam emittance measurement.

INTRODUCTION
In the AWAKE experiment, a 19 MeV electron bunch getsaccelerated in the wakefield of a rubidium plasma [1]. The
plasma wake is driven by a 400 GeV proton bunch producedin the CERN Super Proton Synchrotron.
In order to measure the characteristics of the accelerated
electron beam, a magnetic spectrometer was installed downstream of the plasma cell. The development of the magnetic
spectrometer is reported in [2–4].
The core functions of the electron spectrometer are tomeasure on the accelerated electron beam:
1. The beam peak energy and energy profile
2. The beam charge
3. The beam emittance
In AWAKE Run 1 the resolution proved to be too limited
to measure the beam emittance; a study was launched to improve the spectrometer system for AWAKE Run 2. This paper discusses possible technical implementations to improve
the spectrometer resolution, focusing on a high resolution
optical system. Possible modifications of the spectrometer
magnetic lattice to transport the accelerated beam will be
discussed elsewhere, and go beyond the scope of this work.

SPECTROMETER DESIGN
Beamline Components
The magnetic components of the spectrometer have notchanged since AWAKE Run 1 [5, 6]. The electron and proton∗ eugenio.senes@cern.ch

beams exit the plasma, pass through a quadrupole doublet
and then through a dipole magnet. A dipole field up to 1.5 T
separates the electron and proton beams, introducing a spatial dependence for the electron beam energy in the range
30 MeV-8.5 GeV. After the dipole, the accelerated electrons
traverse a 2 mm thick vacuum window, and impact on a
1 m-long plastic scintillator. The scintillator currently in
use is a 0.5 mm thick DRZ-High, a terbium-doped, gadolinium oxysulfide (Gd2 O2 S:Tb) screen produced by Mitsubishi
Chemical. The particle transport through the spectrometer
was simulated and validated through measurements withpartially stripped ions [7, 8]. A layout of the spectrometersystem is shown in Fig. 1.

Optical System
Information on the electron beam structure is reconstructed from the emitted light pattern produced by the scintillator. An accurate imaging of the scintillator emission isessential for a correct measurement of the electron beam
parameters.
During AWAKE Run 1, the scintillation light was propagated through a 17 m-long optical line, to reach an intensified
camera in a dedicated dark room outside the high radiation
area [9, 10]. The camera is equipped with a 400 mm photographic lens, in order to resolve the whole scintillator.
Although this system worked, the optical resolution proved
to be too limited for emittance measurements.
Recent developments in the CMOS camera infrastructure
for AWAKE allowed for placing cameras inside the experimental hall. A camera array was installed within 1.5 m ofthe scintillator, to record its light. This system is describedin detail later on.

Beam Charge Measurement
In order to measure the total accelerated charge, the scintillator light yeld is calibrated against a known electron
charge. In the past, the calibration was performed by installing the scintillator and the camera in the CLEAR test accelerator [11], and using electron beams with known charge.
To allow for the in-situ calibration, a commercial Integrating
Current Transformer (ICT) from Bergoz [12] was installedin the beamline upstream of the dipole. The charge signal isdetected with custom electronics within a range of 750 pC.

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Quadrupole
doublet

ICT

Protonbeam

Electron beam
Scintillator screen
CMOS camera array

EnergyEnergyFigure 1: Layout of the spectrometer system in AWAKE. The inset shows a sample image of an accelerated beam from oneof the cameras in the array.

DIGITAL CAMERA SYSTEM
Digital Camera Array
A general description of the digital camera infrastructure for AWAKE was given in Ref. [13]. The
measurements shown in this work are performed with
Basler acA1920-40gm cameras. The camera specifications
are shown in Table 1. These devices showed a good resistance to modest radiation environments, and are remotely
powered, allowing for reboot to recover from single event
upset failures. The cameras are hardware triggered with a
750 µs delay, to capture the decay of the scintillation light.
As the light emission is peaked at 545 nm, each camera isequipped with a 525 ± 46 nm bandpass filter to reduce thebackground.
A resolution in the order of 100 µm can be reached with
sufficient camera magnification. This is a tradeoff between
the distance to the scintillator and the objective focal length.
The limitation, arising from high magnification, is that eachcamera can image only a part of the spectrometer. A camera array is set up to allow the imaging of a larger area ofthe spectrometer. The larger spectrometer image is thenreconstructed by combining the camera images. The recomposition algorithm is described later on. The camera arrayis placed within 1.5 m from the scintillator, with an angle of30◦ below the beam plane. This allows both for removing
the cameras from the beam plane, where the radiation ishigher, and for leaving free space in the optical line towards
the intensified camera.
The selected camera lenses are Fujinon HF75HA1S witha focal length of 75 mm and an aperture of f/2.8. The camera array was placed at 110 cm from the scintillator, thatis the minimum focusing distance of the lenses. The setup
was reproduced in the lab, where USAF1951 targets wereused to measure the point spread function (PSF) and hence
infer the resolution. Additionally, a different camera model(Basler acA2040-35gm) was tested, featuring smaller pixel
size and smaller sensor format. Both cameras showed that

Table 1: Comparison of parameters for the tested cameras.
The top half shows camera specifications. The bottom halfshows the measured resolution with 75 mm lens at 110 cm
distance from the target.
Model

acA1920-40gm

acA2040-35gm

Resolution[px]
Pixel size[µm]
Sensor format

1920 × 1200
5.86 × 5.86
1/1.2"

2048 × 1536
3.45 × 3.45
1/1.8"

Lens
Magnification
Field of view [cm]Pixel object size[µm]
PSF 𝜎
[µm]

HF75HA1S - 75 mm f/2.8
0.076
14.9
9.5
77
45
77
48

the resolution is dominated by the pixel size of the camera, and that optical resolutions as low as 77 and 48 µm arereachable at 110 cm distance, respectively. The camera parameters and resolutions are reported in Table 1. These tests
showed that high resolution can be achieved at the expenseof the field of view. To resolve the full 1 m-long scintillator,
the array would require 7 and 11 cameras, respectively. Additionally, the use of sensors with smaller pixel size allows
for increasing the distance to the scintillator, reducing theradiation damage.

Camera Calibration
Camera calibration is an essential part of understanding the three-dimensional aspects of objects from twodimensional images. Extracting metric information fromthe 2D images acquired by the AWAKE spectrometer camera array forms the basis of the accelerated electron bunch
analysis.
The mapping of a 3D point X in the real-world, to a 2D
pixel coordinate, x, can be described via Eq. (1):
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Dipole magnet

231

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where P is the projection matrix – a combination of the
intrinsic camera calibration parameters and extrinsic camera
parameters of position and orientation.
The process of camera calibration in this work employs
Zhang’s method, where the camera observes a planar pattern at different translations, rotations and tilts [14]. This
technique offers a simplified, easily reproducible calibration
procedure compared to the multiple, orthogonal planes usedin 3D reference object-based calibration methods.
The procedure begins by mounting a pattern of known
structure and size, in this case a chessboard, on a flat surface.
The camera is fixed and the chessboard is imaged at different
orientations. The OpenCV-Python [15] library is used and,for every image, a coordinate system is defined in which
the chessboard forms the XY plane; all points lying on thechessboard have Z = 0. With this approach, the mapping
between coordinate systems is simplified as the last columnof the rotation matrix within P can be eliminated.
The corners of the chessboard pattern are located as key
points through a function based on the Harris corner detection algorithm [16]. With each point representing anequation in the coordinate mapping system, the camera calibration parameters are obtained from the homography ofreal-world points to image points. Figure 2 shows the chessboard calibration target repeatedly imaged through one camera on the spectrometer array, with the corners successfully
identified.

Figure 3: Examples of radial lens distortion, where lightrays deviate from rectilinear projection.

introduces a perspective distortion to the acquired images.
By looking up at the screen, a foreshortening effect is created, where shapes higher up in the image appear condensed
compared to shapes of the same size at the bottom.
Figure 4 demonstrates this effect. The left figure showsan uncorrected USAF1951 resolution target viewed througha spectrometer camera. The right figure is the same image
but corrected for the perspective distortion. In addition to
the rectified scaling offered by the perspective correction, as
evidenced by the lengthened bars and increased target height,
the image is rotated to create a ‘head on’ view of the target.
The perspective correction was achieved by mapping thecoordinates of the target corners in the unprocessed image
to the corresponding corners in a scale, technical drawingof the same target.

Figure 4: USAF1951 resolution target as seen from an unprocessed spectrometer image 30◦ below the beam plane(left) and a perspective corrected image to view ‘head on’(right).

Figure 2: Chessboard calibration pattern captured at different positions and orientations. The corners are located with
OpenCV.
Lens distortion effects, such as the ones illustrated inFig. 3, arise from the lens optical design. The calculationof the camera matrix, distortion coefficients, translation androtation vectors in the camera calibration procedure allows
for the compensation of this effect.

Perspective Correction
As described earlier, the camera array views the spectrometer scintillator screen from an angle of 30◦ below thebeam plane. Whilst it has practical advantages, this setup

Emittance measurements with the camera array involve
extracting the transverse size of the electron beam waist.
Without perspective correction, the measured beam size will
differ from reality, impacting the validity of emittance results.
It is therefore essential to apply the appropriate perspective
correction prior to analysis.
To correct the scintillator screen for perspective distortion,a screen target with commensurate dimensions with the scintillator is temporarily placed across the screen and imagedby all cameras in the array. Similarly to the resolution target
perspective correction, key coordinates from the acquired
images are mapped to the equivalent features in a technicaldrawing of the screen target. This can be seen in Fig. 5,
where corrected images are sampled up to the same size ofthe drawing.

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reduces the maximum field of view achievable compared torecombination by image overlaying for a camera array of agiven size.

Resolution

Figure 5: Perspective corrected images of the sub region
within the field of view for each camera, with a technicaldrawing of the spectrometer screen target (bottom).

Image Recombination
By matching the size of the sub region images from eachcamera to the reference target, the images can be easily
combined to create a single image of the target, representing
the field of view of the entire camera array.
The four sub region images are overlaid by mapping themin-turn onto the reference target. Empty pixels are discarded,
and any regions of overlap are accounted for by taking the
average intensity value of the shared pixels.
Figure 6 shows the initial recombination result from overlaying two neighbouring camera images with this method.
Whilst the geometric features of the target are successfully matched, it is clear this recombination suffers fromvignetting effects.

Figure 6: Two adjacent sub regions of the screen target
overlaid for image recombination. Due to vignetting, thedistinct images and seams are apparent.
By imaging an approximately uniform light source
through one of the array cameras and the 75 mm lens, a calibration image is obtained, essentially creating a vignetting
map. By dividing the screen target images by the normalised
vignetting map, the variations in brightness across the target
images are compensated.
Figure 7 is the result of complete image recombinationof the four spectrometer cameras, correcting for vignetting,lens and perspective distortion.

It is worth noting that the lens distortion and perspective
corrections performed in this work change the underlying
pixel grid of the images, and are therefore interpolations. For
this reason, it is important to assess the resolution both prior
to and after these corrections. The reference image is sampled upwards, creating a reference with substantially more
pixels than the images from the camera array. This reduces
any loss of information during the perspective correction
from the original image grid to the reference.
The image resolution is measured by imaging a resolution target and calculating the Modulation Transfer Function(MTF). In the case of the resolution targets in Fig. 4, the
0.5 MTF value before correction is ~3.9 line pairs per mm.
After distortion and perspective correcting the image, the
0.5 MTF value is calculated as ~3.8 line pairs per mm. Thisshows a negligible change in the overall resolution due to image processing. It therefore remains advantageous to correct
for the lens and perspective distortions prior to emittanceanalysis.

CONCLUSIONSThe AWAKE experiment continues studying the plasma
acceleration of electrons in a wakefield generated by a proton
bunch. To allow the measurement of the accelerated electron
beam, improvements in the spectrometer system are required.
This work discusses the improvements of the spectrometer
optical system, with the aim of high-resolution imagingof the 1 m-long scintillator. The use of a camera array is
proposed, and the hardware selection is presented. The
image processing algorithm to recombine the single camera
images into the full scintillator image is described, as wellas the impact on the overall resolution of the process.

ACKNOWLEDGEMENTS
The authors would like to thank Mr. Itaru Sakamoto andthe Mitsubishi Chemical Corp. for kindly providing the test
scintillator samples. The authors would like to acknowledge the SPS and AWAKE operation teams, M. Gasior and
T. Levens for the ICT support and S. Burger for the stimulating discussions.

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Figure 7: All four spectrometer camera images combined
into a single image of the screen target.

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ISSN: 2 6 7 3 - 5 3 5 0
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