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

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 - W E 3 C 0 2

DEVELOPMENT OF A PRECISE 4D EMITTANCE METER USINGDIFFERENTIAL SLIT IMAGE PROCESSING
Bokkyun Shin†, Chang-Kyu Sung, Garam Hahn, Pohang Accelerator Laboratory, Pohang, Korea
Moses Chung, Ulsan National Institute of Science and Technology, Ulsan, Korea
SLIT DESIGN

Abstract
We have developed a highly precise 4D emittance meter
for X-Y coupled beams with 4D phase- space (x-x', y-y',x-y', y-x') which utilizes an L-shaped slit and employs
novel analysis techniques. Our approach involves twotypes of slit-screen image processing to generate pepperpot-like images with great accuracy. One which we call the"differential slit" method, was developed by our group.
This approach involves combining two slit-screen images,
one at position x and the other at position x + the size ofthe slit, to create a differential slit image. The other method
we use is the “virtual pepper-pot (VPP)” method, which
combines x-slit and y-slit images to produce a hole (x, y)
image. By combining that hole images, we are able to takeextra x-y' and y-x' phase-space. The "differential slit"
method is crucial for accurately measuring emittance.
Through simulations with 0.1 mm slit width using Geant4,
the emittance uncertainties for a 5 nm rad and 0.2 mm size
electron beam were 5% and 250% with and without the“differential slit”, respectively. In this presentation, we provide a description of the methodology, the design of slit,
and the results of the 4D emittance measurements.

The development and application of an L-type slit scanner, which moves to scan X and Y-axis images in a singleaxis movement. This slit-scanner design is intended forelectron beam testing at PAL-eLABs (or PAL-ITF) [2].
PAL-eLABs utilizes RF photo-cathode guns to generate
electron beams with energy up to 70 MeV.
Beam profile imaging is achieved by capturing images
through the slit and subsequently viewing them on scintillation screens and/or Optical Transition Radiation (OTR)
detectors. These screens and detectors are positioned at distances of 3.5 meters away from the slit.

INTRODUCTION

Two critical aspects of the slit design have been considered: the slit width size and the thickness of the slit plateincluding its material. Figure 1 shows slit design and manufactured prototype slit. The distance between the X and Y
slit centers (Dcenter) is determined considering 15 mm by
the beam size to prevent overlapping X and Y slit image ina shot. A thickness and material of slit plate 50 μm stainlesssteel has been selected to minimize background from70 MeV beam scattering with plate and slit punching byetching. And 5 pin holes at bottom and top of slit to get insite position calibration. The prototype slit was succeeded
manufacture with 140 μm slit width and 100 μm diameter
calibration pinhole.

In the accelerator the accurate measurement of 4D emittance, the phase space parameters (xx', yy', xy', yx’) is important optimizing x-y coupled particle beam properties.
This measurement has been accomplished through two primary methods: the pepper-pot and dual slit scanning techniques. However, each of these methods comes with its
own set of limitations, such as resolution and time-consuming.
The recently developed 4D emittance measurement is
so-called “the Virtual Pepper Pot (VPP)” method, which isimage processing to generate a pepper-pot-like image using x and y axis slit scan data [1].
However, it is important to note that both the dual slit
scanning and VPP methods have uncertainties related to
the width of the slits. To further enhance the capabilities of
the VPP method, this paper proposes an additional improvement: the “Differential Slit” method. This method
employs image processing is utilized to generate virtual
narrower slit images by the positions of the slit at x and x +slit width. This approach aims to minimize the uncertainties associated with slit size, enhancing the accuracy ofemittance measurements.
We describe the method of “differential slit” image processing with VPP method and show precision efficiency
using the simulation.
____________________________________________† bkshin@postech.ac.kr

Figure 1: Slit design and manufactured slit scanner.

SIMULATIONThe simulation data is generated using the Geant4 [3]and image analyzer OpenCV [4]. Electron interaction withslit and screen are simulated by Geant4. And from energy
deposition in the screen to image data by Open CV. The
beam condition is used PAL-eLABs [2], 70 MeV and emittance ~10 nm rad and beam is artificially rotate for 4Demittance for the 4D phase space study. And Beam Profilescreen using the YAG (Y3Al5O12) is located 3.5 meters
away from a slit. The slit is moved every 20 μm from fully
open which is equivalent as 10 Hz beam repetition ratewith continuously moving a velocity of 0.2 mm/s. The images vertical resolution is 12 bit which is common resolution of CMOS camera, and they are stored in a scaled 16bit video format.

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03 Transverse Profile and Emittance MonitorsIBIC2023, Saskatoon, Canada
JACoW Publishing
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 - W E 3 C 0 2

In simulation setup, background noise sources are a critical consideration. two primary sources of backgroundnoise are identified: beam scattered by the slit plate andcamera sensor noise. beam scattered noise is produce using
the Geant4. The noise with 50 μm stainless steel with 3.5 m
way screen does not significantly affect by analysis. It is
uniformly distributed in the screen and its signal-to-noise
ratio (SNR) is smaller than 1/1000. The camera noise was
randomly generated in simulation, which is approximately
5% of the camera sensor cavity.

ANALYSIS
Analysis involves four steps: position calibration, thedifferential slit image process, VPP image process [1], andphase space reconstruction.

Figure 3: Differential Slit mage processing: a) beam profileat screen and slit position; b), c) screen image by the green
and blue dot boxes; d) differential slit image using minimum pixel pick up using the image b) and c).
The VPP Method is employed as the third step in theanalysis process. The image processing steps here are similar to those of the Differential Slit Method, utilizing min(cx, cy), where 'c' represents photon counts in the pixel, and'x' and 'y' denote positions in the x and y axes. Figure 5 describes process of VPP method. It is also using the minimum pixel count selecting. Using the VPP method in combination with the differential slit images yields images thatresemble those obtained from a pinhole, enhancing thequality of data.

Figure 2: Frame ID vs Sum of count in sensor, left highmesa is signal when slit is opened and four peaks represent
calibration pin holes and slits.
In this initial step, position calibration is performed bycorrelating frame numbers with the sums of signals in thecaptured images. Figure 2 shows the relationship between
frame number and the sum of the signal in the image. InFig. 2, the highest mesa corresponds to when the slit is fully
opened. And the first and last peaks obtained from this calibration process are linked to the position calibration pinhole, enabling the determination of the slit's speed and thematching of frame IDs with positions.
The second step, the Differential Slit method is introduced as an innovative approach in this research to enhance
precision in data analysis. This method involves the use of
two images captured at different slit positions. Specifically,images are selected based on the condition slit position difference is smaller than slit moving step. For this study ith
frame and i+7th frame is chosen for differential slit image.
The image processing is then performed using the minimum pixel pick up using the two images. An example of adifferential slit image is show in Fig. 4. Figure 4 a) shows
beam particle distribution and slit position green and bluebox, and b) and c) are the image on the screen at 3.5 meteraway from slit. Figure 3 d) displays the result of the differential slit image obtained by selecting the minimum pixel
count, in contrast to images b) and c).

Figure 4: VPP image processing: a) beam profile at screen
and slit position; b), c) differential image by the green andblue dot lines; d) pin hole image from the VPP image process using b) and c).
In the final step, phase space reconstruction is carried out
which is shows in Fig. 5. This involves using the x and ypinhole images obtained from the previous steps. Phase
space was reconstructed. And through 2D-Gaussian fitting,emittance and Twiss parameters are analyzed in the Fig. 5
red ellipse.

RESULTAs a result of these analysis steps, the entire process
takes approximately 10 seconds using GPU processing.
Table 1 shows the input and reconstructed parameters ofeach phase-space. In the result, emittance uncertainties
from input are less than 10%. But without the difference
slit method, the result of emittance uncertainty higher
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319

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

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 - 3IBIC2023, Saskatoon, Canada
JACoW Publishing
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 - W E 3 C 0 2

than 250%. Overall, these four steps enable a comprehensive and precise analysis of the experimental data, providing valuable insights into the behaviour of the system understudy.

Figure 6 shows the lattice of the PAL-eLABs. The slit
will be installed experiment space and the screen will be
used at before Dipole Magnet. The x-y coupling beam for
4D emittance such as rotation beam is tuning by bucking
coil or skew quadrupole magnet after the accelerator. Afterthe successful measurement. The slit will be applying in
Korea-4GSR LINAC to measure x-y coupling.

SUMMARY

Figure 5: Phase reconstruction and emittance and Twiss
parameter analysed by 2D Gaussian fitting.
Table 1: The Result of Input and Reconstructed
Phase Space

X-X’ in
X-X’ out
X-Y’ in
X-Y’ out
X-X’ in
X-X’ out
X-Y’ in
X-Y’ out

α4.4673.871-0.379-0.367-0.479-0.467
1.914
1.821

β
4.390
4.549
1.377
1.651
0.826
1.000
2.161
2.496

ε
7.449
7.650
23.73
21.24
23.87
22.16
9.137
8.932

EXPERIMENT PLAN
The experiment plan the slit-based analysis method at
PAL-eLABs is shown below.

The VPP method with L type slit is a significant advancement of measurement time efficiency compared totraditional dual slit scan method. And the differential slit
method introduced in this paper represents an additional
step forward in improving measurement precision. Only
applying one step of analysis without any instrument improves uncertainty from slit width. In this simulation study
using 100 μm slit width of 50 μm stainless steel with thismethod, the uncertainty is dramatically improved from250% to 10% which is small step but great achievement.
Consequently, this method is becoming a crucial techniquein any slit-screen experiment.

ACKNOWLEDGEMENTS
This research was supported in part by the Korean Government MSIT (Multipurpose Synchrotron Radiation Construction Project) (RS-2022-00155836) and UNIST CHEA
by the National Research Foundation of Korea (Grants No.
NRF-2016R1A5A1013277 and No. RS-2022-00154676).

REFERENCES[1] G.Z. Georgiev and M. Krasilnikov, “Virtual Pepper-Pot
Technique for 4D Phase Space Measurements”, in Proc.
IBIC'19, Malmö, Sweden, Sep. 2019, pp. 586-590.
doi:10.18429/JACoW-IBIC2019-WEPP029[2] S.J. Park et al., “Construction of Injector Test Facility (ITF)
for the PAL XFEL”, in Proc. IPAC'13, Shanghai, China, May
2013, paper WEPWA043, pp. 2220-2222.
[3] S. Agostinelli et al., “Geant4-a simulation toolkit”, Nucl. Instrum. Methods Phys. Res., Sect. A, vol. 506, no. 3, July 2003,
pp. 250-303, 2003.
doi:10.1016/S0168-9002(03)01368-8[4] OpenCV, https://opencv.org

Figure 6: PAL-eLABs lattice layout.

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