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JACoW Publishing
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MEASUREMENT OF SLICE EMITTANCE WITH
DEFLECTING CAVITY AND SLIT*I. Pinayev†, Brookhaven National Laboratory, Upton, NY, U.S.A.
Abstract
In this paper we describe the system for the of measurement slice emittance utilizing transverse deflecting cavity
and slit. The image of the beam passing through the slit isused to measure slice intensity and uncorrelated angular divergence. Beam size at slit location is measured by scan of
the beam across the slit with calibrated trim. The angularkick by the trim is taken into the account during calculations. Data processing and the experimental results are presented.

INTRODUCTION
Coherent Electron Cooling experiment carried out at
RHIC [1] requires small slice emittance of 15 MeV electron beam with high peak current.
Many applications require knowledge of the local properties of the beam distribution. These properties include
slice emittance and/or slice energy spread. In most casesthe emittance is measured by a quadrupole scan when beamis streaked with transverse deflecting cavity [2, 3]. For alow-energy beam with high peak current the measurements
are affected by space charge forces. If beam is not centeredwith quadrupole, then it will be steered during the scan
what can mix different slices.
To overcome these difficulties slit scan be used [4]. The
slit is moved across the beam and image is observed on adownstream profile monitor. The angular distribution is estimated from the spatial distribution observed on thescreen, and the transverse beam size at the location of theslit is found from the dependence of image intensity fromslit position. In our experiment we utilize a fixed slit, andwe are scanning the beam with help of calibrated trim.

streak of the beam [6]. Beam can be observed with three
profile monitors. The first profile monitor is placed before
the 45-degree dipole and used for measurement of the longitudinal profile of the beam. This screen is also used forslice emittance measurement using quadrupole scan. Second profile monitor is placed before the high-power dump
and is preceded by a vertical slit. This combination is used
for slice emittance measurement described in this paper.
The third profile monitor is placed after the 45-degree dipole and is used for measurement of the energy slew andslice energy spread.
The scan of the beam across the slit is performed withtwo calibrated horizontal trims. The first trim changes position of the beam on the slit and the second trim is restoring initial beam angle (it has the same kick of oppositesign) thus performing parallel scan of the beam over theslit.
Beamlet intensity 𝑤 is estimated from the image brightness inside of the region of interest (ROI). The beamlet angle 𝛼 is found from its center of gravity on the profile
monitor and uncorrelated divergence 𝜎 from the beamlet
width. The measured beam size is corrected for the final
resolution of the system defined by point spread functionof the optical system and slit width. The result of the scanis shown in Fig. 2.

EXPERIMENTAL SET UP
Beam parameters are measured in the dedicated beamline shown in Fig. 1 [5].

Figure 1: Layout of the diagnostics line. There are fourquadrupoles to adjust the beam optics for measurement,followed by a transverse deflecting cavity. Beam can beobserved on three profile monitors.
The beam is matched with four quadrupoles. The transverse deflecting cavity tuned to 1.3 MHz provides vertical___________________________________________

Figure 2: The result of scan of the beam parameters. On theleft side one can see phase space plot for the slice at 2 psfrom the beam center. Negative sign of the correlated angular spread indicates the beam convergence. R.m.s. beamsize is calculated from the right plot, the uncorrelated angular divergence from the left plot. Both values are used
for calculation of the slice emittance.
The slice emittance is calculated using conventional formula:𝜀〈𝑥 〉〈𝑥′ 〉

〈𝑥𝑥′〉

(1)

* Work supported by Brookhaven Science Associates, LLC under Contract No. DE-SC0012704 with the U.S. Department of Energy† pinayev@bnl.gov.

MOP04607 Machine Parameter Measurements

129

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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 - 3

IBIC2023, Saskatoon, CanadaJACoW 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 4 6

The average values for position and angle are supposed
to be equal to zero. The displacement of the beam by trimis ∆ and beamlet r.m.s. size is:〈𝑥 〉∑ ∆ 𝑤 ⁄∑ 𝑤

(2)

and beam divergence is:〈𝑥′ 〉∑ 𝛼𝜎𝑤 ⁄∑ 𝑤

(3)

Similar, the correlation term is found:〈𝑥𝑥′〉∑ Δ 𝛼 𝑤 ⁄∑ 𝑤

(4)

The slice is defined by region of interest on the profile
monitor. The location of the ROI is fixed during the measurement and temporal scan is done by changing the phaseof deflecting cavity. Such approach does not require
knowledge of the cavity voltage and calibration of deflecting angle. The calibration, however, is used for setting thevertical size of ROI.
The emittance of the whole bunch is calculated from theslices data. The weight of each slice is sum of the beamlet
intensities. The position and angle of each slice is weighted
average. Uncorrelated angular divergence is used from theslice data. Emittance calculation is based on Eqs. (1-4). The
emittance of the whole beam is displayed upon completionof the last slice measurement as shown in Fig. 3.

CONCLUSION
We have developed and applied the slice emittancemeasurement system utilizing fixed slit and deflecting cavity. The system allows to obtain slice parameters such assize, uncorrelated angular spread, and correlated divergence. The parameters for whole beam are calculated fromthe slices’ data. The system is regularly used for CeC accelerator operation and tuning.

REFERENCES[1] I. Pinayev et al., “Performance of CeC PoP Accelerator,” inProc. 10th Int. Particle Accelerator Conf. (IPAC’19), Melbourne, Australia, May 2019, pp. 559-561.
doi:10.18429/JACoW-IPAC2019-MOPMP050[2] M. Roehrs, C. Gerth, M. Huening, H. Schlarb, “Slice Emittance Measurements at FLASH”, in Proc. 10th European Particle Accelerator Conf. (EPAC’06), Edinburgh, Scotland, UK,June 2006, pp. 77-79.
[3] J. Rudolph, M. Abo-Bakr, T. Kamps, J. Teichert, “Slice Emittance Measurement at ALBE/SRF Injector”, in Proc. DIPAC’11, Hamburg, Germany, May 2011, pp. 416-418.
[4] R. Niemczyk et al., “Slit-based Slice Emittance Measurements Optimization at PITZ”, in Proc. IBIC’19, Malmo,Sweden, Sep. 2019, pp. 318-321.
doi:10.18429/JACoW-IBIC2019-TUPP013[5] T.A. Miller et al., “Overview of Beam Instrumentation andCommissioning Results from the Coherent Electron CoolingExperiment at BNL”, in Proc. IBIC'18, Shanghai, China, Sep.
2018, pp. 43-47.
doi:10.18429/JACoW-IBIC2018-MOPA09[6] S.A. Belomestnykh, V.D. Shemelelin, K.W. Smolenski,
V. Veshcherevich, “Deflecting Cavity for Beam Diagnostics
in ERL Injector”, in Proc. 22nd Particle Accelerator Conf.
(PAC’07). Albuquerque, New Mexico, USA, Aug. 2007,
pp. 2331-2333.

Figure 3: The snapshot of the last slice emittance measurement with displayed emittance of the whole beam.

MOP04613007 Machine Parameter Measurements