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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
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DIAGNOSTICS FOR A HIGH EMITTANCE AND
HIGH ENERGY SPREAD POSITRON SOURCE
N. Vallis1∗ , F. Marcellini, R. Zennaro, M. Zykova,
P. Craievich, R. Fortunati, R. Ischebeck, E. Ismaili, P. Juranic, G.L. Orlandi, M. Schaer
Paul Scherrer Institute, Villigen, Switzerland
1
also at EPFL, Lausanne, Switzerland
Abstract
This paper is an overview of a diagnostics setup for highly
spread e+e- beams, to be installed at the PSI Positron Production (P3 or P-cubed) experiment. To be hosted at the SwissFEL facility (PSI, Switzerland) in 2026, P3 is e+ source
demonstrator designed to generate, capture, separate anddetect nano-Coulombs of secondary e+ and e- bunches, inspite of their extreme tranverse emittance and energy spread.
The experiment will employ an arrangement of broadband
pick-ups (BBPs) to detect simultaneously the time structureof secondary e+e- bunches. A spectrometer will follow theBBPs and deflect the e+ and e- onto two unconventional
faraday cups that will measure their charge. In addition,
the energy spectrum of e+ and e- distribution will be reconstructed through scintillating fibers.

INTRODUCTION
The SwissFEL facility [1] (PSI, Switzerland) will host thePSI Positron Production (P3 or P-cubed) experiment [2] in
2026, a e+ source demonstrator with potential to improve byan order of magnitude the present state-of-the-art e+ yield.
The experiment layout is shown in Fig. 1, featuring a e+source based on a 6 GeV electron (e-) beam and 17.5 mmthick (or 5 times the radiation length) amorphous Tungsten
target. A capture system will follow the target, consistingof a solenoid system and 2 RF accelerating cavities. The
remarkable e+ capture capabilities of P3 are enabled to greatextent by the usage of high temperature superconducting(HTS) solenoid around the target region, as well as a novel
standing wave solution for the RF cavities that provides alarge iris aperture to maximize e+ capture.

Figure 1: Simplified layout of the P3 [2].
The experiment diagnostics, whose layout is shown inFig. 2, will be located directly downstream from the 2𝑛𝑑 RF
cavity, as illustrated in Fig. 1. This setup will measure thee+e- bunches before and after species separation, equippedwith an arrangement of broadband pick-ups (BBPs), 2 Faraday Cups (FCs) and a variety of scintillating detectors. The∗ nicolas.vallis@psi.chFigure 2: Concept design of the P3 diagnostics.
BBPs will detect, simultaneously for both particle species,
the time structure of the captured e+e- bunches. The FCsand scintillators will be installed in the same vacuum chamber, and will measure the charge and energy spectrum of
separate e+ and e- streams. Separation will be provided bya spectrometer, a dipole magnet based on four copper coils
and an iron yoke, which will be fed at a maximum currentof 340 A in order to reach magnetic fields up to 0.25 T. Additional diagnostics such as the arrangement of screens andcameras are still under discussion.

e+e- Dynamics in P3
200 pC e- at 6 GeV will impinge upon the 17 mm-thickTungsten target, yielding a e+e- beam in the multi-MeV andnano-Coulomb range. Table 1 shows the evolution of thee+ bunches along the P3 capture line. These values clearly
indicate that e+ dynamics are are heavily dominated by an extremely high transverse emittance. In the transverse plane, e+collection relies on the so-called Adiabatic Matching [3,4], awell-known e+ capture technique based on transforming thetransverse phase space of newly generated e+ (moderate 𝜎𝑥and large 𝜎𝑝𝑥 ) into the acceptance of the capture line (large𝜎𝑥 and moderate 𝜎𝑝𝑥 ). Thus, emittance is not damped but
matched and transported up to the diagnostics section. As forthe longitudinal beam dynamics, RF fields will generate time
structures of many consecutive e+ and e- bunches separated
by the S-band 𝜆/2 (167 ps or 50 mm in the ultrarelativisticregime), with the most particle population concentrated overthe first two RF buckets. Two RF working points of interest

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12th Int. Beam Instrum. Conf.
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Table 1: Main parameters of the e+ bunches for RF working
points of interest. Simulated with ASTRA [5].
𝑄𝑒+ [pC]

Φ = (120,-70)

Φ = (70,-110)

1246

1155

𝜎𝑥 , 𝜎𝑦 [mm]𝜎𝑝𝑥 , 𝜎𝑝𝑦 [MeV/c]
𝜖𝑛𝑜𝑟𝑚 [𝜋 mm mrad]
𝜎𝑡 [ps]ΔE𝑅𝑀𝑆 [MeV]

70.5≈4000100≈3395(see Fig. 3) were found in previous optimization studies:Φ = (120,-70), which provides maximum e+ capture andΦ = (70,-110), which will provide maximum energy compression [2]. The quality of measurements must therefore
be particularly high for these RF phase configurations.

was optimized to avoid intrinsic resonances up to frequencies
in the range of a few tens of GHz, while providing a relatively
high peak voltage. According to a preliminary simulation
based on a gaussian approximation of the P3 bunches (seeFig. 5), the BBPs would detect a ±4.5 V peak voltage signal
with very small distortion. Notice that this simulation does
not take into account wakefield effects, cable distortion orenvironment noise.
Two BBP assemblies shown in Fig. 6 have been developed and fabricated, based on broadband feedthroughs of
27 GHz [6] and 65 GHz [7]. The hardware acquisition setup
will consist of low attenuation, broad band cables [8] anda high-end oscilloscope of at least 40 GHz pass band and10 GS/s sampling. Similar solutions based on ultra fast pickups are currently used in accelerator facilities such the fast
BPMs at the SuperKEKB e+ linac [9, 10] and the Bunch
Arrival-Time Monitors at SwissFEL [11].

Figure 4: Mechanical design of the 27 GHz BBP assembly.

Figure 3: Simulated e+e- distributions near the exit of the
2𝑛𝑑 RF cavity (z ≈ 2.8 m) for RF working points of interest:
Φ = (120,-70) (top) and Φ = (70,-110) (bottom).

SIMULTANEOUS e+e- DIAGNOSTICS:
BROADBAND PICK-UPS
An arrangement of 4 broadband pick-ups (BBPs), shownin Fig. 4, will follow the second RF cavity and detect thewake voltage generated by the captured e+ and e- bunches.
This will allow to reconstruct the e+ and e- time structure
simultaneously, including measurements of charge, lengthand separation bunch by bunch. The e+e- time structure will
heavily depend on the RF phase, yet the typical distribution
will consist of alternating e+ and e- bunches of 33 ps length,
and separated by 167 ps, namely half S-band period (seeFig. 3 and Table 1). Therefore, the geometry of the pick-ups

Figure 5: Detected voltage signal by one pick-up using
27 GHz feedthrough arrangement. Signal generated by gaussian bunch of Q𝑏𝑢𝑛𝑐ℎ = 1 nC and 𝜎𝑡 = 33 ps. Simulated withCST [12].

DIAGNOSTICS AFTERSEPARATION OF e+ AND eFaraday Cups
As illustrated in Fig. 2, the spectrometer will deflect thee+ and e- streams onto two faraday Faraday cups (FCs).
Due to the extremely large energy (and p𝑧 ) spread, the main
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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 1 0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.
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challenge for both FCs is to collect a beam with a hightransverse dispersion introduced at the spectrometer, forwhich each FC will follow a different principle. Note thatboth e+ and e- could delivered to either FC through a sign
inversion of the spectrometer polarity.

Table 2: Reference spectrometer strength and measured energy ranges for different channels of FCs. Values based onzero-emittance particles.
12.5 Ω FC50 Ω FC

Mag. strength [T]E. range [MeV]

0.053

9 - 75

0.212
0.120
0.068
0.038
0.021
0.012

50 - 90
28 - 50
16 - 28
9 - 16
5-9
3-5Figure 6: RF (top) and mechanical (bottom) design of
12.5 Ω (left) and 50 Ω (right) FCs, including tungsten blocks,
PEEK supports and vacuum space.
The first FC is tuned at 12.5 Ω in pursuit of a large transverse area (260x90 mm), which will maximize the collectionof charged particles in a wide energy range of 9-75 MeV.
The coaxial impedance, a factor 4 smaller than the 50 Ωstandard, can be easily matched to standard circuits throughthe use of parallel 4 coaxial cables. A second, relatively
compact FC (80x80 mm) tuned at 50 Ω will detect charged
particles in a larger energy range of 3-90 MeV. Although thesmaller transverse size does not allow for single-shot charge
measurements in broad energy spectra, the 50 Ω FC will perform energy discriminating measurements by adjusting thespectrometer strength, which determines the energy rangeof the particles routed towards the FC. A scan of 6 magnetic field values indicated in Table 2 would cover the above
mentioned 3-90 MeV range. Figure 7 shows the frequency
response of both FC arrangements with four diagonal PEEK
supports (as seen in Fig. 6), in both cases above 1 GHz.
Error estimations of the measured e+ charge are shown inFig. 8 for both FC layouts and all RF phase configurations,
showing a reasonably low errors in the vicinity of the RF
working point of interest. These error studies are based onASTRA [5] particle tracking simulations. At Φ = (120,-70),
the point for maximum captured e+ charge, the 12.5 Ω and50 Ω FCs would read -13.6% and -9.4% with respect to theexpected 1246 pC. 60 mm-thick W blocks were considered

Figure 7: Frequency response of 12.5 Ω (top) and 50 Ω(bottom) FCs. Based on HFSS [13] simulations.
for both FCs in order to maximize charge deposition. Backscattering effects due to the high Tungsten density are disregarded as they are negligible above 10 MeV. On the other
hand, poorer charge measurements are expected at lower
energies, as particle divergence will have a greater impact in
the final transverse position. However, this occurs in regionswith relatively small importance for the experiment.

Scintillators in Diagnostics Chamber
Aside from the FCs, the diagnostics chamber will accommodate at least two additional instrumentation setups basedon scintillator materials. First, the front face of the FCs will
have scintillating screens that will allow cameras mounted
outside of the chamber to look at the collected e+ and e- distributoins. These scintillator screens are particularly usefulduring beam commissioning, as counterpart of the FC signals. However, the large size of the FCs and the transverse
emittance of the beam result in a very poor energy resolution, which precludes any usefulness of performing spectral
measurements through these devices. The scintillator caneither be a coating deposited on the face of the FCs, a screen
mounted to the front of the FCs, or a free-standing screen

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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 1 0and signal acquisition. In addition, this spectral detection
is supposed to be relative at the moment. Calibration of thefibers is required for absolute charge measurements and is
still under investigation.

Figure 8: Error (in %) of measured charge by large 12.5 Ω(top) and compact 50 Ω (bottom) FCs over 2D RF phasescan. Estimated through particle tracking simulations withASTRA [5]. Φ = (120,-70) and Φ = (70,-110) marked in
red.
that can be inserted in and out of the chamber. The most
likely materials for the screen would be Cr-doped Alumina(Chromox), Biomax, or YAG, which have been used for
scintillation in accelerators in the past [14].
An high-resolution spectroscopic setup consisting of atleast one pair of scintillator fibers will reconstruct the longitudinal momentum (𝑝𝑧 ) spectrum of the e+ and e- distributions. The fibers, vertically oriented, will be hit by a small
fraction of the particles corresponding to a narrow divisionof the energy spectrum. Following the basic principle ofbeam loss monitors, scintillator fibers will emit a signal proportional to the ionisation energy released by the incident
charges. This will allow to scan the dipole field strength
over as many points as desired. The baseline location of thefirst scintillator fiber pair is considered at x = -150 mm andz = 3520 mm (320 mm downstream from the center of thespectrometer). Figure 9 shows a reconstruction of the 𝑝𝑧
spectrum for both RF working points of interest, computed
through multiple particle tracking simulations. Despite thenotoriously good agreements, the reconstruction below does
not consider important factors such as scintillator response

Figure 9: Estimation of 𝑝𝑧 reconstruction for Φ = (120,70) (top) and Φ = (70,-110) (bottom). Computed
through through 61 ASTRA simulations ranging dipole fieldstrengths from 0 to 0.3 T and scintillator fibers located at
x = -150 mm and z = 3520 mm.

CONCLUSION
This paper introduced the diagnostics for P3 experiment,
consisting of an arrangement of broadband pick-ups (BBPs),
2 types of Faraday cups and a variety of scintillating detectors. The BBPs are currently assembled and under test.
Preliminary measurements with ≈10 ps e- bunches are scheduled before the end of 2023. However, despite their design
being less advanced,
despite their technical design is still in progress the Faraday cups were studied in greater detail as they will arguably
provide the most accurate detection of the captured e+ charge.
Thus we performed a preliminary error study based on simulations, envisaging reasonably high quality measurements of
the Faraday cups, specially at RF working points of interest.

ACKNOWLEDGEMENTThis work was done under the auspices of the CHART
Collaboration (Swiss Accelerator Research and Technology,
http://www.chart.ch).

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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 - 3IBIC2023, Saskatoon, Canada
JACoW Publishing
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