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NON-INVASIVE PROFILERS FOR THE COLD PART OF
ESS ACCELERATOR
J. Marroncle, P. Abbon, F. Belloni, F. Bénédetti*, B. Bolzon, N. Chauvin, D. Chirpaz-Cerbat,
M. Combet, M. Desmons, Y. Gauthier, T. Hamelin, C. Lahonde, P. Legou, O. Leseigneur,
Y. Mariette, JP Mols, V. Nadot, M. Oublaid, G. Perreu, F. Popieul, B. Pottin, Y. Sauce, L. Scola,
F. Senée, J. Schwindling, G. Tauzin, O. Tuske, S. Tzvetkov, CEA-Saclay, Gif-sur-Yvette, France
I. Dolenc Kittelmann, A. Gevorgyan, H. Kocevar, R. Tarkeshian, C. Thomas, ESS Lund, Sweden
Abstract
Several Non-invasive Profile Monitors are being installed along the accelerator to support the commissioning,tuning and operation of the powerful proton based ESS linear accelerator. In the low energy parts of the ESS linac(3.6 MeV to 90 MeV), the residual gas pressure is highenough to measure the transverse beam profile by using
fluorescence induced by the beam on the gas molecules.
However, in the ESS linac sections above 90 MeV, protons
are accelerated by superconductive cavities working at cryogenic temperatures and high vacuum. Therefore, the signal based on the fluorescence process is too weak, while
ionization can counteract this drawback.
We have provided five IPM (Ionization Profile Monitors) pairs for energies ranging from 100 to 600 MeV. The
design of such monitors is challenging due to weak signal(as a result of high proton energy and low pressure<10-9 mbar), tight space constraints inside the vacuum
chamber, space charge effect, ISO-5 cleanliness requirement, and electrode polarization at ±15 kV.
This publication will detail the development we followed to fulfil the ESS requirements.

section. These transverse beam profile monitors will cover
proton energies ranging between 90 MeV and 600 MeV
and are conceived to deliver one profile/pulse at a residual
gas pressure of 10-9 mbar with an uncertainty on the beamwidth of less than 10% of its dimension (see Refs. [2] and[3] for a profiler detailed overview).
At such high proton energies and low residual gas pressure, the NPMs located on the superconductive beam line
are based on the ionization of the residual gas. In the warm
sections, transverse beam profiles are measured by using
the fluorescence of the gas molecules induced by the beam.

ESS IPM DESCRIPTION
Two parts can be considered for describing an IPM(Fig. 1): the inner, situated inside the vacuum chamber, andthe outer part. The beam traverses the cage approximately
at its center.

INTRODUCTION
The European Spallation Source (ESS) [1], presently under construction at Lund (Sweden), will consist of a 537 mlong linear proton accelerator delivering a 2 GeV proton
beam with a 5 MW power to a tungsten target, equippedwith a highly optimized neutron moderator capable ofproviding a bi-spectral (thermal and cold) neutron beam to
42 beam ports, 22 of which are followed by a flight pathleading to faraway measuring stations.
A perfect knowledge of the proton beam is critical for
maximizing the number of protons on target and minimizing beam losses. Transverse beam profile monitors are
therefore essential to support the tuning of the beam forgood operation of the facility, both during its commissioning and in everyday operation.
Due to the high beam power, all beam profilers to be
used at nominal operating conditions will be non-invasive
type. In the frame of the in-kind contribution agreement
signed with ESS, CEA has delivered to the European Spallation Source five Non-invasive Profile Monitors (NPMs)
to be installed in the Cold Linac section: one in the Spoke
section, 3 in the β-medium section, and one in the β-high*Figure 1: An ESS IPM.
Then, the ions and electrons, by-products of beam-residual gas interactions, drift under the influence of the electricfield applied between both electrodes through a slit for ions
before collection onto an MCP (Micro-Channel Plate). The
MCP is equipped with a phosphorescent screen, allowing
electric signal conversion to an optical one. To enhance theelectric field uniformity, degraders positioned perpendicular to the electrodes are designed.

Presently at LIPAc Rokkasho

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On the outer part, the read-out is done with an opticalsystem consisting of a lens and a CMOS camera. The HVconnectors are used for the MCP and the cage polarization,linked with copper wires (1 mm). The control of the camera
daq and the HV system is fully implemented in EPICS.

FEASIBILITY STUDYWhen the project was initiated in mid-2016, several
challenges were encountered that necessitated preliminary
studies to assess the feasibility of such IPMs. These challenges can be summarized as follows:• Ionization signal strength.
• Limited space inside the vacuum chamber.
• The space charge effect.
• Background particles that may hide or distort the profile, but were not inquired.
• ISO-5 cleanliness.
The following sections will be devoted to these topics.

Ionization Signal Strength
The nominal beam conditions for IPMs are low pressure,
likely below 10-9 mbar, with a residual gas composition ofH2 (79%), CO (10%), CO2 (10%) and N2 (1% in mass),
62.5 mA beam intensity, and a beam width σX,Y = 2 mm.
During beam commissioning, the intensity may decrease to
1 mA, which will require an excess of read-out sensitivity.
Bethe’s relation ([4] original) gives the mean energy
losses by a charged particle per unit of media length; thus,
knowing the average energy for producing ion/e pairs, we
deduced their numbers in Table 1.
Table 1: e/ion Pairs Produced at Various Beam Energies
E (MeV)e/ion (pairs/cm)

97
1.0 105280
4.9 104628
3.4 104

The electrode slit allowing the ions to pass toward the MCP
is 2 cm large.

IPM Vacuum Chamber
The vacuum chamber, hosting a pair of Wire Scanners
and a pair of IPMs, appears quite crowded, as illustrated inFig. 2. The CF200 flanges (yellow) highlight their overlap.
To address this issue, an adjustment is made by shifting thecenter of the IPM-X cage by 36 mm relative to the flange,
enabling its insertion into the chamber.

Figure 2: The Linear Warm Unit (LWU) comprises a vacuum chamber that includes 2 IPMs and 2 Wire Scanners.
In such a constrained space, two 100 mm cubic IPM
cages, along with conductive discs, are securely attachedto the chamber, perpendicular to the beam direction, and

03 Transverse Profile and Emittance Monitors

located 33 mm upstream and downstream from both IPM
edges. The diameter of the hole in each disc is 100 mm,
similar to that of the beam pipe. The purpose of these components is to ensure and improve the uniformity of the electric field inside the cage by grounding them as much as
possible. In Fig. 3, a simulation conducted using COMSOL
software [5] shows the original Gaussian profile achievedby a perfect field (green) and the reconstructed profile obtained by tracking (blue) ions into a realistic electric fieldtaking into account the discs, which are in good agreement.

Figure 3: Gaussian profiles reconstructed by tracking forthe original (green) and for the realistic profile (blue).

Space Charge Effect
One of the ESS requirements is to measure the beamwidth with a maximum total error in the RMS beam extension of less than ±10%. This becomes difficult to achieve
in ESS conditions due to the space charge effect on the
beam profile measurement.
The effects of space charge are twofold: they affect the
charged particle beam and any other charge in its proximity. Our focus is on the later.
A charge generated at rest between two parallel plates
kept at different voltages drifts towards the electrodes travelling parallel to the electric field lines. In an ideal case ofa perfectly uniform electric field, the point where thecharge meets the electrode will simply be the projection of
its initial position on the electrode. In IPMs, charges are
created via gas ionization, and the beam profile is reconstructed this way. However, the presence of a charged particle beam, necessary to create ionization charges, induces
an electromagnetic field that modifies the trajectories ofthe electrons and the ionized gas molecules and thus introduces a shift between the point where they should have ideally met the electrode and the point where they reach it.
The measured beam profile, therefore, will differ from thereal one by an amount that depends on the beam intensity,
the beam size, the beam energy, and the strength of theelectric field applied between the electrodes.
Therefore, a simulation code was developed at CEA Saclay [6]. The momenta distributions for electrons are calculated using Garfield++ [7] while for ions the assumption𝑝 ⃗𝑝 ⃗ was applied.
Calculations presented in Fig. 4 are performed for both
ionization by-products, electrons and ions, versus the injected beam size into an IPM with a 300 kV/m electricfield.
We observe that at ESS, the precision is consistently exceeds the required level for electrons. For ions, even at
90 MeV and a beam size larger than 2 mm, |∆ | is below
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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

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5%. The influence of the beam pulse diminish at higher energies due to pulse particle velocities, which reduce theirimpact on the scattered particles. As a result, IPMs mustmeet the requirements for beam width measurement, which
can be achieved by establishing a high electric field insidetheir cages, around 300 kV/m.

Figure 4: Space charge effect on the measured profiles forelectrons and H2+ ions versus injected beam size. The IPM
electric field is 300 kV/m.

ISO-5 Cleanliness
Due to their proximity to the superconductive cavities,
IPMs must be delivered at ESS with a high cleanlinesslevel, namely ISO-5, to prevent dust contamination andlimit field emission. This implies that their designs must
integrate compliant materials and that the test bench must
be installed inside a laminar air flux.
The deployed logistics are pretty heavy, as listed below:• Washing and rinsing all components in an ultrasonicbath,• Transporting them in a basin filled with dezionizedwater to an ISO-7 clean room to prevent dust deposition,• Moving them into an ISO-5 room for drying. Oncecompleted, the integration of an IPM pair can start. An
ionization gun with pure nitrogen gas is used for detaching particles, and a particle counter is used tocheck the ISO-5 level. All elements were mounted except the MCP and their copper wire connections.
• Finally, both IPMs are inserted into their vacuum
chambers and wrapped inside a double ISO-5 plastic
bag.
• The 5 IPM pairs follow these procedures and arestored in a laboratory.
• Then, we started integrating one pair, followed by another, into the test bench inside a laminar air flux.
Baking is necessary as soon as the MCP comes in contact with air. Several mounting, dismounting are necessary due to the narrow space devoted to IPMs,
which are finally characterized.
• The IPM survey and insertion inside its vacuum
chamber are performed in a laboratory to pump andensure that the IPM can travel in a static vacuum.

Feasibility Conclusion
The identified challenges have been largely addressed,with only a few minor difficulties encountered. Following
a thorough review, the design of prototypes started. In thesubsequent sections, we present some results from the prototype testing using a 3 MeV proton beam at the IPHI accelerator located at CEA Saclay [8].

PROTOTYPE TESTS AT IPHI
Three types of IPM were designed: A) equipped with an
MCP featuring a phosphorescent screen and an opticalread-out (lens + CMOS); B) incorporating conductive
strips engraved on an electrode with the read-out of the
strip current; C) similar to B) but with the addition of asimple MCP set before the strip plate for amplifying thestrip current.
The optimal MCP can be polarized in asymmetric mode,
one grounded electrode and the other at the HV, or in symmetric mode, meaning electrode polarization at ±HV.
We also have solid scintillators for comparison. All these
instruments were inserted into a large vacuum chamber,
moved in 2018 to IPHI, delivering a proton beam at 3 MeV
working between 0.7 mA to 40 mA in pulse mode (0.1 to1 ms) at 1 Hz during our tests.

Figure 5: Strip and CMOS profile comparison (a). Profile
measured for every pulse with MCP amplifying current (b).
First conclusion: A more sensitive integration-electronic
for sampling the strip current is needed. However, it works
very well when the physical signal is amplified by a simple
MCP, which is quite similar to the optical read-out profile,as shown in Fig. 5a.
Second conclusion: We have opted for the optical readout system due to the potential issues arising from the connections inside the beam pipe associated with the strip option, which may be challenging to resolve. Additionally,
the strip option necessitates the use of an MCP, introducing
a second fragile element in the system.

Figure 6: Correlations between IPM and BPM.
Figure 5b shows the ability to measure the beam profile forevery pulse using the IPM equipped with an MCP amplifierof the strip currents, similar to the optical IPM.
During the experiment, we noticed a “slow” shift of thebeam profile, which came back swiftly to the same position
after a while. Initially, some electronic charge loading anddischarging were suspected. However, a strong correlation
between IPM and beam position monitor (BPM) favoured
an objective observation of beam position shift, as shown

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in Fig. 6, with a small disagreement due to monitor calibrations.
A few other interesting results were obtained, such as:• Beam width convergence, refers to the electric field
threshold above which the beam width stays constant.
The optical IPM achieved this at around 100 kV/m fora 10 mA beam intensity and 125 kV/m at 30 mA.
• The beam size variation was studied with an IPM
when the perpendicular electric field of a second IPM
was varied from 0 to 200 kV/m. The beam size increases by less than 75 µm. The same study, but forthe beam position, shows a displacement of around
0.4 mm. For both studies, there was no correction of
the beam deviation induced by the upstream IPM in
the downstream one.
• The uniformity of the electric field was assessed in thecenter of the IPM by sweeping the beam inside thecage. Results are consistent with expectations. To illustrate, a thin conductive 100 µm weft mesh wasplaced above the slit to enhance the electric field. In
Fig. 7, a CMOS image captures the fine grid weft,confirming the uniformity of the electric field.

Figure 7: Beam signal observed through the mesh.
The main conclusion of this test is the extrapolation of
IPHI data to ESS experimental conditions. For doing this,we requested a stable low beam intensity of 0.7 mA by
50 µs pulses (1 Hz) with 4 10-8 mbar residual gas pressure.
Taking all these conditions, the cross-section ratios and thegas composition, results in a signal higher at 97 MeV andat 1.6 GeV, but 0.9 times lower at 2 GeV compared to ESS.
Thus, we decided to modify the optical IPM mainly by:• Taking a double MCP with a P43 phosphorescentscreen for increasing the gain and their lifetimes• Polarizing the IPM in symmetrical mode, -15 kV and+15 kV which improves the electric field uniformity• Modifying the mechanics to give easier access to the
MCP (weakest element) and having the same pieces
for X and Y thanks to a rotating trick.
The final IPM versions take into account these remarks.

IPM INTEGRATION AND TESTThe first phase of ISO-5 integration is time consuming,
mainly due to achieving ISO-5 standards with Macor materials. During the second phase of integration finalizationand HV tests, several issues arose, with the most notableones including:• Correctly crawling the connection cables into the narrow chamber to set them at reasonable distances fromgrounded items. Several short circuits were induced,
implying the need to break the vacuum, fix wire position, and bake necessitating at least one week.
• For minimizing matter, MCPs are screwed on 3 pillars, which must be tightened by fingers, but this was

not mentioned by the provider. Therefore, a few ofthem were broken, particularly after baking. Finally,
the solution was found by releasing slacks at screw
level (Fig. 8). A small spring was added on each screw(Fig. 8(b)) to give longitudinal slack, while screws
were filed on the top thread part for releasing transversal slack (Fig. 8(c)).

Figure 8: MCP fixed on 3 pillars (a), spring for longitudinalslack (b), filed thread for transverse slack (c).
To check the MCP gain, we used a UV lamp, which isfixed on the opposite side of the IPM vacuum chamber. UVphotons have to cross a quartz viewport, and a small slit
centred in the +15 kV electrode, the second slit (-15 kV)
before reaching the MCP. Such a result is shown in Fig. 9.
Our acceptance criteria was IPM is validated when it isable to sustain ±15 kV with a ΔMCPIn-Out ≳ 1.3 kV for
more than 24 hours. This means that MCP is polarized at 16 kV for In, -14.7 kV for Out, and -10.7 kV for the Screen.

Figure 9: MCP gain measured with a UV lamp.
To complete the IPM processing, it is surveyed with a
3D Faro arm before being inserted into a vacuum chamber.
Then, it is pumped once again at around 10-7 mbar, readyto be sent to ESS Lund under static vacuum. Finally, the10 IPMs were delivered in pairs, the latest in August 2023.

CONCLUSION
The processes involved in the design, assembly, testing,and validation of the 10 IPMs for the cold section of the
ESS accelerator have been succinctly described.
The challenge of operating the IPMs in a symmetric polarization mode at ±15 kV was encountered. The read-outsof the IPMs are facilitated by a CMOS camera utilizing an
MCP equipped with a phosphorescent screen. One of themost demanding requirements was the achievement of
ISO-5 cleanliness quality, which necessitated a significant
logistical effort complicated by the limited space within thevacuum chamber.
All IPMs have been delivered to ESS Lund and are currently undergoing testing or installation stages.
The authors wanted to thank Stéphane Berry, Pierre Bosland and Catherine Madec, who kindly taught, advised andhelped us achieve ISO-5.
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