A dielectric pickup for short bunches
Eugenio Senes, V. Bencini, C. Davut, W. Farabolini, P. Karataev, P. Korysko,
M. Krupa, K. Lasocha, S. Liu, T. Lefevre, T. Manson, S. Mazzoni, B. Moser,
C. Pakuza, E. Poimenidou, A. Schloegelhofer, A. Topaloudis, B. Spear,
L. Verra, V. Verzilov, M. Wendt, G. Zevi Della Porta
International Beam Instrumentation Conference IBIC 2023, Saskatoon, Canada
11 September 2023

OutlineMotivation• The AWAKE experiment• Co-propagating beams diagnostics
Coherent Cherenkov diffraction radiation
Pickup design
Test results
Operational electronics
Future outlook

E. Senes | A dielectric pickup for short bunches

11 September 2023

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Motivation – AWAKE Experiment
AWAKE: Proton-driven plasma acceleration
SPECTROMETER

ELECTRON
LINAC

Electron BPMs

LASER
PULSE

DUMP
SPS
protons

Plasma cellProton BPMs

E. Senes | A dielectric pickup for short bunches

Accelerated Beamdiagnostics

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Motivation – AWAKE Experiment

1 INJECTORSThe AWAKE ingredients
SPECTROMETER

ELECTRON
LINAC

Electron BPMs

LASER
PULSE

Proton beam 48 nC, 250 ps-σ
Electron beam 600 pC, 4 ps-σ
DUMP

SPSprotons

Proton BPMs

E. Senes | A dielectric pickup for short bunches

Laserbeam
120 fs,Beam450 mJ
PlasmacellAccelerated
diagnostics
Rb vapour

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Motivation – AWAKE Experiment

2 PROTON MICROBUNCHING AND
ELECTRON ACCELERATION

The proton bunch is broken into a
train of microbunches via the self
modulation process in the plasma.

See AWAKE Collab., Phys. Rev. Lett. 122, 054802 (2019)
E. Senes | A dielectric pickup for short bunches

Captured electrons in the plasma
wakefields are accelerated.

See AWAKE Collab., Nature 561, 363-367 (2018)
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Motivation – Multiple beams

ELECTRON
LINAC

Electron BPMs

LASER
PULSE

SPSprotons

Proton BPMs
Commonbeamline

E. Senes | A dielectric pickup for short bunches

The trajectory control of thelaser, electron and proton
beam
SPECTROMETER
is essential to avoid instabilities
and study reproducibility !
DUMP
Due to the larger intensity, thePlasma
cell beam
Accelerated
Beamall theproton
saturates
diagnostics
other diagnostics
when present.
Need for an electron position
measurement in the commonbeamline.

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Coherent Radiation
For any radiation mechanism, we know that theradiation intensity spectrum isAt low frequencies, in the coherent region ofthe spectrum, one can assume the single
particle emission in quasi-phase, hence:Bunch
intensity

Bunch form
Singlefactorparticleterm (here Gaussian)E. Senes | A dielectric pickup for short bunches

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Coherent Radiation

Protons

Different beams feature different bunch
form factors. For the AWAKE case:!

Gaussian beams assumed here !
Real life is not that simple !

E. Senes | A dielectric pickup for short bunches

Electrons
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Coherent Radiation
DOMINANT PROTON AREA < 2 GHz
BPMs for p+ detect at 20 MHz!

Gaussian beams assumed here !
Real life is not that simple !

E. Senes | A dielectric pickup for short bunches

11 September 2023

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Coherent Radiation
DOMINANT PROTON AREA < 2 GHz
BPMs for p+ detect at 20 MHz
BPMs for e- detect at 404 MHz!

Gaussian beams assumed here !
Real life is not that simple !

E. Senes | A dielectric pickup for short bunches

11 September 2023

10Coherent Radiation
DOMINANT PROTON AREA < 2 GHz
BPMs for p+ detect at 20 MHz
BPMs for e- detect at 404 MHz
DOMINANT ELECTRON AREA > 2 GHz!

Gaussian beams assumed here !
Real life is not that simple !

E. Senes | A dielectric pickup for short bunches

11 September 2023

11Coherent Radiation
DOMINANT PROTON AREA < 2 GHz
BPMs for p+ detect at 20 MHz
BPMs for e- detect at 404 MHz
DOMINANT ELECTRON AREA > 2 GHz
Ideal BPMs for e- detect at 20-30 GHz!

Gaussian beams assumed here !
Real life is not that simple !

E. Senes | A dielectric pickup for short bunches

11 September 2023

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Coherent Radiation
DOMINANT PROTON AREA < 2 GHz
BPMs for p+ detect at 20 MHz
BPMs for e- detect at 404 MHz
DOMINANT ELECTRON AREA > 2 GHz
Ideal BPMs for e- detect at 20-30 GHz
PLASMA FREQUENCY > 100 GHz!

Gaussian beams assumed here !
Real life is not that simple !

E. Senes | A dielectric pickup for short bunches

11 September 2023

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p+ average
Waterbag fit
Gaussian fit

Non-Gaussian Beams
The Gaussianity of the beam profile determines theextension at high frequency of the beam spectrum.
For the AWAKE case, a low-tailed distribution sucha waterbag fits better the p+ beam profile measured
with a streak camera analysing the OTR light.
This has a longer extension in the frequency spectrum.
To select the detection frequency for the electrons,some margin must be accounted for toaccommodate proton non-Gaussianity.

ChDR
BPM

WATERBAG FUNCTION

E. Senes | A dielectric pickup for short bunches

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E. Senes | A dielectric pickup for short bunches

11 September 2023

15Cherenkov Diffraction radiation
Cherenkov Diffraction Radiation is a type ofpolarization radiation induced in dielectrics
when a charged particle passes in proximity tothe surface
Very well-defined properties, including the radiation
front direction at the Cherenkov angle θCh

ChDRemission

εr (ω)

See: M. V. Shevelev and A. S. Konkov, JETP 118, 501-511 (2014)

Recent interest to realise non-intercepting
diagnostics

θCh
hBeam

See: A. Curcio et al., Phys. Rev. Accel. Beams 23 (2020) 022802
R. Kieffer et al., Phys. Rev. Lett. 121 (2018) 0548802
T. Lefevre et al., Cherenkov Diffraction Radiation as a tool for beamdiagnostics, Conf. Proc. IBIC 2019 (2019), Malmo, Sweden

E. Senes | A dielectric pickup for short bunches

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ChDR position dependence
The position dependence was tested with alarge PTFE radiator in air at the CLEAR facility
to compare different theoretical models.

PTFEtarget

To detection

e- beamback side

The radiation was coupled with a horn antenna,and measured with a Schottky diode at 30 GHz
with 300 MHz bandwidth.
Details were presented in IPAC’22
See: K. Lasocha et al., Experimental Verification of Several Theoretical
Models for ChDR Description, Conf. Proc. IPAC’22 (2022), Bangkok, Thailand

For the latest results with direct E-fieldmeasurement with EO techniques checkout the poster TUP022 !
E. Senes | A dielectric pickup for short bunches

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Pickup design – Mechanical description
Cylindrical Radiators are inserted in avacuum chamber.
A brazed collar assure them to be vacuum tight.
The dielectric bar is left protruding outside, toease the transition into waveguides.

E. Senes | A dielectric pickup for short bunches

11 September 2023

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Pickup design – Radiation propagation
Although broadband radiation is generated at thesurface, the radiator dimensions determine theoutput radiation properties.
For cylindrical radiators surrounded by metallic
walls, the base mode cutoff frequency isRadiatorradiusMaterial
selection

Below cutoff, the electric field propagation isexponentially damped.
E. Senes | A dielectric pickup for short bunches

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Pickup design – Radiation propagation simulation
Analytical formula

CST simulationCST Studio

E. Senes | A dielectric pickup for short bunches

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Pickup design – Material selection
A large choice of dielectric materials can beused as radiators, such as PTFE, PEEK,
Fused silica, Alumina, …

PTFE
εr = 2.1

PEEKεr = 3.2

Al2O3
SiO2εr = 3.8 εr = 9.6

As εr increases:• Larger Cherenkov angles• The low cutoff frequency decreases

BeamComparison of different dielectric materials
for cylindrical ChDR radiators. The radiatorradius is scaled to maintain the same
cutoff frequency.

E. Senes | A dielectric pickup for short bunches

11 September 2023

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2019 – PTFE Proof of Principle

2020/21 – In-air alumina prototype

2022 – Vacuum version

E. Senes | A dielectric pickup for short bunches

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Pickup design – Real case
Beampipe diameter 60 mm
Alumina 99.5% pure (εr = 9.4)
Cylindrical rods 90 mm x Ø 6 mm
Metallic coating ~10 µm thickLow cutoff frequency choice
fc,TE11 = 9.7 GHz

E. Senes | A dielectric pickup for short bunches

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Test setup
Beam-based test with 100-600pC single-bunch electron.
Multi-shot measurement, using
the remote-controlled attenuator
to keep a constant diode input
power and limit non-linear
response effects.
Data recorded with a 6 GHz
scope in the alcove ~20 m away.

E. Senes | A dielectric pickup for short bunches

11 September 2023

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Test results (1)Sensitivity to electrons only
Similar response to high and low
charge with AWAKE parameters

Linear response in a 5 mm range

Limited test time in AWAKE,
preliminary test for the operational
electronics

E. Senes | A dielectric pickup for short bunches

11 September 2023

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Test results (2)Proton signal rejection
Proton-only signals below noiselevel for 1e11 protons per bunch

No signal shape change forsimultaneous protons + electrons
Some signal with 3e11 protons perbunch, however depressed withrespect to the electrons. Under
investigation.
E. Senes | A dielectric pickup for short bunches

11 September 2023

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Operational electronics
A collaboration with TRIUMF was started to realise
high-frequency BPM electronics.
The elected operation frequency is 30.5 GHz witha bandwidth of 1 GHz.
The system was realised on the model of theexisting electron BPMs, to ease integration.
First module installed for tests in 2022.

E. Senes | A dielectric pickup for short bunches

11 September 2023

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Operational electronics

BEAM

Initial tests in 2022 currently ongoing.

ChDR
BPM
Pickup

10 MHzREF

BP FILTER
f0 30.5 GHz
BW 1 GHzTUNNEL SIDE

WR28 WAVEGUIDECOAXIAL CABLEIFRFLO

28 GHzGenerator

Stripline
e- BPM

2.5GHz28 GHzDigitiser
ALCOVE
E. Senes | A dielectric pickup for short bunches

AnalogFront-end

ChDR
BPM

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Future outlook
ChDR pickups are a promising but young technology !
PROs:Large signal output. Can measurecharges as low as few pC.

See: C. Pakuza et al., Electron Beam studies on a Beam Position Monitor basedon Cherenkov Diffraction radiation, Conf. Proc. IPAC’23 (2023), Venice, Italy

E. Senes | A dielectric pickup for short bunches

11 September 2023

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Future outlook
ChDR pickups are a promising but young technology !
PROs:Large signal output. Can measurecharges as low as few pC.
Very large bandwidth.
High cutoff frequency defined
only by the bunch length.
Potential for optical detection
with short bunches.
Low cutoff frequency defined byconstruction.
Allow for beam distinction based onfrequency discrimination.
E. Senes | A dielectric pickup for short bunches

Example for driver + witness bunch with beam parametersfrom Pompili et al., Nature Vol 605, 659 (2022)

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Future outlook
ChDR pickups are a promising but young technology !
PROs:

CONs:

Large signal output. Can measurecharges as low as few pC.
Very large bandwidth.

Difficult to test in the GHz regime.
Pickup quality control and pairing.
No lab setup to find the electrical centre.

High cutoff frequency defined
only by the bunch length.
Potential for optical detection
with short bunches.
Low cutoff frequency defined byconstruction.
Allow for beam distinction based on
frequency discrimination.
E. Senes | A dielectric pickup for short bunches

11 September 202336

Future outlook
ChDR pickups are a promising but young technology !
PROs:

CONs:

Large signal output. Can measurecharges as low as few pC.

Difficult to test in the GHz regime.

Very large bandwidth.

Complicated RF front-end design.

High cutoff frequency defined
only by the bunch length.
Potential for optical detection
with short bunches.
Low cutoff frequency defined byconstruction.

Need careful front-end design withwaveguide hardware and broadband
emission.
A broadband transition between
radiator and standard waveguidemust be engineered.

Allow for beam distinction based on
frequency discrimination.
E. Senes | A dielectric pickup for short bunches

11 September 2023

37

Future outlook
ChDR pickups are a promising but young technology !
PROs:

CONs:

Large signal output. Can measurecharges as low as few pC.

Difficult to test in the GHz regime.

Very large bandwidth.

Complicated RF front-end design.

High cutoff frequency defined
only by the bunch length.
Potential for optical detection
with short bunches.
Low cutoff frequency defined byconstruction.

Impedance control: so far tests withsingle bunch single pass beamlines.
To be assessed for rings.

Allow for beam distinction based on
frequency discrimination.
E. Senes | A dielectric pickup for short bunches

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Conclusions
Cherenkov Diffraction Radiation is a useful
tool for building non-interceptive beamdiagnostics, and it is increasingly gaining
relevance in the community in recent years.
Dielectric pickups are a promising young
technology, as they can achieve a larger
bandwidth than other pickup types, even withlarge beampipe apertures.
Potential impact for any accelerator withshort bunches, including new acceleration
technologies (PWFA, LWFA, xFELs, …).

E. Senes | A dielectric pickup for short bunches

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Thanks for your attentionAcknowledgements: V. Bencini, P. Bestmann, P. Burrows, A. Cherif, N. Chritin, V. Clerc,
C. Davut, W. Farabolini, F. Galleazzi, E. Gschwendtner, E. Guran, P. Karataev, J. Kortesmaa,
P. Korysko, M. Krupa, K. Lasocha, S. Liu, T. Lefevre, T. Manson, S. Mazzoni, B. Moser,C. Pakuza, E. Poimenidou, P. Muggli, A. Pardons, C. Pasquino, C. Saury, A. Schloegelhofer,
P. Schwartz, E. Senes, A. Topaloudis, B. Spear, L. Verra, F. M. Velotti, C. Vendeuvre,V. Verzilov, M. Wendt, B. Woolley, G. Zevi Della Porta

Questions :eugenio.senes@cern.ch