WIR SCHAFFEN WISSEN – HEUTE FÜR MORGEN

Nicole Hiller :: Group Leader for SwissFEL Operation :: Paul Scherrer Institute

Overview of Beam Diagnostics for Different Accelerator
Types: Longitudinal Profile
IBIC’23 - TU2T01

Some Questions to the Audience
1. Who works at an electron accelerator?
2. Who works at a proton or ion accelerator?
3. Who works with longitudinal diagnostics?
4. How many of would need/like better longitudinal diagnostics?
5. How big is the panic level at your accelerator if certain
longitudinal diagnostics are not available?

2

A few disclaimers…
• My background is in electron beam diagnostics at storage rings and FELs
• At PSI we have one shared control room for 4 accelerators which we operate with one team
• Proscan - cyclotron for proton therapy & irradiation• HIPA - 2 cylclotrons (generate muons, pions, (ultracold) neutrons and also isotopes)• SLS - Storage ring light source → will now go into dark time for the upgrade (SLS 2.0)
• SwissFEL - soft- & hard-X-ray FEL• While I mainly work on SwissFEL, I also get to operate the other machines• Our proton accelerators at PSI have CW beams, so I have no real first-hand experience withlongitudinal diagnostics for proton beams… but there were measurements done during thecommissioning (HIPA is turning 50 next year) → our expert R. Dölling is here
• I do spend a lot of time in the control room using diagnostic devices• to set up the machine → standard measurements• to maintain a stable operation → feedbacks and empiric tuningsa
Sorry, this look• during machine development shifts → characterise as much as possible
bit like a license
agreement…
• My tutorial is most likely a bit biased towards FELs and storage rings.
3

E

Things to consider…

t
What do you
want tomeasure?
What type ofbeam do you
have?
Measurement
types

electronarrival time /beam phasefilling
patterncurrent /chargeprotononlineofflineabsolute

relative

directindirectnoninvasive

invasivesingleshot

averagingion

beam energy

bunch length

temporalprofilelong. phasespace(bunch)chargerepetition
rate

temporalstructure

How can I calibrate?

In which location?
Do I need an expert to measure?
arge and time resolution)
(ch
?
ed
ne
I
do
ge
ran
c
mi
na
dy
at
Wh
Do I need every shot?
Online measurement?
DAQ &controls?

Do I want to run a feedback on this?
Commercial option?
What is my budget?
Automated readout/procedure?
How much space do I need?
Reliability & robustness?
4

Who wants what?

protonelectron

ion

Linac

long. phasespace

Synchrotron
temporalprofileA very roughoverview!
ably
There are probns
some exceptiokewhich I don’t ta
into account!

LWA/PWA
FEL

bunch length

Storage Ring

arrival time /beam phase

Cyclotron
filling
pattern

1 fs

1 ps

1 ns

5

What will this tutorial cover?
• Some basics• Feschenko monitors, fast Faraday cups, laser wire measurements, transverse
deflectors, passive streakers, spectral diagnostics, fast detectors, streak cameras,
electro optical methods• What am I leaving out? - A lot, unfortunately!
• Most of the mathematical descriptions & details on the electronics → I’ll give yousome great references to read up on them.
• Phase probes, filling pattern monitors, beam arrival monitors (BAMs), wall current
monitors, zero phase RF, fluctuation analysis to retrieve the bunch length, scanningspectrometers, diagnostics for laser wakefield & plasma wakefield accelerators• Probably a lot of proton / ion diagnostics, I have no clue about → hoping to learn
more about this here6

Some Basics• The electric field of a relativistic bunch has approximately* the same temporal profileas the charge distribution → if you want to detect non-relativistic bunches, you need
another method
Coulombfield
at rest✓⇠Lorentz boost

1

relativistic ➜“pancake”• Direct time-domain detection is typically limited by the readout of the electronics→ can push this to about 100 GHz bandwidth → measuring shorther signals definitely
needs a different approach• There are several ways of transforming longitudinal characteristics into transverse
ones → longitudinal diagnostics frequently exploit them (more about this later)* If you want to measure sub 50 fs and don’t have a very high ɣ you will need to think about this!

7

More Basics - Different Types of Radiation

me references
I will give you so
ad up moreat the end to ree different
th
t
u
o
b
a
s
il
ta
e
d
on !
types of radiati• For longitudinal diagnostics (esp. electrons) quite often,
radiation generated by the beam is used → non-invasive• This radiation can also make your life difficult…
• The radiation typically has the same temporal profile as the charge distribution• Synchrotron radiation (SR)• is emitted if charged particles are deflected in a magnetic field (bending magnet,
wiggler, undulator…)• ranging from IR to X-rays depending on ɣ, bending field → not really usable for proton/
ion beams unless you have really high energies (e. g. LHC)
• Cherenkov radiation → emitted if charged particles travel faster than the speed of lightin a medium (e. g. in fused silica)• Diffraction radiation → can be generated rather easily by putting a holed foil in the
beam pipe and then coupling the radiation out (non-invasive to the beam)• Transition radiation (often called OTR as it is usually in the “optical” range)
8Some more basics

Brillance / Photons/s/mrad^2/0.1%BW

10

tic dependence
bunch charge

dependent onbunch length

10
10
10
10
10
10
10

17

quadratic dependence
on bunch charge

1615dependent onbunch length1413• The spectrum of coherent synchrotron radiation (CSR) is a Fourier transform of thecharge distribution → shorter bunches → wider spectrum.
Simulation Parameters:
precisely,
you can retrieve the pulse shape (some• If you can measure the spectrum
Electron Energy:
1.3 GeVbunch length: 2 ps relation
assumptions are required) RMS→ Kramers-Kronig
8
Charge: 16 pC (10 Electrons)• You will have to take the transfer function of your beam transport and detection
system into consideration• The same also goes for coherent transition and diffraction radiation (CTR and CDR)

dependent onbunch substructure

dependent on
particle energy

linear dependence
on bunch charge

12

The Spectrum can be computedfor the different radiation types
and beam properties.

11
10

10
10

Simulation Parameters:
Electron Energy: 1.3 GeV
RMS bunch length: 2 ps
8Charge: 16 pC (10 Electrons)

9
8

Example for (C)SR at 10-1
the KARA storage ringdependent on

0

1

2

3

4

5

10
10
10
10
10
10
Frequency of emitted Synchrotron Radiation / THz

6

10

But beware, the more complex
your longitudinal beam shape, thetrickier the reconstruction of thebunch shape from a measured
spectrum…
9

What concepts do you need to understand?
Detect directly in thetime domain

Highbandwidth
pickupsFast radiation
detectors &readout

Exploit link between thecoherent spectrum andthe bunch profile

Spectrometers

Compression
monitors

Use the electric field to“modulate”

Electro-opticaltechniques

Fast Faraday
cups

Passivestreakers

Turn longitudinal
characteristics intotransverse ones

RF deflectors

Generate secondary
particles to be analysed

Laser wire

Streak Camera
Feschenko
monitor10

What to do with non-relativistic ion beams?
• Fast Faraday Cup → invasive• Feschenko Monitor → but need to scan
• Time of flight (ToF) detection → insert a foil for scattering and use particle detectors
with a fast rise time (e. g. synthetic diamond)• Use coherent transition radiation in the GHz range → can be an option if the pulses
are “short” enough (< 1 ns) [1]visit S. Klapro
TUP011 to th at
find out
how to cou
ple theradiation out[1] R. Singh doi: 10.1103/PhysRevAccelBeams.25.032801
11

t thereg.

Fast Faraday Cup

iasing• Cup is inserted into beam directly
e FFC• Works up to about 100 MeV/u
as pernd the• Secondary particles are a problem
shown→ DC bias voltage on central conductor
iasing11th Int. Beam Instrum. Conf.
IBIC2022, Kraków, Po
nd 3 nsISBN:
978-3-95450-241-7
ISSN: 2673-5350
helps
pplicaion ofectrons
Image from P. Forck (JUAS 2023, see end of talk for link)
Figure 6.17: Head of a coaxial Faraday cup. The grid for shielding of the advanced field and suppresits thesion of secondary electrons is shown in the middle. The collector including the ceramic cooling watern emisfeed-through is seen on the right-hand side.
hole ascity ofe blindThere
wheneragedecond
ion asFigure 6: The longitudinal charge profile measurement averFigure 6.18: Observed bunch signal with 2 ns/div from a coaxial Faraday cup. Left: withoutaged
for a macro-pulse with application of various DC bias
R. Singhsuppresdoi:10.18429/JACoW-IBIC2022-TUP25
beam
sor voltage on the grid, right: -500 Volts on the grid.
voltages on the radially coupled FFC central conductor.

achine12

Figure 8: T
aged for a m
voltages on

Feschenko Bunch Shape Monitor (BSM)• A wire (~-5 kV) is inserted into thebeam → generate secondary electrons
with same time structure as primarybeam
Vfocus• Electrons pass through a set of slits anddeflecting plates
the electrons• Only(cup,Detector
electron close to the RF zeromultiplier…)
crossing are transmitted to the detector
Vfocus(e. g. electron multiplier) → this allows
to measure at a defined bunch phase• Scan the deflector phasewire• Can also scan the wire to reconstruct at• Accelerated beam strikes a wire (at ~ ‐5kV)
different
locations• Produce secondary electrons, which same bunch
structuretransverseas
primary beam.
• Deflecting plates are biased to focus on• Electrons pass through a set of slits and RF deflecting plate.
2ndareslit• Only those electrons close to the RF zero crossing
Sketch from R. Scrivens CERN ABIWorkshop,
12/2008transmitted(and measured
in a cup/electron multiplier).
https://twiki.cern.ch/twiki/pub/SPL/Minutes19January2009/CAREWorkshopOnBSMs.pdf
Hence measure at a defined beam/bunch phase.

13Bunch Shape Monitor – Linac 2 – p+ @ 10MeV and 30MeVFeschenko
3D Bunch Shape Monitor– Linac Monitor
2 – p+ @ 50MeV• BSMs are widely used at proton
and ion linacs

ens, CERN, ABI Workshop, 12/2008

Image of a BMS monitor at CERN linac 2 from R. Scrivens CERN ABI Workshop, 12/2008
14

must maintain attribution to the author(s), title of the work, publisher, and DOI

ISBN: 978-3-95450-230-1

ISSN: 2673-5350

doi:10.18429/JACoW-IBIC2021-W

Comparison of Feschenko BSM and Fast Faraday Cup• Colleagues at GSI performed tests tobenchmark the performance of the
BSM and the FFC• At HELIAC Advanced Demonstrator
beamline (He1+ with 100 μA average
current and 1.4 MeV/u kinetic
energy)Figure 6: FFC is moved by ±2 mm and the profile
sured at each step.

[2] A. V. Shemyakin et al., “Experimental Study of
Dynamics in the PIP-II MEBT Prototype”, in Pro
R.
Singh
et
al.,
doi:10.18429/JACoW-IBIC2021-WEPP16ICFA Advanced Beam Dynamics Workshop on
Figure 5: Comparison of the normalized longitudinal pro15
Intensity and High-Brightness Hadron
Beams (Hlongitudinal profile of the measured ion beam.
The outcome of the photodetachment, denoted as thedensity of photodetached electrons npd , is directly proportional to the product between photon and particle beam
densities as

and T is the period of the ion bunch. For a demonstration of
the basic principle, we ignore the depletion of the ion beam
density due to photodetachment and further assume thatboth ion and laser beams have a Gaussian distribution in all
directions, i.e.,

Laser Wire with Virtual Slit at SNS

linacsH-• Laser neutralises some of the H- ionsLaser
→ detached electrons are deflectedElectronpulse
detectorand detectedH!
• Laser beam delay is scanned0!
He• Taking measurements at different
magnet currents creates a “virtualLensslit” and reduces
thePROFILE“effective
LONGITUDINALBUNCH
MEASUREMENT …
transverse
beam size”
LONGITUDINAL BUNCH PROFILE MEASUREMENT …
PHYS. REV. ACCEL. BEAMS 26, 042801 (2023)

non-invasive

averaging

Magnetin Fig. 11. Also plotted are the fitted curves based on theGaussian function. Obviously, both profiles do not have aperfect Gaussian distribution, which is quite normal forhigh-power small size beams. The horizontal profile in
Y.
−
−
L
iu
F
FIG. 2. Schematicaof
beam longitudinal bunch profile measurement. H beam propagates in the z direction andralaserdaywire–based
Cup AwHard
has a significant tail. The rms beam sizes weretainsetlk box shows an example of the distribution of the photodetached electronsparticular
the laserin the y direction. The
in theMbeam
O2I0propagates1
calculated
from the experimental data.
“
N
o
n
-i
nterceptive Be
laser-ion beam interaction region.
Prior to each longitudinal profile measurement, theamDiagnostics ina H- Linac Duridipole magnet for the electron detector was optimized tongOperations Usi
obtain a longitudinal profile with the maximum detection
ng Laser Comb
efficiency, and the corresponding magnet current I 0 wasand
042801-3
Virtual Slit”
used as the reference. The magnet current was then adjusted
so that a transition slope such as the one shown in Fig. 5
was observed. Figure 12 shows six longitudinal profiles
measured at different magnet currents. Each dot in the
figure represents an average from ten measurement sam16
FIG. 10. Laser wire measurement setup in the SNS superples. Clearly, all measured profiles were well fitted with!

Y. Liu et al., doi: 10.1103/PhysRevAccelBeams.26.042801~10 ps long
H- bunches

ion with sub-fs resolution. Furthermore, this feature allows the measurement of special beam props the slice emittance in di↵erent transverse planes. Figure 2 shows a schematic of a longitudinal
sed on the variable polarization of the TDS, the Polarizable X-band (PolariX) TDS.19, 24 In recent

Transverse Deflecting Cavities (TDCs)

Vs at the LCLS HXR and SXRlinac, FEL

electron

The famous XTCAVat LCLS

vides non-invasive, single-bunch snapshots of thee of the electron beam and is used for routine
ulator and electron beam parameters.

m of the undulator also allows reconstruction of the Xch can be streamed to the user experiments on a
20 Hz.

Image from P. Krejcik’s talk WE3D2 at FLS’23

deflecting
cavities/ structures(TDC/TDS)
have
been
usedTDS.
for longitudinal beam profile• Transverse
e 2. Schematicof a longitudinal
diagnostic
based
on the variable
polarizationof thePolariXn 8 times
stronger for
kick
than
theatoriginal
Mulitmeasurements
many
years
linacs S-band
PolariX TDSs were built in total by PSI and installed at various facilities. Currently, the two
4times
wavelength
twice
the→used
field
bunch
operateup to 12andGHz(X-band)highand• Nowadays
FLASH
II shorterand the one
at FLASHForward
areroutinelyin frequencies
operation whilethehigh
two gradients
installed give a good resolution
arecommissioning
as RFsoon youas thesystemavailable.
Thescreen
remaining
twoandwerespecial beam opticsTuning
Toforgeta good resolution
willRF
also
needwillanbeexcellent
setup(smallr ofready2• wasgainedwithpulse
compression
after YAG

he post-undulator diagnostic section in the Athos soft X-ray beamline at SwissFEL, with the aim ofbeam size when not streaking and 90 DEG phase advance between TDC and your screen
b-fs resolution for soft X-ray pulse profiles. In early 2022, the X-band power source for the TDS in
mpletedand P.commissioning
the wholesystem1258106-1
has begun.25 This contribution summarizes
Sketch:Craievich et al.ofProc.
of SPIETDS
Vol. 12581
17
achieved at PSI, focusing on the commissioning of the RF
system with the On
electron beam and theOff
On and lasingPolarix Collaboration - CERN, DESY & PSI

he middle plots display single-shot longitudinal phase space images images for lasing-o↵
wer plots showX-band
the average
FEL power
profile(in bold) and 10 single-shot reconstructions.
deflector
withvariablelse durations (rms values obtained from Gaussian fits) are 19.1 ± 1.4 fs.

polarisation (can streak in x and y)

Lasing on

Lasing off
Polarix installed at SwissFEL (post undulator in Athos)Observe the beam beam on a screen
inPicturea section
withperpendicular
the streaking
Figure 4.
of thetwodispersion
PolariX TDSs
installed in thetopost-undulator
diagnost
direction (e. g. the beam dump spectrometer)→ direct access to the long. phase space
3. SETUP OF THE MEASUREMENT SYSTEM→ even the FEL power profile
can beofextracted
thePS2energyspreadwith3,andThe phasing
the three whenphase comparing
shifters, PS1,
and PS3
in Fig.
was the first
without “lasing”
electron beam. This was carried out at low RF power from the klystron (300 kW), wh

P. Craievich
al. Proc.
of SPIE
Vol.beam
12581
1258106-1of the etbeam
centroidon thepositionmonitors (BPMs) placed downstream of th18

Polarix TDC - Measurement Results
How to do a time calibration scan: Vary the RF phase and measure how the beam shifts

Figure 12. Example of calibration and resolution measurement. Top: Single-shot streaked images for di↵erent phases at
the zero-crossing. Bottom left: single-shot unstreaked image. Bottom right: measured and fitted beam centroid variation
as a function of the beam phase.

P. Craievich et al. Proc. of SPIE Vol. 12581 1258106-1

19
the presence of jitter, we may exploit the jitter itself. As shown in Fig. 13 for an example at the spectrometerbration coefficient can be estimated with reasonable accuracy by means of a linear regressionof the beam centroid atPolarix
the screen andthe -RFphase jitter values. Results
TDC
Measurement

How to do a time calibration scan if
your RF phase is not very stable andyour beam wobbles off the screen if
you try and do a normal phase scan?
→ measure beam synchronously thecentroid position and the RF phaseand correlate it linearly

on in the presence of beam jitter. The calibration can be obtained from a linear regression between
m centroid at the screen and the RF phase jitter values.
P. Craievich et al. Proc. of SPIE Vol. 12581 1258106-1
20

Polarix TDC - Streaking in Different Directions
www.nature.com/scientificreports/

r deflecting
becausebreakdowns
the phase
limitedThis
the ispower from the klystron. This is
eakdowns
in voltage
the phaseshifters
limited theinpowerfrom shifters
the klystron.
se RF
conditioning
stillofhas
be performedon some of the polarization planes.
beperformed
on somethetopolarization
planes.

The main advantage of the variable polarisation is that you
can do slice emittance measurements in both planes.

The images with the red headings were done witha slightly lower RF power
P. Craievich et al. Proc. of SPIE Vol. 12581 1258106-1

Figure 3. Streaked beam at multiple angles. The figure shows a composit
visualized at 11FLFXTDS have been combined. Each image has been takephase shifter, as shown in Supplementary Movie 1. The profiles have been
taking as reference the center of charge of the distributions for a better vis
streaking.

B. Marchetti et al.
doi: 10.1038/s41598-021-82687-2

tion8.angle.
polarization.
Bottomure
ImagesTopon left:screenhorizontal
S1 as a function
angle. S1Top
horizontal
polarization.
BottomFigure of
8. polarization
Imageson right;
screenas aleft:functionof polarization
angle.
Top right;
left: horizontal polarization. Bottom right;k polarization
angles:integrated
deflecting
voltage
85 MW,
red a rotation
izontalpolarization
after
a rotationof
180 degree.
Darkpolarization
angles: ofintegrated
deflecting
voltage 85 MW,
redintegrated deflecting voltage 85 MW, redhorizontal
polarization
after
180 degree.
Dark polarization
angles:For 6 MW peak input power in the PolariX structure a deflection v
arization angles: 72.8 MV.
polarization angles: 72.8 MV.

3 Arrival time jitter and beamloading time jitter and beam loading
3.3 Arrival

which allows achieving sub-10 fs resolution for the highest values of beresolution has been achieved during longitudinal phase-space measuresioning of the prototype structure, and the results are going to be presen
article we will focus on the first experimental realizationof slice emitta
21

concerns the stabilization of the EEHG mode in order tproject, but was not part of the baseline design for financial turn this mode from an experimental demonstration into
reasons. The upgrade of the Athos line to enable laser-based work horse for regular user experiments. This will requirseeding of the FEL process was then realized during the both improvements to 3the seed laser systems as well as
initial user operation phase, in two stages. In a first step a feedback loop keeping constant the overlap between laser an
Lasing on seed laser, with central
Lasing off
wavelengths of 790 and 390 nm, and electron beam, based on suitable instrumentation monitorin
a 200 mm period magnetic modulator
wereinstalledin the had
Werecently
run at inSwissFEL
atThelowpreviously mentione• Left:the adifference
arrival time.
3375 of the so-called “HERO” project (named after
frameworkcleaningof the longitudinal
electron current
charge10ResonatingpC (200 pC
is standard)
to produce
very profile is als
the funding ERC project “Hidden andEntangled
an important prerequisite for the stable operation of EEHG
Order” [18]).
was alreadyshort
X-raysufficientpulses In the longer term we are considering moving the firs
3370 This part of the upgrade
to demonstrate the generation of attosecond pulse trains by bunch compressor further upstream in order to reduce thenergy-modulating the electron beamthroughoverlap with
Polarix
in action for enhanced SASE (ESASE)• Right:
the seed3365
laser and converting the energy modulation into
generating “attosecond X-ray pulse trains”
a density modulation in a chicaneoperation
following the→ESASE
scheme [19]. An alternative method making use of a strong
0
10
20
0
10
20
t (fs)t (fs)was also applied with success. The
undulator taper [20]FEL power profile
electronlongitudinal phase space after energy modulation,measured with the X-band rf deflector and shown in Fig. 4,“dead fish”
clearly exhibits the periodicity expected from the wavelengthof the seed laser. In a next step, mode locking between theattosecond pulses by applying delays to the electron beamthat match the laser wavelength was attempted, but no clean
verification was possible at this point, given our lack of phasesensitive diagnostics.

E (MeV)

3375

3370

3365

P (GW)

10

5

E (MeV)

Polarix TDC - making use of the sub-fs resolution

The second stage of the upgrade comprises a second seed0
1 incoupling2
laser withanothermodulator (also 200 mm pet (fs)riod), and a larger magnetic chicane, all necessary to put into
CourtesyofE.
Prat
etforal.a photon
paper
submitted
APL3. Spectral
Photonics
practiceecho-enabled
harmonic
generation(EEHG)[21].100 single-shot measured spectra for
FIG. 2. Time-resolved results
energyof 645eV. Top: to
FIG.
results. Top:22 (energy versu
Figure 4: Electron longitudinal phase space
single-shot image of the electron beam LPS for lasing-on (left) and
0

-2

-1

TDCs

onlineabsolute

single-shot

invasive• Good for• State of the art method• Easy access to the long. phasespace• Resolutions just below 1 fs can beachieved• Nowadays streaking in x and y ispossible → slice emittance in bothplanes• Non-invasive if used after theundulators at an FELMAX IVE. Mantsen WE1I02 “Thevity”
Transverse Deflecting Ca

linac, FEL

bunch length

temporalprofilelong. phasespace• Challenging• Synchronisation with the beam needs
to be excellent, otherwise your beam
will just wobble around/off thescreen and cause losses• Quite costly• Cannot easily go to very high rep.
rates
• Need to go to higher frequencies forbetter resolution → THz-TDC• Don’t forget that you need an
excellent screen as well and suitablebeam optics
23

Passive streaker general / a

Passive Streakers

linac, FEL

bunch length

temporalprofilelong. phase
space

Beam on a dispersive screen after the SwissFEL
Screen inAramis
the dispersive
section after
theinstreaker
passive(y)streaker,
actingX.
Longitudinal wakes=10 mmEnergy=500 μm=250 μm

Transverse wakes
Time

●
●
●

●

Vacuumwith double-side,
movablecorrugatedjaws. electric field excites strong
a “corrugatedpipe” close to
the beam→ the bunch’s• Bringchamber
Moving one of the plates close to the beam excites strong transverse wakefields.
transverse wake-fields that “streak” the beamUse dipole wakefield model by Bane, Stupakov, Zagorodnov, PRAB 19, 084401 (2016) to convert• Observe the beam on a screentransverse to longitudinal coordinates in an iterative procedure.
Structure calibration, i.e., obtaining the beam position relative to the plates:
Original idea from SLAC A. Novokhatski doi: 10.1103/PhysRevSTAB.18.104402
1) Independentcalibration
example with TDS): beam position (in calculations) is varied until
Images courtesyof P. Dijkstal (fordoi: 10.1103/PhysRevResearch.4.013017
24
reconstructed current profile agrees with the independent measurement.

rotated structures are employed for
Passive Streakers - streaking.
an example from PSI
Energy• Image of a structure installed at
SwissFEL used for “dechirping” before
Transverse wakes
the undulators→ a similar structureTime
acting in x is installed for “streaking”
after the undulators
nsverse wakefields.

PRAB 19, 084401 (2016) to converte.
to the plates:dsition (in calculations) is varied until themeasurement.
nt beam positions relative to the corrugatedbetween these beam positions must benance, no cooling water, no space for a25

Passive Streakers - What to do with them?
“Normal SwissFEL beam”

2 Color mode

Short pules

Lasing offI(t)Lasing on

Reconstruct
FEL power
profilesImages courtesy of P. Dijkstal doi: 10.1103/PhysRevResearch.4.013017

26

ost effective, comparable kick strength, easier to water cool for highPassive
Streakers
New Developments
ower beam operation(~1
kW/m at-LCLS-II)

adopt •the
DESY innovation to use an L-shaped bar to
L-Shaped structures for choosing the
Y-kick
X-kickkickdirection
(DESY
and
now
SLAC)
ctively kick in X- or Y-planes• SLAC is proposing an anodized
aluminum dielectric bar as analternative to machined corrugations(LDRD Proposal D. Bohler)• Cost effective, comparable kickstrength, easier to water cool for highpower beam operation (~1 kW/m atancels quadrupole
wakes and doubles the kick.
LCLS-II)combined kick in both planes
posal for aD. K. Bohler TUP030 “Proctor for
Low-Cost Wakefield Defle
CW X-ray FEL Operation”

Twice the kickFrom P. Krejcik’s talk WE3D2 at FLS’23
27

Passive Streakers

onlinesingle-shot

invasive

absolute• Good for• Cheap (compared to an RF deflector)• Self-synchronised to the beam
• Gives access to the long. phasespace and even the FEL power
profile• The closer you bring the structure(or the beam), the stronger the
streaking• Can also be calibrated with a TDC or
be used with a TDC to streak evenmorelinac, FEL

bunch length

temporalprofilelong. phasespace• Challenging• Poor resolution in the head of thebunch• Reconstruction method is difficult• Can generate losses if used at full
repetition rate and/or strong
streaking• High beam energies require
longer structures (e. g. structure
at DESY is ~5 m long)• Could work for high rep. rates but
will need cooling
28

WIDELY TUNABLE TWO-COLOR X-RAY FREE-ELECTRON …

PHYSICAL REVIEW RESEARCH 4, L022025 (2022)

Other methods at an FEL - Dispersion Streaking

Pulse energy ( J)

103

RON …

915 eV, OK
915 eV, no OK
350 eV, OK
350 eV, no OK• If you have a beam with linear energy chirp: Streak with a quadrupole → you will not
10
easily get an absolute measurement, but it is a very easy way of telling which part of thebunch is lasing (at SwissFEL we use it when losses are a problem) → e. g. 2 color
101
operation (make sure that different parts of the beam generate the colors) [1]100 • Also a great option for slice emittance measurements (you might want them also in
PHYSICAL REVIEW RESEARCH 4, L022025 (2022)
locations where you do not have a TDC) [2]
2

10-1

1

2

3

4

5

6

7

8

Undulator module

FIG. 2. Measured FEL pulse energy along one undulator section for 915 and 350 eV with and without OK.

8

ration of the Athos beamline for the OK mode of operation.
The 915 eV photons were produced in the first undulator
section, the 350 eV ones in the second. Chicane delays andphase shifters were optimized as described in Ref. [25]. For[1] E.
et al.
doi: 10.1103/PhysRevResearch.4.L022025
both photon energies,
wePratalso
optimized
the linear taper
FIG. 3. Top: Longitudinal phase space of the electrons for
[2]
E.
Prat
and
M.
Aiba
doi:
10.1103/PhysRevSTAB.17.032801
amplitude of the last undulator modules. For the
OK cases,lasing-off (a) and lasing-on (b) conditions. Green lines 29are image

8th Int. Beam Instrum. Conf.
ISBN: 978-3-95450-204-2

Bunch Compression Monitors

relative
onlinenoninvasive• At every compression stage of an FEL
you want to run a robust feedback
controlling your RF phases to keep yourbunch length constant• Bunch compression monitors make useof a coherent part of the spectrum(e. g. CSR or CDR)
• Pick a suitable spectral range for your“standard” beam parameters and find a
detector.
• The shorter your bunches the more you
will go towards the visible light domain.

Visit G. L. Orlandi at TUP026 tolearn more about BCMs and how to
extract even the bunch length

0.6 THz filter

IBIC201

FEL / linac ISSN:
bunch
length
2673-5350

0.3 THz filter

2: Top(a): Estimated spectral energy of measured
F. FreiFigure
et al., doi:
10.18429/JACoW-IBIC2019-WEPP026

longitudinal electron bunch profile at the working
30 point af-

m=0
m=2

0.2
0.0

0.4

0.6

0.8

attached to the CTR beam line. The detector arrays are not yet installed, hence fourreflection gratings, G0–G3, are visible. G4 is just outside the photo but its mirror
can be seen. P is the polarizer, and M1, M2 are the input alignment mirrors.

Spectrometers for Coherent Radiation

1.0
1.2
λ/d

1.4

1.6

1.8

Fig. 2. Efficiency curves of a gold-plated reflection grating for radiation polarized
perpendicular to the grooves, computed with the codes PCGrate (solid curves) andGSolver (dashed curves). Top graph: first-order diffraction m¼ 1. The wavelength
range 0:78 o l=d o 1:31 marked by the shaded area is used as a basis for the
spectrometer layout providing an almost flat efficiency. Bottom graph: diffraction
orders m¼ 0 and m¼ 2. Above l0 =d ¼ 1:33 all radiation is directed into the zerothorder which simply means that the grating acts as a plane mirror. For small
wavelengths ðl=d o 0:78Þ all three orders m ¼ 0; 1,2 contribute to the diffraction
pattern. In order to avoid ambiguities this wavelength range must be removed byfiltering the incident radiation.

Table 1
Parameters of the gratings. The distance between two grooves is called d. For lightwith l Z l0 the grating acts as a plane mirror. The minimum and maximum
wavelengths of the free spectral range for first-order diffraction are called lmin andlmax . All dimensions are quoted in mm. The coarse gratings with d Z 58:82 mm are
gold-plated and were custom-made by Kugler GmbH, the fine gratings withd r 33:33 mm are aluminium-plated and were purchased from Newport Corporation.

• You can also detect the full spectrum of the coherent
radiation and reconstruct the longitudinal bunch
profile using Kramers-Kronig relation.

lmin

lmax

Mid-infrared configuration 5.1–43:5 mm
G0
4.17
–
G1
6.67
5.13
G2
11.11
8.56
G3
20.0
15.4
G4
33.33
27.5

–
8.77
14.6
26.3
43.5

5.5
8.8
14.7
26.4
–

Far-infrared configuration 45.3–434:5 mm
G0
33.33
–
G1
58.82
45.3
G2
100.0
77.0
G3
181.8
140.0
G4
333.3
256.7

–
77.4
131.5
239.1
434.5

44
77.6
132
240
–

Gratingdl0

Pyro detector line array

S. Wesch et al. / Nuclear Instruments and Methods in Physics Research A 665 (2011) 40–47

Fig. 3. Principle of the staged spectrometer equipped with five reflection gratings.
Explanations are given in the text. To avoid FIR absorption in humid air, thespectrometer is mounted in a vacuum vessel (not shown). The detector
Fig.arrays
6. Arrangement of the grating, the ring mirror and the array of 30 pyroelectric
mounted above the focusing mirrors are not displayed.
detectors. The light dispersion and focusing have been computed with a ray
tracing code. For clarity only 5 of the 30 wavelength channels are shown.

Fig. 8. Photo of the 30-channel pyroelectric detector
preamplifiers and twisted-pair line drivers.

Example: CRISP Spectrometer at DESY - S. Wesch et al., doi: 10.1016/j.nima.2011.11.037

pyroelectric detector (for G0 and G4) which are needed for31Spectrometers for Coherent
Radiation• Comparison of the measured spectra with Fouriertransforms of the TDC measurements show good
agreement
TDC Measurements

Example: CRISP Spectrometer at DESY - S. Wesch et al., doi: 10.1016/j.nima.2011.11.037
32

Abstract

allows to sample a pulse with high accuracy. Figure 1 shows
the result of such a time scan for a short detector pulse.

The KAPTURE system (KArlsruhe Pulse Taking andUltrafast Readout Electronics), developed at the Karlsruhe
Institute of Technology (KIT), was designed to digitize detector pulses during multi-bunch operation at the KIT storage
responseof Schottky
barrier diodes
is• The signalring
KARA (Karlsruhe
Research Accelerator).
KAPTURE
provides digitization for pulses at rates of 500 MHz using upvery fast, but
has apoints“tail”→
you
can still study
10th Int. Particle Accelerator Conf.
Melbourne,
JACoW
to 4 samplingIPAC2019,perpulseAustralia
to record each bunch andeachPublishing
ISBN: 978-3-95450-208-0
doi:10.18429/JACoW-IPAC2019-WEPGW017
turn for potentiallyunlimitedtime. The new KAPTURE-2
microbunchingat a storagering
system now provides eight sampling points per pulse, including baseline sampling between pulses, which allows
improved reconstruction of the pulse shape. The advanced
reconstruction of the pulse shape is realized with a highly
parallelized implementation on GPU. The system will be
used for the investigation of longitudinal beam dynamics
e.g. by measuring instability induced CSR fluctuations or
arrival time oscillations. This contribution will report on Figure 1: Time scan of a Schottky detector pulse of a single
first results of the KAPTURE-2 system at KARA.
input channel. A Gaussian and an exponentially modified

KAPTURE-2

n of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

Fast Radiation Detectors & Fast Readout

Gaussian (EMG) curve are fitted to the data.

The KAPTURE system has proven its great potential inseveral measurements in the last years. Among others, itenabled a snapshot technique [1, 2] to instantaneously map
machine parameters to the resulting bursting spectrogram
PULSE RECONSTRUCTION
and was used to buildasingle-shot
spectrometerrecordGraphs: J. L. Steinmann et al., doi:10.18429/JACoW-IPAC2019-WEPGW017
naAnten- “THz
4: (from
left to03
right):
spectrogramsof theextractedspectra
amplitude,per
arrivaltime andof a bunch signal over charge.
WE1C
6. KAPTURE-2- 8-sample-point
configuration.
Yadav
R.Figureing 500
millionsecond[3].width
A synchronization
TheFiguretime scanis needed0221/12/01/C01040
toproperly set the
sampling points
2 effect
readout
M. Caselle
et al.,
doi:
10.1088/1748The behaviour of the amplitude is typical for the microbunchingKAPTURE
instability. The
on the arrival
time and width
could
schememakes
itpossibletotakemeasurements,
which
are
Diode
tky
Schot
for
the
different
readoutchannels.
Then,
each
Bias
ZeroCoup
not beledstudied before.
Even
faster YBCO
detector
P. Thoma et al., doi: 10.1109/TTHZ.2012.2234209 channel cansynchronizedtootherexperimental
stations[4].
However,
set to a differenttime[7].
pointof thepulse
sampleit. Toreq
erators”
4003 laminate
for beRF/Microwave
substrate
Bothanalogandtodigital
signals
Detectors for Particle Accelwith YBCO
Rousselet al., doi:the10.1103/PhysRevLett.113.094801
due to the limitation Microbunchingof four readout channels,
atransmission
pulseE.reconreconstructoriginal
pulse
from
these
eight
pointsanbe
lines and well-controlled time skew signals, therefore33all lines havethe signal fluctuations in dependence of the bunch current

support by the Helmholtz International Research School forStreak Camera (SC) - Working Principle

ΔxImage on screen
Fasttime axis

Fluorescentscreen800 psSynchroscan in 2nd plane

Spatial distributionof fluorescence

Bunch 2

Sweep field

Bunch 1
Streakimage
Photoelectrons

Sinusoidal
sweep voltage(f = 250 MHz ! T = 4 ns)

Δt
Photocathode
Incident
light pulse

100 nsSlowtime axis

Bunch 1
Bunch 2

Delay relative tosweep changed

Shortest bunches measured with a SC ~300 fs FWHM at SACLA
see Y. Otake et al., doi: 10.1103/PhysRevSTAB.16.042802
34

Streak Camera

visible lightradiation

non-invasive

absolute

averaging(typically)• Good for• a few ps - 100s of ps• need light in the UV-NIR (typicallysynchrotron radiation, but also
possible with, OTR or Cherenkov
radiation)• can be bought commercially• Working horse at storage rings (bunch
length wise, they are also good forthe new DLSR, but getting visible lightout of the small apertures might be a
bit of a challenge)storage rings

temporalprofilebunch length• Challenging• signal from a single shot for 1
bunch is relatively weak
→ often averaging is required• fully automated remote readout
and setup is challenging (but not
impossible)• short pulses, substructures → noteasy to resolve

35

Peak signal

Electro-Optical Sampling
Laser: typically around ~800, ~1000 or ~1500 nm
variabledelay

EO-crystal
(GaP)

1.2

1

0.8

Laser
(short pulses,
in sync to
Polarizer
E-field)
electron bunchstrength ofelliptical pol. ~ E!/2• Data acquisition witha photodiode• Very sensitive• Instead of “nearfield” also the “farfield” (e.g. CSR in theTHz range) can beused to modulate!

delay

0.6

0.4

Detector

0.2

0

-0.2

-2

-1

0

1

1 .2

2

!/4

Crossedpolarizer

1

0 .8
1.2
1.2

0 .6

1

1

0.8
0.8

0 .4
0.6

0.6

0.4

0 .2

0.4

Strong electric field of the bunch makes
the crystal birefringent (Pockels effect) -> modulation ofpolarisation can be turned into an intensity modulation for easy
detection.
Beam arrival monitors (BAMs) also make use of this effect

1.2

1.2

1

1

0.8

0.2

0.2

0.6

0.8

0.6

0.4

0.4

0

0

-0.2

-2

-1

0

1

0.2

0

0.2

0

0

-0.2
-2

2

- 0 .2

-2

-1

0

1

- 0 .2

2- 0 .2

-2

-1

0

1

-2

-1

0

1

-1

0

1

2

2

2

36

Electro-Optical Spectral Decoding (EOSD)
Int.

t
laser
(long chirped pulse)

EO-crystal
(GaP)

Pol.

!/4

electron bunch
E

t

!/2

Crossedpol.

• Calibration needs to be done
(move laser relative to beam andsee how signal shifts)• Bottlekneck: Data acquisition witha spectrometer (commercial line
arrays are “slow” ~100 Hz)
Spectrometer• Option 1: Develop a very fastline array → KALYPSO at KIT &DESY with a line array from PSI[1]“Photonic• Option 2: Stretch yourtime stvisit S. Biela retch”
modulated laser pulse in time
wski
TUP008 tosee how to
until you can detect it with apush this fu
rther!
fast photodiode and osilloscope[2] or with dedicated ultry-fastsampling DAQ [3][1] L. Rota et al. doi: 10.1016/j.nima.2018.10.093[2] “Photonic time stretch” E. Roussel doi: 10.1038/s41598-019-45024-2[3] M. Caselle et al. doi: 10.1117/12.2607818

37

Example EOSD Setup at ANKA/KARA
Radiation
protection wall

Detection E
O-Laser

Storagering
long cable (26 m)
to get BPM signal

BPM
EO-Monitor
positionlong SMfiber (25 m)
to transport
signal backfor detection

long PM fiber (35 m)
to stretch pulses andtransport them to the
EO-Monitor

loss rate
counter
38

EOSD at KIT
Impedance
protection

Laser
beam
path

Yb-doped fiber laser system(1030 nm) developed at PSI(PhD topic F. Müller) now in
use at KIT and DESY

32 mm

Electrons flying into plane of view
crystal(laser is reflected
at HR coatedback surface)

Electron beam
direction

Designed at
PSI & DESY
39

EOSD at a Storage Ring - Why?
• Why not just use the streak camera?
• Micro-bunching instability does not only
lengthen the pulses → substructures
emerge “bursting” behavior of theemitted CSR• Now imagine if you can observe every
round-trip for µs to ms → tomographic
phase space reconstruction becomes
possible40

Tomographic Phase Space Reconstruction at KARA
Reconstruct the longitudinal phase space from EOSD data to study the evolution of microbunchingfrom S. Funkner et al. doi: 10.1038/s41598-023-31196-5
41

EOSD at EuXFEL (DESY)• Full system installed in racks below beamline (fully remote controlled)• All sub-systems in 19’’ crate standard with a compact design (25 HU total)

Material courtesy of B. Steffen → visit him at TUP013
42

EOSD at EuXFEL (DESY)

visit B. Ste
ffe
TUP013 to ns
how to pu ee
sh thisfurther!

• Bunch arrival times measured are in good agreementwith BAM measurements• Bunch length measured are in agreement with beam
dynamics simulations for given RF settings
Material courtesy of B. Steffen

43

EO-Techniques

onlineabsolute
noninvasive
singleshot• Good for• 50 fs to 50 ps• high repetition rates can beachieved with fast DAQ• non-invasive• single-shot

arrival timebunch length

temporalprofilelong. phasespace• Challenging• generally a challenging setup• costly (needs a fully synchronised laser
system and the optical setup)• pushing the resolution below 50 fs(resonances in crystal)• near-field measurements → impedancecontrol (esp. at a storage ring)• Push this technology to 1.5 µm“standard” lasers?

Check out: E. Roussel et al., “Phase Diversity Electro-optic Sampling: A new approach to
single-shot terahertz waveform recording” doi: 10.1038/s41377-021-00696-2
44Time for discussion & questions• I hope, I managed to give you an overview over the different techniques in use!
• What interesting new developments did I miss?

45

This is not the end - further reading• General tutorials / talks on longitudinal diagnostics:• A. Gillespie (CAS 2018)
Part 1: https://cas.web.cern.ch/sites/default/files/lectures/tuusula-2018/cernschool2018pt1gillespie.pdf
Part 2: https://cas.web.cern.ch/sites/default/files/lectures/tuusula-2018/cernschool2018pt2gillespie.pdf• P. Forck (JUAS 2023) https://indico.cern.ch/event/1214547/contributions/5109013/attachments/
2596536/4482436/juas_script.pdf• R. Thurman-Keup (USPAS 2019) https://drive.google.com/file/d/192pJRMjWsV1C3bOgxPVVrYkFIBJRO7bX/view• T. Lefevre (CAS 2019)https://indico.cern.ch/event/780638/contributions/3371129/attachments/1865863/3068169/CAS19_longitudinal.pdf• M. Ferianis (CAS 2008)
Part 1: https://cas.web.cern.ch/sites/default/files/lectures/dourdan-2008/ferianis-longit-1.pdf
Part 2: https://cas.web.cern.ch/sites/default/files/lectures/dourdan-2008/ferianis-longit-2.pdf

46

Thank you!
My thanks go to• All the colleagues who
supplied me withmaterial anddiscussion• The many, greatscientists who
prepared tutorials
from which I could get
inspiration

47