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12th Int. Beam Instrum. Conf.
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BUNCH LENGTH MEASUREMENT SYSTEMDOWNSTREAM THE INJECTOR OF THE S-DALINAC∗
A. Brauch† , M. Arnold, M. Dutine, J. Enders, R. Grewe, L. E. Jürgensen,
N. Pietralla, F. Schliessmann, D. Schneider
Technische Universität Darmstadt, Dept. of Physics, Institute for Nuclear Physics,
Schlossgartenstr. 9, Darmstadt, Germany
Abstract
The S-DALINAC is a thrice recirculating electron accelerator with a continuous-wave beam at a frequency of2.9972(1) GHz. Short bunches are crucial to enable tuning of the machine for operation as an energy-recovery
linear accelerator. Currently, measurements of the bunch
length are accomplished by using the radio-frequency zerocrossing method. Since this method is time consuming, anew setup for these measurements using a streak camera
is developed. Optical transition radiation emitted from analuminum-coated Kapton screen is used to map the bunch
length information to a light pulse which enables an accurate
measurement compared to a scintillating screen. The lightpulse can then be evaluated with the streak camera. This contribution will present the current status of the measurement
setup as well as its design and properties.

BUNCH LENGTH MEASUREMENTS
DOWNSTREAM THE INJECTOR
The current method for bunch length measurements isbased on the radio-frequency zero-crossing method [3]. An
RF (radio-frequency) cavity is used to impose a momentum
chirp on a bunch.

INTRODUCTION
The superconducting Darmstadt electron linear accelerator S-DALINAC was originally established as a twicerecirculating accelerator in 1991 operating at a radio frequency of 2.9972(1) GHz. An upgrade of the machine
adding a third recirculation beamline in 2015/16 was performed. This beamline also features an adjustment system
for the path length of the electrons in the second recirculation beamline with a range of 10 cm which is identical to
the wavelength of the operation frequency (see Fig. 1). It
is possible to recover the energy of this beam by passing
the main accelerator a second time if this system is set toan 180° phase shift and the beam passing the main accelerator for the first time is led to the respective beamline. This
operation mode called 1×ERL (1× Energy-Recovery Linearaccelerator) was achieved in 2017 for the first time at the SDALINAC [1]. Four years later a 2×ERL mode was demonstrated by accelerating and decelerating the beam twice [2].
More than 87 % of the beam energy were recovered in thismode. The efficiency was observed to decrease with theincrease of the initial current at the electron gun. The effectis partly caused by the greater bunch length in high current
operation. Therefore, the minimization of this parameter
is important for reaching higher currents and efficiencies
in ERL operation. This requires a setup for measuring thebunch length.
∗ Work supported by the State of Hesse within the Cluster Project

ELEMENTS (Project ID 500/10.006) and by DFG (GRK 2128AccelencE).
† abrauch@ikp.tu-darmstadt.deFigure 1: The layout of S-DALINAC including the beam
path for operation in 2×ERL mode (following [2]). The top
image shows the acceleration in this mode. After passing
the injector in (1) the beam is led to its main accelerationin (2). Following the first recirculation beamline (3) thebeam is accelerated a second time in the main accelerator (4).
The beam then enters the second recirculation beamline (5).
The bottom image shows the deceleration of the beam in
2×ERL mode. Because of the path length adjustment system
the beam receives a phase shift of 180° in (6). The first
deceleration takes place in (7) and the beam is again led
through the first recirculation beamline in (8). The second
deceleration stage (9) reduces the beam energy to the energythat was originally provided by the injector linac. At thisenergy, the beam is dumped (10). The intended bunch length
measurement setup location for commissioning is also shownin purple.

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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 - T U P 0 0 9Figure 2: Current setup for measuring bunch lengths via OTR screens and a streak camera. The emitted light pulses fromthe screen are transported and focused via two off-axis parabolic mirrors on the entrance slit in front of the camera. All
electrical devices for the streak camera will be stored in rooms that are protected from radiation.
After a dispersive section the transverse beam profile ofthis bunch is measured with a scintillating screen. Becauseof the dispersive section the size of the beam on the target
is connected to the induced momentum chirp which again
is dependent on the bunch length. Using this method toevaluate the bunch length has some significant drawbacks:• The initial transverse beam size and the scintillating
screen itself cause the measurement to be an upper
estimate of the bunch length only.
• The measurement is only valid at the position of theused RF cavity and must be extrapolated otherwise.
• It is impossible to determine the bunch length value
when all RF cavities are operated in intended operation,that is, when all RF cavities are operated at their designvalues.
These issues make it impossible to monitor the bunch
length parameter in ERL operation properly. Therefore, a
new setup for bunch length measurements downstream theinjector has been developed. All components are currently inthe procurement phase. Figure 2 shows a sketch of measuring bunch lengths with this new setup. An optical transition
radiation (OTR) screen is used to create light pulses withthe same length as the electron bunches. These pulses aretransported via optical elements to a streak camera whichthen measures the length of these pulses. Next to the streak
camera the setup requires an OTR screen, an electrical reference signal, a radiation shielding and optical elements.
Details of the shielding will be determined from upcom-ing measurements. The electrical reference signal will beproduced by the master oscillator.
The main goal with this setup is to provide an accurate androbust method for measuring the bunch length just before
the main accelerator with a time resolution of better than
1 ps. This is required for specific machine settings and ERL
operation. However, smaller values of the bunch length arefavorable to further improve ERL operation [2]. In non-ERL
operation the bunch length is 2 ps and will be monitoredwith this setup as well [4]. In the following section thecontributions to the total time resolution of the setup will be
discussed and estimates for the main contributions from theoptical setup in front of the streak camera and the electricalreference signal will be investigated.

TIME RESOLUTION OF THE NEW BUNCHLENGTH MEASUREMENT SETUP
Several uncertainties contribute to the total time resolution 𝑅t of the streak camera setup. Assuming no correlations
between these contributions, the total resolution can be approximated by:
2 + 𝑅2 + 𝑅2
2𝑅t = √𝑅ttd
jitter + 𝑅rest .
slit

(1)

Here, 𝑅ttd is the contribution from the transit time dispersion of the photoelectrons and the electrical deflector inside
the streak camera. This is the lower limit to the resolutionof streak cameras and can be estimated to be ≈ 100 fs forcommercial devices [5]. This limit is assumed to be valid
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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 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U P 0 0 9

for the procured streak camera C10910 from Hamamatsu
Photonics.
The beam profile of the light pulses reduces the resolutionof the streak camera. It can be controlled with a slit in frontof the device so that its contribution to the total resolution
is 𝑅slit . However, it is better to reduce the spatial intensity
profile of the light pulse prior to entering the slit in the firstplace. This allows for more photons to be collected for themeasurement and increases its accuracy. The optics in frontof the streak camera and their impact on the resolution will
be shown in the following section.
The electrical reference signal used to synchronize thestreak camera with the S-DALINAC will be supplied by themaster oscillator of the machine. Its phase noise creates atime jitter 𝑅jitter that reduces the resolution of the setup. A
measurement of this value at the location of the setup andits influence on the resolution is discussed below.
Finally, 𝑅rest contains all contribution from minor sources
to the total resolution. These are e.g. dispersion in theoptics of the camera, misalignment of the orientation of
the OTR screen and fluctuations of the beam. Although
this value is assumed to be negligible, it can be estimatedby measuring the total resolution and subtracting the otherthree contributions from it.
It follows that for achieving a total time resolution of atleast 1 ps the condition below must be true:
2 + 𝑅22√𝑅slitjitter + 𝑅rest < 995 fs.

(2)

OPTICS OF THE NEW BUNCH LENGTH
MEASUREMENT SETUP
A preliminary design assumed an optical system with atelescope and periscope in front of the streak camera [6].
The former reduced the beam profile of the light pulses andcollimated them while the latter prevented a direct line ofsight to the streak camera from the OTR screen. At the sametime the streak camera could be located close to the floor of
the accelerator hall which additionally protects the camera
from radiation. Still, a radiation shielding around the streakcamera was required to prevent gamma rays and neutrons
from damaging the camera sensor at the end of the device.
After further simulations with the software OSLO (Optics
Software for Layout and Optimization) [7] the optics werechanged. Two main conditions must be met by the setup to
be used in front of the streak camera:• The light pulse created by the OTR screen has a large
opening angle (14.6° in the lowest energy scenario).
Collimating the light for transportation and focusing
on the slit of the streak camera is required as soon as
possible behind the screen window.
• The light pulse must be focused as small as possiblethrough the input slit in front of the streak camera.
These conditions led to the current setup shown in Fig. 3.
The refractive telescope was replaced by a mirror design because its focus on the streak camera slit was larger than withthe purely reflective setup. At the same time the reflective
setup can focus all wavelengths of the incident light and improve the accuracy of the bunch length measurement setup.
The input optics of the streak camera include an achromatic
relay lens which allows to make use of the independence
from the wavelength. However, the input optics may be removed entirely. The alignment of the two off-axis parabolic
mirrors in Fig. 3 was chosen so that aberrations caused by
off-axis light beams could be counteracted [8]. At the focal point the spot size radius 𝑟 of the on-axis light beams is
1.36 µm while the spot size radius of the off-axis light beams
is 1.37 µm.
The contribution of the optical setup to the total resolution
can be estimated with the acquired results. The screen atthe end of the camera has a diameter 𝑑 of 18 mm [9]. The
slowest time the camera needs to streak over the screen in
one cycle, 𝑇, is 100 ps. The contribution from the opticalsetup 𝑅slit can be calculated following [10]𝑅slit =𝑇𝑟,𝑑(3)resulting in a contribution to the time resolution of approx.
8 fs with the slowest streak speed. Further analysis of thesetup in regards to tolerances in the components will beconducted before the necessary components are procured.

TIME JITTER FROM THE
MASTER OSCILLATOR
The signal of an RF oscillator contains random fluctuations in phase. These are dependent on the offset frequencyof the supplied carrier signal. An electrical device like thestreak camera accepts a passband from the RF oscillator thatcontains these random fluctuations. This leads to a time jitter(or phase jitter) in the device which degrades the resolutionof the measurement. A measurement of the time jitter from

Figure 3: Sketch of the current optics setup in front of the streak camera. The rays shown are drawn by OSLO [7] andvisualize the on-axis beams (green) and off-axis beams (blue). The latter are at a distance of 0.5 mm from the center of theelectron spot on the target which is to be expected at the setup. Both mirrors are the same off-axis parabolic mirror with afocal length of 101.6 mm.
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the 2.9972(1) GHz master oscillator of the S-DALINAC at
the commissioning location close to the main accelerator
entrance was performed with a spectrum analyzer (Rohde& Schwarz FSV). A graph of the results from 120 min ofmeasurements is shown in Fig. 4. The time jitter caused by
the master oscillator is computed using the integration of
the measured phase noise 𝐴 following [11]𝑅jitter =10𝐴/20.
√2𝜋𝑓0(4)Hereby, 𝑓0 is the frequency of the master oscillator. Boundaries must be set for the integration. The upper integration
limit is 200 kHz and is set by the passband of the streak camera. The lower limit is 100 Hz and was chosen because ofanomalous behavior close to the carrier frequency. This ismost likely caused by the phase noise of the reference oscillator inside the spectrum analyzer. A proper measurement
method for phase noise close to the carrier frequency will beneeded since this area has the most significant contribution
to the time jitter. An average of 0.403(6) ps for 𝑅jitter was
calculated from the 120 min of measurements. While thisvalue is still lower than the required total time resolution itis larger than the contribution from the optical setup. It isalso a lower limit to the expected time jitter since the phasenoise close to the carrier frequency could not be measured.
This may increase the time jitter to a value over 1 ps. A newmaster oscillator with lower phase noise was procured. The
time jitter of this device is yet to be measured and is expected
to be smaller.

in front of the streak camera and master oscillator of theS-DALINAC will have an impact on the total time resolution of the bunch length measurement. The optical setup
was revised and will not significantly degrade the targeted
resolution of 1 ps. Contributions from the master oscillator’stime jitter have been estimated to amount to about 400 fs;
however, future measurements close to the carrier frequency
will need to demonstrate that the required resolution of better than 1 ps can be reached. A new master oscillator wasprocured and is expected to perform better than the current
device [12].

REFERENCES[1] M. Arnold et al., “First operation of the superconducting
darmstadt linear electron accelerator as an energy recovery
linac,” Phys. Rev. Accel. Beams, vol. 23, no. 2, p. 020 101,Feb. 2020.
doi:10.1103/PhysRevAccelBeams.23.020101[2] F. Schliessmann et al., “Realization of a multi-turn energy
recovery accelerator,” Nat. Phys., vol. 19, no. 4, pp. 597–602,
Apr. 2023. doi:10.1038/s41567-022-01856-w[3] D. X. Wang, G. A. Krafft, and C. K. Sinclair, “Measurementof femtosecond electron bunches using a rf zero-phasing
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1998. doi:10.1103/PhysRevE.57.2283[4] F. Hug, “Erhöhung der Energieschärfe des Elektronenstrahls
am S-DALINAC durch nicht-isochrones Rezirkulieren,”
Ph.D. dissertation, Technische Universität Darmstadt, Darmstadt, Germany, 2013.
[5] P. A. Jaanimagi, “Breaking the 100-fs barrier with a streak
camera,” in Fourth-Generation X-Ray Sources and Ultrafast X-Ray Detectors, International Society for Optics andPhotonics, vol. 5194, 2004, pp. 171–182.
doi:10.1117/12.512747[6] A. Brauch, M. Arnold, J. Enders, L. E. Jürgensen, N. Pietralla,
and S. Weih, “Bunch Length Measurement Systems at SDALINAC,” in Proc. IBIC’22, Kraków, Poland, Dec. 2022,
pp. 96–99. doi:10.18429/JACoW-IBIC2022-MOP26[7] Lambda Research Corporation, “OSLO - optics software
for layout and optimization,” EDU Version 2023. https://lambdares.com/Figure 4: Measurements of 𝑅jitter at 2.9972(1) GHz with aduration of 120 min. The phase noise for each measurement
was integrated from 100 Hz to 200 kHz. The error band
displays the standard deviation of the extracted average fromall measurements.

CONCLUSION
A new setup for evaluating the bunch length behind theinjector of the S-DALINAC is procured to improve 2×ERL
operation by minimizing the bunch length. The optical setup[8] C. Brückner, G. Notni, and A. Tünnermann, “Optimal arrangement of 90° off-axis parabolic mirrors in THz setups,”
Optik, vol. 121, no. 1, pp. 113–119, 2010.
doi:10.1016/j.ijleo.2008.05.024[9] Hamamatsu Photonics K.K., “Universal streak camera
C10910 Instruction Manual,” May 2018.
[10] M. M. Shakya and Z. Chang, “Achieving 280fs resolution
with a streak camera by reducing the deflection dispersion,”
Appl. Phys. Lett., vol. 87, no. 4, p. 041 103, Jul. 2005.
doi:10.1063/1.2001732[11] W. Kester, “Converting oscillator phase noise to time jitter,”
Tutorial MT-008, Analog Devices, 2009.
[12] Rohde & Schwarz, “R&S SGS100A SGMA RF Source Specifications,” Jan. 2019.

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