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DEVELOPMENT OF AN ACTIVE BEAM-STABILIZATION SYSTEM FOR
ELECTROFISSION EXPERIMENTS AT S-DALINAC∗
D. Schneider† , M. Arnold, U. Bonnes, A. Brauch, M. Dutine, R. Grewe,L. E. Jürgensen, N. Pietralla, F. Schließmann, G. Steinhilber
Technische Universität Darmstadt, Institute for Nuclear Physics, Darmstadt, Germany
Abstract
The r-process fission cycle terminates the natural synthesis of heavy elements in binary neutron-star mergers. Fission
processes of transuranium nuclides will be studied in electrofission reactions at the S-DALINAC. Due to the minuscule fissile target, the experimental setup requires an active
electron-beam-stabilization system with high accuracy anda beam position resolution in the submillimeter range. In
this contribution, requirements and concepts for this system
regarding beam diagnostics elements, feedback control andreadout electronics are presented. The usage of a beam position monitor cavity and optical transition radiation targets tomonitor the required beam parameters will be discussed indetail. Additionally, various measurements performed at theS-DALINAC to assess requirements and limits for the beamstabilization system will be presented. Finally, the option
to use advanced machine learning methods, such as neuralnetworks and agent-based reinforcement learning, will bediscussed.

INTRODUCTION
The superconducting Darmstadt linear electron accelerator (S-DALINAC) at the institute for nuclear physics at
the Technische Universität Darmstadt is used for scientificresearch in nuclear spectroscopy and meteorology, nuclear
astrophysics and accelerator science [1] and can be operated
in energy-recovery modes [2, 3]. The current layout of theS-DALINAC is shown in Fig. 1. Due to its superconducting
design the accelerator is able to provide a continuous-wave
electron beam at a frequency of 2.997 GHz with a kinetic
design energy of up to 130 MeV. The beam quality can befurther improved using the scraper systems located in boththe low and high energy sections to reach its minimum design energy spread of 10−4 with bunch lengths of smaller
than 2 ps [4]. The electron beam can be utilized at various
experimental setups including the QCLAM magnet spectrometer. Multiple high-resolution (e,e’) and (e,e’𝛾) coincidence experiments have been conducted successfully at theQCLAM [5, 6].
Within the cluster project ELEMENTS a new seriesof measurements based on electron-induced fission at
S-DALINAC is proposed to aim at a better understandingof the natural synthesis of heavy elements in our universe.
While the details of the proposed experiment are provided∗ Work supported by DFG (GRK2128), BMBF, the state of Hesse withinResearch Cluster Project ELEMENTS and the LOEWE Research Group
Nuclear Photonoics† dschneider@ikp.tu-darmstadt.dein the following chapter, the requirements for the accelerator
facility are challenging due to the miniscule radioactive target of 1 mm, the request of ultra-short bunch lengths in theorder of 1 ps and the demand for stable beam conditions forseveral hundreds of hours of measurement. In particular, themonitoring and control of the beam spot size and position
are of importance.
Therefore, a new active beam-stabilization system is being designed for electrofission experiments at the QCLAMspectrometer at the S-DALINAC. This project includes thecommissioning of beam diagnostics elements and correction
magnets including an in-house developed beam position andphase monitor. Additionally, the electronics layout for signal
readout and processing will be designed. The focus is on
utilizing existing in-house developed electronics and commercially available solutions. As feedback control algorithm
both the performance of traditional PID (Proportional–integral–derivative) controllers and machine learning methods
will be assessed. To test and optimize the control mechanism beforehand, the creation of a virtual test environment
based on neural networks and beam dynamics simulations
are planned.

ELECTROFISSION AT S-DALINAC
In order to investigate fission reactions of actinides
and their dependence of the excitation energy of the nucleus, electron-induced fission will be employed at theS-DALINAC. It allows the determination of the excitationenergy of the actinide target nucleus. Additionally, higher
multipoles can be excited and studied.
A schematic representation of the proposed electrofission
setup is shown in Fig. 2. The electron beam will be deliveredby S-DALINAC with a beam current of 3 µA, electron energies of the order of 100 MeV and a 3σ beam spot size smaller
than 1 mm at the electrofission interaction point. The electrofission target has a diameter of 1 mm and a target thicknessof approximately 350 μg/cm2 . After its interaction with anelectron the target nuclei can decay via fission. An accurate description of this process, which is not available up todate, requires the measurement of both fission fragments incoincidence. While e.g. photon-induced fission primarily
excites E1 transitions, in electrofission higher multipole orders such as E2 and E3 can be exited. As electron scatteringexperiments allow the decoupling of excitation energy andmomentum transfer, a model-independent multipole decomposition using the electrofission cross section [7]MOP03808 Feedback Systems and Beam Stability

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d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - M O P 0 3 8Figure 1: Floorplan of the S-DALINAC. The electrofission experiment as well as the beam stabilization system will be set
up at the QCLAM spectrometer. The OTR setups marked in red will be upgraded with fast cameras to allow high-precision
beam stability measurements.

∞𝑑 2 𝜎f𝑑𝐵= 𝜎M ∑ 𝑃f (𝐸𝜆, 𝜔)|𝐹(𝐸𝜆, 𝑞, 𝜔)|(𝐸𝜆, 𝜔)𝑑𝜔 𝑑Ωe𝑑𝜔𝜆=0and mean stability of the stabilization system is aimed to besmaller than 500 μm. The design layout of the active beamstabilization system is outlined in Fig. 3.

is possible. Here, 𝜔 is the excitation energy, Ωe the solidangle of the spectrometer, 𝜎M the Mott cross section, 𝜆the multipole order, 𝑃f the fission probability, 𝐹 the form
factor and 𝐵 the strength function. Due to its relatively low
cross section, measurement periods in the order of several
hundreds of hours are planned.
Figure 3: Sketch of the beam stabilisation system. OTR lightemitted by both the electrofission target and OTR target isevaluated to determine the beam profile. Additionally, a nondestructive beam position measurement is deducted with aresonator cavity. The signal will be processed by the control
algorithm to adjust the correction magnet currents.

Beam Diagnostics
Figure 2: Schematic overview of the electrofission setup
design. After interacting with an electron the target nucleus
decays via fission. Both fission fragments are detected incoincidence with double sided silicon strip detectors.

DESIGN OF THE BEAM
STABILIZATION SYSTEM
The proposed electrofission experiment demands a highcontrol and stability of beam parameters over a large measurement period. As the long-term stability of the beam
position and size has not been studied at the S-DALINAC,
several existing OTR (optical transition radiation) screen stations (see Fig. 1) are upgraded by ultra-fast cameras that canreach 300-600 fps (frames per second). Within the upcoming beamtime the beam stability and frequency of distortions,and therefore the required frequency of the beam stabilization system, is examined. The desired position resolution

To receive a transverse center beam position signal forbeam spots of smaller than 500 μm two methods are proposed and pursued in parallel. As preferred method theobservation of OTR directly emitted at the electrofission
target is proposed. Transition radiation is emitted when arelativistic charged particle crosses the boundary between
two media with different permittivity as the electric field has
to rearrange at the boundary [8]. The radiated power for anelectron with Lorentz factor 𝛾 ≫ 1 can be described by [9]𝑊o (𝜃) =𝑒2
𝛽2 sin2 𝜃
34𝜋 𝜖0 𝑐 (1 − 𝛽2 cos2 𝜃)2with 𝑊o = 𝑑 2 𝑊 /𝑑𝜔𝑑Ω the spectral energy distribution
and 𝛽 = 𝑣/𝑐 the electron velocity. Additionally, the totaltransmitted intensity is proportional to 𝛾 and the plasma
frequency of the target material. The emitted OTR light canbe observed with a fast camera. The analysis of the target
image gives access to the beam profile parameters right atMOP038
11208 Feedback Systems and Beam Stability

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the electrofission interaction point. Here, the resolution
and frequency of the transverse beam profile signal is only
dependent on the OTR intensity and camera resolution andframe rate as the OTR is emitted instantaneously and nobroadening effects occur. A beam position resolution of≪ 100 µm at a frequency of 35 Hz has been measured at
OTR setups at the S-DALINAC [10]. The observation ofOTR at the target material has been used before to improvethe experimental analysis at the QCLAM [11].
Depending on the target material and the QCLAM interaction chamber geometry it cannot be guarantied thatOTR at the electrofission target is accessible. Therefore, analternative method is pursued in parallel. The beam will
be monitored non-destructively with an in-house developed
cavity beam position monitor shortly before the interaction
chamber [12]. As the cavity beam position monitor is currently decommissioned, a first test of its performance has
been conducted (see Fig. 4). With the existing electronics
beam positions with an increment of 1 mm could be resolved
at a beam current of 500 nA. In order to derive the beam
position at the interaction point, an OTR screen will be installed behind the electrofission target and the emitted OTR
light will be evaluated as in option 1. A position resolutionof 100 μm of the cavity beam position monitor has beenshown for beam currents of 1 μA [12]. The resulting position resolution at the interaction point will be examinedwithin the commissioning phase.

will be substituted with state-of-the-art low-noise amplifiers
and an adjustable attenuation. For the signal processingof the amplified 2.997 GHz signal, in-house developed rf
boards are available. They consist of an IQ (In-phase andquadrature) demodulator, an amplitude detector and a FPGA
(Field Programmable Gate Array). The transverse centerbeam position is calculated directly from the IQ vector onthe FPGA.
The power control boards of the correction steerers are
in-house developed power supplies with an input signal processing of up to 7 Hz. If a faster signal processing is required,
new power control boards have to be acquired. The hardware implementing the control algorithm depends on thecontrol cycle repetition rate. The EPICS-based control system (Experimental Physics and Industrial Control System)
allows scanning frequencies of up to 10 Hz [13, 14]. For
higher frequencies FPGA boards with implemented controlalgorithm configurations can be used.

Feedback Control
A first test of a feedback control using a Python-based
PID controller with EPICS interface has been conducted.
A steerer has been used to manipulate the beam position
while a second correction steerer was controlled to stabilize
the beam to a fixed position on a downstream beryllium
oxide screen. Both steerer magnets were operated with afrequency of 5 Hz. The horizontal center beam position wascalculated by Gaussian fits on the vertically projected image
profile. As shown in Fig. 5 the correction steerer was able
to stabilize the beam within a 2σ-interval of 0.84 mm. Yet,
beryllium oxide has broadening effects and the strong distortion of the beam most likely does not model the noise of
the S-DALINAC electron beam sufficiently. A beam stabilization test measurement using an OTR target is scheduled
for the upcoming beamtime where a higher beam stability isexpected.

Figure 4: Left: Image of an in-house developed cavity beam
position monitor for non-destructive phase and beam position measurements. Right: Test of a decommissioned cavity
beam position monitor with existing readout electronics at abeam current of 500 nA. Beam positions with an incrementof 1 mm can be resolved.

Readout Electronics
The choice of readout electronics depends strongly on
the required repetition rate of the beam stabilization system.
The OTR cameras will be chosen to have the maximum
resolution for the given system repetition rate.
For the cavity beam position monitor, the extracted signal
power is 𝑃 ∝ 𝑥 2 𝐼 2 where 𝑥 is the transverse beam shift
and 𝐼 is the beam intensity. To be sensitive to 𝑥 < 1 mmfor beam currents of 100 nA to 2 μA while protecting thefollowing electronics of large 𝑃 induced by 𝑥 ≫ 1 mm in thebeam tuning process, the existing amplification electronics

Figure 5: First test of a beam stabilization with a PID controller and implemented fast target analysis algorithm. Despite the strong simulated distortion, the correction magnet was able to keep the center beam position within a 2σinterval of 0.84 mm.
As surrogate models of accelerator subsystems have been
proven useful in analyzing and refining the underlying system, it is planned to create such a system based on bothMOP038

08 Feedback Systems and Beam Stability

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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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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 - M O P 0 3 8simulations and machine learning techniques like neural networks or polynomial chaos [15]. The constructed surrogate
models will serve as training and testing environments fordifferent control algorithms, such as PID controllers andsupervised learning agents.

CONCLUSIONThe S-DALINAC is well suited to provide a high-quality
electron beam for the upcoming electrofission experiments.
Yet, due to its challenging requirements including a beam
spot size of 1 mm and a position stability of 500 μm, an
active beam stabilization system is essential for the successof the electrofission project. Multiple OTR setups are currently being upgraded to allow a systematic investigation of
the beam stability within the upcoming beamtime. The presented design of the stabilization utilizes existing in-house
knowledge while remaining flexible to match the electrofission setup requirements.

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