Development and Implementationof an Active Beam-Stabilization
System for Electrofission
Experiments at the S-DALINAC**Work supported by DFG (GRK2128), BMBF, the state of Hesse within the Research Cluster Project ELEMENTS (Project ID 500/10.006) and the LOEWE ResearchGroup Nuclear Photonics
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

Beam Diagnostics Elements

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 inelectrofission reactions at the S-DALINAC. Due to the minuscule fissile target, theexperimental setup requires an active electron-beam-stabilization system with high accuracy
and a 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 opticaltransition radiation targets to monitor the required beam parameters will be discussed indetail. Additionally, various measurements performed at the S-DALINAC to assess
requirements and limits for the beam-stabilization system will be presented. Finally, theoption to use advanced machine learning methods, such as neural networks and agent-based
reinforcement learning, will be discussed.

Backwards angular distribution of OTR
at a 45° tilted diagnose screen forseveral beam energies. The resolution
and frequency of the transverse beamprofile signal only depends on the OTR
intensity and both the camera
resolution and frame rate.

Intensity in a.u.

Electrofission @ S-DALINAC

Left: Image of an in-housedeveloped cavity beam
position and phase monitor.
Right: Test of this monitor
for a beam current of500 nA. Beam positions
with an increment of 1 mm
can be resolved.

Source: M. Fischer, master thesis, „Design und Aufbau
eines Messplatzes zur Strahlqualitätsmessung basierend
auf optischer Übergangsstrahlung am S-DALINAC “, 2019.

Angle of emitted radiation

Readout Electronics
Ultra-fast Camera - OTR Setup
Floorplan of the S-DALINAC. To investigate fission reactions of actinides and their dependence
on the excitation energy of the nucleus, electron-induced fission will be employed at theQCLAM magnetic spectrometer. The electron beam will be delivered by the S-DALINAC with abeam current of 3 μA, a frequency of 2.997 GHz, electron energies of the order of 100 MeV
and a 3σ beam spot size smaller than 1 mm at the electrofission interaction point.
Design of the electrofission
setup at the QCLAM. After its
interaction with an electron
the target nuclei can decay
via fission. An accurate
description of this process,which is not available up todate,requiresthemeasurement of both fission
fragments in coincidence.

rf boardcontrol
algorithm

correction steerer

electronbeampositionand phase
monitorFPGA - Feedback Control▪ 0.4-6 GHz input signal▪ Faster signaltransfer than via
control system▪ IQ demodulator, power
detector, IQ modulator▪ 1 MS/s internal
processing rate

▪ Calculate beam position
from cavity BPM via
IQ vector▪ Can implement
variety of control
algorithms▪ In-house developed

Feedback Control
First test of a beam stabilization
with a simulated beam shift(steerer), a correction steerer and aPID controller by monitoring adownstream located beryllium oxide
screen. The correction magnet kept
the center beam position within a
2σ-interval of ca. 0.8 mm. It isplanned to test supervised learning
agents as an alternative to
traditional PID controllers.

OTR
screenrefocusing
quadrupoles

beam
dump

electrofission
targetConcept of the beam stabilization setup. If the emitted optical transition radiation (OTR) of
the electrofission target can be observed directly, the beam width and center position can bemonitored with an ultra-fast camera (option 1). Alternatively, two independentmeasurements, deduced with a beam position monitor (non-destructive) and a downstream
located OTR screen, can be used to approximate the beam profile at the interaction point(option 2). The signal will be processed by the readout electronics to generate a feedback
signal for the correction magnets. The feedback control algorithm can be based on a PID
controller, a trained AI model or simulations.
†email contact: dschneider@ikp.tu-darmstadt.de

RF-Board - Cavity BPM

OTR imageanalysiscameracorrection
quadrupoles▪ Fourier-transformation of OTR signal to investigate
beam-stability at the S-DALINAC with up to 300 Hz

Surrogate models have been used topredict the beam position x and width
σx in subsystems of the accelerator. It isplanned to create a surrogate
environment for the beam stabilization
setup based on both simulations andmachine learning to train, test andrefine the selected feedback control
algorithms.

Surrogate model

power supply boards▪ 500-600 frames per second can be reached

Surrogate model

Beam Stabilization Setup▪ Blackfly S GigE monochrome camera with 0.4 MP

Accelerator

Accelerator