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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW 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 - W E 2 C 0 2

SOFTWARE DEFINED RADIO BASED FEEDBACK SYSTEMFOR TRANSVERSE BEAM EXCITATION∗
P. J. Niedermayer∗∗, R. Geißler, R. SinghGSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany
SOFTWARE-DEFINED RADIOS

Abstract
Controlling stored beams in particle accelerators requires
specially designed RF signals, such as needed for spill control via transverse excitation. The software-defined radio(SDR) technology is adopted as a low cost, yet highly flexible setup to generate such signals in the kHz to MHz regime.
A feedback system is build using a combination of digital
signal processing with GNU Radio and RF Network-on-Chip(RFNoC) on a Universal Software Radio Peripheral (USRP).
The system enables digitization of signals from particle detectors and direct tuning of the produced RF waveforms via
a feedback controller – implemented on a single device. To
allow for triggered operation and to reduce the loop delay
to a few ms, custom OOT and RFNoC blocks have been implemented. This contribution reports on the implementationand first test results with beam of the developed spill controlsystem.

INTRODUCTIONRadio Frequency Knock Out (RF-KO) extraction [1] is astandard method used to extract stored particle beams fromsynchrotrons through transverse excitation, providing spillsof particles for experiments and medical therapy. It uses abeam optics near a third order resonance driven by sextupole
fields [2]. The non-linear amplitude and phase detuning
effects create a separatrix in phase space [3]. Transverse
electromagnetic RF fields are applied to increase the betatron amplitude of particles in a controlled manner. This
process, referred to as excitation, drives the particles into
the separatrix. As a result, the motion becomes unbound,and the particles can be extracted at a septum.
The excitation system consists of an RF signal generator,
RF amplifiers, and a stripline kicker inside which the RFfields act on the beam. Here, the signal generator is thecentral element by which the extraction process, and thus
the spill, can be controlled.
The primary goal is to maintain a constant spill rate, that is,
to extract an equal number of particles per unit time. To reachthis goal, two complementary strategies exist: On timescales
above about 50 ms, the signal amplitude is controlled by acomplex function or a feedback system based on measured
spill rates [4]. On smaller timescales not accessible by thefeedback system, the signal waveform is optimized to reduce
statistical fluctuations [5].
∗ This project has received funding from the European Union’s Horizon

2020 Research and Innovation programme under GA No 101004730.

∗∗p.niedermayer@gsi.de

Software-defined radio (SDR) is an RF transceiver technology where signal processing is implemented in softwareon computer processors (CPUs) and/or field-programmablegate arrays (FPGAs). It is widely used in radio communication systems, but has potential applications in manyfields. An SDR typically consists of a frontend with ADCsand DACs, and a backend performing the digital signal processing (DSP). Here, a Universal Software Radio Peripheral (USRP) is used as off-the-shelf frontend to generate
the RF signals. For implementation of the DSP chain, theopen source framework GNU Radio [6] is used in combination with the RF Network-on-Chip (RFNoC) technology [7],which enable a flexible, graphical design of signal processing flow graphs by combining predefined and custom blocks.
With SDRs being adopted by an increasing number of users
in the accelerator community [8–10], collaboration and sharing of algorithms between institutes is easily possible [11].

IMPLEMENTATIONThe excitation, feedback and spill control system builds
on the development of a beam excitation system with applications in beam diagnostics [12]. The device does not
only generate the excitation signals required for RF-KO extraction, but also processes the signals of a spill detector
monitoring the extraction process. A feedback controller
adjusts the excitation signal amplitude in order to maintaina constant spill rate on the detector. Statistical fluctuations
on timescales below about 50 ms down to 10 µs – which are
not accessible by spill feedback systems – are reduced by anautomatic optimization of the used excitation signal waveforms. The implementation and realization of the system is
described in detail in Refs. [11, 13].

COMMISSIONING
The spill control and feedback system was commissioned
at the Cooler Synchrotron (COSY) accelerator in Jülich. For
spill detection, a Low Gain Avalanche Detector (LGAD)
developed by Ref. [14] for the High Acceptance Di-Electron
Spectrometer (HADES) was used, which was being testedby the HADES LGAD group at COSY at the time.
Figure 1 shows a 20 s spill recorded with the developedsystem. Excitation with a constant signal level shows – asexpected for this kind of slow extraction – a significant driftof the achieved spill rate over the course of the extraction.
The feedback system, if switched on, is able to adjust theexcitation level such as to maintain a constant spill rate on
the detector. More details are available in Refs. [11, 13].

WE2C02306

09 Data Acquisition and Processing Platforms

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 - W E 2 C 0 2Controller
outputSpill ratein particles/s

1e6
Feedback off
Feedback on
Target rate

1.5Res., Sect. A, vol. 795, pp. 92–99, 2015.
doi:10.1016/j.nima.2015.05.054[5] P. J. Niedermayer, R. Singh, G. Franchetti, E. C. Cortés García, E. Feldmeier, and T. Haberer, “Investigation of micro
spill in RF KO extraction using tailored excitation signals”,in Proc. IPAC’23, Venice, Italy, 2023, pp. 2398–2401.
doi:10.18429/JACoW-IPAC2023-TUPM094

1.0
0.50.00.2

tcount = 50 ms
0.0
0.0 2.5[6] GNU Radio project, GNU Radio, version 3.10, 2007. https://www.gnuradio.org

5.0

7.5 10.0 12.5 15.0 17.5 20.0
Extraction time / s

Figure 1: Measured spill rate and controller output with andwithout active feedback. Plot resolution: 50 ms.

ACKNOWLEDGEMENTS
The authors would like to express our gratitude to thewhole team of the COSY accelerator in Jülich for their support during the preparation and execution of the beamtime.
We are very thankful to Jerzy Pietraszko and the HADESLGAD group for providing us with an excellent detector
signal. Bastian Bloessl and Eike Feldmeier are thanked fortheir support with GNU Radio and advice on SDRs. Finally, we acknowledge support by the beam instrumentation
department of GSI.

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[7] M. Braun, J. Pendlum, and M. Ettus, “RFNoC: RFNetwork-on-Chip”, in Proc. GRCon16, Boulder, CO,USA, 2016. https://pubs.gnuradio.org/index.php/grcon/article/view/3[8] E. Feldmeier et al., “Upgrade of the Slow Extraction Systemof the Heidelberg Ion-Beam Therapy Centre’s Synchrotron”,in Proc. IPAC’22, Bangkok, Thailand, 2022, pp. 2509–2512.
doi:10.18429/JACOW-IPAC2022-THPOST029[9] R. Steinhagen et al., “GNU Radio 4.0 for real-time signalprocessing and feedback applications at FAIR”, in Proc.
IPAC’23, Venice, Italy, 2023, pp. 4637–4640.
doi:10.18429/jacow-ipac2023-thpl099[10] F. Kuehteubl et al., “Investigating alternative extraction methods at MedAustron”, in Proc. IPAC’23, Venice, Italy, May
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niedermayer/exciterhttps://git.gsi.de/p.

[12] P. Niedermayer and R. Singh, “Novel Beam Excitation System Based on Software-Defined Radio”, in Proc. IBIC’22,
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doi:10.18429/JACOW-IBIC2022-MOP36[13] P. J. Niedermayer, R. Singh, and R. Geißler, “SoftwareDefined Radio Based Feedback System for Beam Spill Control in Particle Accelerators”, in Proc. GRCon23, 2023, unpublished.
[14] J. Pietraszko et al., “Low Gain Avalanche Detectors for theHADES reaction time (T0 ) detector upgrade”, Eur. Phys. J.
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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3