The International Fusion Materials Irradiation Facility (IFMIF) is an international project to study and qualify candidate materials for the construction of a future fusion reactor. One of the objectives of the IFMIF-Engineering Validation and Engineering Design Activity Project is to build a linear prototype accelerator (LIPAc) to validate the final IFMIF accelerator concept. LIPAc, which is currently under construction in Rokkasho (Japan), will generate a 9-MeV deuteron beam of 125-mA current with 100% duty cycle. CIEMAT (Spain) is in charge of providing the RF power system, including the low-level radio frequency (LLRF) system. Most of the developed LLRF systems are not completely digital, as they use analog front ends for intermediate frequency conversion before or after digitalization. However, the LIPAc LLRF system is a fully digital system: no analog frequency conversion is performed, the radio frequency (RF) signals are directly digitally synthesized and sampled by means of high-speed digital-to-analog converters and analog-to-digital converters. This is a clear advantage in terms of flexibility, reliability, reconfigurability, cost, and response time, as all the signal processing is performed in the digital domain. The other main advantages and novelties are the use of White Rabbit (WR) for timing synchronization and master oscillator distribution (distributed RF over WR). The LIPAc LLRF system is the first LLRF based on WR, and it has been designed and fabricated using the most advanced technology. This paper presents the detailed description of the LIPAc LLRF system and its advantages, performance evaluation, and verification.

Fully Digital and White Rabbit-Synchronized Low-Level RF System for LIPAc

The high resolution scattering experiments conducted at the superconducting Darmstadt electron linear accelerator S-DALINAC call for a small energy spread of (ΔE/E) ≈ 1×10⁻⁴ of the beam. This requires stabilization of amplitude and phase of the electric field inside the accelerating cavities to (ΔA/A)ᵣₘₛ = 8×10⁻⁵ and (Δφ)ᵣₘₛ = 0.7°. The design and the commissioning of a new digital rf control system is the subject of this thesis.

At the S-DALINAC two types of cavities are in use. The normal-conducting chopper and buncher cavities only need corrections for slow temperature drifts and can be controlled by a generator-driven resonator control algorithm. The superconducting accelerating cavities have a very high quality factor and thus are very susceptible to vibrations. Therefore they are operated in a self-excited loop.

The rf control system is based on in-house developed hardware that converts the rf signal down to the baseband, digitizes it and feeds it into an FPGA. Inside this FPGA, a soft digital signal processor executes the control algorithm. The resulting correction is modulated onto the rf signal again and sent back to the cavity.

All accelerator components are remote-controlled from a central room via an accelerator control system. Since complex and re-programmable devices are not supported well by the existing in-house developed control system, the design and implementation of a new accelerator control system is also subject of this thesis. Further important aspects are expandability, usability and maintainability of the system. Therefore the new accelerator control system uses the EPICS framework as a basis since it already provides much of the basic functionality like graphical user interfaces and flexible control servers that can be customized rapidly. This allowed the implementation of more advanced functionality like extensive read-out and diagnostics for the rf control system.

The read out data can be visualized with a software oscilloscope and a spectrum analyzer software. Additionally the system provides on-line rms errors that can be used to optimize the control parameters very precisely and to monitor the performance of the controllers.

Measurements show that the performance of the rf control system has been improved by one order of magnitude compared to the analog system, yielding a phase stability of (Δφ)ᵣₘₛ = 0.8° and an amplitude stability of (ΔA/A)ᵣₘₛ = 7×10⁻⁵ and thus meeting the specification.

The described rf control system has been commissioned and successfully used for beam operation for two years. During this time the system has proven to be significantly more stable and reliable than the old analog system.

Development and commissioning of a digital rf control system for the S-DALINAC and migration of the accelerator control system to an EPICS-based system

This paper presents a detection system for high-accuracy amplitude and phase measurements on RF signals. It is typically used to set up the low-level RF system for particle accelerators. The proposed detector employs the principle of a heterodyne receiver and uses the digital IF architecture with a high oversampling rate to increase the SNR. A discrete Fourier transform-based quadrature demodulator is used to suppress the analog-to-digital converter (ADC) quantization noise and IF harmonics. The system has been evaluated at an IF of 1 MHz and in an RF range of 500-3000 MHz. A graphical user interface is developed to display and record the measured values. The detection accuracy achieved on 500-MHz signals in a 250-kHz bandwidth is better than ±0.0051% for the amplitude and better than ±0.0032 ° for the phase. Experimental results at different frequencies prove that the detection system can meet the demands of state-of-the-art RF control systems for modern particle accelerators.

High-Accuracy Amplitude and Phase Measurements for Low-Level RF Systems

Der erste Teil dieser Dissertation thematisiert Erweiterungen an der digitalen Hochfrequenzregelung des Elektronenbeschleunigers S-DALINAC. Diese umfassen die Inbetriebnahme von piezoelektrisch angetriebenen Frequenzabstimmern der supraleitenden Beschleunigungsstrukturen. Hierfur wurde ein mit der vorhandenen Elektronik kompatibles Netzteil entwickelt, welches die Piezoaktuatoren versorgt. Weiterhin wurde die Flexibilitat der Hochfrequenzregelung an supraleitenden Viertelwellenresonatoren des Ionenbeschleunigers ALPI demonstriert. Diese von den Beschleunigungsstrukturen des S-DALINAC sehr verschiedenen 160-MHz-Resonatoren konnten erfolgreich mit Regelabweichungen in Phase und Amplitude von ∆A/A ≤ 2·10⁻⁴ und ∆φ ≤ 0,1° betrieben werden.

Teil zwei beschreibt die Migration des Beschleunigerkontrollsystems auf ein EPICS-basiertes System. Eine Vielzahl von Geraten mit unterschiedlichen Schnittstellen und Protokollen wurde integriert und allgemeine Funktionalitat hoherer Ebene wurde vereinheitlicht. Im Rahmen des Einbaus der dritten Rezirkulation wurde eine Motorisierung der Strahl-Weglangenverstellung vorgenommen.

Optimierung und Test der digitalen Hochfrequenzregelung und Entwicklungen für das EPICS-basierte Beschleunigerkontrollsystem am S-DALINAC

In dieser Arbeit wird die longitudinale Teilchendynamik des Elektronenstrahls im supraleitenden, rezirkulierenden Elektronenbeschleuniger S-DALINAC detailliert untersucht und systematisch optimiert. Ziel ist dabei eine Erhohung der Energiescharfe des Elektronenstrahls, um die Prazision kernphysikalischer Elektronenstreuexperimente zu steigern. Im Gegensatz zur gelaufigen Konvention an Linearbeschleunigern wird hierzu eine Beschleunigung auf der Flanke des Hochfrequenzfeldes zusammen mit einer nicht-isochronen Teilchendynamik in den Rezirkulationsstrecken verwendet. Durch optimale Anpassung des longitudinalen Betatronphasenvorschubs kann eine Unterdruckung des Einflusses der Regelungsgenauigkeit der supraleitenden Hochfrequenzstrukturen auf die Energiescharfe erreicht werden, so dass die resultierende Energiescharfe letztlich unabhangig von Fehlerbeitragen der supraleitenden Beschleunigungsstrukturen im Hauptbeschleuniger ist.

Zur Umsetzung wurden im Rahmen dieser Arbeit zuerst Simulationsrechnungen im longitudinalen Phasenraum durchgefuhrt, um den optimalen Arbeitspunkt fur den S-DALINAC einzugrenzen. Die Ergebnisse dieser Simulationen lassen dabei neben einer hoheren Energiescharfe des Elektronenstrahls auch eine grosere Stabilitat gegenuber Fehlereinflussen erwarten. In einem weiteren Schritt musste die Strahldynamik in den Rezirkulationsbogen des S-DALINAC fur einen nicht-isochronen Strahltransport mit Hilfe weiterer Simulationsrechnungen optimiert werden, bevor diese Bogen gemas der Berechnungen umgestaltet wurden.

Die Quantifizierung der durchgefuhrten Anderungen erfolgte durch Flugzeitmessungen mit einem eigens entwickelten Messaufbau, der in dieser Arbeit prasentiert wird. Zum Abschluss konnte der optimale Arbeitspunkt am S-DALINAC mit Hilfe systematischer Messungen aufgefunden und eingestellt werden. Die Energiescharfe des nicht-isochron rezirkulierten Strahls war mit 1.23 EXP -4 um einen Faktor 5.4 besser als diejenige mit isochron rezirkuliertem Strahl. Dieser Wert stellt gleichzeitig die bislang hochste gemessene Genauigkeit fur einen rezirkulierten Strahl am S-DALINAC dar.

Erhöhung der Energieschärfe des Elektronenstrahls am S-DALINAC durch nicht-isochrones Rezirkulieren

The low level rf system for the superconducting Darmstadt electron linear accelerator (S-DALINAC) developed 20 years ago and operating since converts the 3 GHz signals from the cavities down to the baseband and not to an intermediate frequency. While designing the new, digital rf control system this concept was kept: the rf module does the I=Q and amplitude modulation/demodulation while the low frequency board, housing an field programmable gate array analyzes and processes the signals. Recently, the flexibility of this concept was realized: By replacing the modulator/demodulators on the rf module, cavities operating at frequencies other than the one of the S-DALINAC can be controlled with only minor modifications: A 6 GHz version, needed for a harmonic bunching system at the S-DALINAC and a 324 MHz solution to be used on a room temperature cavity at GSI, are currently under design. This paper reviews the concept of the digital low level rf control loops in detail and reports on the results gained during first operation with a superconducting cavity.

https://link.aps.org/pdf/10.1103/PhysRevSTAB.13.082801

3 GHz digital rf control at the superconducting Darmstadt electron linear accelerator: First results from the baseband approach and extensions for other frequencies

Araz Araz

Papers

3 GHz digital rf control at the superconducting Darmstadt electron linear accelerator: First results from the baseband approach and extensions for other frequencies