Optische Übertragung phasensynchroner Taktsignale unter Verwendung des Wellenlängen-Multiplex-Verfahrens

Es wird ein System präsentiert, das Zeitinformationen an räumlich getrennten Punkten mit einer Genauigkeit im Picosekunden-Bereich bereitstellen kann. Diese Zeitinformationen sollen dazu genutzt werden, die Beschleuniger- und Speicherringe von FAIR (Facility for Antiproton an Ion Research) zu synchronisieren. Zur Ansteuerung der Hochfrequenz-Kavitäten dieser Ringbeschleuniger werden Signale mit unterschiedlichen Phasenlagen und Frequenzen (0,4 bis 5,4 MHz) benötigt. Einige Frequenzen dieser Signale sind aufgrund der sogenannten Rampenansteuerung während der Beschleunigung variabel. Um dies zu ermöglichen, hat das hier entwickelte System die Aufgabe, mindestens zwei unterschiedliche Taktsignale phasensynchron an verschiedenen Stellen der Anlage, die bis zu 1 km auseinander liegen können, bereitzustellen. Mit Hilfe dieser Taktsignale können dann Frequenzgeneratoren synchronisiert werden, die die eigentlich benötigten Signale für die Kavitäten erzeugen. Aufgrund des universellen Charakters der bereitgestellten Zeitinformation können mit ihrer Hilfe neben der Ansteuerung der Kavitäten auch andere Prozesse synchronisiert werden. Zur Übertragung der Taktsignale wird ein optisches Netzwerk mit DWDM (Dense Wavelength Division Multiplex)-Verfahren verwendet. Die Laufzeit der Taktsignale wird gemessen und ein Referenzgenerator erzeugt mit Hilfe der Laufzeitinformation am Ende jeder Übertragungsstrecke eine phasensynchrone und –stabile Zeitreferenz. Da aufgrund von Umwelteinflüssen die Laufzeiten der Taktsignale nicht konstant sind, müssen sie regelmäßig erfasst werden. Die Eigenschaften des Systems werden eingehend untersucht und es werden Wege zur Optimierung der verschiedenen Teilfunktionen aufgezeigt. Mit einem Prototyp des Systems konnte eine Genauigkeit der Zeitinformation von 21,2 ps im Mittel erreicht werden. Die kurzzeitigen Schwankungen weisen eine Standardabweichung von 7,57 ps auf. Neben der Beschreibung eines konkreten Systems enthält diese Arbeit viele allgemeingültige Informationen und neu gewonnene Erkenntnisse zum Thema Zeitübermittlung, insbesondere zu Phasenschwankungen, Rauschen, Reflexionen und Veränderungen der Signallaufzeit in einem Glasfaserkabel durch Umwelteinflüsse. Außerdem sind hier erstmals die bisher eingesetzten Methoden zur Verteilung phasenstabiler Referenzsignale in den Hauptanwendungsbereichen Teilchenbeschleuniger und Radioteleskope zusammenfassend beschrieben worden

Multipacing in an RF Window: Simulations and Measurements

Electron guns are used in the accelerators of the European XFEL and FLASH. They are operated at 1.3 GHz. The RF peak power is 5 MW at 650 s pulse width and 10 Hz repetition rate. In order to understand the multipacting that occurs during conditioning, it was simulated in the RF window type that is used for the electron gun in the XFEL. The reduction in secondary emission yield associated with conditioning was taken into account. Since the RF windows are tested with high power on a test stand before their use, without the electron gun, measurement results are available which are compared with the simulation results. The main advantage of the simulation compared to the measurement is that the locations of multipacting can be determined in the RF window. This could be helpful for the development of high-power RF components in the future, in order to detect pronounced multipacting resonances even before production and to avoid them by design changes

Planning and Controlling of the Cold Accelerator Sections Installation in XFEL

The installation of the main linear accelerator in the 2 km European XFEL (X-Ray Free-Electron Laser) tunnel is currently under way. The accelerator consists of nineso-called cryo-strings. A typical cryo-string comprises 12 accelerator modules, which will be fed by three RF stations. Furthermore, the installation of electronic racks,cables, power and water supply etc. takes place. To enable a most effective installation of the accelerator components, planning and controlling methods, whichhad first been developed for the RF system work package, were adapted for the entire main linear accelerator. As a first step, a process plan was developed in cooperationwith the work package leaders. On the basis of this plan, the installation process is promoted by several measures: The status of the installation is precisely registered byweekly queries which enable monitoring of the Progress and feedback to everyone involved. With this Information at hand, the installation process can be controlled andplan deviations can be corrected. Furthermore, the experience gained at one cryo-string is used to optimise the plan for the next cryo-string installation

Solving the USB Communication Problem of the High-Voltage Modulator Control System in the European XFEL

Since the commissioning of the modulators in the European XFEL in 2016, it happened from time to time that the modulator control system hung up. The reason for the problem was unknown at that time. Initially, the MTBF (Mean Time Between Failure) was 10⁴ days, which was so rare that other problems with the RF system clearly dominated and were addressed first. Over the next 2 years, the error became more frequent and occurred on average every 18 days. After the winter shutdown of the XFEL in 2020, the problem became absolutely dominant, with an MTBF of 2 days. Therefore, the fault was investigated with top priority and was finally identified. Two units of the control electronics communicate via USB 2.0 with the main server. Using special measurement technology, it was possible to prove that weak signal levels in the USB signal led to bit errors and thus to the crash of the control electronics. This article describes the troubleshooting process, how to measure the signal quality of USB signals and how the problem was solved in the end

Optical Synchronization and Electron Bunch Diagnostic at ELBE

The continuous wave electron accelerator ELBE is upgraded to generate short and highly charged electron bunches (~200fs duration, up to 1 nC) . In the last years a prototype of an optical synchronization system using a mode locked fiber laser has been build up at ELBE which is now in commissioning phase. The stabilized pulse train can be used for new methods of electron bunch diagnostics like bunch arrival time measurements with the potential of femtosecond resolution. At ELBE a bunch arrival time monitor (BAM) has been designed and tested at the accelerator. The contribution will show the design of the BAM and first measurement results at the ELBE accelerator

Operational Experience with the European XFEL SRF Linac

The European X-ray Free Electron laser (EuXFEL) is a 3.4 km long research facility which generates ultrashort Xray flashes of outstanding brilliance since 2017. Up to 27000 electron bunches per second are accelerated in a 1.3 km long superconducting radio frequency (SRF) linac to a maximum energy of 17.6 GeV. Within this time, operational experience with a pulsed RF machine has been gained and new operation modes simultaneously delivering electron bunches to 3 different SASE undulator beamlines have been successfullyimplemented. Recent activities on increasing the linac availability, power efficiency and duty cycle are discussed

Operational Experience with the European XFEL SRF Linac

The European X-ray Free Electron laser (EuXFEL) is a 3.4 km long research facility which generates ultrashort X-ray flashes of outstanding brilliance since 2017. Up to 27000 electron bunches per second are accelerated in a 1.3 km long superconducting radio frequency (SRF) linac to a maximum energy of 17.6 GeV. Within this time, operational experience with a pulsed RF machine has been gained and new operation modes simultaneously delivering electron bunches to 3 different SASE undulator beamlines have been successfully implemented. Recent activities on increasing the linac availability, power efficiency and duty cycle are discussed