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Report on my stay at the Paul Scherrer Institute sponsored by the CALIPSOplus program

13th February 2019

I’m working at the Helmholtz-Zentrum Dresden-Rossendorf e.V. at the linear electron accelerator ELBE. In November 2018, thanks to the sponsorship of the staff exchange program of the EU project CALIPSOplus, I had the opportunity to travel to the Paul Scherrer Institute (PSI) located in Switzerland in order to gain hands-on experience at the recently built X-ray free-electron laser SwissFEL, which implements similar systems as ours.

My current task at ELBE is to design and install a new diagnostics tool, called Bunch Arrivaltime Monitor (BAM). Such devices are used to measure the arrival time of the electron bunches shot-to-shot and non-destructively with a very high precision in the fs range. The measurement principle is shown in Figure 1.

Figure 1- Working principle of the BAM

The BAMs are using a reference signal which consists of a pulse train of very short optical pulses (<200 fs), emitted by a phase-stabilized mode-locked fiber laser oscillator. The reference pulses are delivered to the BAM-Stations by single-mode optical fiber links which are actively stabilized, so that their drift over hours is very low (< 10 fs pk-pk). When an electron bunch passes the BAM-pickup, it generates an electrical transient signal with a steep slope on an antenna as shown on Figure 1. By using an electro-optic modulator (EOM), each of these pickup signals modulates one single optical pulse from the reference pulse train, thus encoding the bunch arrival-time in its amplitude. By measuring the intensity of the modulated pulses, it is possible to determine the time stability of the electron bunches passing the BAM device.

At the moment, we are in the planning stage of the new ELBE-BAM-system. So I wanted to use the CALIPSOplus program as an opportunity to visit a facility that has already implemented a similar system. Through this I wanted to exchange practical and operational experience which allows to incorporate this knowledge into the ELBE-BAM-design. My first choice for that visit was the SwissFEL at PSI because there a newly developed BAM system has been installed recently. Therefore, I asked the PSI BAM-system responsible, Dr. Vladimir Arsov, for a possibility to visit him at the PSI and to learn about his BAM-design. Additionally I wrote an application for the CALIPSOplus staff exchange program. I asked for a longer stay than the suggested 3 to 5 days because in my opinion, this time is too short to get a deeper insight into a complex project such as the BAM project. Fortunately this was permitted and so I was able to travel to PSI for 9 days from 19th to 29th of November 2018.

Thanks to the thorough planning of my host, the SwissFEL was in a shutdown period during the first days of my visit. In this way I was able to enter its accelerator hall and to see the beamline and its components. This was a great opportunity to compare the SwissFEL to ELBE and to learn about the similarities and differences between both facilities.

Figure 2- The SwissFEL Beamline

Even more importantly, because of the shutdown, we could install and commission a new BAM inside the accelerator hall during my visit. The following list shows a summary of the fulfilled tasks:

  • Optical Alignment of one BAM-Box.
  • Installation of this BAM-Box at its final position at the SwissFEL-Beamline.
  • Electrical and optical Wiring of this BAM-Box at the beamline and debugging of the electrical wiring at the control rack.
  • Calibration of the BAMs Analog-Digital-Converters and optical sensors.
  • Final functionality test of the BAM.

Additionally, we managed to execute some additional tasks during my visit:

  • Optical Alignment of another BAM-Box.
  • Measuring of the optical spectrums of several BAMs to investigate potential error sources.
  • We spliced and commissioned a distribution-box for the optical reference signals of two BAMs.

These activities extended over the major part of my visit and because I was involved in the most critical steps of the installation of a BAM were very interesting and instructive. I also got the possibility to learn more about the design of this diagnostics tool and to decide which components and design solutions would make sense for the ELBE-BAM, and which not.

Figure 3 – A look inside the BAM-Box which was installed during my visit

Retrospective I am sure that the visit at the PSI was very helpful for me and for the ELBE-BAM-project as well. A very important role for this success played my host Dr. Arsov. He spent a lot of time working with me and patiently explained many details of his BAM-design, so that I´m very grateful to him. Fortunately, I was also able to help Dr. Arsov at the installation and could give him some hints how we manage some tasks at ELBE. So I believe my visit was interesting and helpful for him as well.

Concluding this report I want to say that I am grateful that I was allowed to visit the SwissFEL due to the CALIPOplus program. The profit of this travel was huge and I´m happy that such a program was created. So I hope, that in the future many more technicians, engineers and scientists can profit from this program to visit other facilities.

© Report kindly provided by Andreas Schwarz (HZDR)