Abstract
FERMI, the seeded free electron laser (FEL) in operation in Italy, is providing the User Community with unique fully coherent radiation, in the wavelength range 100–4 nm. FERMI is the first FEL fully synchronized by means of optical fibers. The optical timing system ensures an ultra-stable phase reference to its distributed clients. Several femtosecond longitudinal diagnostics verify the achieved performance; the bunch length monitor (BLM) and the bunch arrival monitor (BAM) will be presented in this paper. Feedback systems play a crucial role to guarantee the needed long-term electron beam stability. A real-time infrastructure allows shot-to-shot communication between front-end computers and the servers. Orbit feedbacks are useful in machine tuning, whereas longitudinal feedbacks control electron energy, compression and arrival time. A flexible software framework allows a rapid implementation of heterogeneous multi-input–multi-output (MIMO) longitudinal loops simply by selecting the appropriate sensors and actuators.
Highlights
FERMI is a seeded free electron laser (FEL), routinely providing the User Community with unique state-of-art coherent radiation, in the wavelength range 100–4 nm[1, 2].In this paper, after an introduction to the seeded FEL scheme, we will present the engineering effort implemented to provide the required femtosecond time stability to the electron bunch needed to ensure a stable generation of the coherent radiation, in short to make it a user facility
FERMI has been conceived on a different scheme, called high-gain harmonic generation (HGHG)[10, 11], in which an external laser, named ‘seed’, interacts with the relativistic electron beam when the latter goes through an undulator, called ‘modulator’
We focused our attention on the system developed by LBNL (continuous wave (CW) optical timing in the following)[19] and the one developed[20] by Research Laboratory in Electronics (RLE) at MIT (Pulsed optical timing)
Summary
FERMI is a seeded free electron laser (FEL), routinely providing the User Community with unique state-of-art coherent radiation, in the wavelength range 100–4 nm[1, 2]. In order to verify the achieved performance of the timing system during the commissioning and to monitor the effective stability of the whole facility in day-by-day operation, several longitudinal diagnostic instruments with femtosecond precision have been developed and installed in FERMI. These instruments provide, in a nondestructive way for the electron beam, the required femtosecond measurement accuracy at several machine locations, like the relative bunch length monitor (BLM) and the bunch arrival monitor (BAM). We present the development of the above-mentioned systems as well as the obtained results from field measurement, with special emphasis on the engineering and technological aspects
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