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This contribution describes design and results obtained with newly installed longitudinal diagnostics at the MAX IV linear electron accelerator: a Bunch Compression Monitor (BCM) and an Electro-Optical Arrival Time Monitor (EO-BAM). Both are part of a concerted effort to improve longitudinal stability to a level commensurate with the shortest available bunch lengths of about 3 fs. As MAX IV is a small lab with limited resources, these diagnostics build on existing, proven designs.
The BCM measures the intensity of diffraction transition radiation generated by a copper radiator. Holes allow the electron bunch to pass undisturbed. A ZnSe vacuum window, a number of THz filters and a LiTaO3 pyroelectric detector yield sensitivity for bunch lengths from hundreds down to a few femtoseconds. The installation location allows absolute comparison against a high-resolution transverse-deflecting cavity.
The EO-BAM is based on an 40 GHz bandwidth electrode pick-up design from DESY and can use fibre-transported reference laser pulses from both the recently commissioned 1550 nm laser-synchronisation system and from the 800 nm pump-probe experimental laser oscillator. At 800 nm a Treacy pre-stretcher counteracts dispersion in the fibre connections, at 1550 nm the laser link is already dispersion compensated. Free-space optical delays control temporal overlap in two commercial EO modulators, one for each wavelength.
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