FLASH operates two distinguished undulator sections driven by one linear accelerator. In the 11th year of user operation the grown demands for detailed photon beam performances are doubled approached. The more complex machine settings and setup times require a more and more efficient determination of its characteristics concerning electron- and photon-beams. The photon diagnostics systems, e.g. gas monitor detection, photon-ion spectroscopy, or diffractive tools, not only have to deal on a regular basis with fundamental wavelengths between 4nm and 90nm, also they have to be reliable from 1µJ up to 1mJ of average single pulse energy. For the success of the experiments the error bars of many diagnostics measurements need to be pushed into their current limits and developments to go further are always issued. Especial, the pulse duration in conjunction with the spectral width has been accessed in the last year. Direct approaches of fundamental wavelengths below the Nitrogene K-edge and higher harmonics in and below the water window were achieved. While in principal distinguished to each other, the photon diagnostics tools of FLASH1 and FLASH2 add-up to a more complete understanding of the other. Together they allow for a better perspective towards further developments and a more suitable use of beam times. The intermingled knowledge of electron- and photon-beams is essential for an FEL particular in simultaneous operation mode. Examples out of regular user operation and distinguished FEL-studies are given to illustrate the current state of the photon diagnostics at FLASH.

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Marion Kuhlmann, Rolf Treusch, Elke Plönjes-Palm, Bart Faatz, Kai Tiedtke, Markus Braune, and Barbara Keitel, "Opportunities and challenges for photon diagnostics at the soft X-ray FEL FLASH in simultaneous operation mode (Conference Presentation)," Proc. SPIE 10237, Advances in X-ray Free-Electron Lasers Instrumentation IV, 1023707 (Presented at SPIE Optics + Optoelectronics: April 26, 2017; Published: 16 June 2017); https://doi.org/10.1117/12.2269835.5463398586001.