Photon diagnostics

The SQS instrument provides diagnostics to characterize the properties of the FEL beam. There is a beamline section dedicated to diagnostics upstream of the KB mirror system, which contains an intensity monitor as well as two imagers. In a second section downstream of the focus, a refocusing mirror, a grating spectrometer, a wavefront sensor, a timing tool and additional imagers will be installed. At present, only the timing tool is available downstream.

The FEL (yellow) is entering the SQS experiment from the left, passing through destructive diagnostics (red) and/or parasitic diagnostics (green).

FEL imagers

Different imagers are distributed throughout the instrument to measure the FEL beam position, shape and pointing. These imagers consist of scintillator screens (Ce:YAG or pBN) and cameras. We are using several imager designs to fit the requirements of the individual locations in the beamline. The imagers are destructive, non-parasitic devices and can only be used during single-bunch operation. The FEL imagers have been developed in collaboration with the X-ray Photon Diagnostics group of European XFEL.

Intensity monitor

The X-ray Gas Monitor Detector (XGMD) measures the total pulse energy of each individual FEL pulse in a train. It contains a dilute rare gas target, which the X-ray pulse ionizes when passing through. The created ions and electrons are guided to two Faraday cups by an electrostatic field, and their currents are directly proportional to the number of photons per pulse. Typical operating pressures are of the order of 10-6 mbar, such that the XGMD is almost completely transparent to the X-rays. It can therefore be operated without influencing the experiments. Note that relative intensity information is available pulse-resolved, and the absolute (calibrated) intensity is measured integrated over all pulses of each train. The XGMD was developed by and will be installed in collaboration with the group of K. Tiedtke at DESY.

Wavefront sensor

A wavefront sensor employing a Hartmann plate can be placed about 2 m behind the experimental interaction point for focus diagnostic purposes. It is capable of characterizing both the focus size and shape, as well as the wavefront of the X-ray beam. This device is only suitable for single-bunch operation and for very low pulse energies, such that it is not usable parasitically for most experimental conditions, but it may be used for optimization before or during the experiment.  The wavefront sensor is available in collaboration with the groups of E. Plönjes at DESY and K. Mann from the Laserlab Göttingen.

Timing diagnostics

For pump-probe experiments, the relative arrival time of the optical laser pulse with respect to the X-ray pulse can be characterized on a single-shot basis by a transmissive online monitoring system. It is based on measurements of photon-induced changes of the refractive index on a semiconductor surface. It is designed for pulse-resolved diagnostics at the full repetition rate and has been developed in collaboration with the X-ray Photon Diagnostics group of European XFEL.