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 focusing mirror system, which contains an intensity monitor. Downstream of both focus points, a refocusing mirror and a grating spectrometer can send the beam into a wavelength spectrometer, an array of electron time-of-flight spectrometers, and onto a timing tool. In the future, a wavefront sensor will also become available.

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

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.

Spectral diagnostics

A spectral diagnostics device is available for the characterization of the SASE spectrum delivered to SQS. The spectrometer is located in the DIAG3 diagnostic section and accepts the beam downstream of the interaction region, such that for experiments performed on dilute samples with negligible impact on the beam transmission it can work fully parasitically and in parallel to the timing and temporal diagnostics described below. The spectrometer consists of a VLS diffraction grating and a  fluorescence screen. The screen can be imaged by a Gotthard-II camera allowing the single shot spectral characterization of the beam with capability of resolving multiple pulses within a train. Typical resolution values under optimal imaging conditions are around E/ΔE = 3000, and careful alignment can give up to E/ΔE = 4500. Single shot characterization capability for pulses down to few 100s of μJ has been demonstrated for photon energies up to 1.2keV. Development is ongoing for extending this range to shorter wavelengths.

X-ray pulse duration diagnostics

For experiments with x-ray pulses of few fs or below, a setup can be employed for obtaining the full spectro-temporal characterization of the individual pulses. The setup is installed in the DIAG3 diagnostics section and accepts the beam downstream of the interaction region, such that for experiments performed on dilute samples with negligible impact on the beam transmission it can work fully parasitically and in parallel to the spectral diagnostics described above. The setup is based on the detection of x-ray induced photoemission from a probe gas by an array of 16 time-of-flight electron spectrometers. The photoelectrons are streaked by a circularly polarized laser field, such that their energy and angle of emission encodes the spectral and temporal profile of the x-ray intensity. The setup has been successfully commissioned with a streaking field of 1030nm, allowing the characterization of short pulses (< 3.5 fs). An extension to longer pulse durations (>12 fs) employing OPA-generated mid-IR streaking fields is under development. At the moment, this diagnostics is only available if the optical laser is not required in the experiment and when the FEL is delivering < 3.5 fs pulses (special mode).

Laser/X-ray pump-probe diagnostics

For pump-probe experiments, the relative arrival time of the optical laser pulse with respect to the X-ray pulse can be characterized with femtosecod precision. It is based on spectrally encoded measurements of photon-induced changes of the refractive index on a semiconductor surface through a temporally stretched optical laser pulse. 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. This is mainly used at a 10Hz train-to-train (train average arrival) basis, but can also monitor the intra-train jitter (pulse-resolved timing) at up to 47 kHz. At the moment this pump-probe diagnostics is only available for FEL photon energies of 1keV and above. Commissioning of the pump-probe diagnostics for lower photon energies is subject to future studies and upgrades.

Wavefront sensor

A wavefront sensor employing a Hartmann plate is planned to be installed in the DIAG3 diagnostic section. It is designed to allow the characterization of the beam spatial distribution in the focal position as well as the wavefront of the X-ray beam for any of the three interaction regions F1, F1' and F2, thanks to a large surface area and high density pitch Hartmann plate. This device will only be suitable for characterization under single-bunch operation and for very low pulse energies conditions such that it will be used for optimization before or during the experiment. The wavefront sensor is delivered by Imagine Optic. It will be installed in the second half of 2025 and will be commissioned in the first half of 2026.