Fig. 2: Design of the laser-assisted measurement and feedback system.

a Layout of the X-ray beam path (purple) and the IR laser optical guidance system (red and green). The X-ray beam is reflected by the optics M1–M3 at a height 25 mm below the optical guide beam and passes the other IR optics through apertures in their posts. M1 and M2 each consist of a Bragg reflector for the X-ray beam and a mirror for the IR laser beam. For each X-ray/IR optics pair, the respective optics are mounted on top of each other within a rigid mechanical holder, mounted on a 3-axis piezo steering stage. M3 reflects both the X-ray and IR wavelength. The guidance system laser beams have a frequency offset of 4096 Hz from each other and their interference generates a beat note at this frequency. Two interferometers are formed from them, and their detection is highlighted in yellow (reference interferometer) and blue (measurement interferometer) shapes. The reference interferometer signal is used to subtract common mode noise from the measurement signal. The compound refractive lens focuses the X-rays onto the profile monitor for beam pointing measurements. b Schematics of the data acquisition and control flow for the interferometer readout and motion control feedback architecture. All photodiode elements are first converted to voltages with transimpedance amplifiers (TIAs) and digitized with an ADC. The signal processing is performed with a LIGO style Control and Data System (CDS). The length changes in the interferometer are measured via phase tracking to a digital oscillator at the beat note frequency and control signals computed and distributed to the piezoelectric mirror mounts