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FELs deliver rapid pulses on the femtosecond scale, and high peak intensities that fluctuate strongly on a pulse-to-pulse basis. The fast drift velocity and high radiation tolerance properties of chemical vapor deposition (CVD) diamonds make these crystals a good candidate material for developing a high frame rate pass-through diagnostic for the next generation of XFELs. We report on two diamond based diagnostic systems being developed by a collaboration of a UC campuses and National Laboratories supported by the University of California and the SLAC National Laboratory. For the first of these diagnostic systems, we have developed a new approach to the readout of diamond diagnostic sensors designed to facilitate operation as a passthrough detection system for high frame-rate XFEL diagnostics. Making use of the X-ray Pump Probe (XPP) beam at the Linac Coherent Light Source (LCLS), the performance of this new diamond sensor system has been characterized and compared to that of a commercially available system. Limits in the magnitude and speed of signal charge collection are explored as a function of the generated electron-hole plasma density and compared to results from a TCAD simulation. A leading proposal for improving the efficiency of producing longitudinally coherent FEL pulses is the cavity-based X-ray free electron laser (CBFEL). In this configuration, the FEL pulses are recirculated within an X-ray cavity in such a way that the fresh electron bunches interact with the FEL pulses stored in the cavity over multiple passes. This creates a need for diagnostics that can measure the intensity and centroid of the X-ray beam on every pass around the recirculatory path. For the second of these diagnostic systems, we have created a four-channel, position-sensitive pass-through diagnostic system that can measure the intensity and centroid of the circulating beam with a repetition rate in excess of 20 MHz. The diagnostic makes use of a planar diamond sensor thinned to 43 µm to allow for minimal absorption and wave-front distortion of the circulating beam. We present results on the response and position sensitivity of the diagnostic, again measured using the LCLS XPP beam.

X-ray FELs deliver rapid pulses on the femtoseconds scale, and high peak intensities that fluctuate strongly on a pulse-to-pulse basis. The fast drift velocity, and high radiation tolerance properties of CVD (chemical vapor deposition) diamonds, make these crystals a good candi-date material for developing a multi-hundred MHz pass-through diagnostic for the next generation of XFELs. Commercially available diamond sensors work as posi-tion-sensitive pass-through diagnostics for nJ-level pulses from synchrotrons. Supported by the University of Cali-fornia and the SLAC National Laboratory, a collaboration of UC campuses and National Laboratories have devel-oped a new approach to the readout of diamond diagnostic sensors designed to facilitate operation for FEL-relevant uJ and mJ pulses. Single-crystal diamond detectors have been tested on the XPP end station of the Linac Coherent Light Source beam at SLAC. We present results on the linearity and charge collection characteristics as a function of the density of deposited charge., Proceedings of the 10th International Beam Instrumentation Conference, IBIC2021, Pohang, Rep. of Korea