This Conceptual Design Report describes LUXE (Laser Und XFEL Experiment), an experimental campaign that aims to combine the high-quality and high-energy electron beam of the European XFEL with a powerful laser to explore the uncharted terrain of quantum electrodynamics characterised by both high energy and high intensity. We will reach this hitherto inaccessible regime of quantum physics by analysing high-energy electron-photon and photon-photon interactions in the extreme environment provided by an intense laser focus. The physics background and its relevance are presented in the science case which in turn leads to, and justifies, the ensuing plan for all aspects of the experiment: Our choice of experimental parameters allows (i) field strengths to be probed where the coupling to charges becomes non-perturbative and (ii) a precision to be achieved that permits a detailed comparison of the measured data with calculations. In addition, the high photon flux predicted will enable a sensitive search for new physics beyond the Standard Model. The initial phase of the experiment will employ an existing 40 TW laser, whereas the second phase will utilise an upgraded laser power of 350 TW. All expectations regarding the performance of the experimental set-up as well as the expected physics results are based on detailed numerical simulations throughout.

Results obtained from numerical calculations of and experimental studies on the pulse height distribution inherent in ionizing radiation gallium arsenide sensors as a function of the design features of the devices and electrophysical characteristics of the detector material are presented. It is shown that the pulse height distribution is defined by the distribution pattern of the nonequilibrium charge carrier lifetime and by the electric field profile in the bulk of the sensor. Investigations on the detector sensitivity to X-ray energies in the range between 40 and 150 keV were performed. The sensor polarization was found to produce only a marginal effect compensated by an increase in the bias voltage. Prototype pixel sensors measuring 256 × 256 and 512 × 768 pixels with a 55 µm pitch and a 500 µm thick sensitive layer were produced. The dependence of the photocurrent and count rate on the X-ray radiation intensity and bias voltage applied to the sensor was examined. In the 40-80 keV energy range, the maximum count rate amounted to 800 kHz/pixel for a negative sensor bias voltage of 800 V. The sensors are demonstrated to provide spatial resolution varying with the pixel pitch and to enable high-quality X-ray images to be obtained.

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