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We will report on the ongoing ThomX ring commissioning, its status, its main challenges, our results and our planning. ThomX is a compact Compton-based X-ray source under commissioning at IJCLab in Orsay (France). This facility is composed of a 50-70 MeV linac, a transfer line and a storage ring whose circumference is 18 m. Compton scattering between the 50 MeV electron bunch of 1 nC and the 30 mJ laser pulses stacked in a Fabry-Perot cavity will result in the production of X-rays with energy ranging between 45 keV and 90 keV. We aim at a total flux of about 10 13 X-rays per second. The injector commissioning started in the spring of 2021. The ongoing storage ring commissioning faces many challenges due to the rings low energy, its compactness, its non-linear beam dynamics, the time-limited beam storage and the need to achieve a very accurate and stable geometry of the collision region between the laser pulses and the electron bunch. The commissioning and operational experience is of great importance for the future Compton sources.

An Inverse Compton Scattering (ICS) X-ray source is under development at the ELSA (Electrons Laser X-Sources and Applications) electron RF linac of CEA DAM (Commissariat à l’Energie Atomique et aux Energies Alternatives, Direction des Applications Militaires). X-rays are emitted by the interaction of 30 MeV electron bunches with Nd:YAG laser pulses. The electron bunches duration is 30 ps rms before compression in two alpha magnets. In such a system, electron trajectories are curved with a short radius, resulting in a noticeable degradation of the beam emittance. In the specific case of strongly curved trajectories, the straight trajectory and beam shape assumptions used for space charge calculation in most simulation codes are questionable. Two different approaches to the simulation of electron beam dynamics within the alpha magnets are compared. A specific method to deal with changes from the reference particle frame to the laboratory reference frame, which does not imply any trajectory and beam shape assumptions, is proposed. Calculation results are presented. Along with an important emittance growth, they show that more physical effects can be taken into account in the latter simulation.

The I.FAST CBI is an immersive challenge-based innovation program funded by the H2020 I.FAST project. The 10-day face-to-face challenge brings together students of different disciplines from all over Europe to work together on innovative projects using accelerator technology applied to environmental challenges. We report on the first edition of the I.FAST CBI, the proposed projects and feedback from the students.

A multidisciplinary collaboration within the I.FAST project teamed-up to develop additive manufacturing (AM) technology solutions for accelerators. The first prototype of an AM pure-copper Radio Frequency Quadrupole (RFQ) has been produced, corresponding to ¼ of a 4-vane RFQ. It was optimised for production with state-of-the-art laser powder bed fusion technology. Geometrical precision and roughness of the critical surfaces were measured. Although the obtained values were beyond standard RFQ specifications, these first results are promising and confirmed the feasibility of AM manufactured complex copper accelerator cavities. Therefore, further post-processing trials have been conducted with the sample RFQ to improve surface roughness. Algorithms for the AM technological processes have also been improved, allowing for higher geometrical precision. This resulted in the design of a full 4-vane RFQ prototype. At the time of the paper submission the full-size RFQ is being manufactured and will undergo through the stringent surface quality measurements. This paper is discussing novel technological developments, is providing an evaluation of the obtained surface roughness and geometrical precision as well as outlining the potential post-processing scenarios along with future tests plans.

A new kind of nonresonant optical recirculator, dedicated to the production of γ rays by means of Compton backscattering, is described. This novel instrument, inspired by optical multipass systems, has its design focused on high flux and very small spectral bandwidth of the γ -ray beam. It has been developed to fulfill the project specifications of the European Extreme Light Infrastructure “Nuclear Pillar,” i.e., the Gamma Beam System. Our system allows a single high power laser pulse to recirculate 32 times synchronized on the radio frequency driving accelerating cavities for the electron beam. Namely, the polarization of the laser beam and crossing angle between laser and electrons are preserved all along the 32 passes. Moreover, optical aberrations are kept at a negligible level. The general tools developed for designing, optimizing, and aligning the system are described. A detailed simulation demonstrates the high efficiency of the device.

We have used coherent Smith-Purcell radiation (cSPr) in order to determine the temporal profile of sub-ps long electron bunches at the Facility for Advanced Accelerator Experimental Tests, at SLAC. The measurements reported here were carried out in June 2012 and April 2013. The rms values for the bunch length varied between 356 to 604 fs, depending on the accelerator settings. The resolution of the system was limited by the range of detectable wavelengths which was, in turn, determined by the choice of the grating periods used in these experiments and the achievable beam-grating separation. The paper gives the details of the various steps in the reconstruction of the time profile and discusses possible improvements to the resolution. We also present initial measurements of the polarization properties of cSPr and of the background radiation.

Following previous results which have shown that some components built using additive manufacturing (3D printing) are compatible with ultra high vacuum, we have adapted the design of a stripline BPM to the requirements of additive manufacturing and built it. We report here on the design adaptation and on its mechanical and electrical performances.

We report and discuss high-flux generation of circularly polarized γ -rays by means of Compton scattering. The γ -ray beam results from the collision of an external-cavity-enhanced infrared laser beam and a low emittance relativistic electron beam. By operating a non-planar bow-tie high-finesse optical Fabry-Perot cavity coupled to a storage ring, we have recorded a flux of up to (3.5 ± 0.3) × 10 8 photons per second with a mean measured energy of 24 MeV. The γ -ray flux has been sustained for several hours. In particular, we were able to measure a record value of up to 400 γ -rays per collision in a full bandwidth. Moreover, the impact of Compton scattering on the electron beam dynamics could be observed resulting in a reduction of the electron beam lifetime correlated to the laser power stored in the Fabry-Perot cavity. We demonstrate that the electron beam lifetime provides an independent and consistent determination of the γ -ray flux. Furthermore, a reduction of the γ -ray flux due to intrabeam scattering has clearly been identified. These results, obtained on an accelerator test facility, warrant potential scaling and revealed both expected and yet unobserved effects. They set the baseline for further scaling of the future Compton sources under development around the world.

The Horizon 2020 project EuPRAXIA (European Plasma Research Accelerator with eXcellence In Applications) is producing a conceptual design report for a highly compact and cost-effective European facility with multi-GeV electron beams accelerated using plasmas. EuPRAXIA will be set up as a distributed Open Innovation platform with two construction sites, one with a focus on beam-driven plasma acceleration (PWFA) and another site with a focus on laser-driven plasma acceleration (LWFA). User areas at both sites will provide access to free-electron laser pilot experiments, positron generation and acceleration, compact radiation sources, and test beams for high-energy physics detector development. Support centres in four different countries will complement the pan-European implementation of this infrastructure.

The experimental investigation of the transition radiation (TR) generated by a \"half-bare\" electron having the proper field different from the Coulomb one is proposed. The electrons in half-bare state are intended to be obtained in the result of their crossing of a conducting screen. We propose to investigate the influence of the half-bare state of electron in this process upon TR generated by such electron on a downstream OTR screen situated on some distance along the direction of the electron beam from the upstream screen which \"undresses\" the particle. Calculations are presented for the case of a 45 MeV linac and the distance between the screens in the region between 100 mm and 300 mm. The proposed experiment is expected to reveal new features of TR signal in such process comparing to previous measurements.