One of the main current efforts in high-energy astrophysics and astroparticle physics is the development of multi-messenger astronomy, aiming at studying the universe and exploring fundamental physics through four complementary channels: photons, neutrinos, gravitational waves and cosmic rays. The latter have been observed since more than one century but remain very challenging as cosmic messengers because of their electric charge, which leads to deflections in the intervening magnetic fields between their sources and the Earth that essentially isotropize them and prevent direct (pointing) identification of their sources.
Of particular interest are the ultra-high-energy cosmic rays (UHECRs), with macroscopic energies up to 10²⁰ eV and beyond. At such energies, magnetic deflections are strongly reduced, but the price to pay is an extremely low flux, ~1 particle/m²/billion years, requiring very large observatories such as Auger (3000 km², southern hemisphere) and Telescope Array (700 km² being extended to 2500 km², northern hemisphere). These ground observatories allowed significant progress in the last two decades but proved insufficient to discover the sources and elucidate the origin of UHECRs, which remain the main objectives in the field. A thorough assessment of the situation led the involved international community to publish a detailed and extensive white paper (Coleman, et al., 2023) with key requirements for further progress, stressing the necessity of a complementary approach involving a space-based UHECR observatory to detect the fluorescence light of the so-called extensive air showers induced by the interaction of UHECRs in the atmosphere. 
The JEM-EUSO Collaboration (Joint Exploratory Missions for an Extreme Universe Space Observatory) has been anticipating this situation and developing the space road to UHECR study since ~15 years (e.g. Parizot, et al., 2023). The obvious advantage of a space observatory in low Earth orbit is to operate ~400 km away from the atmosphere (while maintaining low absorption) to cover ~200,000 km² of atmosphere in its field of view, considerably increasing the observation capabilities at extreme energies. Another essential advantage is the quasi-uniform coverage of the entire sky by a single instrument, to solve the existing conflicts between the energy spectra measured from the ground in different hemispheres.
The JEM-EUSO collaboration (10 countries, 160 members) has already developed three stratospheric balloon missions: EUSO-Balloon (CNES, 2014), EUSO-SPB (NASA, 2017), EUSO-SPB2 (NASA, 2023), and one space mission, MINI-EUSO, in operation onboard the ISS since 2019. Because of a significant leak developed in the last Super Pressure Balloon (SPB) provided by NASA, the flight of EUSO-SPB2 was interrupted after 2 days (instead of 100 planned), so its main scientific objectives could not be fulfilled. A new flight has thus been offered by NASA (spring 2027), intended as the final one before a full-scale UHECR space mission. The JEM-EUSO Collaboration will prepare this new mission, EUSO-SPB3, with an upgraded design involving a tiltable telescope with 2 cameras on the same focal surface (a fluorescence camera with MAPMTs to detect UHECR showers, and a Cherenkov camera with SiPMs to detect nearly horizontal showers along their axis, from high-altitude cosmic-ray and so-called “Earth-skimming” neutrinos). This will involve the key participation of the French teams supported by CNES since 2011 and the inaugural EUSO-Balloon mission (CNES 2014).
The selected PhD student will work on the instrumentation associated with the French workpackages in JEM-EUSO, namely the production, tests and absolute calibration of the detection units of the fluorescence camera, as well as the tests and calibration of the integrated instrument and the flight campaign. The optimization of the photodetection and trigger parameters will also take advantage of the analysis of the data of the MINI-EUSO mission, notably concerning the multidisciplinary objectives (TLEs: ELVES and Blues, meteors, SQM). Finally, the PhD student will take part in the development of innovative instrumentation for the search of Strange Quark Matter (SQM), in the framework of the SQM-ISS mission that was selected by ESA within the OSIP reserve pool program, of which a prototype will fly on EUSO-SPB3. Both developments have an important instrumental component in the domain of photodetection, with MAPMTs in single photoelectron counting mode and charge integration (EUSO-SPB3) and with SiPM reading plastic scintillators (SQM-ISS).
The optimal timing of the proposed PhD, given the mission schedule, will allow the PhD student to develop precious instrumental skills and follow all the steps of the mission: shower simulations, instrument response and trigger simulations, production of the detection units, tests and calibration in the lab, integration, field tests, flight campaign and first data analysis.