We review the physics potential of a next generation search for solar axions: the International Axion Observatory (IAXO). Endowed with a sensitivity to discover axion-like particles (ALPs) with a coupling to photons as small as g aγ ∼ 10 −12 GeV −1 , or to electrons g ae ∼10 −13 , IAXO has the potential to find the QCD axion in the 1 meV∼1 eV mass range where it solves the strong CP problem, can account for the cold dark matter of the Universe and be responsible for the anomalous cooling observed in a number of stellar systems. At the same time, IAXO will have enough sensitivity to detect lower mass axions invoked to explain: 1) the origin of the anomalous "transparency" of the Universe to gamma-rays, 2) the observed soft X-ray excess from galaxy clusters or 3) some inflationary models. In addition, we review string theory axions with parameters accessible by IAXO and discuss their potential role in cosmology as Dark Matter and Dark Radiation as well as their connections to the above mentioned conundrums.

This article describes BabyIAXO, an intermediate experimental stage of the International Axion Observatory (IAXO), proposed to be sited at DESY. IAXO is a large-scale axion helioscope that will look for axions and axion-like particles (ALPs), produced in the Sun, with unprecedented sensitivity. BabyIAXO is conceived to test all IAXO subsystems (magnet, optics and detectors) at a relevant scale for the final system and thus serve as prototype for IAXO, but at the same time as a fully-fledged helioscope with relevant physics reach itself, and with potential for discovery. The BabyIAXO magnet will feature two 10 m long, 70 cm diameter bores, and will host two detection lines (optics and detector) of dimensions similar to the final ones foreseen for IAXO. BabyIAXO will detect or reject solar axions or ALPs with axion-photon couplings down to gaγ ∼ 1.5 × 10−11 GeV−1, and masses up to ma ∼ 0.25 eV. BabyIAXO will offer additional opportunities for axion research in view of IAXO, like the development of precision x-ray detectors to identify particular spectral features in the solar axion spectrum, and the implementation of radiofrequency-cavity-based axion dark matter setups.

With the first tokamak designed for full nuclear operation now well into final assembly (ITER), and a major new research tokamak starting commissioning (JT60SA), nuclear fusion is becoming a mainstream potential energy source for the future. A critical part of the viability of magnetic confinement for fusion is superconductor technology. The experience gained and lessons learned in the application of this technology to ITER and JT60SA, together with new and improved superconducting materials, is opening multiple routes to commercial fusion reactors. The objective of this roadmap is, through a series of short articles, to outline some of these routes and the materials/technologies that go with them.

This article describes BabyIAXO, an intermediate experimental stage of the International Axion Observatory (IAXO), proposed to be sited at DESY. IAXO is a largescale axion helioscope that will look for axions and axion-like particles (ALPs), produced in the Sun, with unprecedented sensitivity. BabyIAXO is conceived to test all IAXO subsystems (magnet, optics and detectors) at a relevant scale for the final system and thus serve as prototype for IAXO, but at the same time as a fully-fledged helioscope with relevant physics reach itself, and with potential for discovery. The BabyIAXO magnet will feature two 10 m long, 70 cm diameter bores, and will host two detection lines (optics and detector) of dimensions similar to the final ones foreseen for IAXO. BabyIAXO will detect or reject solar axions or ALPs with axion-photon couplings down to g aγ ∼ 1.5 × 10 −11 GeV −1 , and masses up to m a ∼ 0.25 eV. BabyIAXO will offer additional opportunities for axion research in view of IAXO, like the development of precision x-ray detectors to identify particular spectral features in the solar axion spectrum, and the implementation of radiofrequency-cavity-based axion dark matter setups. 7 BabyIAXO structure and drive system 58 7.1 Requirements 58 7.2 Foundations 60 7.3 Positioner: tower, head and yokes 61 7.4 Mechanical elements of the drive assemblies 63 7.5 Support frame 64 8 BabyIAXO site and infrastructure 65 8.1 Civil engineering 65 8.2 HERA South Hall 66 8.3 Data acquisition and control 67 9 Conclusions 68 10 Acknowledgements 68 * Notice, however, that the recent analysis in Ref. [48] indicates no exotic energy loss.

High temperature superconducting (HTS) materials are widely utilized in various design proposals for fusion magnets, resulting in enhanced performance of the machines compared to the past. However, a reliable quench detection in HTS conductors remains an open issue. Using a co-wound superconducting wire of high normal state resistance as an electrically insulated and thermally coupled sensor provides strongly increased sensitivity for the voltage-based quench detection methods. Furthermore, resistance of the wire can be practically proportional to the size of the normal zone, even though the location of the hot-spot cannot be identified. We present adaptation of this method for fusion conductors by considering various wire options, such as MgB2 wires in a highly resistive matrix, non-stabilized Nb3Sn wires and (K,Na)-Ba122 wires. The insulated wire of a small diameter (<1 mm) can be embedded in the steel jacket, thus barely affecting the conductor design and manufacturing aspects. Alternatively, if installed within the cable space, the wires might even allow monitoring of quench dynamics among the strands. Our first experimental demonstration is planned in a sub-scale ReBCO cable-in-conduit sample, which will be tested in the SULTAN test facility recently upgraded for DC operation in resistive samples with the transport currents up to 15 kA and maximum voltage of 10 V.