# Current Status and Future Developments of Micromegas Detectors for Physics and Applications
## 6 Micromegas Readout of TPCs
### Solar Axion Searches: CAST and IAXO  
Axions are predicted by the Peccei-Quinn mechanism proposed to solve the long-standing strong-CP problem in the Standard Model (SM) of particle physics. More generic Axion-Like Particles (ALPs) appear in diverse extensions of the SM (and notably in string theory). Axions and ALPs are invoked in a number of cosmological and astrophysical scenarios. Most relevantly, axions are very well suited candidates to compose all or part of  
Figure 17: The Micromegas Large Prototype of the LCTPC collaboration at the DESY II beam test facility. cold Dark Matter (DM). ALP fields appear in models of inflation, dark radiation, and even dark energy. A number of long-standing astrophysical anomalies could also be solved by the presence of axions or ALPs [78]. A rapidly growing landscape of experiments are now attempting to detect these particles. The efforts are roughly classified in three areas, depending on the source of axions: laboratory, solar, and dark matter axions. Axion helioscopes looking for solar axions represent the only approach that combines relative immunity to model assumptions (solar axion emission is a generic prediction of most axion models) plus a competitive sensitivity to parameters that are largely complementary to those accessible with other detection techniques. The most advanced axion helioscope is the CERN Axion Solar Telescope (CAST), active for more than 15 years at CERN. CAST has probed some QCD axion models in the 0.1-1 eV mass range. The latest CAST result sets an upper bound on the axion-photon coupling of \(g_{a7}<0.66\times 10^{-10}\) GeV\({}^{-1}\) up to \(m_{a}\sim 0.02\) eV [79]. This value competes with the strongest bound coming from astrophysics. Advancing beyond this bound is now highly motivated, not only because it would mean to venture into regions of parameter space allowed by astrophysics, but also because some of the aforementioned astrophysical anomalies would seem to hint at precisely the range of parameters at reach. IAXO is a new-generation large-scale axion helioscope that aims to search for solar axions with a signal-to-background ratio of about four to five orders of magnitude better than CAST. This translates into a factor of \(\sim\)20 in terms of the axion-photon coupling constant \(g_{a7}\). IAXO follows the conceptual layout of an enhanced axion helioscope, in which all the magnet aperture is equipped with focusing optics. This sensitivity relies on the construction of a large superconducting eight-coil toroidal magnet optimized for axion research. Each of the magnet bores features X-ray optics focusing the signal photons into small spots that are imaged by low background X-ray detectors [80]. BabyIAXO was conceived as a first experimental stage toward IAXO. The BabyIAXO systems will have dimensions representative of the final infrastructure, and therefore they constitute risk-mitigating prototypes for IAXO. The BabyIAXO magnet will feature two 10 m long, 70 cm diameter bores, and two detection lines (optics and detector) of dimensions similar to the final ones foreseen for IAXO. The BabyIAXO infrastructure is described in detail [81]. The physics case of the experiment has been reviewed in detail in [82]. The required BabyIAXO sensitivity imposes very stringent constraints on the background levels needed for the X-ray detectors, i.e., \(10^{-7}\) c/keV/cm\({}^{2}\)/s a factor of 10 better than the current levels. The baseline detection technology in BabyIAXO are small TPCs with 2-dimensional Micromegas readouts built with the Microbulk technology [11]. These detectors have been the object of intensive low-background development within the CAST experiment [21; 83; 84].