The axion, a pseudoscalar particle originally introduced by Peccei, Quinn [1, 2], Weinberg [3], and Wilczek [4] to solve the "strong CP problem", is a well motivated dark-matter (DM) candidate with a mass lying in a broad range from peV to few meV [5]. The last decade witnessed an increasing interest in axions and axion-like particles with many theoretical works published and many new experimental proposals [6] that started a real race towards their discovery. Driven by this new challenge and stimulated by the availability, at the Laboratori Nazionali di Frascati (LNF), of large superconducting magnets previously used for particle detectors [7, 8] at the DAFNE collider, we proposed to build a large haloscope [9] to observe galactic axions in the mass window between 0.2 and 1 µeV [10]. This paper is the Conceptual Design Report (CDR) of the KLASH (KLoe magnet for Axion SearcH) experiment, designed having in mind the performance and dimensions of the KLOE magnet, a large volume superconducting magnet with a moderate magnetic field of 0.6 T. In the first part of this Report we discuss the physics case of KLASH, the theoretical motivation for an axion in the mass window 0.1÷1µeV based on a review of standard and non-standard axion-cosmology (Sec. 1), and the physics reach of the KLASH experiment (Sec. 2), including both the sensitivity to QCD axions and to Dark-Photon DM. The sensitivity plots are based on the detector performance discussed in the second part of the CDR. Here, we summarize the results obtained with calculations and simulations of several aspects of the experiment: the mechanical construction of cryostat and cavity based on the study commissioned to the mechanical engineers of the Fantini-Sud company [11] (Sec. 3); the cryogenics plant (Sec. 4); the RF cavity design and tuning based on detailed simulations with code Ansys-HFSS (Sec. 5); the signal amplification, in particular the first stage based on a Microstrip SQUID Amplifier (Sec. 6). Finally, in Sec. 7, mainly based on the experience of existing experiments [12–14], we discuss the data taking, analysis procedure and computing requirements. The main conclusion we draw from this report is the possibility to build and put in operation at LNF in 2-3 years a large haloscope with the sensitivity to KSVZ axions in the low mass range between 0.2 and 1µeV in a region complementary to that of other experiments with a cost of about 3 MAC. Timeline and cost are competitive with respect to other proposals in the same mass region [15, 16] thanks to the availability of most of the infrastructure, in particular the superconducting magnet and the cryogenics plant. During the writing of this CDR, in July 2019, we were informed about the decision of INFN management to devote the KLOE magnet to the DUNE experiment at Fermilab. The KLOE magnet has always been the preferred choice for several reasons: it was in operation until 2018; its mechanical structure is able to support the several-tons weight of the cryostat and cavity; it is placed in the KLOE assembly-hall that can be used as the experimental area of KLASH. However, another option is given by the FINUDA magnet. The are few aspects to be explored (mechanical strength, move to experimental area, put in operation after more than 10 years), but it has a higher nominal field of 1.1 T in a large volume with an inner radius 1385 mm and length 3800 mm. A preliminary estimate of sensitivity to axions of FLASH, the haloscope built with the FINUDA magnet, gives results similar to those obtained for KLASH. This option will be eventually investigated in another document
