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Conceptual Design of the International Axion Observatory (IAXO)

E. Armengaud, F. T. Avignone, M. Betz, P. Brax, P. Brun, G. Cantatore, J. M. Carmona, G. P. Carosi, F. Caspers, S. Caspi, S. A. Cetin, D. Chelouche, F. E. Christensen, A. Dael, T. Dafni, M. Davenport, A. V. Derbin, K. Desch, A. Diago, B. Döbrich, I. Dratchnev, A. Dudarev, C. Eleftheriadis, G. Fanourakis, E. Ferrer-Ribas, J. Galán, J. A. García, J. G. Garza, T. Geralis, B. Gimeno, I. Giomataris, S. Gninenko, H. Gómez, D. González-Díaz, E. Guendelman, C. J. Hailey, T. Hiramatsu, D. H. H. Hoffmann, D. Horns, F. J. Iguaz, I. G. Irastorza, J. Isern, K. Imai, A. C. Jakobsen, J. Jaeckel, K. Jakovčić, J. Kaminski, M. Kawasaki, M. Karuza, M. Krčmar, K. Kousouris, C. Krieger, B. Lakić, O. Limousin, A. Lindner, A. Liolios, G. Luzón, S. Matsuki, V. N. Muratova, C. Nones, I. Ortega, T. Papaevangelou, M. J. Pivovaroff, G. Raffelt, J. Redondo, A. Ringwald, S. Russenschuck, J. Ruz, K. Saikawa, I. Savvidis, T. Sekiguchi, Y. K. Semertzidis, I. Shilon, P. Sikivie, H. Silva, H. ten Kate, A. Tomas, S. Troitsky, T. Vafeiadis, K. van Bibber, P. Vedrine, J. A. Villar, J. K. Vogel, L. Walckiers, A. Weltman, W. Wester, S. C. Yildiz, K. Zioutas

TL;DR

The paper proposes the International Axion Observatory (IAXO), a forth-generation axion helioscope designed to detect solar axions/ALPs with a large eight-coil toroidal magnet, eight 0.6 m bores with x-ray optics, and ultra-low-background Micromegas detectors, enabling a sensitivity gain of roughly 4–5 orders of magnitude over CAST. It details the magnet design, figure-of-merit optimization, cryogenics, quench protection, and modular infrastructure, including a T0 prototype coil plan and reliable operation under solar tracking of about 12 h/day. The x-ray optics are baselined on segmented, slumped glass with 8 telescopes of 123 nested layers, optimized for a focal length near 5 m to achieve a spot size around 1 mrad and throughput compatible with the solar axion spectrum in the 1–10 keV range; the detectors aim for backgrounds near $10^{-7}$ counts keV$^{-1}$ cm$^{-2}$ s$^{-1}$ or lower. The paper also discusses alternative detectors (GridPix, TES, CCDs) and microwave cavity concepts to broaden IAXO’s physics reach, including searches for dark radiation, relic axions, and other WISPs, positioning IAXO as a multi-purpose facility for axion/ALP research in the coming decade.

Abstract

The International Axion Observatory (IAXO) will be a forth generation axion helioscope. As its primary physics goal, IAXO will look for axions or axion-like particles (ALPs) originating in the Sun via the Primakoff conversion of the solar plasma photons. In terms of signal-to-noise ratio, IAXO will be about 4-5 orders of magnitude more sensitive than CAST, currently the most powerful axion helioscope, reaching sensitivity to axion-photon couplings down to a few $\times 10^{-12}$ GeV$^{-1}$ and thus probing a large fraction of the currently unexplored axion and ALP parameter space. IAXO will also be sensitive to solar axions produced by mechanisms mediated by the axion-electron coupling $g_{ae}$ with sensitivity $-$for the first time$-$ to values of $g_{ae}$ not previously excluded by astrophysics. With several other possible physics cases, IAXO has the potential to serve as a multi-purpose facility for generic axion and ALP research in the next decade. In this paper we present the conceptual design of IAXO, which follows the layout of an enhanced axion helioscope, based on a purpose-built 20m-long 8-coils toroidal superconducting magnet. All the eight 60cm-diameter magnet bores are equipped with focusing x-ray optics, able to focus the signal photons into $\sim 0.2$ cm$^2$ spots that are imaged by ultra-low-background Micromegas x-ray detectors. The magnet is built into a structure with elevation and azimuth drives that will allow for solar tracking for $\sim$12 h each day.

Conceptual Design of the International Axion Observatory (IAXO)

TL;DR

The paper proposes the International Axion Observatory (IAXO), a forth-generation axion helioscope designed to detect solar axions/ALPs with a large eight-coil toroidal magnet, eight 0.6 m bores with x-ray optics, and ultra-low-background Micromegas detectors, enabling a sensitivity gain of roughly 4–5 orders of magnitude over CAST. It details the magnet design, figure-of-merit optimization, cryogenics, quench protection, and modular infrastructure, including a T0 prototype coil plan and reliable operation under solar tracking of about 12 h/day. The x-ray optics are baselined on segmented, slumped glass with 8 telescopes of 123 nested layers, optimized for a focal length near 5 m to achieve a spot size around 1 mrad and throughput compatible with the solar axion spectrum in the 1–10 keV range; the detectors aim for backgrounds near counts keV cm s or lower. The paper also discusses alternative detectors (GridPix, TES, CCDs) and microwave cavity concepts to broaden IAXO’s physics reach, including searches for dark radiation, relic axions, and other WISPs, positioning IAXO as a multi-purpose facility for axion/ALP research in the coming decade.

Abstract

The International Axion Observatory (IAXO) will be a forth generation axion helioscope. As its primary physics goal, IAXO will look for axions or axion-like particles (ALPs) originating in the Sun via the Primakoff conversion of the solar plasma photons. In terms of signal-to-noise ratio, IAXO will be about 4-5 orders of magnitude more sensitive than CAST, currently the most powerful axion helioscope, reaching sensitivity to axion-photon couplings down to a few GeV and thus probing a large fraction of the currently unexplored axion and ALP parameter space. IAXO will also be sensitive to solar axions produced by mechanisms mediated by the axion-electron coupling with sensitivity for the first time to values of not previously excluded by astrophysics. With several other possible physics cases, IAXO has the potential to serve as a multi-purpose facility for generic axion and ALP research in the next decade. In this paper we present the conceptual design of IAXO, which follows the layout of an enhanced axion helioscope, based on a purpose-built 20m-long 8-coils toroidal superconducting magnet. All the eight 60cm-diameter magnet bores are equipped with focusing x-ray optics, able to focus the signal photons into cm spots that are imaged by ultra-low-background Micromegas x-ray detectors. The magnet is built into a structure with elevation and azimuth drives that will allow for solar tracking for 12 h each day.

Paper Structure

This paper contains 40 sections, 8 equations, 21 figures, 2 tables.

Table of Contents

  1. Introduction
  2. The IAXO superconducting magnet
  3. Figure of merit and lay-out optimization
  4. Conductor
  5. Peak magnetic field and forces
  6. Stability analysis
  7. Electrical circuit and quench protection
  8. Cold-mass
  9. Cryostat and its movement system
  10. Cryogenics
  11. Magnet system reliability and fault scenarios
  12. Cryostat assembly procedure and integration
  13. T0 prototype coil for design validation and risk mitigation
  14. The IAXO x-ray optics
  15. Basic considerations
  16. ...and 25 more sections

Figures (21)

  • Figure 1: Conceptual arrangement of an enhanced axion helioscope with x-ray focalization. Solar axions are converted into photons by the transverse magnetic field inside the bore of a powerful magnet. The resulting quasi-parallel beam of photons of cross sectional area $A$ is concentrated by an appropriate x-ray optics into a small spot area $a$ in a low background detector. The envisaged implementation in IAXO (see figure \ref{['fig:IAXO_sketch']}, includes eight such magnet bores, with their respective optics and detectors.
  • Figure 2: Schematic view of IAXO. Shown are the cryostat, eight x-ray optics and detectors, the flexible lines guiding services into the magnet, cryogenics and powering services units, inclination system and the rotating disk for horizontal movement. The dimensions of the system can be appreciated by a comparison to the human figure positioned by the rotating table.
  • Figure 3: Illustration of the two principle angular alignment options considered for the optics with respect to the coils. The rectangles represent the toroid's coil and the circles represent the optics' bores. (a) "Field dominated" alignment: optics behind the coils. (b) "Area dominated" alignment: optics in between the coils.
  • Figure 4: Cross section of the two double pancake winding packs, the coil casing (top) and the conductor with a 40 strands NbTi/Cu Rutherford cable embedded in a dilute Al-0.1wt%Ni doped stabilizer (bottom).
  • Figure 5: Schematic diagram of the electrical circuit and quench protection scheme. Shown are the power convertor, the eight coils, quench heaters (QH 1-8), the slow dump circuit and the quench detection circuit.
  • ...and 16 more figures