(The) first high-temperature superconducting cavities in axion dark matter search

Abstract

Superconducting radio frequency (SRF) technology has played a significant role in the progress of precision measurements in particle physics experiments for decades. Especially dark matter axion haloscope, which utilizes a microwave cavity (> 1 GHz) immersed in a high DC magnetic field (> 8 T), needs high quality (Q) factor superconducting cavities to enhance a scanning speed. High-temperature superconducting (HTS) rare-earth barium copper oxide (ReBCO) tapes are excellent material for realizing the high Q factor superconducting cavities for the axion search due to ultra-low RF surface resistances (< 0.1 mOhm) at high magnetic fields and high depinning frequencies (> 10 GHz). Moreover, the ReBCO surface resistance can be minimized using additional pinning center (APC) which enhances vortex pinning. The Center for Axion and Precision Physics Research (CAPP) successfully fabricated the three generations of HTS cavities made of biaxially-textured ReBCO tapes. The Q factor of the cavities in an 8 T magnetic field have been improved. The first generation 7 GHz prototype cavity (YBCO) showed 330,000 Q factor, and the second generation 2.3 GHz 1.5 L volume cavity (GdBCO) reached a half-million Q factor. Finally, the third generation cavities (EuBCO + APC) reached 13 million Q factor in a 8 T magnetic field. The 13 million Q factor is the highest Q factor among the SRF cavities in a multi-tesla magnetic field. In this thesis, the material selection, rf design, and fabrication procedure will be discussed. The cavity haloscope experiment with a second generation cavity in CAPP-PACE detector also will be discussed. The experiment reached the highest Q factor and the lowest system noise (~ 175 mK) among published axion haloscope searches in phase-insensitive operation using quantum-noise-limited JPA. The data analysis and the signal-to-noise ratio issue in the baseline removal will be discussed.