Dr
Woohyun Chung
(IBS/CAPP)
A high Q-factor microwave resonator in a high magnetic field could be of great use in a wide range of fields, from accelerator design to axion dark matter search. IBS/CAPP in Korea is one of those who would like to enhance the sensitivity to detect dark matter axions by boosting the Q-factor of the resonant cavity, taking advantage of a recent advance of high temperature superconductor (HTS) technology. HTS is a natural choice of superconducting material for the resonator in a high field because of its high upper critical field (~100 T) and strong vortex pinning characteristics. The deposition, however, of a high-quality, grain-aligned HTS film on a three-dimensional surface is technically challenging. We have applied biaxially-textured commercial YBa2Cu3O7−x (YBCO) tapes to the inner surface of a polygon-shaped resonant cavity to overcome this problem. The measurement showed that a clear superconducting phase transition occurred around 90 K and a very high Q-factor of 330,000 (about 6 times higher than the same size cavity with OFHC Cu) at 4 K at 6.93 GHz (TM010 mode) was maintained even in a high magnetic field of 8 Tesla. We demonstrated that the high Q-factor of the superconducting YBCO resonant cavity showed no significant degradation from 1 T up to 8 T with our technique. This is the first indication of the possible applications of HTS technology to the research areas requiring a strong magnetic field at high radio frequencies.
Summary
The axion is a hypothesized particle predicted to resolve strong CP problem in the standard model and is also an excellent cold dark matter candidate. It could be detected as microwave photons converted from axions inside the resonant cavity under a strong magnetic field. It is extremely challenging to detect such a feeble signal (10-24 W) over background noise (mostly from thermal and from amplifier noise). The IBS Center for Axion and Precision Physics Research (CAPP) is conducting various R&D’s to enhance the sensitivities of the experiment and developing a superconducting resonant cavity is one of them. Even though YBCO is a perfect high temperature superconductor (HTS) material for the superconducting cavity, it seems almost impossible to grow YBCO on to the inner surface of the cylindrical shape cavity. We utilize commercially available, grain-aligned YBCO tapes and design the vertically split, polygon cavity to overcome the difficulty. The fabricated cavity with YBCO tapes shows six times higher Q-factor than the normal Cu cavity and sustains its superconductivity without much degradation up to 8 Tesla magnetic field successfully. Considering that there are issues to be improved further, the Q-factor should be better near in the future.
Dr
Woohyun Chung
(IBS/CAPP)
Mr
Danho Ahn
(IBS/CAPP, KAIST)
