Successful Al₂O₃ coating of superconducting niobium cavities with thermal ALD

Abstract

Surface modification of superconducting radiofrequency (SRF) cavities is mandatory to further push the limits in future accelerators. One strategy is the deposition of a multilayer of superconducting and insulating materials on top of the inner surface of a SRF cavity. Here we report on a successful low-temperature coating of a SRF cavity with insulating Al2O3 by thermal atomic layer deposition (ALD) without mitigating its maximum achievable accelerating field of more than 40\,MV/m. Furthermore, an improvement of the surface resistance above 30\,MV/m has been observed, which is likely caused by an enhanced oxygen diffusion during the deposition process. Our results show that ALD is perfectly suited to conformally coat the interior of the cavity and to even modify and improve the properties of such devices.

Reducing the Thermal Effects during Coating of Superconducting Radio-Frequency Cavities: A Case Study for Atomic Layer Deposition of Alumina with a Combined Numerical and Experimental Approach

“…A transient three-dimensional (3D) digital twin is generated mirroring the setup of a homemade 23 ALD system. The model is simulated within the ANSYS Fluent framework.…”

“…In our previous experimental work, 23 the ALD chamber's base pressure was maintained at 10 −1 Pa through continuous operation of a vacuum pump. Nitrogen gas was utilized as a carrier and purge gas, set at a volumetric flow rate of 20 sccm.…”

“…There are handful studies reported on the experimental work of SRF cavities coated with ALD Al 2 O 3 . − Proslier et al utilized thermal ALD to coat cavities with a thin layer of Al 2 O 3 and Al 2 O 3 /Nb 2 O 5 bilayers at ≥200 °C. Their results indicated a significant increase in field emission and multipacting, limiting several cavities to around 20 MV/m and causing an increase in surface resistance .…”

“…20,21 These qualities make Al 2 O 3 a promising material to be used as an insulating layer which has been shown to preserve Nb superconductivity. 22,23 There are handful studies reported on the experimental work of SRF cavities coated with ALD Al 2 O 3 . 23−27 bilayers at ≥200 °C.…”

“…In contrast, our own research leads to a successful low-temperature coating of 1.3 GHz Tesla-shaped SRF cavities at 120 °C with insulating Al 2 O 3 using thermal ALD. 23 The coating process involved pulsing TMA for 500 ms, followed by an exposure time of 60 s, and purging for 120 s during the first half cycle. In the second half cycle, water was pulsed for 500 ms, followed by an exposure time of 60 s and purging for 120 s. Remarkably, this coating did not compromise the cavity's maximum achievable accelerating field, which exceeded 40 MV/m.…”