The Surface Treatment of Niobium Superconducting Reentrant Cavities by Means of High Temperature Nitrogen Plasma Based Ion Implantation

Rogério Moraes Oliveira,Carina B Mello,Elvis Ferreira,Michel Felipe Lima De Araujo,Odylio Denys Aguiar,Graziela Da Silva Savonov,Renata Lopes Gonçalves De Souza,Vincenzo Liccardo,Matheus M N F Silva,Koumei Baba

doi:10.1590/1980-5373-mr-2019-0277

Abstract

High Temperature Nitrogen Plasma Based Ion Implantation (HT-NPBII) has been used to treat the surface of niobium superconducting reentrant cavities, which are part of parametric transducers in a resonant-mass gravitational wave detector. The aim is to enhance the corresponding electrical quality factors (Q-factors) which are closely related with the increase of the sensitivity of the system. In this experiment, the cavities are immersed in plasma and bombarded by energetic nitrogen ions, which are implanted into the surfaces of these heated substrates. The heating temperature of the cavities is controlled during the treatment and its level directly affects the N implantation depth profile due to the diffusion process. Additional tailoring of the nitrogen doping can be performed by the adjustment of the intensity and the duty cycle of the high negative voltage pulses used for ion implantation. For implantations performed at 5 kV /20 µs /300 Hz/ 700 °C, nitrogen atoms occupy interstitial spaces in the crystal lattice of niobium. The treatment of niobium superconducting cavities under these parameters caused the enhancement of two orders of magnitude of respective Q-factors. A set of characterization techniques was performed herein in order to help with the understanding of the underlying mechanism behind this phenomenon.

Highlights

Niobium superconducting reentrant cavities have been used as parametric transducers of the resonant-mass gravitational wave detector Mario Schenberg [MS]1-2, in order to monitor the vibrational state of its massive spherical antenna, which resonates at about 3.2 kHz (f0)
Extensive investigation has systematically been accomplished for the improvement of the Q-factors of superconducting niobium cavities, with especial attention devoted to the superconducting RF (SRF) cavities used in particle accelerators[7,8,9,10,11,12,13,14,15,16,17]
For Nb samples submitted to pulses of 10kV/20μs/500Hz and heated at 1200 °C, for 1h, the attained elemental depth profile shown in Fig. 4 indicates that N-concentration is enough for the formation of the δ phase for a layer of about 400 nm in depth from the outer surface

Summary

Introduction

Niobium superconducting reentrant cavities have been used as parametric transducers of the resonant-mass gravitational wave detector Mario Schenberg [MS]1-2, in order to monitor the vibrational state of its massive spherical antenna, which resonates at about 3.2 kHz (f0). Of upmost relevance in this case is the detector sensitivity, associated with the ability to convert extremely small mechanical vibrations of the antenna into electrical signals Success in this application keeps strong dependence with the performance of the reentrant cavities, measured by the respective quality factors. The objective is to introduce nitrogen atoms as interstitial impurities in the niobium lattice and to avoid the presence of contaminants on the surface of the metal The findings of such R&D efforts can apply to treat the surface of cavities employed in non-accelerating applications, as is the case for the reentrant cavities. HT-NPBII has been proposed with the goal of attaining convenient surface modification, able to render high Q-factors for the reentrant cavities operating under cryogenic environment In this sense, the measured Q-factors were associated with the surface properties of the cavities and with the ion implantation discharge parameters. Result of this research stresses the importance of studying distinct processes for the preparation of the surface of niobium superconducting cavities

Experimental

Results

Summary