The treatment of reentrant niobium cavities by nitrogen plasma immersion ion implantation for the Mario Schenberg gravitational wave detector

Abstract

Reentrant niobium cavities playing the role of parametric transducers of the gravitational wave detector Mario Schenberg (MS), in operation at the Astrophysical Division (DAS) of INPE, have been treated by High Temperature nitrogen Plasma Immersion Ion Implantation (HT-PIII). The aim is to enhance the respective electric quality factors (Q-factors), which are closely related with the increase of the sensitivity of the system. Experiments performed recently by the PIII research group at LAP/INPE revealed an increase of two orders of magnitude for Q values for cavities submitted to ion implantation in comparison with un-implanted cavities [1]. In the process the cavities immersed in plasma are bombarded by positive energetic nitrogen ions with energies varying from 5 to 7 keV, which are implanted into the surface of these heated substrates. The heating temperature is precisely controlled during the process [2] and its level directly affects the depth profile of the implanted nitrogen ions due to diffusion. Maximum nitrogen implantation depth measured by glow discharge optical emission spectroscopy reached about 7 mm for T =1150 °C, against 400 nm for T = 700 °C. It is known that part of implanted N is able to bond with Nb atoms in order to form niobium nitrides. On the other hand, N atoms can also be dissolved into Nb, occupying interstitial spaces in the crystal lattice. In order to correlate the operation parameters of PIII with Q-values of niobium reentrant cavities, it is demanded to perform systematic investigation, which is the aim of this research project. In fact, the search for an optimal condition must take into account the variation of the following parameters: ion energy, substrate temperature, treatment duration and plasma cleaning via argon discharge. The surfaces to be treated under these distinct experimental conditions will be characterized in order to identify the morphology, the formation of phases, the depth profile of implanted elements and respective binding energies. It is worth to note that intensive investigations have been performed by different international research groups aiming the achievement of high Q-values for niobium cavities used in particle accelerators [11-14]. One of the research fronts that have gathered relevant results is concerned with the insertion of nitrogen into Nb surface via thermal treatment [15-18], corroborating with the results attained by PIII/LAP group. Thus, the proposed research project aims to attend a demand from DAS/INPE to augment the sensitivity of the MS detector, assisting in the detection of gravitational waves. In addition, it can also help with the improvement of the performance of particle accelerators, since PIII is an innovative method to treat niobium superconducting cavities. (AU)