High-Q Superconducting Niobium Cavities for Gravitational Wave Detectors L.A.N. de Paula, a,b* S.R. Furtado c , O.D. Aguiar c , N.F. Oliveira Jr b , P.J. Castro d and J.J. Barroso d a Physics Department, Technological Institute of Aeronautics – ITA, Praça Marechal-do-Ar Eduardo Gomes 50, São José dos Campos, Brazil b Department of Mechanics and Material Physics, University of Sao Paulo – USP, Rua do Matão 187, São Paulo, Brazil c Astrophysics Division, National Institute for Space Research – INPE, Av. dos Astronautas 1758, São José dos Campos, Brazil d Associated Plasma Laboratory, National Institute for Space Research – INPE, Av. dos Astronautas 1758, São José dos Campos, Brazil E-mail: leandroifusp@yahoo.com.br ABSTRACT: The main purpose of this work is to optimize the electric Q-factor of superconducting niobium klystron cavities to be used in parametric transducers of the Mario Schenberg gravitational wave detector. Many cavities were manufactured from niobium with relatively high tantalum impurities (1420 ppm) and they were cryogenically tested to determine their resonance frequencies, unloaded electrical quality factors (Q 0 ) and electromagnetic couplings. These cavities were closed with a flat niobium plate with tantalum impurities below 1000 ppm and an unloaded electrical quality factors of the order of 10 5 have been obtained. AC conductivity of the order of 10 12 S/m has been found for niobium cavities when matching experimental results with computational simulations. These values for the Q-factor would allow the detector to reach the quantum limit of sensitivity of ~10 -22 Hz -1/2 in the near future, making it possible to search for gravitational waves around 3.2 kHz. The experimental tests were performed at the laboratories of the National Institute for Space Research (INPE) and at the Institute for Advanced Studies (IEAv - CTA). KEYWORDS: Resonant Detectors; Instrument optimisation; Modeling of microwave systems. * Corresponding author.