A low susceptibility proof mass for the nova satellite

The proof mass for the NOVA satellite must be designed to minimize disturbance forces from the field of a nearby damping magnet. To insure this, proof mass susceptibility must be ≤ 4% that of Ag, and ferromagnetic impurities must be minimized. The magnetic design and testing to meet these specifications may be of interest to others requiring high purity nonferromagnetic materials and the elimination of ferromagnetic contaminants. Low susceptibility was achieved by using laminations of Al and Ag proportioned to balance para and diamagnetic susceptibilities. Susceptibility as low as 3% of Ag was achieved. However, residual magnetization of 2 × 10^-4pole-cm was detected after exposure to 18 kG, indicating ferromagnetic contamination. Attempts to identify the contaminant by atomic absorption and mass spectroscopy were unsuccessful. High speed machine tools were replaced by Stellite alloy tools, resulting in reduction of residual moment by an order of magnitude. A "soft" magnetization of 39 × 10^-5pole-cm was observed in one proof mass. The lowest susceptibility achieved was 2% that of Ag for a Stellite-machined proof mass. Although Stellite tooling reduced residual magnetic moment on the average, the lowest moment was observed in a proof mass machined with tungsten high speed steel tools.