Present status of the NRLM superconducting magnetic levitation system for absolute determination of the magnetic flux quantum

The NRLM superconducting magnetic levitation is based on the concept of quasi-static energy injection process, where the energy injected from external source to the coil system is equal to the sum of the increase in magnetic energy and gravitational potential energy. The levitation system uses a niobium coil and floating body with mass of 25 g. The measurement consists of two processes: (1) to increase the magnetic flux through the coil in terms of the Josephson voltage which we call flux-up, (2) to measure the coil current and the vertical displacement of the center of gravity of a floating body in equilibrium. By the repetition of these processes, the relation which we call equilibrium trajectory is obtained. The absolute value of the magnetic flux quantum is calculated in terms of the energy equation using the trajectory data. Hence, a quasi-static process is not vital. The levitation system has been developed at NRLM and the reproducibility of magnetic flux and vertical displacement in equilibrium at 0.1 ppm level was confirmed including Meissner effect of the superconductive material. In this paper, we report a recent improvements concerning mechanical energy determination as well as the changes to the electrical system.