Yukinobu Miki

Balance for Measuring Mass Under Microgravity Conditions

A method for efficiently and accurately measuring mass under conditions of weightlessness is proposed using the law of conservation of momentum. In this method, the velocities of two different uniform motion states of an object and the reference mass are measured highly accurately using an optical interferometer. For the preparatory experiments on earth, an instrument, with which linear motion of constant velocity is realized using pneumatic linear bearings, is developed. The combined standard uncertainty in mass measurements from 4 to 18 kg by a single collision is estimated to be about u c = 0.012 kg, which corresponds to 0.07% (7 x 10 -4 ) of the maximum value of 18 kg. For a mass-measurement instrument employed under microgravity conditions, a design for the instrument and a measurement procedure are proposed.

Absolute determination of the magnetic flux quantum using superconducting magnetic levitation

The current status of our superconducting magnetic levitation experiment for determining the magnetic flux quantum is described. The flux-up system has been improved significantly by using a Josephson voltage standard. Studies are also in progress to improve the mechanical measurement relevant to the floating body with the goal of reducing the uncertainty to less than 1 ppm. >

Evaluation of equilibrium trajectory of superconducting magnetic levitation system for the future kg unit of mass

The superconducting magnetic levitation system for absolute determination of magnetic flux quantum /spl Phi//sub 0/, which is aimed at the future replacement of the kilogram unit of mass, has been developed at the National Research Laboratory of Metrology (NRLM) with a small floating body of approximately 25 g mass. The consistency of the relation between potential energy of the floating body and the electromagnetic energy of the superconducting magnetic levitation system has been evaluated with 1 ppm order resolution. In this paper, the results of the evaluation as well as the improvements made in the apparatus in recent years at NRLM are described.

Vertical displacement determination of a levitated superconducting mass

The determination of the vertical displacement of the center of gravity of a levitated superconducting body in the National Research Laboratory of Metrology (NRLM) superconducting magnetic levitation project, which is aimed at establishing a new definition of the unit of mass based on the fundamental constants, is discussed. The translation displacement (three degrees of freedom) and the attitude change (three degrees of freedom) of the measuring point, which is the optical center of the cube corner prism O/sub C/ of the floating body, are measured using a newly developed optical measuring system. To determine the vertical displacement of the center of gravity of the floating body G/sub C/ the relative position of G/sub C/ with respect to O/sub C/ is required, which is determined using the energy relation between the electromagnetic energy and the gravitational potential energy under levitating conditions.

A buoyancy-free mass comparator with precision better than 10/sup -6/ for use in a superconducting magnetic levitation system

A system for measuring mass without the necessity of correcting for the buoyancy of air with a relative standard uncertainty better than 10/sup -6/ has been developed. The system is intended for use in determining the mass of the floating body in the superconducting magnetic levitation system.

Mechanism for levitated superconductor experiment

A mechanism for a levitated superconductor experiment has been proposed and is being developed with the final target of defining the kilogram in terms of fundamental constants. Improving the stability of the levitated object by means of introducing a superconducting linear bearing is the aim of the work reported in this paper. The strategy of this development, results of the preparatory experiments, and further prospects are discussed.

Frequency Mixing of 50 GHz Microwave and 4.25 THz FIR Light by a Josephson Point Contact

The harmonic mixing of 50 GHz microwave and 4.25 THz far-infrared (FIR) light, the highest order mixing ever reported in this frequency range, has been achieved using a Josephson point contact made of Nb. The conditions under which the mixing was performed are described. The conversion efficiency of the incident FIR light to the beat signal is evaluated and the performance limit of the Nb Josephson point contact is also discussed.

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.

Point-contact-type Josephson harmonic mixer for frequency measurement of far-infrared lasers

Point-contact-type Josephson junctions have been developed as a harmonic mixer to measure far-infrared (FIR) laser frequencies in a frequency chain up to the infrared region. The junctions were formed by mechanically pressing a niobium wire of 70 μm in diameter with a sharply etched tip against a niobium post of 2 mm in diameter. The voltage-current characteristics of the mixers were investigated under the irradiation of FIR lights from an optically pumped methyl-alcohol laser. The Shapiro steps for FIR lights up to 3.1 THz were observed. The mixing properties of the mixers were studied through harmonic mixing experiments of 50-GHz microwaves and the FIR lights up to the frequency of 3.1 THz. The conversion efficiency of the beat signals to the incident FIR power was found to vary as the — 4th power of the FIR frequency. From the mixing properties, the combination of the critical current Ic of the junction and the product IcRn where Rn is the normal resistance of the junction, was found important to optimize the mixing performance.

Liquid helium transfer using a mass-flow controller

An automated liquid helium transfer method using a mass-flow controller, by means of which an empty cryostat at room temperature can be filled with liquid helium without thermal shock, has been developed. Before liquid helium collection is initiated, mass flow is controlled to be constant. After the start of liquid helium collection, mass flow is controlled so that the differential pressure of the empty cryostat against the storage Dewar containing the liquid helium is of the proper value to overcome the height difference due to the transfer tube. A mass-flow controller, a vacuum pump, a differential pressure sensor, a liquid helium level sensor and a computer with suitable software are required to replace the conventional manual transfer by this method.