Jun Luo

A scientific case study of an advanced LISA mission

A brief status report of an ongoing scientific case study of the Advanced Laser Interferometer Antenna (ALIA) mission is presented. Key technology requirements and primary science objectives of the mission are covered in the study. Possible descope options for the mission and the corresponding compromise in science are also considered and compared. Our preliminary study indicates that ALIA holds promise in mapping out the mass and spin distribution of intermediate mass black holes possibly present in dense star clusters at low redshift as well as in shedding important light on the structure formation in the early Universe.

Folded pendulum tiltmeter

The application of the folded pendulum (FP) as a tiltmeter is proposed and some features of it have been studied both theoretically and experimentally. First, FP could have a quite low resonance frequency due to its mechanical structure. The period of our prototype FP is 6.2 s and the amplification factor is about 100. Second, FP is not sensitive to the environmental temperature variation but seriously affected by the temperature gradient. The experiment with a temperature gradient modulation shows that the static equilibrium position of the FP will change by 2.5 μrad if the temperature difference between the horizontal platform and the base of the FP is 0.1u200a°C. With a prototype FP, we have observed obvious tilt tides, and the calibration result shows that the resolution of the FP is about 1.2 nrad.

Status of measurement of the Newtonian gravitational constant G

A precise knowledge of the Newtonian gravitational constant G has an important place in physics and is of considerable metrological interest. Although G was the first physical constant to be introduced and measured in the history of science, it is still the least well known. The 1998 CODATA recommended value for G, (6.673±0.010)×10-11 m3 kg-1 s-2, has an uncertainty of 1500 parts per million (ppm), which is much larger than that of all other fundamental constants. Here we review the status of our knowledge of the absolute value of G, nine experiments for measuring the absolute values of G within the last five years, the experiments in progress and being planned, and the systematic error due to the inelasticity, the nonlinearity and the thermoelasticity of torsion fibre.

Performance measurements of an inertial sensor with a two-stage controlled torsion pendulum

A novel two-stage electrostatically controlled torsion pendulum has been developed to simultaneously investigate the performance of a translational and a rotational degree of freedom of an electrostatic inertial sensor on ground. The motions of the proof mass (PM) relative to the electrode frame are monitored by a high-precision capacitance transducer, and are synchronously controlled by electrostatic actuators. The parasitic stiffness induced by capacitance transducers and the effect of the magnetic field are measured. Both translational and rotational motions of the PM succeed to be simultaneously controlled, and the cross-coupling effect between both controlled degrees of freedom is also preliminary measured. The experiments show that the scheme obviously suppresses the translational to rotational effect of the PM, and then effectively improves the torque resolution compared with the single-stage torsion pendulum. The noise floors of the controlled torsion pendulum come to 1.2 × 10 −11 NH z −1/2 along the translational degree of freedom, and 1.4 × 10 −13 NmH z −1/2 along the rotational degree of freedom, near 30 mHz, which are mainly limited by the back action of the capacitance transducer below 0.1 Hz and by the horizontal seismic noise disturbance above 0.1 Hz.

Thermoelastic property of the torsion fiber in the gravitational experiments

The thermoelastic and the nonlinear properties of a torsion fiber were studied. A symmetric disk torsion pendulum was designed to measure the temperature coefficient of the torsion spring constant of a tungsten fiber at room temperature, and the result shows that the ambient temperature fluctuation with ±1u200a°C would introduce a considerable uncertainty about ∓165u200appm in the torsion spring constant of the fiber. It is suggested that the thermoelasticity of the torsion fiber should be measured in a precision torsion pendulum experiment.

New upper limit from terrestrial equivalence principle test for extended rotating bodies

An improved terrestrial experiment to test the equivalence principle for rotating extended bodies is presented, and a new upper limit for the violation of the equivalence principle is obtained at the level of 1.6x10(-7), which is limited by the friction of the rotating gyroscope. This means that the spin-gravity interaction between the extended bodies has not been observed at this level.

Torsion pendulum for the performance test of the inertial sensor for ASTROD-I

A torsion pendulum facility for a ground-based performance test of the inertial sensor for ASTROD-1 has been constructed. The twist motion of the test mass is monitored and servo-controlled. The sensitivity of the electrostatic servo-controlled actuator is calibrated based on the elastic torque of the torsion fibre, and the torque resolution of the servo-controlled torsion pendulum comes to 2 × 10−11 N m Hz−1/2 from 1 mHz to 0.1 Hz, which is likely limited by the seismic noise, electronic noise and the cross coupling between the translation and twist modes.

Analytical error analysis for satellite gravity field determination based on two-dimensional Fourier method

The time-wise and space-wise approaches are generally applied to data processing and error analysis for satellite gravimetry missions. But both the approaches, which are based on least-squares method, address the whole effect of measurement errors and estimate the resolution of gravity field models mainly from a numerical point of view. Moreover, requirement for higher accuracy and resolution gravity field models could make the computation more difficult, and serious numerical instabilities arise. A direct analytical expression between power spectral density of the satellite gravimetry measurements and spherical harmonic coefficients of the Earth’s gravity model is derived based on two-dimensional Fourier description. This method provides a physical insight into the relation between mission parameters, instrument parameters and gravity field parameters. In contrast, the least-squares method is mainly based on a mathematical viewpoint. By taking advantage of the analytical expression, it is efficient and distinct for parameter estimation and error analysis of missions. It is easy to obtain from the analytical relationship that the low-frequency noise affects the gravity field recovery in all degrees for the instance of satellite gradiometer recovery mission, which agrees with the work before by the numerical error analysis methods.

A self-analyzing double-loop digital controller in laser frequency stabilization for inter-satellite laser ranging

We present a digital controller specially designed for laser frequency stabilization in the application of inter-satellite laser ranging. The prototype of controller is developed using field programmable gate arrays programmed with National Instruments LabVIEW software. The controller is flexible, self-analyzing, and easily optimized with build-in system analysis. Application and performance of the controller to a laser frequency stabilization system designed for spaceborne scientific missions are demonstrated.

Switching frequency response characteristics of a low cost wireless power driving and controlling system for electrically tunable liquid crystal microlenses

The essential switching frequency response characteristics of a low cost wireless power driving and controlling system for electrically tunable liquid crystal microlenses (ETLCMs) are obtained. The wireless power system is mainly composed of two coils with different radius and winding as well as a power metal-oxide-semiconductor field effect transistor switch. The voltage response in the small coil, which is connected directly with ETLCMs, is measured and analyzed under the condition of changing some key parameters of the coil system, such as the width of switching frequency region and the duty-cycle of the switching signal ranging from 20% to 80% in intervals of 20%. Through extending the switching frequency range to a few hundreds of kilohertz, an attractive property of only modulating switching frequency to tune precisely the rms voltage in the small coil for ETLCMs is presented. Some interesting phenomena in high frequency regions, for instance, the rms voltage being stable or slightly changed as the frequency, the voltage response cutoff or disappearance after the frequency surpassing a threshold value, and then regeneration after lowering the frequency to lower frequency point than that of generating voltage response cutoff during increasing frequency, are also discovered.