Experimental Study On The Gas Damping Effect In The Space Inertial Sensor

The inertial sensor is one of the core loads of the high precision space gravity experiment.The inevitable material out-gassing will lead to some residual gas molecules in the inertial sensor,which will collide with the test mass surfaces due to Brownian motion,resulting in a gas damping effect on the moving test mass.This effect satisfies the fluctuation dissipation theorem.In the gravitational wave detection project,whether LISA or LIGO,the gas damping effect of inertial sensors is one of the main limitations affecting the gravitational waves detection,so it is crucial to study the law of the gas damping effect on the test mass.Despite the fact that the Trento University and the Washington University have used the torsion pendulums to study the gas effect in LISA inertial sensors,respectively,more investigation into the mechanism of the gas damping effect is still required due to variations in theoretical and experimental results.In order to obtain the law of gas damping effect on space inertial sensors,this paper designs and applies a one-dimensional symmetric adjustable torsion pendulum device to study the mechanism of gas damping effect,including experimental scheme design,torsion pendulum system development and testing,and gas damping experimental measurement.The research contents and main results of this paper are as follows:The influence analysis of the gas damping effect is carried out.Firstly,the model of the gas damping from an infinite volume is analyzed in translational and rotational directions.Secondly,the model of the gas damping in a constrained volume and the random-walk diffusion time are analyzed.Finally,the principle of gas damping measurement is introduced by the torsional pendulum system with an adjustable symmetrical gap of one-dimensional electrodes,and the design of the experimental measurement scheme is described in detail.A torsional pendulum system with symmetrical electrodes and adjustable gaps is developed and tested.Firstly,the free torsional pendulum suspended by a conductive film-coated silica fiber is constructed,and the torque noise is about 7.5 ×10-15 N·m/Hz1/2.A pair of parallel electrodes with the adjustable gap is installed.Based on the AC voltage feedback control method,the test mass in the equilibrium position is realized.The distance between the electrode and the test mass is calibrated,and the maximum gap is about 3 mm.The pressure in the vacuum chamber can be adjusted by a flapper valve between the chamber and the ion pump.The hot-cathode ion gauge is calibrated with another higher precision gauge,which is cosistent within an error of<10%.Finally,the composition of the residual gas in the chamber at different pressures 10-5～10-2 Pa is measured by a residual-gas analyzer.The results show that the main components of gas molecules are different at different pressures.The gas damping effect of the torsional pendulum system is measured at various gaps with different pressures.Firstly,the influence of electrostatic effect and its demand are analyzed.Based on the two methods of DC modulation and AC modulation,the surface potential of test mass is measured about 25 mV when the pressure is 10-5 Pa.A method of measuring and compensating of the stray potential on each electrode is proposed,which effectively reduces the influence the electrostatic effect.Secondly,the gas damping effect caused by the test mass is analyzed.The dissipation angles of the test mass’s free oscillations are measured at different gaps with pressures 1.50×10-5 Pa,2.57 × 10-4 Pa,2.74×10-3 Pa and 1.77 ×10-2 Pa.The new model is being verified for the first time with both elastic collision and inelastic collision,which is more consistent with the actual situation.The relationship between the thermal accommodation coefficient and the pressure is observed,and the corresponding model is preliminarily expressed as an exponential decay function.The physical interpretation is given:the elastic collisions correspond to the simple bouncing of residual gas molecules,while the inelastic collisions correspond to the adsorption and desorption cycle of gas molecules on the surface.The amount of inelastic collisions increases as pressure decreases because there are fewer gas molecules adsorbed on the object’s surface.The acceleration noise of gas damping effect in the inertial sensor for the space gravitational wave detection is preliminarily determined to be 3.71 × 10-16～3.71 ×10-15 m/s2/Hz1/2 at pressure 10-6～10-4 Pa.