Research On Temperature Effect In Superconducting Gravity Instruments

Although superconducting gravity instruments have the advantages of low thermal noise and extreme mechanical stability, they are sensitive to temperature fluctuation. Temperature fluctuation is one of the main sources of low frequency noise of superconducting gravity instruments. The temperature can affect the instrument output in two ways: firstly, the critical current of Josephson junctions in SQUID and thus the transfer function of the instrument are temperature dependent; secondly, the penetration depth of a superconductor varies with temperature. Consequently, the effective inductance of the sensing coil closed to a superconductor will change with temperature. In order to clarify the temperature stability required for superconducting instrument, a systematic measures have been carried out in our research.Precise control and manipulation of temperature are the preconditions to measure the temperature effects in superconducting instruments. The temperature in experimental space can be stabilized within ±200 μK during 24 hours by multiple passive thermal isolating and active PID control. The temperature can be set to random value in the range from 4.2 K to 5 K. Also, the vacuum level can be adjusted to meet the experimental requirement in different stages.Next, the SQUID noise level has been measured in the temperature range from 4.2 K to 4.7 K. It has been found that the SQUID noise lever does not change with its working temperature provided the temperature is stable enough. However, the SQUID out-put will change with temperature. The temperature dependence of the SQUID output has been measured to be(0.14±0.01) Φ0/K, which is equivalent to(0.014±0.001) ng/mK for a typical superconducting accelerator.Next, a set of superconducting circuit has been designed to measured the temperature effect of a superconducting coil. The temperature dependence of coil’s effective induc- tance has been measured to be(2.37±0.07) nH/K. It is equivalent to(1.38±0.04)×10-7 m/K, if the coil is used to measure the displacement of test mass in a superconducting gravity instrument.Based on the above results, it is estimated that the temperature fluctuation should be stabilized within 115 μK for a superconducting gravimeter with 1 ng resolution and 4 Hz resonance frequency and less than 10 μK/m for a superconducting gravity gradiometer with 1 E resolution and 0.2 m baseline.