Sensor Temperature Stability Performance of the Advanced Liquid Helium Dewar

Abstract

The measured sensor temperature stability in the sensor volume of the Advanced Liquid Helium Dewar (ALHD) designed, built, and tested by Ball Aerospace for the Naval Surface Warfare Center Coastal System Station at Panama City, Florida, is described. ALHD is a horizontal, quartz epoxy composite, liquid helium dewar designed to support in-motion testing of a SQUID superconducting magnetic gradiometer. The sensor is cantilevered from the end of the neckplug and resides in an isothermal sensor volume filled with a constant volume helium exchange gas. The sensor temperature stability tests were performed at exchange gas pressures of 30–400 torr such that the peak-to-peak temperature variations were greater than the system temperature measurement noise. The sensor temperature was measured and the rms temperature stability was computed for several cases, namely: 1) stationary and in-motion, 2) stationary as a function of liquid helium level, and 3) stationary for several helium exchange gas pressures. The results are: 1) the in-motion and stationary rms temperature stabilities are similar in magnitude; for the non-optimal, 250 torr exchange gas pressure, the stability is ~1×10^-6 K rms, 2) the stability is essentially independent of fill level, and 3) the extrapolated stability at 5 torr exchange gas pressure in the absence of self-heating is 3×10^-8 K rms.