Invited Paper VT-MoM8
Quantum Pressure Standard in the range 200 Pa to 20 kPa using Superconducting Microwave Cavity

Monday, October 21, 2019, 10:40 am, Room A213

An LNE-INRiM collaboration is conducting proof-of-principle tests of a primary pressure standard operating at pressures 200 Pa< p < 20 kPa at temperatures 4.6 K < T < 5.8 K. The proposed standard is based on precise measurements of the microwave resonance frequencies of a quasi-spherical, helium-filled, superconducting cavity maintained at cryogenic temperatures. Ultimately, the accuracy of this standard will be competitive with the present standard at LNE over the whole pressure range. The proposed standard exploits 4 theoretical and technological advances: (1) recent ab-initio calculations of the microwave-frequency refractive index of helium n(p, T) near 6 K with an uncertainty corresponding to a relative pressure uncertainty u_r(p) < 1.10^-5 (at a 68% confidence level); (2) the commercial availability of cryogen-free, low-cost, pulsed-tube refrigerators, (3) the ability to manufacture superconducting microwave cavities with resonance quality factors on the order of 5 million (4) and the impending change of the SI that fixes the value of the Boltzmann constant, thereby reducing the uncertainty of thermodynamic temperature determinations in the cryogenic range. A crucial requirement for microwave pressure standards is maintaining the purity of the ⁴He sample under test. For the proposed standard, this is facilitated by cryogenic cold traps that effectively remove all impurities except ³He.

The first experimental result will be present with a 2.5 cm radius and with Nobium coated quasi sphere. A particular focus on the hydrostatic head correction and the thermomolecular effect will be presented during the presentation.