Speaker
Description
A flexible liquid helium level gauge has been developed to compensate lack of commercial availability to procure replacement level gauges for the LHC 1.8 K refrigeration unit cold box requiring an active measurement length of about 1 m. The main difficulty for manufacturing the level gauges is related to the insertion capillary requiring an overall length ranging from 1.75 m to 4.30 m. The liquid helium level gauge is based on a NbTi superconducting wire attached to a PCB (Prototype Circuit Board) carrier including two surface mounted heating resistors and a Cernox© temperature sensor at the bottom tip of the gauge.
The superconducting wire selected is made of a single 0.049 mm NbTi filament with a copper cladding resulting in a wire overall diameter of 0.079 mm. The copper cladding is removed by using standard PCB manufacturing chemicals instead of nitric acid, that is extremely hazardous to handle.
The temperature sensor at the tip complements the measurement of absolute pressure of the saturated liquid helium bath (1.8 K cold box phase separator) and provides useful information to assess whether liquid helium is present.
A first batch of 26 liquid helium level gauges with various active measurement lengths has been manufactured and tested. The collected data permitted to optimize the complete etching of the copper cladding and to provide revisions to the PCB layout thus improving the measurement accuracy and addressing minor issues concerning the manufacturing yield.
The liquid helium levels gauges are validated in both normal and superfluid helium bath, the experimental data permits to select a single operating condition that permit a smooth measurement across the liquid helium lambda transition.
The level gauges are characterized by applying a slow electrical current sweep permitting to assess the loss of the superconducting state along the wire. In superfluid helium the penetration of the normal state is qualitatively well correlated with data reported in the literature for the peak flux in horizontal wires.
The paper presents the design requirements, the main basic components, and the manufacturing steps of the superconducting liquid helium level gauge. Experimental data are also presented demonstrating the correct operation of the proposed novel gauges.
| Submitters Country | Switzerland |
|---|
