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Cryogenic Systems

Incorporation of a binary gas analyzer into a dilution refrigerator

The operational of a low temperature (~30 mK) dilution refrigerator (DF) is controlled by the relative mixture of He3-He4 in a closed system. We previously had no knowledge about the composition of the gas in the DFs used on CHRNS instruments that are often used to probe the quantum behavior of materials and require temperatures to be maintained over the experiment. If there was a temperature failure we could not diagnose and solve gas mixture problems while the experiment was on going and it would waste valuable beamtime.

During the unexpected outage, a binary gas analyzer capable of distinguishing He3 and He4 has been integrated into one of the DF gas handling systems. This diagnostic and troubleshooting tool allows in situ 3He-4He mixture analysis enabling a quick and simple diagnosis of any DF issues related to 3He-4He mixture. It also provides a means to further optimize the DF cooling power and base temperature on the fly without risking the neutron experiment.

binary gas analyzer integrated into fridge gas system
Figure – Binary Gas Analyzer integrated into the ICE dil fridge gas handling system

Carbon fiber sample sticks for helium-flow cryostats and top-loading refrigerators

Using the top-loading closed cycle refrigerators (TLCCR) and helium-flow cryostats has involved rather long times to cooling to base temperature after a sample change. For example, for a tall TLCCR it can take several hours for the system to re-cool.

During the unscheduled outage, we have evaluated low thermal conductivity and low heat capacity carbon fiber sample sticks as a replacement for the current stainless-steel ones for both helium flow and TLCCR systems.  

The carbon fiber sample sticks showed excellent performance reducing the sample change times by 20 to 30 minutes depending on the experiment and type of equipment which allows experiments to re-start sooner, enhancing neutron utilization.  This sample stick is now in use on the CHRNS HFBS TLCCR.  A similar stick will be ordered for the 100 mm CHRNS MACS helium cryostat.

Helium Flow Cryostat
Figure 1. Sample change dynamics in the pre-cooled 70 mm helium flow cryostat for a carbon fiber (left) and traditional metal sample stick (right).

Orange cryostat upgrades

The original helium flow ‘orange’ cryostats have a single 1.5 K heat exchanger. This results in cryostat preparation and sample change times on the order of hours.

During the unexpected outage, a cryostat (#13) has been equipped with three heat exchangers resulting in more cooling power, faster preparation and sample changing times (see Figure 1). It now takes under half the time to prepare a cryostat. Based on this successful test, we will continue to retrofit the suite of helium cryostats and anticipate two more will be complete in 2022.

three heat exchangers graph
Figure 1. The preparation performance of upgraded 70mm ILL Orange cryostat #13. The cryostat has been cooled from room temperature down to 2K for 90 minutes (under same conditions, the prep time before upgrade was about four hours).
MACS CHRNS
Created March 30, 2022