Bottom top orange

Liquid helium cryostats

It'll take a few sketches to explain how we moved from "bottom-loading" to "top-loading" liquid helium cryostats, to finally end up with the Orange cryostats, the anti-cryostat.

Bottom-loading cryostats

The old-style bottom-loading cryostats were basically just Dewar (Thermos) flasks, vacuum insulated. The sample-holder below was cooled directly by the liquid nitrogen or helium bath to 77 or 4 K. The temperature of the sample was kept above 77 or 4 K, without straying far from these values as the calories injected would make the cryogenic fluid boil, thus reducing the autonomy of the cryostat (< 1 day).
Whenever the sample needed to be changed, therefore, the cryogen had to be removed and the cryostat re-heated, before inserting the new sample and re-cooling the cryostat again. This was very time-consuming and hampered ILL's quick turnover of experiments.

Top-loading cryostats

The old top-loading variable temperature cryostats had a well within the helium bath. This made it quicker and easier to change the sample, as there was no need to empty the cryostat. We could also vary the temperature of the sample over a wider range, but at the cost of more helium.
The problem with this design was that a small amount of air entered the well and mixed with the helium flow whenever the sample was changed. The humidity in the air migrated to the coolest point in the device, the capillary tube of the cold valve, and blocked it with ice.

The Orange cryostat: the anti-cryostat or anti-Dewar

Unlike the previous two models, the helium bath in an Orange cryostat is not directly exposed to the heat radiated by the sample well, as it is insulated by a screen (annular vacuum space) cooled by the liquid nitrogen bath. It seems that Dominique Brochier, who called it "the anti-cryostat or anti-Dewar", took the idea unknowingly from Air Liquide.
When the user raises the temperature of the sample, the radiation from the well is blocked by the screen; this keeps helium consumption low and significantly increases the cryostat's autonomy, much to the scientists' relief.
The cold helium gas coming from the cold valve leaves not via the sample well but through the annular space around the well. This avoids blockages at the bottom of the well and also reduces consumption even further, as the gas flow cools the wall of the sample well.
The sample sits at the bottom of the well in a "static" exchange gas, which is cooled by an exchanger in which the helium is expanded. The system allows temperatures as low as 1.3 K to be reached and the sample is cooled homogeneously in a large volume of gas.
Apart from its lower helium consumption, the advantages of the orange cryostat are that the sample can be changed rapidly as of 10 K and that it can be heated to 550 K (cryofurnace) without significantly affecting the cryostat's autonomy (because the sample is well clear of the helium bath).
Numerous attempts were made to automatically control the cold valve regulating the helium flow to the sample, but it was only thirty years after the invention of the Orange cryostat that a satisfactory solution was found.