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Cryostat
The superconducting cavity and the control of the blackbody radiation field require a very low temperature environment (below 1K). We achieve this in a $^4$He-$^3$He cryostat.
This picture presents a cut through the cyrostat. The outer dimensions are about 60 cm in diameter, for a height of about 150 cm. Successive cryogenic stages are used to reach a base temperature between 0.6 and 0.8 K.
A shield cooled to liquid nitrogen temperature protects first all the inner parts from the room temperature blackbody radiation. An automatic filling system avoids a fortuitous heating up of this essential shield.
The liquid Helium 4 container has a capacity of 25 litres, enough for more than two days autonomy (the thermal losses are of the order of 250 mW). It is connected to a copper shield surrounding the core of the apparatus.
Liquid $^4$He is pumped out of this reservoir into a small exchanger box, evacuated by a primary vacuum pump. The temperature of this exchanger reaches 1.5 K in continuous operation.
This temperature is low enough to liquefy $^3$He. This gas (rather expensive) is used in a closed circuit. It is injected in a capillary tube solderd onto the 1.5 K box. The impedance of this capillary tube has been adjusted to provide an appropriate flow rate for an injection pressure of about half an atmosphere.
The liquid $^3$He is then injected in an exchanger box, evacuated by a high flow rate vacuum-tight rotary pump. The temperature of the exchanger is then between 0.5 and 0.7 K, depending upon the heat load on the experimental core (a few mW at most).
The final base temperature of the set-up is then between 0.6 K and 0.8 K. This is low enough to ensure that the cavity $Q$ is not limited by the temperature. At the same time, the residual mean blackbody photon number by mode is considerably reduced (0.05 at 0.8 K).
The evacuated $^3$He iscompressed by the rotary pump, filtered and reinjected.
The total mass of the present experimental core is about 40 kg. This represents, particularly at high temperature, and enormous heat capacity, incompatible with a cooling directly by liquid He. The temperature of the set-up is thus first reduced to about 100 K by a heat exchanger, directly attached to the core, in which we feed liquid nitrogen. The nitrogen is then evacuated, and the cooling proceeds with liquid He transferred in the main reservoir.
During this cooling phase, the heat exhanges between the reservoir and the core are provided by thermal switches (two of them are visible on the picture). They contain helium gas and have thus a high thermal conductivity. Below 10 K, however, a small piece of active charcoal adsorbs all the gas in the switches, opening the thermal contact and allowing for the cooling by the $^3$He flux. The total cooling time from room temperature is about 15-20 hours and requires 150 liters of helium.
For some preliminary experiments, a base temperature of 1.5 K is appropriate. We obtain it by establishing a direct thermal contact between the $^4$He box and the core by means of $^3$He gas at a few torrs injected in the pumping line, in direct contact with both the core and the exchanger box.
Temperature cycles are extremely risky, since the cavity (always the coldest point) may trap dirt outgased by other surfaces. The cryostat thus remains cold as long as the experiment lasts (up to 26 months of continuous operation).
This picture shows the assembled cryostat core from the side ( the nitrogen shield and the 4.2K lower shield have been removed). The $^4$He reservoir is on top. The 0.6 K experimental core in its shield is in the bottom. Many coaxial lines and waveguides, with integrated thermal isolation, connect the core of the set-up to the outside.
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