Design

Figure 1 schematically illustrates the principle, which permits the heat to be extracted where it is required (at the level of the detector mosaic) and makes optimal use of the enthalpy.

Figure 1 schematically illustrates the principle, which permits the heat to be extracted where it is required (at the level of the detector mosaic) and makes optimal use of the enthalpy.

Figure 1. Cryogenic principle with detail of the anti-overflow.

Internal pressure (P), forces the LN2 to flow from the storage tank (7) through pipe (8) to and through a heat exchanger (2) which is in direct thermal contact with the mosaic base plate (1). The heat exchanger consists of three parallel bars to ensure good temperature homogeneity across the mosaic. After absorbing the heat load, the gasified nitrogen circulates through a special annular heat exchanger (5), which acts as radiation shield for the storage tank. A final heat exchanger (9) is used to (electrically) heat the gas to room temperature. The gas is captured in a small tank (12) after exiting the instrument. At this point the gas is now warm and perfectly dry and can serve a second purpose and be safely blown over the dewar entrance window to prevent the condensation of air humidity.

The thermal regulation employs a valve (11), which is supervised by a PID controller in order to maintain constant operating temperature of the heat exchanger (2). The refilling of the internal tank is done from a standard 120 liter storage tank via a vacuum-insulated line. When the latter is connected to refilling tube (3), this is detected by a proximity sensor, and a valve (10) is opened in order to depressurize the internal tank so that the filling can begin. The valve is automatically closed when the tank is full (which is reported by a temperature sensor). The refilling port is fitted with a small spring loaded valve which is activated by the end of the refilling tube. This maintains the operating pressure when removing the tube.

The tank has been dimensioned to contain 40 liters for a hold time of 30 hours, making refilling only necessary once per day. Thanks to a special anti-overflow system, which allows the cooling tube (8) to be permanently at the lower position (in the liquid) and the vent tube (6) to be permanently at the highest position (in the gas), 90% of this capacity can be used without spilling (up to the nominal pointing limit of 60 deg zenith distance).

A dedicated thermal clamp has been designed to allow easy separation of the detector head from the cooling system. Figure 2 (left) shows the top of the cooling system with the 12 thermal clamps and (right) the bottom of the mosaic plate with the 12 thermal heat-sinking points.

Figure 2. (left) Thermal clamps and (right) heat sinks.

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