Cryogenics

This section describes the main characteristics of the GIANO cryostat and its cooling system.

The core structure of the cryostat is the aluminum optical bench and liquid nitrogen tank (``BeTank''). This somewhat unusual structure allows having a large (up to 100 liters) reservoir of liquid nitrogen at the very center of the cryostat and in direct contact with the surface of the optical bench.

The upper part of the BeTank, carrying the spectrometer, is enclosed inside a cover made of thick (4 mm) aluminum plate, thus minimizing the thermal gradients in the quite high (up to 400 mm) opto-mechanical components mounted on the optical bench. The large thermal conductivity of the cover guarantees that the temperature of the upper part of the cover remains within <2 K of the bench temperature. Noticeably, this bulky structure cannot be rigidly mounted on the BeTank, otherwise it would deform the optical bench because of its non uniform temperature distribution.

The vacuum shield is a ∅1300 mm X 1900 mm cylinder manufactured of welded steel plates. The structure is vertically cut at 120 mm below the cylinder center in order to have the flange at the same level as the surface of the BeTank. The interfaces to fix the hexapod arms are welded to the lower part of the shield together with the support columns which will be mounted on load-cells.

The inner part of the cryostat includes an efficient radiation shield manufactured of standard multilayer super-insulation aluminized mylar. The radiative heat losses are thus reduced to about 0.5 W/m.

The total weigh of the cryostat, including the opto-mechanical elements mounted on the bench, is about 550 kg, of which 350 kg are cooled.

The vacuum is obtained using a generous two stage system of commercial pumps (i.e. Trivac D-65B and Turbovav 1100C by Leybold) permanently positioned on the cart below the cryostat. A gate-valve is also included to allow an automatic control of the pumps.

Once the instrument is sufficiently cold, the gate valve is closed and the vacuum is maintained by a large reservoir of active charcoal. This is stored in isolated boxes which can be heated even when the BeTank is cold, thus making it possible to out-gas the charcoal without heating the whole instrument.

The vacuum and cryogenic devices are controlled by an industrial PLC which communicates with the instrument PC-system through a serial protocol (modbus) re-routed on the ethernet by a Lantronix terminal server.

We note here the following, special features of the operations performed by the cryogenic control system.

• The flow of out-boiling N₂ gas is regulated by a mass flow control (MFC) valve (FCV-101). This element is servo-controlled by the PLC on the basis of the signal coming from a high precision barometer positioned on the output line of the nitrogen gas (PT-101). This allows achieving an ultra-accurate (0.01 K) long term temperature stability of the spectrometer.

• The input flow of liquid nitrogen is controlled by a valve (LV-101). During the cooling phase, the aperture of this elements is limited by the PLC to a value yielding a maximum flux of 0.3 lit/min. This limitation is maintained as long as the PLC detects a BeTank temperature (TE-101 and TE-102) above a given threshold.

• When the cryostat is cold, the control system monitors the vessel internal pressure (PT-202). If this value exceeds a few  mbar, the PLC activates the turbo-pump (P-202), opens the gate-valve (YV-202) and inhibits the liquid N₂ supply valve.

• The weight of the cryostat is monitored by means of 4 load-cells (WT-101/2/3/4). This parameter provides a direct measurement on the amount of liquid nitrogen present in the BeTank. When the automatic refill procedure is activated, the PLC uses this information to define if and when to open or close the liquid N₂ supply valve.

It is worth stressing that the vessel and all the cryogenic systems will be permanently mounted on the cart. Therefore, the spectrometer cart is a standalone, complete system which can be operated anywhere close to a suitable supply of liquid nitrogen.