Cycling Instructions for SPARO Cryostat

Step 1: Open both heat switches

Under all normal operating conditions with the exception of initial cooldown, the heat switch from the pots to the ⁴He reservoir(marked with red tape) should be open(The function of this switch is to expedite initial cooldown of the pots to 4K.) During a ³He cycle, this heat switch must be in the open configuration in order to thermally isolate the colder pots from the 4K reservoir.

Opening the pumps heat switch (marked by a green tape) serves to thermally isolate the charcoal pumps from the 4K reservoir. This allows them to be heated without simply boiling off the ⁴He in the reservoir.

Step 2: close the valves to the external ³He tanks

Closing these valves serves to prevent the gas from collecting in the external tanks when driven off. NOTE: if the dewar has been cycled previously during the current cooldown, these valves may be already closed.

Step 3: Heat the charcoal pumps

The purpose of heating the pumps is to drive off the gas from the pumps into the condensing chamber(thermally connected to the pumped pot). The recommended voltage is 3V(this should produce a current of about (0.1A). One will begin to see the ³He gauges peak and then turn over. Since the guard(outer) stage contains much more gas than the inner stage it will cycle first followed by the detector(inner) stage. Basically there are two processes going on here. The first is the driving off of the gas from the pumps. This increases the pressure in the condensing chamber. The second process is the condensing of the gas and the corresponding dripping into the pots. This process thereby reduces the pressure. At the beginning of the heating stage, the first dominates and hence the pressure increases. As more gas enters the chamber, more begins to condense. Eventually, there is no more gas entering the chamber and the condensation dominates resulting in a turnover in the pressure. Also, during this process, one will notice a rise in the pumped pot temperature, typically up to 2.0 to 3.5 K.

Depending on the cooling power of the pumped pot during a particular cycle the duration of this stage could last anywhere from 3 hours to 15 hours. The cooling power is indicated by the pressure on the capacitance manometer gauge when the system is in equilibrium, i.e. the pumped pot is neither filling nor draining. A higher pressure indicates higher flow through the capillary and thus a greater cooling power.

A good rule of thumb is to heat until 1 hour after the pressures on the inner ³He stage turns over. One word of caution: each internal ³He system of SPARO has a very small volume and is connected to the external tanks via a pressure relief valve. This valve opens with a difference in pressure of about 20 psi which translates into a pressure difference of about 1 Volt on the Omega pressure gauges. During low cooling power cycles, it may be necessary to back off on the heater voltage if the pressures appear to be approaching this limit.

Past experience indicates that SPARO can be cycled with pumped pot pressures as low as 0.075 Torr. However when the pressure is in the low range(0.075 to 0.200 Torr), it is advisable to use 2.5 V when heating. Below 0.075 Torr, it may be possible to cycle with even less heat, but this has not been tried.

Step 4: Turn off heaters

Turning off the heaters gives the gas an opportunity to condense in the pots. During this phase, the pumped pot is recovering and again, the duration of this phase will vary depending on the cooling power of the pumped pot. The best way to judge how long to condense for is to again watch the ³He pressure gauges. They should go assymptotically to some low value(the vapor pressure of the liquid at the temperature of the pumped pots) as the pumped pot condenses the gas into liquid. Once these gauges indicate that the pressures have settled, step 5 can be taken.

Step 5: Close the pumps heat switch

This step initiates the pumping action of the charcoal pumps on the liquid ³He which has been condensed in the two pots. This serves to lower the temperature of the inner stage to about 0.29K and the guard stage to about 0.39K by lowering the vapor pressure. The closing of the heat switch involves heat sinking the warm(40K) charcoal pumps to the ⁴He reservoir. Thus, a lot of heat is dumped to the reservoir thereby increasing the boil off rate. It is a good idea to disconnect the flow meter temporarily during this process. This simply entails moving the lever on the flow meter selector to the open position. REMEMBER TO TURN THE LEVER BACK TO THE 11 LPM SETTING AFTER ABOUT 15-20 MINUTES. On the data logger, the inner and guard stage temperatures will drop rapidly for about 20 to 30 minutes upon closing the heat switch. After two hours, serious data taking can begin.