1.  SYSTEM EVACUATION AND COOL-DOWN:

         The system is normally pumped out and cooled to liquid helium temperatures prior to delivery and whenever possible we deliver systems filled with liquid helium. This section of the manual is provided in the event that the system needs to be warmed to room temperature for any reason and then the customer needs to check the vacuum and conduct the complete cool down. In the following discussion, we assume that the SRM (Superconducting Rock Magnetometer) is at room temperature and under vacuum.

Figures 1-1, 1-2, 1-3, and 1-4 will assist in understanding the construction and connections of the magnetometer.

1.1 CHECKING VACUUM:

a)     Remove the brass threaded plug from the vacuum valve on the SRM top plate (on the end of the SRM where the SQUID amplifiers are located), see 1-3. Using teflon thread seal, screw the brass hose barb fitting into the vacuum valve until it is tight. Do not over tighten. The brass fitting supplied with the SRM is for 1/2 inch inside diameter rubber vacuum hose. Other size fittings and hose may be used if desired.

b)    Connect the hose to a vacuum pump that will reach a vacuum of at least 5 microns (5 millitorr) and install a vacuum gauge in the pumping line close to the SRM valve.

c)     Pump the rubber hose and gauge down to 5 microns then open the vacuum valve on the SRM and record the pressure. If convenient close the pumping line between the gauge and the vacuum pump and record the initial pressure in the SRM. If the SRM has not been pumped since delivery and it has been allowed to warm to room temperature, then the initial pressure should be between 100 and 1000 microns.

d)    Close the SRM vacuum valve as soon as the pressure is determined. Proceed to the next section if additional pumping is required.

1.2 PUMPING ON THE SRM VACUUM:

a)     If the initial pressure is above 50 microns then it is desirable to pump on the vacuum space to remove the outgassing contaminants that have accumulated since the previous cool down.

b)    A trap MUST be inserted between the vacuum pump and the SRM to prevent pump oil vapor from back streaming into the superinsulation space of the SRM. We strongly recommend a liquid nitrogen cooled trap although a good antiback streaming gauze type trap is adequate if it is clean. If a turbo molecular pump is used the trap is not needed.

c)     With the pump, trap and gauge installed pump out the line to the SRM valve before opening the valve. Now open the valve to the SRM and pump until the vacuum reaches at least 30 microns and preferably 10 microns. This will take anywhere from 1 to 10 days depending on the time since the previous pump down. When the pump down is completed close the SRM valve and remove the vacuum pump. Replace the brass vacuum valve plug.

1.3 PUMPING ON THE HELIUM RESERVOIR:

            When the system is warmed to room temperature it is a good opportunity to pump on the helium reservoir and all of its associated plumbing and oscillation dampers, and then to back fill this reservoir volume with ultra high purity helium gas. This will remove any contaminants that may have collected in the reservoir, which could cause plugging of the fill, vent or safety line when the system is cooled down. To pump, use the same vacuum system described in the previous section for pumping the dewar high vacuum. Remove the brass hex cap from the FILL port and install the brass hex cap with a vacuum valve in place of the pop off valve. Connect the pumping system to the output of this vacuum valve. Close the needle valve at the SRM VENT, and be certain that the SRM VENT valve is also closed. See figure 1-3 and 1-4 for the location of these fittings on the SRM top plate.

            Pump down the line to the FILL vacuum valve then open this valve. The pressure will initially be one atmosphere and it will require several hours pumping to reach the 1000 micron level. Continue pumping until a pressure of below 200 microns is reached (about 6 hours). Now close the FILL port vacuum valve. Connect a hose from a cylinder of ultrahigh purity helium gas (99.999 %) to the VENT valve.  Be certain to flush this hose with the helium gas first to avoid adding contaminants to the reservoir. Let helium gas into the reservoir until the SAFETY pressure reaches about 5 psig. Close the VENT valve and pump the reservoir back down to below 200 microns a second time. Repeat the filling with helium gas to 5 psig SAFETY pressure. The reservoir and all of the plumbing attached to it should now be filled with the ultrahigh purity helium gas.

1.4 LEAK TESTING:

            If a vacuum leak is suspected it will be necessary to connect a helium mass spectrometer detector to the SRM vacuum valve. Pump down the connecting line in the manner described in 1-1-a through c and then open the vacuum valve and measure the background level of helium in the SRM vacuum space. External leaks can be located by normal leak detection techniques of applying small amounts of helium gas on any suspected regions and monitoring the internal helium background. The leak detector response can be very slow because of the large volume of the vacuum region in the SRM and the many layers of thermal insulation, so patience and care will be required in the leak testing process. It is advisable to contact the factory before leak testing the system and we will provide specific instructions depending on the nature of the suspected leak.

1.5 COOL DOWN WITH CRYOCOOLER PRECOOL:

a)     Once it has been confirmed that the vacuum is satisfactory the SRM can be cooled to operating temperature. If time permits (about 7 days elapsed time required) it is more efficient to do the preliminary cooling with the cryocooler, then, when the reservoir and superconducting shield reach about 70 Kelvin the helium transfer and further cooling to 4.2 Kelvin will take only about 30 liters of liquid helium. An alternate procedure uses periodic helium transfers and the cryocooler to cool the SRM to helium temperature from room temperature in about two days. It is possible to do the precool with liquid nitrogen but this requires great care to avoid collecting any liquid in the reservoir and to remove ALL of the nitrogen gas prior to operating the cryocooler. We do not recommend this nitrogen precool. Please contact the factory BEFORE attempting a liquid nitrogen precool.

The cool down using the cryocooler only is shown in Figure 1-5 for a 755 magnetometer. The same procedure applies to the larger 760 system but the time required to reach 50 Kelvin will be about 12 days. If the cooling reaches equilibrium at a higher temperature this indicates that either the magnetometer vacuum is not low enough or that the cryocooler is not performing properly. If the vacuum is not low enough there will be condensation in the sample access as well as a decrease in the cooling rate. The vacuum can be reduced by pumping while the system is cold. It is very important to use a liquid nitrogen trap in the pumping line to prevent back flow of oil vapor. Continue pumping until the cooling rate increases and the access tube warms to room temperature. This will normally take 8 or more hours of pumping.

If the vacuum is OK, that is, if the cooling rate does not improve with pumping, then the cryocooler is probably not performing to specifications. Please contact the 2G factory for further advice.

b)    If the system has warmed up from helium temperature and the fill, vent and safety ports have remained in the operating position so that no chance has occurred for air to enter the system , then close the needle valve between the vent pop off valve and the flow gauge and pressurize the helium reservoir to 6 psig on the safety pressure gauge. Use ultra high purity helium gas through the vent fitting, first flushing the external gas line from the helium supply cylinder to remove all air. Close the vent valve and turn on the cryocooler.

c)     When cooling with the cryocooler, monitor the safety pressure every morning and evening and repressurize when needed to make certain that the SAFETY

d)    pressure does not fall below 1 psig. As the helium reservoir cools the helium gas inside will be at a lower pressure and gas will need to be added about once per day for the first three days of cooling, then every other day for about four more days. It is also helpful to monitor the SQUID temperature (see section 3.2 for the thermometer description). It will require about 7 days for the reservoir and shield assembly to reach 70 Kelvin at which time it is efficient to complete the cool down with a liquid helium transfer.

e)     All liquid helium transfers follow the procedures given in  the following section 2. There are two very important aspects of this first transfer after a cryocooler cooldown that must  be carefully reviewed to conserve helium and to prevent a solid air plug.

f)     The first transfer of liquid helium is used to reduce the temperature from that produced by the cryocooler to below 10 Kelvin. It is much more efficient to use the heat capacity of the helium gas to provide this cooling rather than the heat of vaporization of the liquid, thus, we recommend that the first transfer be done slowly and stopped when the SQUID temperature reaches about 1.8 volts or about 10 Kelvin.

            Starting the transfer: Record the temperatures and safety pressure readings and the helium level in the storage dewar. If the dewar pressure is below about 0.5 psig it will be necessary to increase it by adding UHP helium gas through the VENT valve. Be certain to completely flush the gas hose with helium gas before connecting it to the VENT. Add gas to reach a safety pressure of 1.0 psig and close the VENT valve.

            Flush the helium transfer line with welding grade helium gas holding the line as an inverted U to keep air from counterflowing into the line. Slowly insert the line into the storage dewar keeping the storage pressure at one atmosphere. Tighten the seal around the line to storage dewar. Now apply a small helium gas pressure (about 1/2 psig) to the storage dewar and observe the gas flow out the end of the transfer line. Welding grade gas is adequate for the transfer. When the flow is steady but still warm, open the flip valve on the line adapter, remove the rubber stopper from the FILL PORT and quickly but smoothly insert the line into the PORT. The line will precool by gas flowing though the line and out the flip valve. When the flip valve starts to get cool (but before it gets frosty) close the flip valve. Now slowly increase the pressure on the STORAGE dewar to about 2.5 psig. When the safety pressure reaches 2.0 psig open the magnetometer VENT valve. Cold helium gas is now being transferred into the RESERVOIR.

            Backdrafting is certain to occur during this transfer and the precautions given in section 2-2-e MUST be followed to prevent plugging of the vent port. The back drafting will occur when the SQUID diode reaches about 1.15 volts (55K)  and the pressure will quickly decrease (in less than one minute so it is important to continuously monitor the safety pressure) to a negative value of 2 to 4 inches of mercury on the safety gauge. It is crucial that the vent valve be closed before this reservoir pressure becomes negative. By monitoring the SQUID temperature you can tell when the back drafting is about to start, i.e. when the SQUID temperature reaches 1.15 volts. This will leave just enough time to warm the vent valve and close it when the safety pressure begins to fall. If the transfer pressure is left constant at about 2 psig during the back drafting it will take about 5 minutes for the reservoir to reach equilibrium and for the safety pressure to rise to a positive 1 psig. During this time the SQUID temperature will fall rapidly to 1.6 to 1.8 volt and some liquid may collect in the reservoir.

The helium transfer can be stopped quickly by pulling the helium transfer line up in the storage dewar so that the line tip is abvove the liquid helium level of the storage dewar. It is important to keep the pressure on the storage dewar during this operation and until the safety pressure reaches about 1 psig. At this time the transfer pressure can be reduced and the line removed from the magnetometer fill port. as described below.

            If the transfer line is left fully inserted into the stporage dewar then wait until the safety pressure has increased to about 1 psig, stop the transfer as follows:

a)     Close the VENT valve on the SRM dewar when the safety pressure, Ps,  reaches 1 psig.

b)    Loosen the hose clamp on the large adapter hose at the FILL PORT and remove the transfer line and adapter from the fill port. Immediately plug the fill port with the solid black rubber stopper. It is most likely that some liquid helium was collected during the back drafting since the negative pressure produces a very rapid transfer of liquid. Check the lower helium level gauge (Section 3-1) and record its reading.

c)     The SRM will now continue to cool for about 8 hours during which time the boil off rate will be in the 5 liter per day range. Open the needle valve between the pop off valve and the flow gauge so that this boiloff gas can vent through the pop off. Due to the high boil off rate the pressure will increase in the reservoir and it is important to monitor the safety pressure and keep it below about 6 psig by venting excess gas out the vent valve. This venting is to be done slowly and the SRM must NEVER be left unattended during this venting since the pressure may reduce to one atmosphere and then it would be possible for air to cryopump in the vent line and plug it.

d)    When the superconducting shield temperature reaches the same temperature as the SQUIDS it is time to complete the helium fill. As noted above, this will require about 8 hours from the conclusion of this transfer. It is NOT critical when the next transfer is made since the cryocooler will hold the reservoir-shield assembly below 20 Kelvin continuously. The next transfer will proceed as a normal fill except it is much more likely that  the back drafting will occur. Therefore, follow the instructions given in section 2 and fill the reservoir to whatever level is desired. After completing the transfer it will take about 6 hours for the SQUIDS and shield to reach equilibrium and for the SRM to be ready to operate. The helium loss rate and SQUID noise will continue to improve for several days before final operating conditions are reached. This is a good time to trap a low measurement field if desired.

1.6. COOLDOWN WITH HELIUM TRANSFER AND CRYOCOOLER:

            The procedure to cool the magnetometer from room temperature starting with the helium transfer and the cryocooler operating is essentially as described in the above procedure except three separate helium transfers are required. A typical cooling time for this procedure is shown in Figure 1-6  for a 755 system. A 760 will require about 50% longer time at each step. These data show the minimum cool down time needed when time is very critical. It is more efficient and less stressful on the user to wait overnight between each helium transfer. This allows the superconducting shield temperature to reach equilibrium with the reservoir. For example, after the first transfer the reservoir is at 55 K and the shield is at 210 K. By waiting 9 or 10 hours before starting the second transfer, the shield will cool to about 60 K and this will save a few liters of helium on the second transfer. It is even more important to wait

until the shield and reservoir reach the same temperature after the second transfer since any extra cooling of the shield will save on the final amount of helium collected after equilibrium is reached when the system is filled.

1.6.1  Beginning the cooldown:

            The vacuum in the magnetometer should be checked to ensure that it is below about 25 microns, and if it is suspected that any air may have entered the reservoir, then the reservoir should be pumped and back filled with ultra high purity helium gas. (See section 1.3 for this pumping and back filling procedure.)  Pressurize the reservoir to about 1 psig and remove the helium gas line from the magnetometer vent valve.

            Flush the helium transfer line with helium gas holding the line as an inverted U to keep air from counterflowing into the line. Slowly insert the line into the storage dewar keeping the storage pressure at one atmosphere. Tighten the seal around the line to storage dewar. Now apply a small pressure (about 1/2 psig) to the storage dewar and observe the gas flow out the end of the transfer line. When the flow is steady but still warm, open the flip valve on the transfer line adapter, remove the rubber stopper from the FILL PORT and quickly but smoothly insert the line into the PORT. The line will precool by gas flowing through the line and out the flip valve. When the flip valve starts to get cool (but before it gets frosty) close the valve. Now slowly increase the pressure on the STORAGE dewar to about 2.5 psig. When the safety pressure reaches 2.0 psig open the magnetometer VENT valve. Cold helium gas is now being transferred into the RESERVOIR.

            Turn on the cryocooler and measure the 4 temperatures (SQUIDS, superconducting shield, inner and outer cryocooled shields) using box 1 and 3 as described in section 3.2. The voltages should be about 0.4 volts for all diodes indicating room temperature.

            Keep the transfer pressure steady at about 2.5 psig, but do not continually adjust the pressure regulator. The objective is to reach a steady transfer pressure since up and down changes will result in excess loss of liquid helium and make the transfer more difficult to monitor. Keep a record of the 4 temperatures, the safety pressure, and the transfer pressure taking readings every 10 to 15 minutes.

            This first transfer will take about 4 hours during which time the SQUID temperature will decrease from room temperature to about 80 K (0.4 volt to 1.0 volt). The shield will cool much slower so when the SQUIDS reach about 1.0 volt the transfer is to be stopped to allow time for the shield and SQUIDS to reach equilibrium. The reservoir thermal mass is much larger than that of the shield so the shield will cool  about 150 K while the reservoir warms only about 20 K. Also, the cryocooler is now continuing to cool the two thermal shields and after about 2 hours the inner shield will be colder than the superconducting shield so additional superconducting shield cooling will occur by radiation and conduction between these two shields. When the SQUID temperature reaches about 1.0 volt stop the transfer by slowly reducing the storage dewar pressure to 0.5 psig , then close the VENT VALVE on the magnetometer, remove the TRANSFER line from the FILL PORT and seal the PORT with the black rubber stopper. Continue to reduce the pressure on the storage to one atmosphere then remove the transfer line from the storage. Measure the helium level in the storage when convenient. It should require about 25 liter for a 755 SRM and 30 liter for a 760 to reach this 1.0 volt SQUID temperature.

            Open the needle valve to the magnetometer flow gauge and continually monitor the safety pressure. The gas flow will be high and the normal magnetometer vent will not be able to maintain a stable pressure. When the safety pressure reaches about 5 psig, reduce it back to one psig by opening the VENT valve. This pressure reduction will need to be done about every 30 minutes for the next few hours.

            After about 8 hours the superconducting shield and the reservoir will both be at temperature of about 100K and the second transfer can be started. At this time the reservoir and shield will both start to cool slowly because of the cryocooler, so if you are not ready to start the next transfer it will be important to monitor the safety pressure to be certain that it does not reduce below one atmosphere due to reservoir cooling. The cooling will be very slow so if the reservoir is at 1 psig it will require about 6 hours for the pressure to fall to zero.

Second and third transfers:

The second transfer is used to cool the reservoir to 10 K then to let the shield and reservoir reach equilibrium at about 30 K. This transfer is identical to the first one described in the previous section 1-5.e where the initial cooling to 80 K was done only with the cryocooler.

The third helium transfer will be done following the procedure given in the next section 2 for a normal helium fill. The one difference is that this third transfer will start with the helium reservoir slightly above 4.2 K so backdrafting is certain to occur very soon after the transfer begins. Use extreme caution and constantly monitor the SAFETY pressure and close the VENT valve as soon as the SAFETY pressure starts to fall near 1psig.