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
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
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
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
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
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
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
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.