Cool Down Procedure for SPARO
- Perform electrical checks on all four electrical feedthroughs using
DVM set on Ohms - autoranging scale. It is advisable to record the resistances
between pins corresponding to common internal devices and record these resistances
in the logbook. The pinouts for the preamp connectors can be found in this
documentation book. The pinout for the cycling connector can be found in logbook 4.
- Throughout the cooldown process, it is necessary to periodically manipulate all of the
mechanical feedthroughs EXCEPT for the grid adjuster. This process insures that
differential contraction will not cause the devices to seize. However, the grid
adjuster has been set in the lab to the best alignment between the two arrays and
should not be adjusted.
- Put the dewar case on the turbo pump. This is accomplished via a 1.33" conflat
seal connecting to a silver metal seal valve which requires a torque wrench
to open and close(Incidently, closed is about 20 lbs. of torque). It is
a good idea to put a kf o-ring valve in series with this valve since the number
of open/close operations of metal seal valves are limited.
- when pressure is low enough, move to leak checker and leak check
(a pressure of 1 x 10-4 mmHg should be low enough)
- Leak checking should be done on 10-9 scale of Alcatel
- Make sure to blow He down into reservoir and pumped pot line
- Put the dewar case back on the turbo pump.
- After ~15 hours on turbo-pump, pressure should go to ~3 x 10 -5 mmHg
- Set up the pump such that it is possible to pump both the reservoir and the pumped
pot line. This setup should include the possibility of both backfilling the reservoir
and flushing the capillary. See diagram(Use the reservoir pumping fixture as part of this
setup. Located on the wall in the SPARO closet)
Photos of fixtures used during cooldown
- Begin pumping on the reservoir with the pumped pot line valved off.
- When the reservoir pressure gets to 20 mTorr(Note: it should take 10 minutes to
get the valve completely open and up to an hour to get down to 20 mT), begin pumping on the pumped pot
line while continuing to pump on the reservoir.
- Pinch off a section of the tygon flushing line and open the valve so
as to pump out the pinched off section of the line.(make sure that before having done
this, the tygon helium line has been purged of other gasses) This step has the importance
of clearing the valve volume of any gases other than helium which may be trapped.
- Reclose this valve and unpinch the hose.
- Close the valve connecting the pump to the pumped pot line and begin flushing
UHP He gas through the pumped pot(use a pressure of ~10 psi).
- valve off pump to reservoir (but keep back pressure on pumped pot)
- gently back fill reservoir with dry gas (N2 or UHP He work fine)
- fill with LN2 using the LN2 fill fixture (pump liquid in directly from transfer dewar)
- let soak at LN2 for ~ 5 hours or more
- 3 hours after beginning the nitrogen fill:
- flow ~ 50 on 11 LPM flowmeter;
- P ~ 2-3 x 10-6 (with some large upwards jumps)
- 20 hours after beginning the nitrogen fill:
- flow ~ 8 on 11 LPM flowmeter;
- P ~ 1-2 x 10-6
- dewar is near 77K when charcoal pump diode reads around 1V
- Blow out LN2, using the LN2 blowout fixture and
compressed gas, (either N2, or UHP He will work).
- Make sure gas pressure is less than flushing
pressure supplied to the pumped pot line. (2 lbs. of pressure on the
LN2 blowout fixture should be sufficient to blow out the
Also, here one must be careful to slowly increas blowout pressure
so as to give pressure gradients in the capillary time to dissipate.
- (OPTIONAL)Allow the reservoir to warm up to about 100 K in order to
ensure that almost all of the nitrogen has evaporated.
- pump on reservoir for about 2 hours using the reservoir pumping fixture.
Pressure has gone to 10-100 mT in the past.
Note: at this stage, if the previous step has been skipped, the low pressure on the reservoir causes the liquid nitrogen
to freeze and thus may take longer to pump out. It is important to get as much
nitrogen out of the reservoir as possible.
- gently back fill reservoir with He gas. keep flushing pressure
on pumped pot line.
- Valve off the dewar case. Remove from pump. Put tape on valve and blank
off the valve with a 1.33" Conflat bankoff. This will ensure that in case the
valve leaks slightly, not much air will be allowed to enter the dewar.
- Begin LHe transfer
- Maintain flushing pressure on pumped pot line
- monitor and record diode voltages and 3He gas pressures
- depending on speed of transfer, dewar may be cold before reservoir is
- Flow meter readings:
- 2 hours after beginning fill: flow is 30-50 on 30 LPM flowmeter;
- 5 hours after beginning fill: flow is 15-20 on 30 LPM flowmeter;
- 15-20 hours after beginning fill: flow is 25-30 on 11 LPM flowmeter,
depending on the flow through the capillary;
- dewar is near 4.2K(when at sea level) when 4He surface diode reads around 1.6V
- SPARO has a 4He hold time of 3-4 days indoors
- Using the plexiglass laboratory battery fixture(C-clamped to dewar ring)
apply 18 Volts to the JFET heaters via the "Heat" BNC's on the small boxes
which attach to the 15 pin connectors on the preamps.
- Be careful not to short the leads on this fixture. Also, avoid touching
the metal grounding plate to the dewar. Note that battery life for this
fixture is at least a week.
- As soon as dewar is cold, you may valve off back pressure and
begin pumping on the pumped pot.
As measured on the capacitance manometer gauge, the pressure settles at
some value between 0.25 and 1.6 Torr. Higher values indicate larger cooling power.
Last update February 7, 1999 DTC