15.9.99
Before any
cooling :
-
the
outer jacket should be pumped
-
make
sure to purge the whole circuits with helium (to avoid blocking the needle
valve in particular)
-
make
sure that the capillary tube through the coils is well opened (flush helium
through it)
-
pre-cool
the magnet with nitrogen for 2 days so that the coil is at 100K
Be careful, when the cryostat is hot, nitrogen evaporates quickly and you have
to fill it at least twice a day (you may use the « Tempotacs »)
-
Once
the cryostat is pre-cooled, you must pre-cool the coil down to 5K with helium
-
DO NOT
directly transfer helium, it is hopeless, use the transfer rod with can be
screwed onto the capillary which flows through the coils.
-
Transfer
helium slowly at a rate of about 30-50 l/min.
-
Monitor
the temperature on the sensors (Coils). First look at the Pt sensors, it will
steadily decrease down to 20K. Below that, the reading is meaningless, Look at
the Carbon sensor, its resistance will rather quickly increase, from 200 to
350.
It should then rise rather quickly (1 min) to 390 ohms and the gas flow should
drop to 15 l/min. => the transfer has started
-
As a
general rule, the gas output should barely freeze.
-
The
steady state is obtained for 414 ohms and the helium flow is then of about
4l/min.
The
temperature is automatically recorded as soon as the temperature controller is
connected to the GPIB port.
To cool
down the sample down to helium temperature, force some helium flow through the
needle valve by partly closing the red valve. Be careful, if you completly
close the red valve, there might be an overpressure and the transfer hole cap
may pop out. No more helium will flow throughthe needle valve and the
temperature of the sample will increase.
There is an
diameter adapter. To make sure that the cap will not pop out, take out the
adapter and put the big one. It holds much better and the sealing is better.
Bad luck
you’ve frozen the sample rod in the sample space. Heat up the sample up to 80K,
to unfreeze the air. It should be ok then
?? (heat
with which heater?)
Take the
“versatile black cable”. Connect it to the output named “Bobines”.
There are
two small resistances stuck on the two super-conducting shields. If there are
vortices trapped into these shields, you have to reset them to expel the
vortices otherwise the flipping ratio
Find a
small 20V power supply.
Set the
flippers to -+, if there are vortices, the flipping ratio should be very bad
(close to 1 or 2).
Slowly
apply some current (max 100mA) into both resistors (total resistance 200 ohms
if mounted in parallel. At some stage when the shields are heated above their
Curie temperature, there is a sudden switch to good flipping ratios (if
everything was properly set before)