The tower acts as a bridge and waystation for signals going
between the electronics sitting at cryogenic and room temperatures.
Information from the detectors
(in the 75 fridge, they sit at 20 mK -- or 0.02 Kelvin) comes in
through the basement,
is amplified, and then sent outside the cryostat to computers
and other devices (sitting at room temperature) which record the
data. Instructions can then be sent back to the detectors and other
instruments inside the cryostat.
The design of the tower is based on the need
for hardware to operate inside the cryostat at different
temperatures in close proximity, without interfering and heating up
each other. Within several inches the temperature of the tower
ranges from 0.02 K to 4K. This is done by attaching the different
"floors" of the tower to parts of the cryostat maintained at
different temperatures. This way, for example, the SQUIDs,
connected to the 600
mK spool (which is connected to the 600 mK floor), can run
efficiently at ... you guessed it, 600 mK.
This picture of a tower,
minus the SQUIDs and FETs, is annotated -- click on it to load a
larger version and get a closer look.
On this page:
600 Km and 4 K heatsinks
In this picture you can see the
600 milliKelvin (mK) and 4 K heatsinks which connect to the parts of
the tower that need to be maintained at 600 mK and 4 K,
respectively. The heatsinks consist of two copper
plates, connected by copper wiring. One plate screws into the
tower at the appropriate temperature and the second plate screws
into the cryostat, at the appropriate
temperature. The copper heatsink then acts as a thermal conductor,
cooling the part of the tower to which it is connected.
20 mK floor
The first in a series of four
"floors" at 20 mK, 50 mK, 600 mK, and 4 K. Hardware is attached to
the different floors to be maintained at their proper temperatures.
For instance, the FETs
need to be kept closer to 4 K, so they are attached to the 4
In the picture to the right a tower is attached to
the IR Dewar, so the setup is not the same as it is inside the
cryostat. However, one can see the different floors, connected to
each other via heatstraps. Inside the cryostat, in the chamber where
the tower would sit, the floors are individually connected to the
inside of the chamber at appropriate places, so that they are
maintained at their respective temperatures. The
floors are all connected to each other in the picture, because in
this IR Dewar test all the hardware is being tested at the same
600 mK spool and 4 K floor
A view of the IR shield (the
copper sheet), the 600 mK spool (the innermost circle with screw
holes), and the 4 K floor (surrounding the spool), from under the
cryostat looking up. The spool is where the SQUIDs will be connected
and the 4 K floor is where the FET cards will be connected, along
with a thermometry card.
infrared (IR) shield
Without the IR shield (a simple
sheet of pure copper) infrared radiation would get inside the
cryostat and heat up parts inside it, reducing its efficiency among
SQUID board and gusset
Here you can see the SQUID (Superconducting Quantum Interference
Device, not the monster from 20,000
Leagues Under the Sea) connected to a FET
card via a flyover
cable. The entire thing is called a SQUET. A SQUID has two
parts: the gusset (where the amplifying
electronics are located) and the board,
where the gusset sits and the flyover is connected.
If you're interested, you can look at this block-diagram
of the SQUID's electronics.
A picture of an unfinished flyover
cable connected to a SQUID board. Information received from the
detectors is amplified by the SQUID and then, via the superconducting wires in the flyover cable, sent
to the FET card and then outside the cryostat. Although the flyover
cable runs between a 600 mK SQUID and a 4 K FET, this is not a
problem and does not affect the functioning of either hardware.
Click on the picture to see the flyover cable close-up.
Six SQUETs attached to a tower can
be seen in the picture to the right. Facing the camera are the FET
(Field Effect Transistor) boards,
each with a SQUID
attached to it via a flyover
cable. The FETs require higher temperatures than the SQUIDs to
function properly, so they are maintained closer to 4 K.