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The Tower

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.

thumbrbt.jpgThis picture of a tower, minus the SQUIDs and FETs, is annotated -- click on it to load a larger version and get a closer look.

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600 Km and 4 K heatsinks

thumbsnk.jpgIn 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.

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20 mK floor

thumbird.jpgThe 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 K floor.
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 temperature.

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600 mK spool and 4 K floor

thumbspl.jpgA 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.

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infrared (IR) shield

thumbshd.jpgWithout 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 other things.

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SQUID board and gusset

thumbqt2.jpgHere 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.

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Flyover cable

thumbfly.jpgA 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.

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FET board

thumbsqt.jpgSix 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.

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Comments:  jrembaum@cosmology.berkeley.edu
Updated: (JDR) 05/24/02

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
 
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