ESTRUCTURA Y CONFORMACIÓN DEL CUADRO OFDM

In our first cool down test we were primarily concerned about leaks into the vacuum chamber as increases in the internal pressure will create a thermal short between cryogenic layers which can drastically affect cryogenic hold times. Leaks from the helium tank and its various cryogenic seals were the biggest concern as any leak in the tank detected with helium gas would be a factor of 1000 worse with liquid helium. However, we do expect a steady background leak level in the cryostat from the various gaskets. The steady state leak rate can be estimated from the largest contributor, the rubber O-rings used to seal all non-cryogenic interfaces. Equation 7.3 is taken from Parker Hannifin Corporation (2007) which calculates the constant leak rate through O-rings.

L= 0.7×F ×D×P ×Q×(1−S)2 = 6×10−8mbar L/s (7.3)

F describes the permeability rate of the gas through the O-Rings with F = 0.2∗10−8std. cc cm

cm2_{s bar}, D is the inner diameter of the O-ring which for the largest seal

parametrize the squeeze of the O-Ring with values of approximately 0.7 and 0.2, re- spectively. The leak rate from this calculation is on the same order to the background level observed when leak checking the cryostat at LN2 temperatures. It should be noted the permeability rate for air is used for F while the leak checker measures the helium leak rate which makes a direct comparison difficult.

While pre-cooling with LN2 during the first cool down of the cryostat, we found a higher background leak rate and a higher loading than predicted which was indicative of a leak. We were unable to locate the leak on the exterior seals of the cryostat which implied the leak was in the helium tank. Our suspicions were ultimately confirmed by pressurizing the tank with helium gas while it was cooled to∼ 80 K which caused a spike in the leak rate. The cryostat was mostly dis-assembled to locate the source of the leak, found in the weld joint of the HWPR motor axle feedthrough tube on the cold plate side of the helium tank. The tank was removed from the assembly and shipped back to Precision Cryogenics for repair.

7.4.1

Cool Down 1

Once the weld was fixed along with a bad indium seal that was discovered in a similar manner, we proceeded with our first liquid helium cool down. We measured boil off flow rates to determine the steady state load of the 4 K stage prior to adding most of the feedthrough components. For the first test we only had the housekeep- ing cables fed through to the cold plate which eliminated many potential sources of loading. At this stage we were still developing our measurement technique which meant we did not recognize the excessive loading on the 4 K at this stage. We did, however, gain a lot of insight into the behavior of the cryostat including the relatively long time scales on which it took the heat exchanger and VCS systems to stabilize, often on the order of 12 hours. During the cool down we adjusted the loading on

the 4 K stage and VCSs with heater elements to gather several data points of loading versus temperature to help assess the performance of the system,. However, the long time scales needed to reach equilibrium limited the amount of tests we could perform before running out of liquid helium.

The cool down also gave us a first look at the effectiveness of the heat exchangers and the plug system. The plug rod for the heat exchangers was inserted to confirm the design performed as expected. We observed a spike in temperature on both heat exchanger thermometers at nearly the same time which was evidence that the spacing between the two plugs was correct. This is an important consideration as we want both plugs to have a similar amount of compression force applied to ensure a good seal on both surfaces. After the plug was inserted we were able to make a measurement of the pressure build up in the helium tank from the impedance of the heat exchangers. The back pressure increases the boil off temperature of the helium which was confirmed to be a small effect. There are two ways we can measure the pressure in the tank, the first is by measuring the pressure at the top of the plug assembly through the pressure relief tube. However, we often lacked a pressure gauge that was sensitive to the small changes in which case a better measure of the pressure in the tank can be made by observing the temperature increase of the liquid helium bath. During this cool down we had difficulty calculating the change in pressure as the diode thermometers had not been fully calibrated but in future cool downs it could be a useful measurement to make.