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- Standby mode
- We need to determine the setting for fridge valves and gate-valve bypass that allows us to keep things cold but uses the least amount of liquid helium as possible.
- The place to start for this study is to overfill the nose to the top of the heat exchanger, crack the run valve, and open the gate-valve bypass just a little so you get something like ~1 slm going through the separator flow.
- The pressure in the nose will build a bit which is what you want. You want to let it rise to maybe a few hundred Torr and find out what the optimal storage temp/flow is.
- This is the mode we should keep the system in when we are not doing something with the fridge.
- It's also possible to let the helium in the nose boil off and just keep the separator cold. This would be the recommended state if we do not use the fridge for many days. However, every time we let the nose empty we will have to cool it down again which is inefficient both in terms of time and helium.
- TE mode
- There is a range of different temperatures we might want to perform TE measurements at. We need to learn what run valve and gate-valve bypass setting correspond to what temperature of interest.
- We need the backing pump running with the gate-valve bypass cracked with the nose being filled by the run valve but at the least amount open to hold the temperature in the nose fixed. We want to be able to do this for various temperatures (1.7K, 2K, 2.5, 3K...).
- 1.7 K ↔ 8.461 Torr — https://qdusa.com/support/helium_vapor_pressure.html
- 2.0 K ↔ 23.48 Torr
- 2.5 K ↔ 76.73 Torr
- 3.0 K ↔ 180.4 Torr
- The run valve setting should be fixed so that it's not introducing helium at a different temperature than the nose reservoir at a fluctuating rate.
- The best temperature sensor we have at the moment is the 100 Torr red brick Baratron. Use that helium vapor pressure to determine the temperature of the nose.
- This could be uniquely challenging because of the way the nose fills (no fill tube just sprays). The time required for the target to thermalize is longer for lower temperatures. You would need to wait at least 3 times the t1 (longitudinal relaxation rate).
- As a starting point, we should demonstrate that we can hold a fixed temperature in the nose for at least 30 minutes at these various temperatures.
- Operations mode
- Operations means that the microwave is running and the beam will be coming once every minute for 4.4s. This is an additional heat load to the system. Here we want our maximum cooling power and lowest temperature possible.
- All roots are running and the run valve is open so we are seeing about 20 slm on the Main Flow. The run valve operating on a PID on Main Flow may be helpful here.
- We should demonstrate the feasibility of this mode over a long period of time.
Procedure for Filling the Fridge with LHe
The fridge is supposed to have no LHe at the beginning. You can/should use this preset page to monitor the key parameters.
A serious failure during this procedure is an overfill of LHe to the fridge (above the separator). When you lose track of the liquid level, you must close the bypass valve (on Fridge_Valve_Main.vi).
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Change the separator flow to 20 SLM (from 7 SLM usually).
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- The auto control is disabled.
- The absolute valve position is 0.08 turns.
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- The level-probe mode is "CONT" (not "HOLD").
- The Interlock with Heat-Ex Top is enabled.
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Open the bypass valve at 2.0 turns. Confirm that the fridge return flow goes up.
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Wait until the Ann-Plate Top resistor goes up to 1400 Ohm, which indicates that the liquid started accumulating. It usually takes 30 minutes from step #3. If it takes longer than 40 minutes, search for unexpected conditions.
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Wait until the Heat-Ex Top resistor goes up to 1400 Ohm, which indicates that the liquid is as high as Heat-Ex Top. It usually takes 10 minutes from the previous step.
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Close the bypass valve.
- Usually the software interlock sets it at 0.5 turns automatically.
- Anyway you set it at 0.0 turns manually.
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Change the separator flow to 7 SLM.
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- The set point for the auto control is 40 Torr.
- The auto control is enabled.
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Fridge Related Procedures
1. Fridge Fill Procedure
Old Procedure for Filling the Fridge with LHe (Until November 2023)
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