It is important to note that TEs take time and depending on the immediate goals, having an very accurate polarization measurement may not be needed. For example if you're just testing equipment or running a functionality test or doing some training. When take physics production data it's important to have high quality TEs. However doing TEs cost valuable beam time with we generally can't afford. So, it good to learn how to do TEs well without wasting too much time. Durning beam time you will need at least three separate TE measurements taking at different times for each coil for every new target load. One single TE can be taken at multiple temperatures to reduce error but this takes more time so it good to plan these things with other beam down time.
- NMR System is tuned and a high statistics baselines (5K sweeps) for the relevant channels have been taken
- Magnet is at full field and in persistent mode, leads are ramped down
- Nose is full and the above the lower heat exchanger
- The run valve is set to replenish nose boil off (most recent studies indicated 0.8 turns with no microwaves)
- Gate vale is closed and the roots backing pump is running with the gate-valve bypass on PID control set to the desired pressure (usually around 20 Torr)
The appropriate time to wait for the spin temperature to reach the lattice temperature is 3t1, if this is not known its good to stablize the fridge pressure for at least 20 minutes. At this point you can start to take TE data and see what type of error you get. The error (standard deviation) will be based on both the variation in area and the variation in pressure. You want both to be as stable as possible but if there is a trend in area as a function of time this is a clear indication that you are not yet at TE. If it's purely stochastic variation over 20 minutes at high temperatures then you are at TE but you may have to wait for 30 or 40 minutes to confirm depending on the temperature of the TE you are doing. Warmer temperatures reach TE faster but have smaller signals to integrate. You get reduced integration error with larger area at lower temperatures but they take much longer. The Q-meter is only designed to do as good as about 1% relative error so trying to get a TE with error better than that is not a good use of time.
1) Set the number of NMR sweeps to 500 and start taking data
2) For a quick and dirty TE you need at least 15 points once the area has stabilized but for a high quality TE you want more like 50 points at no less than 3 different temperatures (usually between 2.4 and 1.4K).
3) Copy the events.csv file on the NMR computer so you don't disturb the writing process to the file.
7) Take the columns "Area" and "Pressure".
8) Use http://twist.phys.virginia.edu/tools/te_calcnew.php to calculate the TE calibration constant for the channel you are taking data.
9) When you press calculate the polarization at the corresponding temperature and the new calibration constant will appear below with errors.
10) You may want to do this a few times to see if the data you are collecting is truly stable in area and the calculated calibration constant is not changing much.
11) Once you have results for various temperature average or fit the resulting calibration constants over the range of temperatures to achieve reduced error
12) Record all information in the logbook and put the final calibration constant in PDP so it will calculate polarization for you.
13) Do the same thing are the other two coils if needed.
If for some reason you get a very different calibration constant at a different temperature, its generally a sign that something shifted in the cable or Q-meter system or in the target cell itself and you may have to start from the beginning.