EPR Continuous Monitoring with Field Feedback

This configuration uses the lock-in amplifier with PI feedback circuit to lock into the EPR resonance frequency and then track its behavior. At the same time a field feedback system is used to stablize the main holding field to a level of 10$^{-5}$. The EPR resonance frequency is the central frequency proportional to the main holding field plus (or minus) a field originated from the $^3$He polarization. By measuring the EPR resonance frequency and the main holding field, one is able to monitor the pumping cell polarization continuously.

The circuit and labview program are the same to that are used for AFP Sweep measurement described in Section , except that field feedback system is needed, as shown in Figure

Figure: Circuit for continuous EPR measurement with field feedback

1. Construct circuit and set parameters. To track the resonance frequency, the only difference between EPR AFP measuremen and EPR continous monitoring is that the modulation source DS 345 amplitude does not need to be large. Typically 0.05 Vpp is enough.

2. Make sure the circuit is tracking the resonance frequency.

3. Use a scope or multimeter to measure the two field feedback signals, the amplitude should be less than 500 mV. If either of them is larger than 500 mV, you must adjust the position of magnetometer sensor such that the signal perpendicular to the main holding field is back to 200 mV or lower. Then adjust the power supply for the compensation coil such that the signal parallel to the main holding field is back to 200 mV or lower.

4. Enable the field feedback system by setting the switch 'bypass/through' on field feedback chassiss to 'bypass'. You should observe that both field feedback signals are stablized to zero.

5. Run LabView program HP3458A_read.vi on computer downstairs to record the current of compensation coil.

6. Open LabView program AFPSweep.vi, enable the program by clicking the white arrow in the upper left corner.

7. Set the paramters, download parameters and start data taking. Do not do sweeping (AFP spin flips).

8. After enough data is collected, stop the program.

9. Disconnect the output of Wavetek 80 to the cable leading to RF coil.