New field controller ITC_FC on the X-band spectrometer

We have put a new field controller, ITC_FC, on our X-band pulsed EPR spectrometer, and we are now measuring how steadily it holds the magnetic field. The controller takes the field reading from an NMR magnetometer and keeps the magnet at the setpoint with a PID loop. The first long test ran for two hours near 3.4658 kG.

The field sensor is a Sibir-1 NMR magnetometer with its probe in the magnet gap. It measures the field from the proton free induction decay, and ITC_FC uses that reading as the feedback signal.

A single free induction decay gives one field point; the points are then followed in time. Over four minutes the field stays within about 50 mG peak to peak, with no visible steps or oscillation of the loop.

The overlapping Allan deviation above summarises the two-hour run. Short averaging times are dominated by the noise of a single field reading: 9.6 mG at 0.25 s, falling to 5.6 mG at 1 s. Averaging keeps helping down to a floor of 3.5 mG at around 22 s, which is about 1 ppm of the working field. Beyond half a minute slow drift takes over and the deviation grows again, reaching 20 mG at the longest averaging times the run can resolve.

The next steps are to push the floor lower and to flatten the long-time drift, and then to run the controller through a real field-swept experiment.