after getting frustrated with the unstable and non-reliable result of the system ( the stability may result from many things, but we checked all measured parameters, they are all the same ), we decide to change the coil, since it is the only moving part in the system and it may cause the instability.
we make a 4mm gap in a new coil, each side is 15 turns, so the laser can hit the sample without blocked by the coil. However, when we insert the coil inside, the resonance frequency of the µw cavity changed. it becomes borden and more reflection. with all effort, the polarization fail.
However, may be it is due to a poor and naive design of the coil. with better design, may be we can satisfy the µw cavity resonance frequency, which is the ESR frequency and able to make a non-moving coil.
there are several changes made on the system when we return to a moving coil setting.
- we checked the turns of the coil, it is 25.
- the NMR coil input impedance is 49.7+0.3i Ω
- the optics was reset and now it hits the center of the sample with laser spot size about 1mm in diameter. and the incident angle is almost perpendicular.
- Reset the µw frequency, now the Gunn Oscillator tuning length is 4.29.
- the µw reflected signal has a noise, and the shape is not right