5. NMR spectro signal coil 

SIMULATION OF NMR SIGNAL COIL

RATIONALE
The resolution of NMR spectrometer depends directly of the magnetic quality of the radio frequency signal coil.
This coil is a micro solenoid made of discrete spires of a conducting material (pure metal, alloy or composite) separated by air or other insulating materials. The coil is wound around the capillary tube, which contains the sample to be analyzed.
During operation, the winding is submitted to the very high permanent operating magnetic field of the NMR Spectrometer (currently 8 teslas and up).
The various materials of winding becomes magnetized according to their own susceptibility. This creates a space dependent small magnetic field, which generates a broadening of the NMR signal.
The problem is to simulate correctly the behavior of winding and then to optimize its structure in order to reduce the magnetic broadening.

Cases shown :
- coil made of 6 spires of 50 µm diameter wire
- copper wire without solid filling (Air is assumed)
- operating DC field 8 teslas (80 000 gauss)
- value of the specific and volume susceptibility (CGS emu 10-6 ):

1- Schematic of the NMR spectro signal coil
    with capillary of 100 µm diameter

RESULTS

case 1 :
copper wire without filling (air assumed)
result : Bz broadening amplitude : 5.9 milligauss

1- 3D graph map of the Bz noise field

2- map of the Bz broadening field on the axis

 
case 2 :
copper wire with perfluorocarbon filling
result : Bz broadening amplitude : 1.1 milligauss

3- 3D graph map of the Bz broadening field

4- map of the Bz broadening field on the axis

  case 3 :
composite copper/aluminum wire without filling
result : Bz broadening amplitude : 0.4 milligauss

5- 3D graph map of the Bz broadening field 

6- map of the Bz broadening field on the axis