The Helmholtz coils are four large interlocking rings of coils that provide a large constant magnetic field. Two sets are necessary so that the magnetic field can point in any direction in the horizontal plane. The larger two coils have an internal diameter of 1.45 m and consist of 272 turns of coil. The smaller set of coils each have a internal diameter of 1.27 m and are made of 256 turns of coil.
The normal holding field for the target is 25 Gauss which corresponds to approximately 7.2 amps of current in the coils. However, when doing an NMR measurement the field gets as high as 32 Gauss, which corresponds to 9.2 amps of current.
The coils are controlled by a DC voltage from two Stanford Research System DS345 Function Generators. These are in turn controlled by a LabView Vi running on a PC. There is a small glitch in the DS345 function generator : there sometimes will be a short drop in voltage before going to the correct voltage. This is a problem when rotating the field, since it causes sudden magnitude shifts which in turn destroy a little of the target polarization. This glitch is compensated for by a specially designed rotation box to which a Wavetek 80 function generator is attached. This function generator compensates for the small decrease in voltage so that the rotation can be performed smoothly.
Attached to the small set of Helmholtz coils are a smaller set of coils. These consist of 20 loops of 16 AWG wire attached to a 150 W power supply. This target occasionally has a problem with a phenomena known as masing. This is a loss of target polarization due to coupling of the helium nuclei in their higher energy states (such as during an NMR measurement) and a coil of wire (the pick-up coils, a Rb ring, or some other source). Masing can be easily identified with EPR, and introducing a small field gradient generally makes it disappear. These coils are used to introduce that field gradient.