NNR IN RAPIDLY ROTATED METALS By - Nottingham eTheses ...

4.3.2
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EXPERIMENTAL DETAILS
At a spinning frequency of 5 kHz the velocity of the rotor
flutes is close to the velocity of sound in air. Both hydrogen
and helium are capable of propelling rotors to considerably greater
speeds, but because sparks are often produced when a spinning rotor
touches the metal stator, helium is the gas used on the grounds
of safety. By using a compressor it would have been possible to
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collect and recycle helium after use. However, in practice
such an arrangement was not found to be very satisfactory because
the gas soon became 'dirty'. With the pulse spectrometer used
in this work it was only necessary to maintain rotation frequencies
for relatively short periods - particularly for metals with their
short spin-lattice relaxation times. Helium could then reason-
ably be taken directly from a cylinder and vented to the atmos-
phere. Unfortunately it proved extremely difficult to start the
rotors spinning with helium as the driving gas. The reason for
this was unclear, but attributed to the low gas density, the large
masses of the metallic, specimens and the lack of dynamic balancing.
Figure 4.2 shows the rotation rates that have been achieved by a
solid nylon rotor when propelled by compressed air and helium.
The conical rotors start more easily with their axes of rotat-
ion vertical, but will rotate about any direction - even upside
down. Consequently a normal electromagnet was employed in this
work and the rotors spun about an axis inclined at 540 44' to the
horizontal magnetic field.
The single radiofrequency coils were wound on nylon formers
and held in position about the rotor superstructure by screws
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