NMR sample rotor cooling technique

#The field of this invention is high speed spinners for nuclear magnetic resonance employing gas bearings and microturbines.

#In U.S. Pat. No. 4,456,882 I disclose a technique for NMR Sample Spinning using cylindrical, ceramic sample containers with press-fit plastic turbines on hydrostatic air bearings. A co-pending application, Ser. No. 07/607,521, discloses novel radial inflow microturbines and other improvements appropriate for high temperature applications. Another co-pending application, Ser. No. PCT/US91/01225 discloses an NMR Sample spinner capable to achieving supersonic surface speeds. Other NMR MAS spinners are disclosed in U.S. Pat. Nos. 4,254,373, 4,275,350, 4,511,841, 4,899,111 and the references cited therein.

#It is well known that considerable sample heating, sometimes in excess of 90.degree. C., results from frictional heating of the bearing gas in high-speed NMR sample spinners.

#FIG. 1 illustrates a prior art high speed NMR sample spinner; and

#FIG. 2 illustrates the use of exhaust gas recirculation.

#The prior art high-speed NMR sample spinner shown in FIG. 1 has hydrostatic air bearing orifices 1 introducing pressurized gas radially to support the ceramic journal or rotor 2 inside a bearing stator 3. The bearing stator 3 is supported inside a housing 4. A bearing manifold 5 provides distribution of air to the bearing orifices. A drive manifold 6 provides distribution of air to the drive turbines 7 to cause high speed rotation of the NMR sample 8. An RF coil 9 excites and detects NMR precession signals form the spinning sample 8. The spinner system is substantially cylindrically symmetric about the spinning axis, and it is often symmetric for a reflection through a plane perpendicular to the spinning axis.

#The drive power supplied by the turbines 7 is approximately equal to the frictional power in the gas journal bearing regions 10 where the clearance is quite small. Other frictional losses are relatively small. Hence the power extracted from the gas expanding through the turbines is approximately equal to the frictional heating in the journal bearings. However the flow rate through the turbines is considerably larger than that through the bearings. Hence, the temperature rise in the bearings is considerably greater than the temperature drop in the turbines. Joule-Thompson cooling in the bearing orifices is negligible under most conditions.

#FIG. 2 illustrates the novel method of negating the frictional heating. The cool exhaust gas 21 is recirculated through the housing and injected through ports 22 adjacent to the bearing regions 10. The cool exhaust gas mixes with the warm bearing gas and flows axially inward over the surface of the rotor. The gas exits near the center of the housing through exhaust port 23. It is further desirable to extend the turbine plugs 24 axially inward beyond the bearing region 10 so that the sample 8 is more nearly uniform in temperature and is located within a more nearly uniform RF field due to the finite size of the RF coil. This minimizes temperature-dependent line broadening and greatly improves spectral resolution. The restricted sample volume has the additional benefit of improving rf homogeneity over the sample, thereby facilitating spinning-sideband Hartmann-Hahn matching and multi-pulse line-narrowing techniques. The sensitivity enhancements from cross polarization at high spinning speeds and improved resolution exceed the decrease expected from the reduced sample volume.