Magnetic field homogeneity

DETAILED DESCRIPTION OF THE INVENTION Portions of a typical NMR data acquisition instrument are schematically illustrated on FIG.
3.
An acquisition/control processor 10 communicates with an RF transmitter 12, modulator 14 and receiver 16, including analog-to-digital converter 18 and a further processor 20.
The modulated RF power radiates an object (not shown) in a magnetic field 21 through a probe assembly 22 and the response to the object is intercepted by probe 22 communicating with receiver 16.
The response typically takes the form of a transient oscillatory signal, often referred to as the time domain waveform.
This transient waveform is sampled at regular intervals and samples are digitized in ADC 18.
The digitized time domain waveform is then subject to further processing in processor 20.
The nature of such processing may include averaging a time domain waveform with a number of similar such waveforms, and transformation of the average time domain waveform to the frequency domain yields a spectral distribution function directed to output device 24.
The latter may take on any of a number of identities for the display of further analysis and data.
The magnetic field 21 which polarizes the sample is established by an appropriate means indicated in FIG.
1 as a cryostat 23 for maintaining a superconducting phase in a solenoid, not shown.
The cryostat comprises a bore 23a in which the probe and sample are housed at room temperature and a cryogen reservoir 26 in which the solenoid is mounted.
The boiling cryogen evolved in reservoir 26 is vented to atmosphere through vent tube 30.
Turning now to FIGS.
2, the effect in question is shown as a set of spectra obtained for an isolated spectral line (chloraform) acquired on a 500 MHz (11.
7 Tesla) instrument.
In FIG.
2a, there is shown the line shape obtained under standard operating conditions with the helium reservoir at approximately 0.
1 psi above atmospheric pressure.
The line width is approximately 0.
5 H.
sub.
z.
This slight pressure is then relieved to atmospheric pressure and the data acquisition is repeated (FIG