Benchtop NMR using a Halbach array

Abstract

Portable NMR is an exciting new branch of NMR that seeks to move NMR experiments from the laboratory to the field. This new branch of NMR demands a compact, strong, yet homogeneous B0 magnetic field in which to do experiments. A discrete, dipolar Halbach array built from neodymium-iron-boron permanent magnets is one means of generating such a B0 field. Whilst some investigation into the magnetic field strength for such arrays has been performed, there has been little investigation concerning how homogeneity may be optimised. By separating the array into a number of sub-arrays, then providing a separation between the central pair of sub arrays, it is possible to find a condition such that: Where Bz is the principal component of the magnetic field produced by the array. This condition represents an array whose homogeneity has been optimised to fourth order in Bz. By minimising the fourth order derivatives of Bz, in addition to setting the second order derivatives to zero, further increases in homogeneity are observed. This is achieved by providing separations between non-central sub-arrays. A Halbach array for which the second derivatives of Bz were zero was successfully constructed; a set of B1 coils were then subsequently designed and constructed. By reducing the noise in the NMR system to that of the Johnson noise of the B1 coil multiplied by the noise figure of the preamplifier, a typical signal to noise ratio of approximately 1000 was observed for the constructed Halbach array and B1 coils. This measurement was typically within a factor of two of calculated values. A spectral width of 2 kHz was observed for a 25 mm diameter B1 coil. Use of a first order shim set did not appreciably reduce this line width due to inappropriate shim set design.