Highly space-efficient active shim designs for small, high-resolution permanent magnets
- NuevoMR, LLC, Albuquerque, United States
The majority of new applications for NMR are first developed and demonstrated on high-field, high-resolution machines, and even when not 100% necessary, the traditional NMR world maintains its high expectations for resolution. To successfully address these applications, developers working with small permanent magnets are challenged to deliver the desired level of spectral resolution (and enough sensitivity) without making the permanent magnet system so large and expensive that it cedes its competitive advantages to simplified, low-cost supercon-based systems.
At least three issues stand out when constructing a real-world high-resolution permanent magnet: raw field homogeneity ("passive shimming"), probe-to-probe and sample-to-sample variations ("active shimming"), and temporal stability of the field strength. Our aggressive goal is to use magnets with 5-8 mm gaps to study samples in standard 3-5 mm NMR tubes. For active and passive shimming, this small size scale simultaneously increases the demands for mechanical precision and severely limits the space available for field correction technologies.
Here, we present active shim designs that occupy less than 0.5 mm of gap space while correcting field errors to higher than second order. The layout of the current carrying wires is very simple: each layer of the shim structure contains wires in one direction only, eliminating all cross-overs and thereby keeping the structure very thin. Wires in only two directions are sufficient for producing all but one of the first and second order correction fields. This two layer structure also can create a useful subset of the higher order corrections. A four layer structure (four wires directions) creates all first and second order corrections fields. Normally, at least 12 layers are required. For a 5 mm gap magnet, the benefit is that more than 4 mm of space remains after active shims are installed, rather than less than 3 mm.
We present maps of all first and second order fields produced by shims of this design, explain how they were achieved, address the peculiar difficulties of our approach to shimming, and discuss the circumstances in which the multi-wire space-efficient shims would be particularly beneficial.
