In the simplest view of NMR, the advantages of higher field (B0) are improved sensitivity and resolution. For NMR spectroscopy, sensitivity and resolution depend, respectively, on amplitude and frequency of measurement. Sensitivity per unit time in signal averaging experiments and resolution for 3D experiments both ideally improve as ?3 and, hence, B03. Thus, increased proton frequency, for example, from 900 MHz, currently the highest frequency at the MIT-Harvard Magnetic Resonance Center, to 1.3 GHz, increases sensitivity and resolution by a factor of 3. This benefit of higher frequency was the basis of our initiative in 2000 to propose a long march towards a 1-GHz NMR magnet by combining low- and high-temperature superconducting magnets, LTS and HTS; in 2007 the proposed frequency was increased to 1.3 GHz. HTS is mandatory at frequencies above 1-GHz, thus, our 1.3-GHz LTS/HTS NMR magnet (1.3G) combines a 500-MHz LTS NMR magnet (L500) with an 800-MHz HTS insert (H800). HTS conductors are used in this 4K application not for their high- temperature capabilities, but rather for their ability to achieve significantly higher magnetic field than can be reached by LTS alone. Despite the best efforts in design and construction, the homogeneity of an ?as-wound? NMR magnet in reality will be more than two orders of magnitude away from required specifications. For field shimming, another critical activity in this Revised Phase 3BZ is field mapping, requiring exact probe positioning along optimized path and accurate measurement, from which to derive the target field gradients that in turn guide the design of appropriate shim coils, in our case, of HTS and room-temperature (RT), both to be designed and built in this Revised Phase 3BZ. Because HTS insert is notorious as a source of ?large? non- uniform field, field shimming our 1.3G will be challenging and laborious, requiring innovative ideas.
The specific aims (SA) of the last phase of this MIT 1.3-GHz LTS/HTS NMR magnet that began in 2000 are to achieve two vital requirements for NMR. In the first two years, we will: 1) replace the H800 damaged in March 2018 test with a new 800-MHz HTS insert (H800N) and 2) combine L500 and H800N to complete a new non-NMR 30.5- T L500/H800N magnet; and in the last two years we will 3) convert the non-NMR 30.5-T field to realize a high- resolution 1.3 GHz NMR magnet (1.3G). To achieve SA3, we will apply two innovative techniques: 1) HTS Z1 and Z2 shim coils, installed in the bore of H800N; and 2) current-sweep-reversal and field-shaking to mitigate the screening-current field (SCF), a non-uniform diamagnetic field, superposed on the main field that severely degrades the spatial field quality particularly for HTS magnets like our H800N. We will also deploy ferro- magnetic passive shimming and RT active shimming, both of our design. We believe that our 1.3G will become a vital force in high-field NMR as well as for drug discovery and development; it will serve the entire U.S. NMR community for decades to come and have a worldwide impact on biomedical sciences. We also believe that our 1.3G will become a model for high-resolution >1-GHz NMR magnets that must incorporate HTS inserts.
In this program, MIT will first build an ultra-high-field 30.5-tesla magnet, comprising low- and high-temperature superconducting magnets, and then complete the first-ever high-resolution 1,300 MHz (1.3-GHz) NMR magnet, the highest-field high-resolution NMR magnet yet. The MIT 1.3-GHz NMR magnet in turn will be a vital force in high-field NMR and will serve the entire U.S. NMR community, impacting medical science and research worldwide for decades to come.
