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MAGNET DEVELOPMENT AND INSTRUMENTATION 2009Special purpose NMR probe for spectroscopy of quadrupolar nuclei at 30 TAlthough Nuclear Magnetic Resonance (NMR) is a wellestablished technique, there is a continuous demand for improvedsensitivity and spectral resolution. This is the basisfor the implementation of NMR techniques in high magneticfields. State of the art NMR spectroscopy is based onsuperconducting magnets with field strengths up to 23.5 T.Higher dc magnetic fields may become possible with furtherdevelopment of superconducting technology, but theprogress is predicted to be incremental and expensive. Asan alternative, dc fields up to 34 T produced by resistivemagnets are already available, and there is considerable interestwithin the NMR community to implement generalpurpose and in particular high resolution NMR in thesemagnets. However, until recently, NMR in resistive magnetswas limited to low resolution spectroscopy due to thestrong field inhomogeneity and the limited stability of thehigh power installations. In order to overcome these intrinsicdrawbacks, the development of tailored and sensitivityoptimized NMR instrumentation is required.In this contribution we report on the commissioning of adedicated probe for NMR spectroscopy of quadrupolar nuclei(nuclear spin I > 1/2) at 30 T. In this case high magneticfield can overcome the problem of sensitivity andline broadening in NMR spectra of low-sensitivity nucleiwith strong quadrupole interactions [O. Pauvert et al., Inorg.Chem. 48, 8709 (2009)]. In order to fully benefit fromthe advantages of higher magnetic fields, the design of theNMR probe has to be optimized for this particular application.A basic origin of low sensitivity is a low gyromagneticratio γ of the examined nucleus (like 91 Zr or 25 Mg),that cannot be compensated as in the case of low naturalabundance, where isotope enrichment increases sensitivity.For a pulsed NMR experiment consisting of an excitation ofthe nuclear transition by a radio frequency (RF) field withstrength B 1 , followed by a detection period, a low value ofγ induces both a low signal (∝ γ 3 ) and a small spectral excitationwidth (∝ γB 1 ). Hence, the efficiency of an NMRexperiment involving low-γ nuclei is strongly reduced. Inresponse to these limiting constraints the design of the RFcircuit of the NMR probe has to ensure that strong RF fieldsB 1 can be applied and that electric losses are minimized.In figure 179 we present our realization of a room temperatureNMR probe meeting these constraints: Its bottomtuned RF circuit consists of the solenoid NMR RF excitationand detection coil of a given inductance L C that isconnected in series with a variable tuning capacitance C T .L C and C T form a tunable series resonance circuit matchedto the source impedance (50 Ω) by a variable matching inductanceL M that is connected in parallel. Since the probeis based on the concept of creating strong B 1 fields by theapplication of high RF power, the probe has to withstandhigh voltages of the order of several kV without electricbreakdown. Therefore the tuning capacitance is realized bya copper cylinder capacitor, that can be continuously filledwith an alumina tube. In addition all conducting parts arerounded to avoid sources of arcing. Low electric loss is ensuredby the usage of high conducting materials (copper,brass). The available tuning range of the probe (30 to 500MHz) covers most quadrupolar nuclei up to 30 T. The outerdiameter of the probe (28 mm) leaves space for future extensionslike passive shimming even in the 34 mm narrowbore M9 magnet of LNCMI-G.In addition, the probe is equipped with a second NMR circuitthat is used to stabilize the 24 MW magnet power supplyduring the experiment by an active, NMR based fieldstabilization (NMR spin-lock). For this purpose the 63 CuNMR signal of a CuCl reference sample is continuouslyrecorded and used for the generation of a control signal. Inorder to avoid crosstalks between the two RF circuits thespin-lock circuit is fully shielded by a metallic cap. Theactive field stabilization enables long time averaging of theNMR signal with reference drifts of less than 1 ppm.After adjustment and optimization the probe was proved tobe fully operational. It withstands pulsed RF power of morethan 1 kW with a B 1 efficiency of 18 G/ √ W for a solenoidNMR coil of 3 mm inner diameter and 5 mm length. At1 kW this corresponds to B 1 field strengths of 570 G. Subsequentlythe probe with the spin-lock option was usedto conduct 91 Zr NMR studies on a series of inorganic Zrcompounds at 30 T. The obtained results provide a systematicand quantitative determination of the relation betweenstructural parameters (bond lengths, bond angles, coordinationgeometry) and NMR parameters (chemical shift andquadrupole tensors) of Zr compounds.Figure 179: Schematic view of the RF part of NMR probe consistingof the main RF circuit (upper right) and the shielded NMRspin-lock part (lower left, shielding cut for clarity).S. Krämer, C. de Vallée, H. Stork, J. Spitznagel, M. Horvatić, C. BerthierF. Fayon, A. Rakhmatullin, O. Pauvert, C. Bessada, D. Massiot (CEMHTI-CNRS, Orléans, France)126