Hardware Development For High-Field NMR

NMR spectroscopy at high magnetic fields enhances both sensitivity and resolution while reducing second-order quadrupolar shift, which greatly facilitates the studies of biosolids, materials and particularly low-y quadrupolar nuclei. This work sets out to research into problems facing hardware development for high field NMR on both superconducting magnets and resistive magnets. Four topics are discussed:(1) A parallel and modular pulse programmer with timing resolution of8.3ns has been realized. Each module consists of an industrial personal computer PCI-104and a FPGA, the PCI-104dedicates to translate pulse sequence elements into48-bit binary words, while the FPGA functions as a sequencer to decode the48-bit words, generate gating and pulse and trigger sampling. NMR console equipped with the proposed pulse programmer features distinct channelizing:each channel is mutual independent and communicates with the host computer directly via Ethernet. This pulse programmer has been applied to our500MHz solution and1.06GHz solid-state NMR consoles, both of which have been working well.(2) A wideband RF transmitter capable of fast modulation has been designed and implemented. Two cascading NCOs are used in the digital IF subsystem, the former one produces baseband modulating signals, the other one which runs continuously at a appointed frequency performs digital quadrature modulation. Our method for fast modulation is more optimal than commercial NMR transmitter, since only an extra48-bit should be provided by the pulse programmer to agilely switch pulse frequency. In addition, the output frequency ranges from10MHz to1100MHz, which meets the requirement of NMR spectrometers operating at the field of25T or lower.(3) Four kinds of NMR coils have been quantitatively evaluated in terms of B1homogeneity A810/A90and electric field strength E/B1by performing electromagnetic field simulations. The results show that cross coil design should be favored for high-field biosolids probes. For the Low-E coil, which is a kind of cross-coil design, the electric field strength as a function of sample dielectric constant and equivalent conductivity has been investigated. Simulation also reveals shortcomings of Low-E. Furthermore, improved Low-E has been presented and studied. Simulation and measurement show that the improved Low-E not only achieves a higher1H B1strength before arcing, but also effectively reduces RF heating caused by the low frequency channel when long-time high-power decoupling applied. Finally, a Low-E720MHz1H/X probe has been built and tested. (4) A method for suppression of field fluctuation of high-field resistive magnets using a set of digital down converter-digital up converter (DDC-DUC) has been proposed. As a way of compensation, flux stabilizer has been used to suppress power supply ripples that cause temporal filed fluctuations, but to what extend the ripples can be reduced depends on its controller design. In order to reduce those large amplitude power supply ripples, a flux stabilization controller using a set of DDC-DUC has been designed and analyzed. In this controller, the DDCs extract the amplitude of each large ripple, and the DUCs restore the ripple frequencies and output to compensate the field fluctuation. By adjusting the phase of each DUC, the close-loop stability problem can, in principle, be avoided, while maintaining a high open-loop gain.