Auto-tuning scheme for NMR probe using LabVIEW
Mentor:Oscar Bernal, Professor of Physics, California State University, Los Angeles
Intermetallic compounds containing rare earth or actinide elements; e.g., Yb3Pt4 or UCu4Ni, typically display broad-band nuclear magnetic resonance spectra. In many cases, the frequency span is in the order of a few MHz. Commercial NMR spectrometers are not designed to handle this type of frequency variation. Broad-band NMR spectrometers can be built in-house to measure these samples, but they too have limitations. One of the limiting factors in the spectrometer design is the bandwidth in the frequency response of the NMR coil where the sample is placed. To measure the full NMR spectrum of a particular sample one is forced to sweep the field at constant frequency or, if the field cannot be swept (as is the case of many superconducting magnets in the persistent mode of operation), to sweep the frequency at constant field. In the latter case the NMR coil has to be manually tuned every time the frequency is advanced by a predetermined amount. This time-consuming method is also prone to errors and drifts, as well as to changes in the rf pulse characteristics (i.e., tipping angle and bandwidth). We are developing an auto-tuning method using LabVIEW to control the probe capacitors in order to keep the system tuned with the changing frequency. The program uses a simplified model of the probe impedance near tune to manipulate the variable capacitors at different frequencies. The capacitor shafts are controlled by a stepper motor through LabVIEW. We describe the method and report our progress towards the development of this auto-tuning scheme. Work supported by NSF-DMR 1105380.