Li Frank X, Voccio John P, Sammartino Michael, Jalali Jalal, Ahn Min Cheol, Hahn Seungyong, Bascuñán Juan, Iwasa Yukikazu
Youngstown State University, Youngstown, OH 44503 USA, and also with the Massachusetts Institute of Technology (MIT) Francis Bitter Magnet Laboratory, Plasma Science and Fusion Center, Cambridge, MA 02139 USA.
Massachusetts Institute of Technology (MIT) Francis Bitter Magnet Laboratory, Cambridge, MA 02139 USA, and now with Wentworth Institute of Technology, Boston, MA 02115 USA.
IEEE Trans Appl Supercond. 2016 Jun;26(4). doi: 10.1109/TASC.2016.2535253. Epub 2016 Feb 26.
This paper presents a high-resolution magnetic field mapping system in development that is capable of collecting spatial magnetic field data for NMR magnets. An NMR probe was designed and built with a resonant frequency of 5.73 MHz. The measured Q-factor of the NMR probe is ~191 with a half-power bandwidth in the range of 5.72-5.75 MHz. An RF continuous-wave technique with magnetic field modulation was utilized to detect the power dispersion of water molecules. The zero-crossing frequency of the NMR dispersion signal corresponds to the magnetic field at the center of the water sample. An embedded system was developed to sweep the frequency and record the reflected RF power simultaneously. A numerically controlled digital oscillator is able to provide a precise frequency step as small as 0.02 Hz, which is equivalent to 4.7 e-7 mT for hydrogen atoms. An RF preamplifier was built to supply up to 4 W of RF power to a bidirectional coupler. The coupler supplies RF power to the NMR probe and channels reflect the RF power back to the detection circuit, which detects the reflected RF power from the NMR probe during the frequency sweep. The homogeneity of an NMR magnet can be determined by magnetic field data.
本文介绍了一种正在开发的高分辨率磁场测绘系统，该系统能够收集核磁共振（NMR）磁体的空间磁场数据。设计并制造了一个共振频率为5.73 MHz的NMR探头。所测NMR探头的品质因数约为191，半功率带宽在5.72 - 5.75 MHz范围内。利用一种带磁场调制的射频连续波技术来检测水分子的功率色散。NMR色散信号的过零频率对应于水样中心处的磁场。开发了一个嵌入式系统来扫描频率并同时记录反射的射频功率。一个数控数字振荡器能够提供小至0.02 Hz的精确频率步长，这对于氢原子相当于4.7×10⁻⁷ mT。构建了一个射频前置放大器，为双向耦合器提供高达4 W的射频功率。该耦合器向NMR探头提供射频功率，并将反射的射频功率通道反馈回检测电路，该电路在频率扫描期间检测来自NMR探头的反射射频功率。NMR磁体的均匀性可通过磁场数据来确定。
