Development of pulsed EPR spectrometer with pulsed field gradient

针对X波段（9.5GHz）脉冲EPR的相干路径选择、饱和实验和扩散测量等应用，研制了一套脉冲磁场梯度系统。脉冲磁场系统由脉冲磁场驱动器、四倍场梯度线圈、介质谐振器、低温氦流低温恒温器和光辐照系统组成。该驱动器可以产生10 A的电流脉冲，开关时间为10 ns。根据Viot-Savart定律，通过对线圈内部磁场的数值计算，设计了一个直径为7 mm，长度为10 mm的四重线圈。线圈的小尺寸使得能够以快速切换时间产生大磁场。线圈可以在样品位置产生100 G/cm的场梯度。用于该应用的EPR谐振器必须很小以放置在小的四极线圈中。为了满足这个条件，环隙谐振器（LGR）和介质谐振器（DR）进行了检查。采用铜材料制作了单间隙五缝瞬态磁场发生器。实验表明，LGR的金属壁将谐振器内的瞬态磁场衰减到空线圈的1/5。DR由CaTiO_3（ε_r=20）制成，其尺寸适合放置在线圈中，并满足9.5GHz的谐振条件。由于材料是绝缘体，脉冲磁场可以在样品位置引入而不会衰减。因此，DR被选为脉冲磁场测量的最佳谐振器。所设计的低温恒温器可方便地实现微波信号与样品交换的耦合调节。还开发了一种高效的光照射方法，使用光纤。

Aiming for applications such as coherence path selection, saturation experiments, and diffusion measurements, at X-band (9.5GHz) pulsed EPR, a pulsed magnetic field gradient system was developed. The pulsed magnetic field system is consists of a pulsed magnetic field driver, a quadruple field gradient coil, a dielectric resonator, a low temperature helium flow cryostat, and photo irradiation system. The driver can produce an electric current pulse of 10 A with 10 ns switching time. A quadruple coil (7 mm φ and 10 mm length) was designed by a numerical calculation of magnetic field inside the coil based on the Viot-Savart's low. The small size of the coil enables one to generate a large magnetic field with fast switching times. The coil can generates field gradient of 100 G/cm at the sample position. An EPR resonator for this application must be small to be placed in the small quadruple coil. To satisfy this condition, a loop-gap resonator (LGR) and a dielectric resonator (DR) were examined. The LGR with single gap and five slits for the introduction of transient magnetic field was made from copper. An experiment showed that the metal wall of LGR attenuates the transient magnetic field inside the resonator to 1/5 with respect to the empty coil. DR was made from CaTiO3 (ε_r=20) to be an appropriate size to place in the coil and to satisfy resonance condition at 9.5 GHz. Because the material is insulator, the pulsed magnetic field can be introduced at the sample position without attenuation. Thus, DR is chosen as the best resonator for the pulsed magnetic field measurements. The designed low temperature cryostat can provide easy coupling adjustment of the microwave signal and sample exchange. An efficient photo irradiation method was also developed with using optical fiber.