Uniform radio frequency fields in loop-gap resonators for EPR spectroscopy
Uniform radio frequency fields in loop-gap resonators for EPR spectroscopy
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DOI：
10.1007/bf03166603
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发表时间：
2007-01-01
影响因子：
1
通讯作者：
Hyde, J. S.
中科院分区：
文献类型：
--
作者：
Mett, R. R.;Sidabras, J. W.;Hyde, J. S.
在高频率下，例如，在Q和W波段，有利的是使环隙谐振器（LGR）的轴向长度至少与自由空间波长一样长。电容和电感与LGR长度的相反缩放表明LGR的长度可以无限制地增加，其中轴向射频（rf）场分布和谐振频率与长度无关。这种缩放对于远小于一个自由空间波长的谐振器尺寸是准确的。当谐振腔长度接近自由空间波长的十分之一时，射频场均匀性降低。从十分之一到一个自由空间波长，计算机模拟和实验测量表明，轴向磁场能量密度分布在LGR的中心达到峰值，从25%逐渐减少到50%，从一个半径的距离结束，并迅速此后。唯名论有两种类型。一种类型，在端部的一个半径附近，是由场从中心环弯曲到端部区域，进入更大的返回环时的张开引起的。另一种类型，在谐振器的中心部分，由LGR两端的阻抗失配引起。LGR可以被视为在两端几乎开放并且支持驻波的强凹入（脊）波导。传输线模型将中心无规性与谐振器两端的边缘电容和电感联系起来。这种无规性可以通过几种方式消除，包括通过添加小金属桥或电介质环来修改LGR的端部。这些均匀性修整元件增加边缘电容和/或减小边缘电感。有了修剪的末端，LGR可以长出许多自由空间波长。最大谐振器长度由基本LGR模式的频率与下一个最高频率模式的接近度以及品质因数确定。该理论的结果进行了比较，并与有限元模拟确认。该理论将均匀LGR与本实验室先前介绍的均匀场腔谐振器联系起来。
At high frequencies, e.g., Q- and W-bands, it is advantageous to make the axial length of loop-gap resonators (LGRs) at least as long as a free-space wavelength. The opposite scaling of capacitance and inductance with LGR length suggests that the length of an LGR can be increased without limit, with the axial radio frequency (rf) field profiles and resonance frequency independent of length. This scaling is accurate for resonator dimensions much less than one free-space wavelength. When the resonator length approaches one-tenth of a free-space wavelength, the rf field uniformity degrades. From one-tenth to one free-space wavelength, computer simulations and experimental measurements show that the axial magnetic field energy density profile is peaked in the center of the LGR, gradually decreases from 25 to 50% at a distance one radius from the end, and rapidly thereafter. The nominiformity is of two types. One type, in the vicinity of one radius of the end, is caused by the flaring of the field as it curves from the central loop to the end region, into the larger return loop(s). The other type, in the central part of the resonator, is caused by impedance mismatch at the ends of the LGR. The LGR may be viewed as a strongly reentrant (ridge) waveguide nearly open at both ends and supporting a standing wave. A transmission line model relates the central nominiformity to the fringing capacitance and inductance at the ends of the resonator. This nominiformity can be eliminated in several ways including modifying the ends of the LGR by adding a small metal bridge or a dielectric ring. These uniformity trimming elements increase the fringing capacitance and/or decrease the fringing inductance. With trimmed ends, LGRs can be made many free-space wavelengths long. The maximum resonator length is determined by the proximity in frequency of the fundamental LGR mode to the next highest frequency mode as well as the quality factor. Results of this theory are compared and confirmed with finite-element simulations. This theory connects the uniform LGR with the uniform field cavity resonators previously introduced by this laboratory.