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首页> 外文期刊>Applied Magnetic Resonance >Uniform Radio Frequency Fields in Loop-Gap Resonators for EPR Spectroscopy
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Uniform Radio Frequency Fields in Loop-Gap Resonators for EPR Spectroscopy

机译：环形间隙谐振器中用于EPR光谱的均匀射频场

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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 nonuniformity 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 nonuniformity to the fringing capacitance and inductance at the ends of the resonator. This nonuniformity 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 cavily resonators previously introduced by this laboratory.
机译：在高频（例如Q和W波段），使环形间隙谐振器（LGR）的轴向长度至少与自由空间波长一样长是有利的。具有LGR长度的电容和电感的相反缩放比例表明LGR的长度可以不受限制地增加，而轴向射频（rf）场分布和谐振频率与长度无关。对于谐振器尺寸远小于一个自由空间波长的情况，此缩放是准确的。当谐振器长度接近自由空间波长的十分之一时，射频场均匀性降低。从十分之一波长到自由空间波长，计算机仿真和实验测量表明，轴向磁场能量密度分布在波峰中达到峰值。距LGR中心的距离逐渐减小，从一端到半径一半径处从25％降低到50％，此后迅速减小。不均匀性有两种类型。一种类型，在端部一个半径附近，是由于磁场从中心回路弯曲到端部区域并进入较大的回路时所产生的。谐振器中心部分的另一种类型是由LGR端部的阻抗不匹配引起的。 LGR可以看作是强折角（脊形）波导，其两端几乎敞开并支持驻波。传输线模型将中心不均匀性与谐振器两端的边缘电容和电感相关联。可以通过多种方式消除这种不均匀性，包括通过添加小的金属桥或介电环来修改LGR的末端。这些均匀修整元件增加了边缘电容和/或减小了边缘电感。端部经过修整，可以将LGR制成许多自由空间波长。谐振器的最大长度由基本LGR模式的频率与下一个最高频率模式的接近程度以及品质因数确定。比较了该理论的结果，并通过有限元模拟对其进行了确认。该理论将均匀LGR与该实验室先前引入的均匀场腔谐振器联系起来。

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