Uniform Field Re-entrant Cylindrical TE$$_{01\text {U}}$$ Cavity for Pulse Electron Paramagnetic Resonance Spectroscopy at Q-band

Uniform field (UF) resonators create a region-of-interest, where the sample volume receives a homogeneous microwave magnetic field (\(B_1\)) excitation. However, as the region-of-interest is increased, resonator efficiency is reduced. In this work, a new class of uniform field resonators is introduced: the uniform field re-entrant cylindrical TE\(_{\text {01U}}\) cavity. Here, a UF cylindrical TE\(_{\text {01U}}\) cavity is designed with re-entrant fins to increase the overall resonator efficiency to match the resonator efficiency maximum of a typical cylindrical TE\(_{011}\) cavity. The new UF re-entrant cylindrical TE\(_{\text {01U}}\) cavity is designed for Q-band (34 GHz) and is calculated to have the same electron paramagnetic resonance (EPR) signal intensity as a TE\(_{011}\) cavity, a 60% increase in average resonator efficiency \(\Lambda _\mathrm{ave}\) over the sample, and has a \(B_1\) profile that is 79.8% uniform over the entire sample volume (98% uniform over the region-of-interest). A new H-type T-junction waveguide coupler with inductive obstacles is introduced that increases the dynamic range of a movable short coupler while reducing the frequency shift by 43% during over-coupling. The resonator assembly is fabricated and tested both on the bench and with EPR experiments. This resonator provides a template to improve EPR spectroscopy for pulse experiments at high frequencies.     

A Single-Ridged Coaxial Hybrid TE011 Coupler with High Mode Purity

We design a single-ridged coaxial hybrid coupler which excites a TE₀₁₁ mode of high mode content in a cylindrical cavity, resonating at 28.2GHz. The coupler consists of a WR-28 rectangular waveguide, a coaxial TE_n11 cavity, and a cylindrical TE₀₁₁ cavity. Both TE₃₁₁ coaxial cavity and TE₄₁₁ single-ridged coaxial cavity are analyzed to examine the TE₀₁₁ mode purity in the central cavity. Mode purity analysis is performed by a field expansion method using Fourier-Bessel orthonormal basis functions. Numerical calculations predict that the TE₄₁₁ single-ridged coaxial cavity excites the TE₀₁₁ mode with mode purity of 98.6%, which is improved by 3% higher compared with the TE₃₁₁ coaxial cavity. Measurements on the single-ridged coaxial coupler show a resonant frequency at 28.078GHz and ohmic and external Qs of 1560, 473 respectively, which are in good agreement with the simulated results of a 3-D finite element electromagnetic code.     

Ein Verfahren zur Messung der komplexen Dielektrizitätszahl an kleinen Flüssigkeitsmengen im Frequenzbereich von 0,7–25 GHz mit Hohlraumresonatoren

A method is described for measuring the complex relative permittivity of liquids. Circular cylindrical TE₀₁₁-, TM₀₁₀- and coaxial-line-resonators are used. The liquid under test is contained in a narrow glass tube (volume<0.5 ml) which is placed concentrically along the axis of the cavity resonator. Besides the knowledge of the diameter of the glass tube and the dimensions of the resonator only frequency measurements are required. The accuracy of the method (error±5% of the absolute value for both, the real- and imaginary part of the permittivity) is comparable to other methods requiring a larger amount of liquid. Furthermore, permittivity variations can be measured very sensitively because these are indicated by shifts of the resonance frequency as well as changes of the quality constant, which can both be determined with an error smaller than 0.5%. Some experimental results for water and an aqueous phospholipid suspension are given to demonstrate the accuracy of the method.

     

Measurement of Low-Loss Dielectric Materials Using Dielectric Rod Resonator

To obtain the complex permittivity of low-loss dielectric materials at 60 GHz, a measurement method is developed. Using a dielectric rod resonator excited by a dielectric waveguide, effective conductivity of conducting plates for short circuiting the resonator is determined. The complex permittivity of the dielectric rod is determined by the resonant frequency and unloaded quality factor of the TE_0m1-mode resonator. Moreover, the complex permittivity of single crystal sapphire, polycrystalline ceramics, and cordierite has been investigated in virtue of numerical simulation. For all the measured specimens in this study, the proposed method is seen to provide much better accuracy for values. This work was supported in part by the National Nature Science Foundation of China, under Grant NSFC 60671009.     

Analysis of a shielded TE011 mode composite dielectric resonator for stable frequency reference

Analysis of a TE₀₁₁ mode composite sapphire-rutile dielectric resonator has been carried out to study the temperature variation of resonance frequency, close to the Cs atomic clock hyperfine frequency of 9.192 GHz. The complementary behavior of dielectric permittivity with temperature of the composite has been exploited to obtain the desired turning point in the resonant frequency. The frequency of the composite structure is found to be independent of the shield diameter beyond four times the puck diameter.     

A New Q-Band EPR Probe for Quantitative Studies of Even Electron Metalloproteins

Existing Q-band (35 GHz) EPR spectrometers employ cylindrical cavities for more intense microwave magnetic fields B₁, but are so constructed that only one orientation between the external field B and B₁is allowed, namely the B B₁orientation, thus limiting the use of the spectrometer to measurements on Kramers spin systems (odd electron systems). We have designed and built a Q-band microwave probe to detect EPR signals in even electron systems, which operates in the range 2 K ≤ T ≤ 300 K for studies of metalloprotein samples. The cylindrical microwave cavity operates in the TE₀₁₁mode with cylindrical wall coupling to the waveguide, thus allowing all orientations of the external magnetic field B relative to the microwave field B₁. Such orientations allow observation of EPR transitions in non-Kramers ions (even electron) which are either forbidden or significantly weaker for B B₁. Rotation of the external magnetic field also permits easy differentiation between spin systems from even and odd electron oxidation states. The cavity consists of a metallic helix and thin metallic end walls mounted on epoxy supports, which allows efficient penetration of the modulation field. The first quantitative EPR measurements from a metalloprotein (Hemerythrin) at 35 GHz with B₁ B are presented.     

Ohmic selection in open coaxial resonators: An experimental study

A study of ohmic selective properties of open coaxial cylindrical resonators has been conducted experimentally and compared with theory. The resonator consists of an inner cylinder made of silicon carbide symmetrically located inside an outer cylindrical, tube shaped waveguide. Several fundamental TE modes were identified over the range 9 to 17 GHz through measurements of the resonant frequencies and the associated quality factors. Mode discrimination is achieved both by exploring selective ohmic effects and examining the electrodynamical properties of the coaxial cylindrical waveguide. The effectiveness of a silicon carbide coaxial insert in providing ohmic mode selection is demonstrated in that the totalQ factors of TE _mp modes with radial indexp2 become well below the quality factors for surface TE _m1 modes. It has been verified that both structure and number of resonant modes are strongly dependent on the diameter and the resistivity of the coaxial insert.     

薄板型介质材料复介电常数的无损测量

采用带法兰结构的TE01n圆柱谐振腔,用无损检测的方法测量薄板型微波介质材料的复介电常数。利用轴向模式匹配法对谐振腔内的电磁场进行了求解,给出了相对介电常数和损耗角正切的计算公式,并利用矢量网络分析仪对几种常用微波介质材料进行了测量,其结果表明:该测量方法对相对介电常数的测量误差不超过1%,而对损耗角正切的测量误差不超过10%。该方法还具备一腔多模的测试能力,测量频率可调,可用于介质材料频率特性的测量。     

新型小铷钟微波腔谐振特性的研究

采用模匹配法对一种陶瓷介质填充的圆柱谐振腔TE₀₁₁、TE₁₁₁模的谐振特性进行了理论分析和软件仿真. 计算了该种谐振腔的谐振频率,并与其他理论方法得到的仿真结果进行了比较,验证了理论方法的正确性. 在此基础上,分析了陶瓷介质尺寸对谐振频率的影响. 与目前铷钟广泛使用的微波腔相比,该种谐振腔腔体的体积在很大程度上得到减小,这对于铷原子钟小型化的实现具有重要的作用.     

A tunable general purpose Q-band resonator for CW and pulse EPR/ENDOR experiments with large sample access and optical excitation

We describe a frequency tunable Q-band cavity (34 GHz) designed for CW and pulse Electron Paramagnetic Resonance (EPR) as well as Electron Nuclear Double Resonance (ENDOR) and Electron Electron Double Resonance (ELDOR) experiments. The TE(011) cylindrical resonator is machined either from brass or from graphite (which is subsequently gold plated), to improve the penetration of the 100 kHz field modulation signal. The (self-supporting) ENDOR coil consists of four 0.8mm silver posts at 2.67 mm distance from the cavity center axis, penetrating through the plunger heads. It is very robust and immune to mechanical vibrations. The coil is electrically shielded to enable CW ENDOR experiments with high RF power (500 W). The top plunger of the cavity is movable and allows a frequency tuning of ±2 GHz. In our setup the standard operation frequency is 34.0 GHz. The microwaves are coupled into the resonator through an iris in the cylinder wall and matching is accomplished by a sliding short in the coupling waveguide. Optical excitation of the sample is enabled through slits in the cavity wall (transmission ～60%). The resonator accepts 3mm o.d. sample tubes. This leads to a favorable sensitivity especially for pulse EPR experiments of low concentration biological samples. The probehead dimensions are compatible with that of Bruker flexline Q-band resonators and it fits perfectly into an Oxford CF935 Helium flow cryostat (4-300 K). It is demonstrated that, due to the relatively large active sample volume (20-30 μl), the described resonator has superior concentration sensitivity as compared to commercial pulse Q-band resonators. The quality factor (Q(L)) of the resonator can be varied between 2600 (critical coupling) and 1300 (over-coupling). The shortest achieved π/2-pulse durations are 20 ns using a 3 W microwave amplifier. ENDOR (RF) π-pulses of 20 μs ((1)H @ 51 MHz) were obtained for a 300 W amplifier and 7 μs using a 2500 W amplifier. Selected applications of the resonator are presented.     

Coaxial resonator for a megawatt 280 GHz gyrotron

To provide the required mode selectivity for a megawatt 280 GHz gyrotron, a coaxial resonator operating in a high order TE mode is considered. Mode discrimination is achieved both by exploring the differences in the transverse structures of the competing modes and investigating a suitable geometry for the coaxial insert. For modes with close eigenfrequencies the associated diffractionQ factors can be widely different in value, thereby ensuring an effective mode selection. In the resonator studied here, the frequency separation between the design mode TE_26,10,1 and its nearest competing mode TE_20,12,1 is about 0.6% and the ratio of the correspondingQ factors is as high as 6.5. Unlike the coaxial resonator, in the hollow cavity without the inner conductor the fundamental spectrum of eigenfrequencies is more dense, and all TE modes within the frequency interval 271–288 GHz have approximately the sameQ factor.     

Effect of Resonator Parameters on Frequency and Quality Factor of Gyrotron Resonators

The influence of some cavity parameters on the resonance frequency and quality factor is investigated with the aim of comparing various methods of optimization. As an example, the geometry of the designed resonator for an experiment on a multifrequency gyrotron that would operate at 140 GHz in the TE_22,8 mode is considered. This is a weakly–tapered conventional cavity resonator with parabolic–roundings. The influence of the roundings on the quality factor is at least as important as that of the cavity length and the output taper angle.     

CONCEPTUAL DESIGN STUDIES OF AN 84 GHz, 500 kW, CW GYROTRON

The feasibility of an 84 GHz, 500 kW, CW gyrotron for ECRH on an experimental tokamak will be presented in this paper. Mode competition and mode selection procedures are carefully investigated by considering various candidate modes and the TE_10,4 mode is chosen as the operating mode. A conventional cylindrical cavity resonator with weak input and output tapers and parabolic roundings is considered for interaction studies. Self-consistent, both single mode and time-dependent, calculations are carried out and power and efficiencies are computed for a typical set of beam parameters. The results show that an output power of well over 500 kW, CW and efficiency around 40% can be reached without a depressed collector.     

High-frequency investigation on nonradiative single-mode dielectric resonators

High-frequency investigations of the recently proposed nonradiative single-mode dielectric resonators are presented. In particular, the TE₀₁₁ mode of high-density polyethylene and single-crystal quartz discs has been experimentally characterized, by means of a simple setup, in a frequency interval around 190 GHz. The obtained unloaded merit factors of 1400 for polyethylene and 2480 for single-crystal quartz lead to magnetic field conversion factors of 15 G/W^1/2 and 49 G/W^1/2, respectively. In the latter case, the obtained value represents the state of the art among the room temperature conversion efficiencies. The close agreement with the theoretical predictions demonstrates that the performances of the proposed resonators are only limited by intrinsic factors, as finite conductivity of metallic mirrors and dielectric losses of the employed materials. Ideal performances are then expected in suitably realized nonradiative resonators, also in the high-frequency regime.     

Investigation of a Broadband Quasi-Optical Mode Converter for a Multi-Frequency 1 MW Gyrotron

A broadband quasi-optical (QO) mode converter for a multi-frequency gyrotron has been designed and tested at Forschungszentrum Karlsruhe (FZK). The launcher is optimized for the TE_22,8 mode at 140 GHz, but the radiated beams present an almost identically focused pattern for all 9 considered modes between 105 GHz (TE_17,6) and 143 GHz (TE_23,8). Combining with a beam-forming mirror system, which consists of a quasi-elliptical mirror and two phase-correcting mirrors with non-quadratic surface contour, further calculations show that efficiencies of more than 94% have been achieved for converting the rotating high-order cylindrical cavity modes into the usable fundamental Gaussian mode. Low power (cold) measurements show a good agreement with theoretical predictions. This QO mode converter can be used for the broadband operation of a multi-frequency 1 MW gyrotron.     

Design of a TE42,7 coaxial cavity for a 1 MW, 280 GHz gyrotron

To provide the required mode selectivity for a 1 MW, 280 GHz gyrotron, a coaxial resonator operating in the high order TE_42,7 mode is considered. Mode discrimination is achieved both by exploring selective ohmic effects and examining the electrodynamic properties of the coaxial cylindrical waveguide. Analytic results indicate the effectiveness of a coaxial insert with a low electrical conductivity in rarefying the mode density of overmoded, megawatt hollow cavities.     

Experimental Study of a Slot Antenna Excited by a TE011 Mode Coaxial Cavity

A slot antenna is developed to excite the high harmonic waveguide mode for generating large-area plasmas. This antenna consists of a TE₀₁₁ mode coaxial cavity with the axial slots positioned on equal interval on the inner wall. The waves radiated from those slots can excite the high harmonic mode in the central area. With the azimuthal symmetric wave field of the TE₀₁₁ mode, the number of the slots can be chosen to match the field pattern of the high harmonic mode. In this report, the dispersion relation of the coaxial waveguide, the coupling scheme and the mode competition of the cavity are studied. A method has been successfully developed to suppress the TE₁₂₁ mode which is the most competing mode to the TE₀₁₁ mode.     

RADIATION PATTERN MEASUREMENTS ON A MILLIMETER-WAVE OPEN RESONATOR

Far-field radiation patterns can be used for identifying different kinds of resonant modes in a gyrotron cylindrical open resonator. The operating TE₀₂₁ mode is identified among its closest competitors TE₂₂₁ and TE₆₁₁ by measuring radiation patterns obtained experimentally based on millimeter wave source for exciting the open resonator. A good agreement between experimental and, theoretically predicted values was found.     

Numerical Study of Processes in the Cavity of the 170 GHz Gyrotron for ITER Operating at the TE25.10 Mode

The opportunity of use of a TE_25.10 operating mode for CW 170 GHz/1 MW gyrotron for ITER is estimated. Parameters are optimized to achieve maximum efficiency of the gyrotron with an acceptable Ohmic load on the cavity. The influence of unwanted mode conversion at the output of the resonator, mode competition, electron beam potential depression, ion compensation of the space charge and beam energy recovery is investigated.     

Electrical Conductivity Measurement Through the Loaded Q Factor of a Resonant Cavity

We present a method for measuring the electrical conductivity of metallic materials that relies on the ratio of two loaded Q factors, Q_R/Q_X, with Q_R corresponding to a TE₀₁₁-mode reference cavity made of aluminum, and Q_X the Q that results upon replacing the aluminum plate with the one fabricated from the material to be examined. Electrical conductivity is mathematically inferred from the ratio Q_R/Q_X where the loaded Q factors are measured by using the transmission-type method. Within a 3.0 percent accuracy, conductivities determined at 8.7 GHz for electrolytic copper (5.6 times 10⁷ S/m) and brass (1.6 times 10⁷ S/m) show to be in good agreement with those reported in the literature.