TE011 Cavity Research Articles

Dielectric resonator in rectangular ТЕ102 cavity for electron paramagnetic resonance study of thin films

The improved compared to a rectangular TE102 cavity geometry of a dielectric resonator (DR) suitable for studying thin films and coatings has been calculated and experimentally verified. It is shown that electron paramagnetic resonance (EPR) signal of SiOx films can be enhanced by using as the DR of two rectangular parallelepipeds fabricated from BaTi4O9 + 8.5% ZnO ceramics (ε=36) with dimension of 5.64×5.5 × 5.9 mm3 and a gap of up to 0.5 mm between them. Located inside a standard rectangular metal TE102 cavity of the X-band EPR spectrometer, the DR increases the filling factor by 5–12 times depending on the size of a sample studied. The experimental use of the DR allows to increase the EPR signal of the 950 nm SiOx thin film by a factor of approximately 4.

3D Printed, Self-Temperature Compensated Microwave Cavity Notch Filters

Utilizing 3D printing to create custom microwave filters has promising aspects such as light weight and low cost, most importantly including the fact that complicated structures can be printed quickly and inexpensively. Some SLA resins have very high Coefficient of Thermal Expansion (CTE) compared to metal, so SLA 3D printed microwave filters can have frequency responses that are strongly dependent on temperature—a serious problem for narrow band filters. The presented technique overcomes this limitation, and in fact yields an SLA 3D printed filter with better temperature stability than an uncompensated metal 3D printed or a conventionally machined metal filter. Two 3D printed, temperature compensated microwave cavity notch filters have been fabricated using stereolithography (SLA) resins. Both use two different values of CTE for compensation. The 3D printed plastic parts are electroplated with copper. One filter consists of a resonant cavity with the TE011 mode coupled to a TE10 waveguide. The other is a TE101 resonant cavity coupled to TE10 waveguide. As temperature increases, the side walls of the cavities grow, lowering the resonant frequency. At the same time, for the TE011 cavity, an end-wall tuning plunger made of stereolithography resin with a higher CTE than the cavity walls expands at a faster rate. The end wall expansion increases the resonant frequency. In the case of the TE101 cavity, a metallic plunger expands much more slowly than the cavity and effectively withdraws as temperature increases, which increases the resonant frequency.

Extruded dielectric sample tubes of complex cross section for EPR signal enhancement of aqueous samples

This paper builds on the work of Mett and Hyde (2003) and Sidabras et al. (2005) where multiple flat aqueous sample cells placed perpendicular to electric fields in microwave cavities were used to reduce the RF losses and increase the EPR signal. In this work, we present three novel sample holders for loop-gap resonators (LGRs) and cylindrical cavity geometries. Two sample holders have been commissioned and built by polytetrafluoroethylene (PTFE) extrusion techniques: a 1mm O.D. capillary with a septum down the middle, named DoubleDee, and a 3.5mm O.D. star shaped sample holder, named AquaStar. Simulations and experimental results at X-band show that the EPR signal intensity increases by factors of 1.43 and 3.87 in the DoubleDee and AquaStar respectively, over the current TPX 0.9mm O.D. sample tube in a two-loop–one-gap LGR. Finally, combining the insight gained from the constructed sample holders and finite-element solutions, a third multi-lumen sample holder for a cylindrical TE011 cavity is optimized, named AquaSun, where simulations show an EPR signal intensity increase by a factor of 8.2 over a standard 1mm capillary.

Frequency shifts in NIST Cs primary frequency standards due to transverse rf field gradients

A single-particle Green's function (propagator) is introduced to study the detection of laser-cooled Cesium atoms in an atomic fountain due to RF field gradients in the Ramsey TE011 cavity. The detection results in a state-dependent loss of atoms at apertures in the physics package, resulting in a frequency bias. A model accounting only for motion in one dimension transverse to the symmetry axis of the fountain is discussed in detail and then generalized to two transverse dimensions. Results for fractional frequency shifts due to transverse field gradients are computed for NIST F-1 and F-2 Cesium fountains. The shifts are found to be negligible except in cases of higher RF power applied to the cavities.

Demonstration of a high-performance pulsed optically pumped Rb clock based on a compact magnetron-type microwave cavity

We demonstrate a high-performance pulsed optically pumped (POP) Rb vapor-cell clock based on a magnetron-type microwave cavity of only 44 cm3 external volume. Using optical detection, an unprecedented 35% contrast of the Ramsey signal has been obtained. Both the signal-to-noise ratio (of 30 000) and the estimated shot-noise limit of 1.7 × 10−14 τ−1/2 are at the same level as those found with a bigger cylindrical TE011 cavity (100 cm3 inner volume) and are sufficient for achieving excellent clock stability. Rabi oscillations are measured and indicate a sufficiently uniform microwave magnetic field distribution inside the cavity. The instability sources for the POP clock's performance are analyzed. A short-term stability of 2.1 × 10−13 τ−1/2 is demonstrated which is consistent with the noise budget.

Spectral shape deformation in inverse spin Hall voltage in Y3Fe5O12|Pt bilayers at high microwave power levels

We report on the deformation of microwave absorption spectra and of the inverse spin Hall voltage signals in thin film bilayers of yttrium iron garnet (YIG) and platinum at high microwave power levels in a 9.45-GHz TE011 cavity. As the microwave power increases from 0.15 to 200 mW, the resonance field shifts to higher values, and the initially Lorentzian spectra of the microwave absorption intensity as well as the inverse spin Hall voltage signals become asymmetric. The contributions from opening of the magnetization precession cone and heating of YIG cannot well reproduce the data. Control measurements of inverse spin Hall voltages on thin-film YIG|Pt systems with a range of line widths underscore the role of spin-wave excitations in spectral deformation.

Resonant cavity mode dependence of anomalous and inverse spin Hall effect

The direct current electric voltage induced by the Inverse Spin Hall Effect (ISHE) and Anomalous Hall Effect (AHE) was investigated in the TE011 and TE102 cavities. The ISHE and AHE components were distinguishable through the fitting of the voltage spectrum. The unwanted AHE was minimized by placing the DUT (Device Under Test) at the center of both the TE011 and TE102 cavities. The voltage of ISHE in the TE011 cavity was larger than that in the TE102 cavity due to the higher quality factor of the former. Despite optimized centering, AHE voltage from TE011 cavity was also higher. The reason was attributed to the E-field distribution inside the cavity. In the case of the TE011 cavity, the DUT was easily exposed to the E-field in all directions. Therefore, the parasitic AHE voltage in the TE102 cavity was less sensitive than that in the TE011 cavity to decentering problem.

Numerical Analysis of the Microwave Heating of Compacted Copper Powders in Single-Mode Cavity

Recently, microwave single-mode cavities have been used to isolate and study the effects of electric and magnetic field components on the temperature and microstructure of samples. This paper discusses the conditions affecting the heating of metal powders in this kind of experiments. In addition, it studies general trends in the interaction of microwaves with powders of different sizes and densities. As an example, it numerically analyses the microwave heating of copper powders in single-mode WR284 TE102 cavity. The optical and thermal parameters of metal powders are estimated by means of Bruggeman's effective medium approximation combined with Mie theory. Analysis shows the strong influence of the device on the temperature of samples located in the electric field maximum. In addition, the study demonstrates the existence of standing waves inside samples. In this case, high temperature is attained easily when samples are placed in the magnetic field maximum.

Spectroscopic and Functional Characterizations of Cyanobacterium Synechocystis PCC 6803 Mutants on and near the Heme Axial Ligand of Cytochrome b559 in Photosystem II

The functional role of cytochrome (cyt) b(559) in photosystem II (PSII) was investigated in H22K alpha and Y18S alpha cyt b(559) mutants of the cyanobacterium Synechocystis sp. PCC6803. H22K alpha and Y18S alpha cyt b(559) mutant carries one amino acid substitution on and near one of heme axial ligands of cyt b(559) in PSII, respectively. Both mutants grew photoautotrophically, assembled stable PSII, and exhibited the normal period-four oscillation in oxygen yield. However, both mutants showed several distinct chlorophyll a fluorescence properties and were more susceptible to photoinhibition than wild type. EPR results indicated the displacement of one of the two axial ligands to the heme of cyt b(559) in H22K alpha mutant reaction centers, at least in isolated reaction centers. The maximum absorption of cyt b(559) in Y18S alpha mutant PSII core complexes was shifted to 561 nm. Y18S alpha and H22K alpha mutant PSII core complexes contained predominately the low potential form of cyt b(559). The findings lend support to the concept that the redox properties of cyt b(559) are strongly influenced by the hydrophobicity and ligation environment of the heme. When the cyt b(559) mutations placed in a D1-D170A genetic background that prevents assembly of the manganese cluster, accumulation of PSII is almost completely abolished. Overall, our data support a functional role of cyt b(559) in protection of PSII under photoinhibition conditions in vivo.

Cryogenic 35 GHz pulse ENDOR probehead accommodating large sample sizes: Performance and applications

The construction and performance of a cryogenic 35GHz pulse electron nuclear double resonance (ENDOR) probehead for large samples is presented. The resonator is based on a rectangular TE102 cavity in which the radio frequency (rf) B2-field is generated by a two turn saddle ENDOR coil crossing the resonator along the sample axis with minimal distance to the sample tube. An rf power efficiency factor is used to define the B2-field strength per square-root of the transmitted rf power over the frequency range 2–180MHz. The distributions of the microwave B1- and E1-field, and the rf B2-field are investigated by electromagnetic field calculations. All dielectrics, the sample tube, and coupling elements are included in the calculations. The application range of the probehead and the advantages of using large sample sizes are demonstrated and discussed on a number of paramagnetic samples containing transition metal ions.

A Homebuilt ESE Spectrometer on the Basis of a High-Power Q-Band Microwave Bridge

We present a Q-band spectrometer which was built recently at the Institute of Physical Chemistry of the University of Stuttgart. It allows us to perform the field-sweep electron spin echo (ESE), pulsed electron–nuclear double resonance (ENDOR), relaxation and electron spin echo envelope modulation experiments both at room and low (down to 1.5 K) temperatures. The spectrometer consists of an electromagnet, digital field controller, pulsed microwave bridge, probehead, cryostat, radio frequency unit, pulse programmer and data acquisition electronics. The Q-band microwave bridge with 10.8 W output power is based on a two-stage IMPATT-diode pulse amplifier. The commercial Varian electromagnet system is controlled by a 24-bit home-built digital controller. The external devices are interfaced to the two PCs via GPIB and LAN. The spectrometer control software was developed in Visual C++. It consists of two programs running synchronously on the control PCs. The spectrometer is equipped with a cylindrical TE011 cavity constructed both for ESE and for pulsed ENDOR. The cavity fits into a liquid He cryostat thus allowing low-temperature experiments. An 8-bit data acquisition digitizer is used to collect the echo signals, and the PBESR-PRO-400 digital word generator orchestrates the pulse experiments and sets pulse sequences of the microwave bridge. The spectrometer performance is demonstrated on nitrogen impurities in a polycrystalline synthetic diamond, on silver clusters supported on NaA zeolite and electron-irradiated tooth enamel.

Targeted Resonance Control in a TEM Cell

We investigate extending the upper frequency limit of an asymmetric ldquoCrawford-style'' transverse electromagnetic (TEM) cell with targeted resonance control. Focusing on the fundamental TE cavity resonance, an active feedback cancellation system is applied with the aim of reducing the resonant field amplitude without affecting the desired TEM mode. A half loop J-dot current probe is positioned with a vertical polarization on one wall of the TEM cell to measure the wall currents generated from the TE resonance. An identical probe is placed on the facing wall that is connected to the first through a negative feedback chain. The second loop's purpose is to excite a field that has the appropriate magnitude and phase to attenuate the TE resonance amplitude. Experimental application achieved a 17-dB reduction in the amplitude of the resonance field. The method is also outlined with respect to the TE resonance. This system is planned for full-scale implementation into a hybrid reverberation/TEM whole-aircraft electromagnetic vulnerability testing facility.

Probing the Role of PrP Repeats in Conformational Conversion and Amyloid Assembly of Chimeric Yeast Prions

Oligopeptide repeats appear in many proteins that undergo conformational conversions to form amyloid, including the mammalian prion protein PrP and the yeast prion protein Sup35. Whereas the repeats in PrP have been studied more exhaustively, interpretation of these studies is confounded by the fact that many details of the PrP prion conformational conversion are not well understood. On the other hand, there is now a relatively good understanding of the factors that guide the conformational conversion of the Sup35 prion protein. To provide a general model for studying the role of oligopeptide repeats in prion conformational conversion and amyloid formation, we have substituted various numbers of the PrP octarepeats for the endogenous Sup35 repeats. The resulting chimeric proteins can adopt the [PSI+] prion state in yeast, and the stability of the prion state depends on the number of repeats. In vitro, these chimeric proteins form amyloid fibers, with more repeats leading to shorter lag phases and faster assembly rates. Both pH and the presence of metal ions modulate assembly kinetics of the chimeric proteins, and the extent of modulation is highly sensitive to the number of PrP repeats. This work offers new insight into the properties of the PrP octarepeats in amyloid assembly and prion formation. It also reveals new features of the yeast prion protein, and provides a level of control over yeast prion assembly that will be useful for future structural studies and for creating amyloid-based biomaterials.

Electron Paramagnetic Resonance Spectroscopy of Site-directed Spin Labels Reveals the Structural Heterogeneity in the N-terminal Domain of ApoA-I in Solution

Apolipoprotein A-I (apoA-I) is the major protein constituent of high density lipoprotein (HDL) and plays a central role in phospholipid and cholesterol metabolism. This 243-residue long protein is remarkably flexible and assumes numerous lipid-dependent conformations. Consequently, definitive structural determination of lipid-free apoA-I in solution has been difficult. Using electron paramagnetic spectroscopy of site-directed spin labels in the N-terminal domain of apoA-I (residues 1-98) we have mapped a mixture of secondary structural elements, the composition of which is consistent with findings from other in-solution methods. Based on side chain mobility and their accessibility to polar and non-polar spin relaxers, the precise location of secondary elements for amino acids 14-98 was determined for both lipid-free and lipid-bound apoA-I. Based on intermolecular dipolar coupling at positions 26, 44, and 64, these secondary structural elements were arranged into a tertiary fold to generate a structural model for lipid-free apoA-I in solution.

Multipurpose EPR loop-gap resonator and cylindrical TE011 cavity for aqueous samples at 94GHz

A loop-gap resonator (LGR) and a cylindrical TE(011) cavity resonator for use at W band, 94 GHz, have been designed and characterized using the Ansoft (Pittsburgh, PA) high frequency structure simulator (HFSS; Version 10.0). Field modulation penetration was analyzed using Ansoft MAXWELL 3D (Version 11.0). Optimizing both resonators to the same sample sizes shows that EPR signal intensities of the LGR and TE(011) are similar. The 3 dB bandwidth of the LGR, on the order of 1 GHz, is a new advantage for high frequency experiments. Ultraprecision electric discharge machining (EDM) was used to fabricate the resonators from silver. The TE(011) cavity has slots that are cut into the body to allow penetration of 100 kHz field modulation. The resonator body is embedded in graphite, also cut by EDM techniques, for a combination of reasons that include (i) reduced microwave leakage and improved TE(011) mode purity, (ii) field modulation penetration, (iii) structural support for the cavity body, and (iv) machinability by EDM. Both resonators use a slotted iris. Variable coupling is provided by a three-stub tuning element. A collet system designed to hold sample tubes has been implemented, increasing repeatability of sample placement and reducing sample vibration noise. Initial results include multiquantum experiments up to 9Q using the LGR to examine 1 mM 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in aqueous solution at room temperature and field modulation experiments using the TE(011) cavity to obtain an EPR spectrum of 1 microM TEMPO.

Electron paramagnetic resonance field-modulation eddy-current analysis of silver-plated graphite resonators

Magnetic field modulation is often introduced into a cylindrical TE011 electron paramagnetic resonance (EPR) cavity through silver plating over a nonconductive substrate. The plating thickness must be many times the skin depth of the rf and smaller than the skin depth of the modulation. We derive a parameter that quantifies the modulation field penetration and find that it also depends on resonator dimensions. Design criteria based on this parameter are presented graphically. This parameter is then used to predict the behavior of eddy currents in substrates of moderate conductivity, such as graphite. The conductivity of the graphite permits improved plating uniformity and permits use of electric discharge machining (EDM) techniques to make the resonator. EDM offers precision tolerances of 0.005 mm and is suitable for small, complicated shapes that are difficult to machine by other methods. Analytic predictions of the modulation penetration are compared with the results of finite-element simulations. Simulated magnetic field modulation uniformity and penetration are shown for several elemental coils and structures including the plated graphite TE011 cavity. Fabrication and experimental testing of the structure are discussed. Spatial inhomogeneity of the modulation phase is also investigated by computer simulation. We find that the modulation phase is uniform to within 1% over the TE011 cavity. Structures of lower symmetry have increased phase nonuniformity.

Optimization of ferroelectric resonators for enhanced EPR sensitivity

A new optimized geometry of ferroelectric resonators (FR) is proposed for increased signal-to-noise ratio in EPR spectrometers. To develop optimization criteria, the distribution of the microwave magnetic field in the FR is computed and analyzed. The computations, based on the solution of Maxwell's field equations, were made for two types of cylindrical FRs: a hollow sample hole and a blind sample hole. To introduce the samples, each hole was drilled through the resonator along the axis of the FR. Microwave coupling is achieved by placing the FR into the center of a standard cylindrical TE011 cavity, with the FR coaxial to the microwave magnetic field. The results indicate that the size of a sample hole affects the microwave magnetic field immensely: 1) the smaller the sample hole diameter, the higher microwave field will be; 2) the depth of the sample hole of the blind type plays an important role in the efficiency of the FR and its optimized value is half the length of the FR. © 2005 Wiley Periodicals, Inc. Concepts Magn Reson 26B: 46–55, 2005.

Phase variations in microwave cavities for atomic clocks

We analyse the phase variations of the microwave field in a TE011 microwave cavity and how these variations affect the frequency of an atomic clock. We analytically solve for the microwave fields in a TE011 cavity. These analytic solutions show significant new terms that are not present in previous two-dimensional treatments. The new terms show that cavities with small radii, near 2.1 cm for a 9.2 GHz cavity, have smaller phase shifts than cavities with larger radii. We also show that the three-dimensional phase variations near the axis of the cavity can be efficiently calculated with a rapidly converging series of two-dimensional finite element calculations. The cavities used in atomic clocks have holes in the endcaps, and we use finite element methods to study the large fields and phase shifts near these holes. The effects of the phase variations on atoms traversing a cavity are analysed using the sensitivity function, and we present a cavity design that has small phase shifts for all atomic trajectories. For two π/2 pulses, the proposed cavity has transverse variations of the effective phase that are within ±0.1 µrad and produce no systematic frequency error for a nearly homogeneous and expanding cloud of atoms.

Axially uniform resonant cavity modes for potential use in electron paramagnetic resonance spectroscopy

This article is concerned with cylindrical transverse electric TE011 and rectangular TE102 microwave cavity resonators commonly used in electron paramagnetic resonance (EPR) spectroscopy. In the cylindrical mode geometry considered here, the sample is along the z axis of the cylinder, dielectric disks of 1/4 wavelength thickness are placed at each end wall, and the diameter of the cylinder is set at the cutoff condition for propagation of microwave energy in a cylindrical waveguide at the desired microwave frequency. The microwave magnetic field is exactly uniform along the sample in the region between the dielectric disks and the resonant frequency is independent of the length of the cylinder without limit. The rectangular TE102 geometry is analogous, but here the microwave magnetic field is exactly uniform in a plane. A uniform microwave field along a line sample is highly advantageous in EPR spectroscopy compared with the usual sinusoidal variation, and these geometries are called “uniform field” modes. Extensive theoretical analysis as well as finite element calculation of field patterns are presented. The perturbation of field patterns caused by sample insertion as functions of the overall length of the resonator and diameter of the sample is analyzed. The article is intended to provide a basis for design of practical structures in the range of 10 to 100 GHz.

Dielectric Resonator-Based Side-Access Probe for Muscle Fiber EPR Study

We present a novel dielectric resonator (DR)-based resonant structure that accommodates aqueous sample capillaries in orientations that are either parallel (i.e., side-access) or perpendicular to the direction of an external (Zeeman) magnetic field, B0. The resonant structure consists of two commercially available X-band DRs that are separated by a Rexolite spacer and resonate in the fundamental TE01δ mode. The separator between the DRs is used to tune the resonator to the desired frequency and, by appropriately drilled sample holes, to provide access for longitudinal samples, notably capillaries containing oriented, spin-labeled muscle fibers. In contrast to the topologically similar cylindrical TE011 cavity, the DR-based structure has distinct microwave properties that favor its use for parallel orientation of lossy aqueous samples. For perpendicular orientation of a dilute (6.25 μM) aqueous solution of IASL spin label, the S/N ratio was at least one order of magnitude better for the side-access DR-based structure than for a standard TE102 cavity. EPR spectra acquired for maleimide spin-labeled myosin filaments also revealed ca. 10 times better S/N ratio than those obtained with a standard TE102 cavity. For the side-access DR with sample capillaries oriented either parallel or perpendicular to the external magnetic field, the Q- and filling factors are in good agreement with the theoretical estimates derived from the distribution of magnetic (H1) and electric (E1) components.