RF Current Distribution Research Articles

Quantifying spin-torque efficiency and magnetoresistance coefficient by microwave photoresistance in spin-torque ferromagnetic resonance

During the spin-torque ferromagnetic resonance (ST-FMR) measurement, the magnetization precession driven by the microwave field yields the radio frequency (rf) oscillating magnetoresistance and its time-averaged change (photoresistance). Here, we find that the strength of photoresistance can be directly determined by using dc bias current Idc modulating the symmetric component VS of the ST-FMR voltage spectrum. By measuring the angular dependence of photoresistance, we can quantify the in-plane and out-of-plane precession angles of ST-FMR, the actual rf current distribution in the magnetic and non-magnetic sublayers, and the magnitude of spin-torque and various magnetoresistance coefficients. These experimentally obtained values and analysis methods can more accurately quantify the spin-torque efficiency of both in-plane and out-of-plane spin polarizations by self-consistent calculation of the precession angle without harsh assumptions. And, we further confirm this universal method in three spintronic systems: the prototypical Pt/Py bilayer with anisotropic magnetoresistance (AMR), Py/Cu/Co20Tb80 spin valve trilayer with AMR and giant magnetoresistance, and [Co/Ni]3/Co/Pt multilayer with AMR and anisotropic interface magnetoresistance. This method eliminates potential deviation in calculating spin-torque efficiency by previously reported line shape analyzation and linewidth modulation methods of the ST-FMR technique and significantly extends its application range in characterizing spintronic materials and nanodevices.

Imaging collective behavior in an rf-SQUID metamaterial tuned by DC and RF magnetic fields

We examine the collective behavior of two-dimensional nonlinear superconducting metamaterials using a non-contact spatially resolved imaging technique. The metamaterial is made up of sub-wavelength nonlinear microwave oscillators in a strongly coupled 27 × 27 planar array of radio-frequency Superconducting QUantum Interference Devices (rf-SQUIDs). By using low-temperature laser scanning microscopy, we image microwave currents in the driven SQUIDs while in non-radiating dark modes and identify the clustering and uniformity of like-oscillating meta-atoms. We follow the rearrangement of coherent patterns due to meta-atom resonant frequency tuning as a function of external dc and rf magnetic flux bias. We find that the rf current distribution across the SQUID array at zero dc flux and small rf flux reveals a low degree of coherence. By contrast, the spatial coherence improves dramatically upon increasing the rf flux amplitude, in agreement with simulation.

Design of a high efficiency relativistic backward wave oscillator with low guiding magnetic field

A high efficiency relativistic backward wave oscillator working at a low guiding magnetic field is designed and simulated. A trapezoidal resonant reflector is used to reduce the modulation field in the resonant reflector to avoid overmodulation of the electron beam which will lead to a large momentum spread and then low conversion efficiency. The envelope of the inner radius of the slow wave structure (SWS) increases stepwise to keep conformal to the trajectory of the electron beam which will alleviate the bombardment of the electron on the surface of the SWS. The length of period of the SWS is reduced gradually to make a better match between phase velocity and electron beam, which decelerates continually and improves the RF current distribution. Meanwhile the modulation field is reduced by the introduction of nonuniform SWS also. The particle in cell simulation results reveal that a microwave with a power of 1.8 GW and a frequency of 14.7 GHz is generated with an efficiency of 47% when the diode voltage is 620 kV, the beam current 6.1 kA, and the guiding magnetic field 0.95 T.

Imaging Coherent Response of Superconducting Metasurface

We study microwave response of the individual metaatoms of a superconducting metasurface formed by a two-dimensional array of superconducting quantum interference devices (SQUIDs). In our experiment, RF currents in the metasurface are directly imaged by using the laser scanning microscopy (LSM) technique. We tested a sample with 21 × 21 SQUID array in a waveguide cavity designed to achieve a uniform microwave distribution over the entire array. The demonstrated tunability of 2D SQUID metasurface resonance frequency by external magnetic field is about 56%, covering 8-12.5-GHz range. The obtained LSM images of the RF current distributions over the SQUID array confirm a high degree of coherence of the entire metasurface. The SQUID-based metasurfaces combine low losses and frequency tunability and can be useful for designing compact cryogenic RF systems.

Electrodynamics of planar Archimedean spiral resonator

We present a theoretical and experimental study of electrodynamics of a planar spiral superconducting resonator of a finite length. The resonator is made in the form of a monofilar Archimedean spiral. By making use of a general model of inhomogeneous alternating current flowing along the resonator and specific boundary conditions on the surface of the strip, we obtain analytically the frequencies fn of resonances which can be excited in such system. We also calculate corresponding inhomogeneous RF current distributions ψn(r), where r is the coordinate across a spiral. We show that the resonant frequencies and current distributions are well described by simple relationships fn=f1n and ψn(r)≃ sin[πn(r/Re)2], where n=1,2... and Re is the external radius of the spiral. Our analysis of electrodynamic properties of spiral resonators' is in good agreement with direct numerical simulations and measurements made using specifically designed magnetic probe and laser scanning microscope.

Electromagnetic Analysis of the EAST 4-Strap ICRF Antenna with HFSS Code

A new ICRF antenna has been designed in EAST, whose aims are to reduce the parallel RF electric fields E‖ and to investigate the current drive using the fast magnetosonic wave. This antenna consists of four toroidally spaced radiating straps. The electrical characteristics of the new antenna are estimated by using a three-dimensional electromagnetic commercial code. The S-parameters, RF current distribution and electromagnetic field distribution on and near the 4-strap antenna are analyzed, and the RF potentials influenced by antenna phasing and radial position are investigated.

Inductance and near fields of a loop antenna in a cold magnetoplasma in the whistler frequency band

The influence of a magnetoplasma on the inductance of a circular loop antenna oriented perpendicular to the ambient static magnetic field and operated in the whistler frequency band is studied. Based on a strict electrodynamic approach, the analytical treatment of the antenna reactance is performed for a uniform rf current distribution along the antenna wire. Calculations are made for plasma parameters and operating frequencies typical for active ionospheric experiments and laboratory rf (helicon) sources of dense magnetized plasmas. It is shown that the plasma influence on the inductance of the loop antenna remains relatively weak, even for antennas with dimensions close to half of the longitudinal whistler wavelength, when the rf field distribution in the antenna near zone is strongly different from that in vacuum. The theoretical predictions are confirmed by measurements performed on the large KROT plasma device. The results obtained are of crucial importance for the preparation of active ionospheric experiments and for the matching of loop antennas used in laboratory rf sources of dense magnetized plasmas.

Unconventional rf photoresponse from a superconducting spiral resonator

Superconducting thin film resonators employing strip geometries show great promise in rf/microwave applications due to their low loss and compact nature. However, their functionality is limited by nonlinear effects at elevated rf/microwave powers. Here, we show that by using a planar spiral geometry carrying parallel currents in adjacent turns, this limitation can be minimized. We investigate the rf current distributions in spiral resonators implemented with Nb thin films via laser scanning microscopy. The rf current density profile along the width of the individual turns of the resonators reveals an unconventional trend: maximum current in the middle of the structure and decaying toward its edges. This unusual behavior is associated with the circular nature of the geometry and the cancellation of magnetic field between the turns, which is favorable for handling high powers since it allows the linear characteristics to persist at high rf current densities.

Electromagnetic Field Simulation for ICRF Antenna and Comparison with Experimental Results in LHD

Ion cyclotron range of frequencies (ICRF) heating antennas in LHD are numerically simulated and analyzed by HFSSTM finite-element electromagnetic wave field calculation code. The model includes an accurate vacuum chamber wall of LHD and ICRF antenna structure and a simple model of plasma in a helical configuration. Antenna coupling with plasma is simulated by an artificial freshwater volume with enhanced high permittivity of e = 500-2000. RF current distribution and electromagnetic field distribution on and near the ICRF antenna are analyzed and well elucidated through a comparison with the experimental results. The frequency dependence of experimental loading resistance can be simulated by the calculation, and the RF dissipation on the antenna structure is studied and compared with experimental results. The local high heat load around gaps between the carbon side protectors is well explained, and the effect of gap distance is studied. Comparison with the experimental results reveals that the ICRF heating in the LHD, including the antenna and helical plasma, is well simulated by commercial HFSSTM code analysis. It will also be useful for future improvements in ICRF antenna design in helical devices.

Local thermal bistability in MgB2 microwave coplanar resonators: Opposite jumpwise response to weak-link switching and to vortex avalanches

We report on specific nonlinear features in the response of microwave coplanar resonators obtained from magnesium diboride thin films. Different jumpwise behaviors observed in the resonance curves are found to be generated by either weak-link switching or vortex avalanches. Both mechanisms can be explained in terms of local-heating-induced thermal bistability, but they result in opposite effects, i.e., opposite signs in the abrupt variations of the quality factor and of the resonance frequency, due to the rf-current distribution. The weak-link-switching response shows promising characteristics (jumpwise signal, bistability, and hysteresis) for applications of MgB2 films in nonlinear devices.

Microwave Current Imaging in Passive HTS Components by Low-Temperature Laser Scanning Microscopy (LTLSM)

We have used the Low-Temperature Laser Scanning Microscope (LTLSM) technique for a spatially resolved investigation of the microwave transport properties, nonlinearities, and material inhomogeneities in an operating coplanar waveguide YBa2Cu3O_{7‐δ}(YBCO) microwave resonator on a LaAlO3 (LAO) substrate. The influence of twin-domain blocks, in-plane rotated grains, and micro-cracks in the YBCO film on the nonuniform radiofre-quency (rf) current distribution was measured with a micrometer-scale spatial resolution. The impact of the peaked edge currents and rf field penetration into weak links on the linear device performance were also studied. The LTLSM capabilities and its future potential for nondestructive characterization of the microwave properties of superconducting circuits are discussed.

RF current distribution and topology of RF sheath potentials in front of ICRF antennae

The 2D (radial/poloidal) spatial topology of RF-induced convective cells developing radially in front of ion cyclotron range of frequency (ICRF) antennae is investigated, in relation to the spatial distribution of RF currents over the metallic structure of the antenna. This is done via a Green's function, determined from the ICRF wave coupling equations, and well-suited to open field lines extending toroidally far away on both sides of the antenna. Using such formalism, combined with a full-wave calculation using the 3D antenna code ICANT (Pécoul S. et al 2000 Comput. Phys. Commun. 146 166–87), two classes of convective cells are analysed. The first one appears in front of phased arrays of straps, and depending on the strap phasing, its topology is interpreted using the poloidal profiles of either the RF current or the RF voltage of the strip line theory. The other class of convective cells is specific to antenna box corners and is evidenced for the first time. Based on such analysis, general design rules are worked out in order to reduce the RF-sheath potentials, which generalize those proposed in the earlier literature, and concrete antenna design options are tested numerically. The merits of aligning all strap centres on the same (tilted) flux tube, and of reducing the antenna box toroidal conductivity in its lower and upper parts, are discussed.

Influence of LaAlO3 surface topography on rf current distribution in superconducting microwave devices

A laser scanning microscope with a thermal spot size of about 4 μm is used to measure a quantity proportional to the rf current density in an operating superconducting coplanar waveguide microwave resonator. The twinning of the LaAlO3 substrate produces a meandering of the current at the edges due to irregularities in the wet etching of the YBa2Cu3O7−δ film associated with substrate twin domain blocks, and an (∼20%) enhancement of the rf photoresponse at these locations. These irregularities are candidates for enhanced nonlinear response from the device.

Radio frequency power deposition in a high-density helicon discharge with helical antenna coupling

A high-density helicon discharge (ne⩽1020 m−3) produced through a new m=1 helical antenna is investigated. Various diagnostics are applied to measure the discharge parameters and the radio frequency (rf) quantities like the plasma resistance and the rf field distribution. Special attention is paid to the axial asymmetry of the discharge, which is characteristic for helicon devices with helical antennas. The axial profiles of the rf wave fields, as well as the energy deposition profiles, reveal that the rf power is mainly transferred and absorbed via the m=+1 helicon mode traveling in the positive magnetic-field direction. The experimental findings are compared with numerical results obtained from a fully electromagnetic model, which takes into account the rf current distribution of the launching antenna, as well as the finite size of the plasma column. The antenna–plasma coupling, as well as the total rf power deposited in the plasma, can be explained satisfactorily if the measured profiles are taken in the computations. In particular, the axial asymmetry of the helicon discharge can be understood in terms of the radial inhomogeneity of the plasma column. Furthermore, the calculations show that the small-scale Trivelpiece–Gould waves may be excited near the plasma edge. These waves would carry a considerable fraction of the absorbed rf power and may thus be important for the rf power coupling.

ICP diagnostics with magnetic probes

Magnetic probe (B-dot) diagnostics of inductively coupled plasmas are reviewed in this lecture. The diagnostic fundamentals, the relationships between the measured B-dot signals and the rf electric fields and currents in an ICP, the design of B-dot experiment and the results of B-dot measurements carried out over a wide range of ICP parameters are discussed here. It is shown that commonly used B-dot probes enclosed in a dielectric shell significantly disturb the measured B-dot signals and may lead to erroneous results. Some new results on the electromagnetic field and rf current distribution measured in a cylindrical ICP with a flat inductor coil are discussed in terms of anomalous skin effect.

Electromagnetic field structure in a weakly collisional inductively coupled plasma

Two-dimensional, phase resolved magnetic probe measurements have been performed in a low pressure inductively coupled cylindrical plasma source driven with a planar coil. The rf electric field and current density distributions determined from these measurements exhibit an abnormal nonmonotonic spatial evolution. Formation of a second current layer, phase bifurcation and a reversal of the rf field phase velocity have been found and are attributed to spatial dispersion of the plasma conductivity due to collisionless electron thermal motion typical of anomalous skin effect.

RF current distribution across metal-semiconductor ohmic contacts in mm-wave IMPATTs

Improved boundary conditions are derived for RF current flow across an interface between two different conducting media. The deduced conditions are used for skin effect analysis of silicon-gold ohmic contact of a mm-wave IMPATT diode. The resulting RF current flow exhibits refraction at the interface and the skin effect resistance (SER) exhibits monotonic increase with semiconductor thickness. The SER behaviour, although different from earlier published theoretical results, is shown to be more realistic and conforming to experimental observations.

Penetration depth and critical current in NbN resonators: predicting nonlinearities and breakdown in microstrip

Parallel plate and microstrip resonator experiments show that NbN films have large magnetic penetration depth and a peak RF critical current density which either matches or exceeds the DC critical current density, depending on the temperature. The large penetration depth, /spl lambda/, suggests that narrow NbN transmission lines will have increased inductive nonlinearities, when compared with other superconductors. The relation between the DC and peak RF critical current densities indicates that by modeling the RF current distribution a simple DC breakdown measurement can be used to estimate a transmission line's RF current carrying capacity.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Silicon Floating Zone Process: Numerical Modeling of RF Field, Heat Transfer, Thermal Stress, and Experimental Proof for 4 Inch Crystals

This paper is a result of several years of work on the silicon floating zone (FZ) process. Starting with numerical calculations of RF current density distribution, heat transfer, and thermal stresses, theoretical results are compared with FZ experiments. The simplified numerical models show qualitatively and quantitatively useful results for designing RF inductors under the assumption of steady‐state growth and for sufficiently low disturbances by the field of the inductor slot. Above all, the thermal influence of modifications of the inductor profile can be predicted more accurately.

Helicon wave coupling to a finite plasma column

Inductive helicon wave coupling to a plasma column is studied numerically. In our theoretical model, the RF current distribution of the launching antenna is taken into account as well as the finite size of the plasma cylinder. Computational results based on the data of present-day helicon devices are shown. The efficiency of various types of antennae is studied for a wide range of experimental parameters. In particular, we discuss the role of magnetic-field-aligned electron Landau damping for the helicon wave absorption. In many cases, the numerical findings can be understood reasonably in terms of the wavenumber spectra of the antenna and the helicon wave dispersion relation. In general, however, the full electromagnetic treatment is necessary in order to describe and to understand the inductive coupling in the helicon wave regime.