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Journal of Physics D: Applied Physics - latest papers
Latest articles for Journal of Physics D: Applied Physics
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Reconstructing the Hall drift current by neural network
Reconstruction of the Hall drift current (HDC) is important for monitoring the performance of a Hall thruster. One of the noninvasive reconstruction methods, which utilises an external magnetic sensor array to measure the magnetic field induced by the HDC and reconstruct the HDC according to the law of electromagnetic induction, has great application potential. In previous research, the relationship between the HDC and the measured magnetic flux densities was formulated with a linear magnetostatic model, and the HDC was reconstructed by solving a magnetostatic inverse problem using the least-squares (LS) method. However, the reconstruction results were prone to large errors. Because the regularisation term is introduced to reduce the ill-posedness of the inverse problem, subjective errors are inevitable when selecting the regularisation parameters. In this study, a method that utilises a Bayesian regularised backpropagation neural network to realise noninvasive reconstruction was developed. Compared with the traditional LS method, the proposed method overcomes the linear limitations of the magnetostatic model and avoids the subjectivity of the regularisation parameters. The results show that the reconstruction accuracy improved by more than 90%, and the recognition capability for different HDCs was significantly strengthened. This study provides strong support for the noninvasive detection of HDC and is also of great reference value for reconstruction research of other complex plasma currents.
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Dual-polarized absorptive frequency selective reflector with reconfigurable absorption intensity and tunable reflection band
One absorptive frequency selective reflector (AFSR) for dual-polarization and wideband, is proposed and characterized by tunable reflection notch and reconfigurable absorption intensity. By placing a lossy frequency-selective surface with four PIN diodes above the metal plane, a wideband absorptive structure with reconfigurable absorption intensity is constructed, and the capacitance of the varactor diode within the lossless layer can be efficiently modulated through an external voltage, enabling rapid and dynamic tuning of the notch frequency. The mechanisms of reconfigurable absorption and adjustable notch are revealed by investigating on the equivalent circuit models and surface currents. The measured experiment results show that the AFSR presents tunable notch band from 5.12 to 7.52 GHz, strong absorption band from 4.31 to 6.88 GHz and 7.96–11.28 GHz. By varying resistance of the loaded PIN diodes, the reflectivity within the absorption band can be dynamically tuned, ranging from approximately 0 to −10 dB. It offers significant insights for the design of antenna reflector systems aimed at reducing radar cross-section out of band.
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Tri-axial time-dependent magnetic field calibrated in-situ by harmonic analysis of adiabatically evolving atomic spins
We introduce a methodology to calibrate in situ a set of coils generating bi- or tri-axial magnetic fields, at frequencies where a calibration performed under static conditions would be inaccurate. The methodology uses harmonic analysis of one component of the magnetization of an atomic sample whose spins adiabatically follow an ad hoc applied time-dependent field. The procedure enables the identification of phases and amplitudes of the coil currents required to produce a dynamic magnetic field with the assigned polarization. This determines coil constants that can be subsequently used to produce arbitrary three-dimensional time-dependent fields.
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Surface Brillouin light spectroscopy of high-frequency guided elastic waves in CoFeB-multilayers on lithium niobate
CoFeB alloy is a ferromagnetic material that is often considered to produce synthetic antiferromagnets (SAF) used in spintronic devices. SAF host spin waves with high tunability, making their coupling with surface acoustic waves (SAWs) a promising avenue for next-generation communication devices. This coupling offers potential advantages such as SAW tunability and non-reciprocal operation in the multi-gigahertz range. Epitaxial lithium niobate (LN) thin films on sapphire substrates have emerged as a promising solution for achieving high SAW phase velocities. The combination of LN thin films with SAF, for instance based on CoFeB-based multilayers, hence offers new possibilities for engineering acoustic wave propagation. As the elastic properties of CoFeB depend on the composition of the alloy, their determination is a requisite to evaluate SAW dispersion. We investigate elastic wave dispersion in Co40Fe40B20-based multilayers on LN/ZX-sapphire, ZX-LN, and SiO2/Si substrates, combining surface Brillouin light spectroscopy and finite element computations. Surface-guided phonons ranging from 9 to 18 GHz are observed, for a wavelength of about 300 nm, and the elastic constants of Co40Fe40B20 are estimated from them.
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On the necessary condition for the generation of atmospheric-pressure electrodeless microwave plasma
This study investigates the necessary conditions for sustaining electrodeless microwave (MW) plasma at atmospheric pressure with efficient power coupling. The analysis focused on two different plasma gases of CO2 and N2, commonly used for hydrocarbon reforming and for hazardous gas decomposition and nitrogen fixation, respectively. First, the plasma temperature and electron number density were measured experimentally to characterize the states of the produced plasmas. Then, chemical equilibrium calculations and MW skin depth calculations in the produced plasmas were performed to determine the necessary conditions for sustaining the plasma. The experiments showed that, regardless of changes in specific energy input, the temperatures of the plasmas in CO2 and N2 were 6300 K and 6500 K, respectively. Additionally, the electron number densities in CO2 and N2 were at nearly the same levels as those for thermodynamic equilibrium, indicating that the produced plasmas are in local thermodynamic equilibrium (LTE). Lastly, from the skin depth analysis, it was found that the temperature of the LTE plasma should be greater than 5300 K for CO2 and N2 to achieve a skin depth that enables 95% power coupling efficiency, a finding that was consistent with the experimental observation.