Xiaohan Sun

In this article, the impact of thermal radiation by the electron beam on the inner thermal state of a traveling wave tube (TWT) under operation is explored. A theoretical model for analyzing heat transfer by thermal radiation from the cylindrical electron beam is derived, in which the electron beam is treated as an equivalent dynamic circular disk with gray-body radiation from its side surface. By using the cathode temperature and the compression ratio of the electron beam in a TWT, the temperature of the equivalent disk is calculated. By considering the Bremsstrahlung radiation and the cyclotron radiation, the power radiated from the electron beam is derived, and the emissivities of the gray-body radiation from the equivalent disk are calculated. Using the parameters of a wideband TWT, we obtain the heat transfer via thermal radiation from the electron beam and conduct thermal analysis by simulation with and without considering thermal radiation from the electron beam, respectively. The result at 11 GHz shows that the temperature rise on the inner surface of the helix tape due to thermal radiation from the electron beam is $0.67~^{\circ }$ C and $0.49~^{\circ }$ C at the beginning/ending point of the helix, respectively. In the working frequency band, the temperature rise at the beginning point of the helix ranges from $0.65~^{\circ }$ C to $0.72~^{\circ }$ C, while that at the ending point ranges from $0.46~^{\circ }$ C to $0.52~^{\circ }$ C.

This letter proposes a sandwich metamaterial window (SMW) for a W-band TE01 mode gyrotron travelling-wave tube (gyro-TWT), which is optimized using machine learning (ML) to achieve high transmissivity and low reflectivity in a wideband range. This SMW has three layers, BN-metal-BN, where the BN has a thickness of 1.05 mm. The metal layer is composed of hexagonal units allowing to improve the cold-test characterization of SMW. The ML optimization makes the SMW have a high simulation performance with S11 less than −20 dB and S21 higher than −0.14 dB within 12.15 GHz. Compared to conventional designs, the proposed SMW provides a thicker one to achieve comparable performance through a simplified fabrication process. Experimental validation confirms alignment with simulation results, demonstrating the feasibility of this approach. This work provides a scalable and cost-efficient methodology for high-frequency metasurface window design.

For converging and switching huge amounts of multisource heterogeneous sensing data in the Internet of Things (IoT), we propose a hierarchical optical access switching networking (HOASN) architecture, the access domain of which is interconnected by passive optical distributing nodes (PODNs), distributed edge nodes (ENs) including sensing accessing interfaces. The structures of PODN, EN and SFM are proposed. And then, a cyclic signaling mechanism and a dynamic bandwidth allocation algorithm are proposed to realize pipelined transmission slot allocation and make full use of the uplink channel bandwidth resources. The simulation shows that the HOASN improves the throughput 6.9% and 10.7%, and reduces the latency 16.4% and 35.3% for converging and switching sensing data in uplink channels, compared with two traditional Ethernet optical access schemes, respectively. Finally, we design and develop an EN prototype based on ARM Cortex-A72 series chip, set up a HOASN experimental system with two prototypes interconnected in the lab, and implement it in the Power IoT Demonstration Project in Tongli, Suzhou, China. All results show that the HOASN scheme can realize converging and switching with high throughput and low latency for sensing data.

To address the security problem of multi-branch tapping attacks in optical access networks, we propose a terminal thermal distribution (TTD)-based detecting and locating scheme. A TTD control module (TCM) including parts of attack detection, branch distinction, and location via effective thermal distribution product (ETDP) is set at the optical line terminal, can detect and locate the tapping attacks simultaneously occurring on multi-branch drop fiber links according to the feedback signals uploaded by each optical network units, and has the advantage of saving a lot of communication and computing resources for the physical layer. Meanwhile, what we believe to be a novel ETDP model is established to improve the efficiency of TTD image processing and judging. An experimental platform with two-branch drop fiber links and the corresponding measurements are set up in the laboratory, in which the single-wavelength OTDR analyzer is combined to verify and evaluate the experimental results of the scheme proposed in this paper. By comparing the experimental results of the TTD-based scheme with that of the OTDR, the detection success rate and location spatial resolution for multi-branch tapping attacks in the TTD-based detecting and locating scheme are around 98% and 0.006 km on average, respectively.

This article presents a gain-equalizing metamaterial pillbox window based on electromagnetically induced transparency effect (EITMPW). The electromagnetic (EM) characteristics and performance of a D -band folded waveguide slow wave structure (FWG SWS) traveling-wave tube (TWT) are analyzed to determine the input and corresponding loss characteristics for achieving equalized output power in band. The matching loss of the proposed EITMPW is designed to fit with the target loss, and the tested results of a W -band EITMPW basically agree with the simulated ones. In addition, the interaction model with EITMPW has lower gain fluctuation, below 0.31 dB, in the frequency range of 145–155 GHz, compared with the gain fluctuation of a TWT with the conventional pillbox window. Moreover, it improves the gain fluctuation of the FWG SWS TWT by integrating the gain equalizer into the pillbox window, which may have further applications in the compact microwave power module (MPM).

Optogenetic-based neuroprosthetic therapies are increasingly being considered for human trials. However, the optoelectronic design of clinical-grade optogenetic-based neuroprosthetic probes still requires some thought. Design constraints include light penetration into the brain, stimulation efficacy, and probe/tissue heating. Optimisation can be achieved through experimental iteration. However, this is costly, time-consuming and ethically problematic. Hence it is highly desirable to have an alternative to excessive animal trials. Thus, a simulation tool for optimising probe design can be an important benefit for the community. The challenge is to understand the interplay between the optical, neural and thermal aspects in the interaction of probe and living neural tissue. In this work, we propose a model which combines these aspects to allow clinically orientated neuroprosthetic teams to design neuroprosthetic probes for optogenetic therapies. Our model provides analyses for optical, thermal and optogenetic electrophysiological processes based on the energy equivalence and exchange among different physical fields. To validate and calibrate the model, optogenetic implantable neuroprosthetic arrayed probes based on miniature LEDs were developed. Then, optical, thermal measurement and neural photocurrent recording experiments were implemented on the probes. We can then provide analysis on exemplar arrayed neural probes.

This article presents a novel approach for achieving a compact high transmissivity mode converter (MC) using a metasurface-based mode converter (MMC). Specifically, the design of Ka-band MCs is proposed using this method, including TE $_{10}^{\mathrm{(R)}}$ –TE $_{20}^{\mathrm{(R)}}$ –TE $_{01}^{\mathrm{(C)}}$ (R denotes rectangular and C denotes circular, RC-MMC) and TE $_{10}^{\mathrm{(R)}}$ –TE $_{30}^{\mathrm{(R)}}$ (RR-MMC). The RC-MMC is designed in a transition metal waveguide with an alumina metasurface and a metallic resonant metasurface, while the RR-MMC is achieved using an alumina metasurface. The simulation results demonstrate that the S $_{21}$ exhibits a loss less than 0.5 dB in the 1.2-GHz bandwidth for RC-MMC and 2.3-GHz bandwidth for RR-MMC, respectively. In addition, S $_{21}$ can reach a maximum value of $-$ 0.1 dB. Finally, a TE $_{10}^{\mathrm{(R)}}$ –TE $_{20}^{\mathrm{(R)}}$ MMC is fabricated and tested. The experimental results show that the $-$ 3-dB bandwidth of this MMC is 2.39 GHz, and S $_{11}$ is less than $-$ 16 dB. Using MMC instead of complex waveguide structures could pave the way for the miniaturization of MC for integration. This advancement could also extent to various applications, including the design of vacuum electronic devices and satellite-specific mode transmitters.

A pseudo-periodic helix slow wave structure (SWS) is proposed to provide high output power with wide bandwidth in Ka -band. The support rods with curved metal provide flat dispersion and high interaction impedance, while a pseudo-periodic SWS suppresses backward oscillation and improves output power. The simulation results show that the traveling wave tube (TWT) with this pseudo-periodic helix SWS produces an average output power of more than 600 W in the frequency range from 26 to 36 GHz with beam voltage and beam current of 11.287 kV and 0.3 A, respectively, with a maximum output power of 700 W at 31 GHz, corresponding to a maximum gain of 42.8 dB and an electronic efficiency of 20.5%. The multiphysics analysis reveals the influence of the high-frequency losses on the temperature distribution in the operating condition. The results indicate that the maximum temperature in the last 15 circles is concentrated at the end of the pseudo-period helix SWS with a peak temperature of 304 $^{\circ}$ C, resulting in better heat dissipation performance, easier structural fabrication, and less deformation.

The Smith–Purcell radiation produced by electrons moving closely to a grating can be enhanced by resonances. Here, we show a method to manipulate the directionality of the resonance-enhanced radiation. Using the rigorous coupled-wave analysis method, we compare the radiation from symmetric and asymmetric gratings, showing that the enhanced Smith–Purcell radiation can become unilateral with a perturbation that breaks the structural symmetry. Our work provides an effective method for frequency-domain calculation of Smith–Purcell radiation and also an approach to realize more efficient use of the radiation.

A novel in-line closed-loop polarization maintaining fiber temperature sensor (CPMF-TS) is designed, which adopts a reciprocal configuration to reduce the susceptibility to optical path noises and environmental disturbances, and uses a closed-loop square-wave modulation and demodulation scheme to improve sensitivity, linearity and dynamic range, while reducing the influence of system power fluctuations on accuracy. A CPMF-TS prototype was developed, whose performances were analyzed and compared with the open-loop scheme. The results show that the relative error (defined as the maximum error divided by the tested measured range) of CPMF-TS using 2.5mm or 3.5mm sensing fiber is 1.1%, which is less than 3% or 1.64% of the open-loop system after third- or fourth-order polynomial-fitting. The closed-loop scheme is shown effective in reducing the impact of sensor fiber cut length errors and optical power dependence on system performance, resulting in a more accurate and stable output over a wider temperature range. Combined with the inherent insulation and anti-electromagnetic interference advantages of optical fiber sensors, CPMF-TS has obvious potential application prospects for accurate temperature measurement in high temperature, high voltage or harsh electromagnetic field environments such as the transformer winding temperature measurement and the aero-engine temperature measurement.