A method to analyze the scattering characteristics of a Laguerre–Gaussian (LG) vortex beam by a low-velocity charge in a steady applied magnetic field is investigated in this paper. Based on the plane wave angular spectrum representation and the electromagnetic radiation theory of a moving charge, the expressions of the scattered field and scattered power per unit solid angle are presented. Considering a low-velocity electron, the distribution of the scattered power per unit solid angle is numerically simulated. The effects of electron motion parameters, beam parameters, and applied magnetic field are discussed in detail. The results show that the longitudinal motion of the electron, which is parallel to the direction of the applied magnetic field, makes the distribution of the scattered power no longer maintain radial consistency but has a concave–convex distribution. Due to the periodicity of the circular motion of the electron in the applied magnetic field, the scattering power distribution pattern consists of many annular lobes. The spectral characteristics of the backscattered field are analyzed by fast Fourier transform. It is confirmed that the scattered field satisfies the rotational and the linear Doppler effects, and the accuracy of the formula provided in this paper is verified. This work lays a foundation for further study of the scattering characteristics of plasma on vortex electromagnetic waves and helps to promote the in-depth development of vortex beams in the field of plasma diagnosis.

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The plasma wake of reentry vehicles has the advantages of extensive space range and long traceability, which provides new possibilities for the detection and monitoring of reentry vehicles. Based on the Zakharov model, this work investigates the excitation and power spectrum characteristics of electromagnetic radiation for the plasma wake of a typical reentry vehicle. With the aid of parametric decay instability, the excitation condition of electromagnetic radiation for a typical plasma wake is evaluated first. The power spectrum characteristics of electromagnetic radiation including the effects of both the flight parameters and incident wave parameters are analyzed in detail. The results show that when the phenomenon of the excited electromagnetic radiation occurs, plasma wakes closer to the bottom of the vehicle and with faster speeds require higher incident frequencies and thresholds of the electric field. As the frequency of the incident wave increases, peaks appear in the power spectra of plasma wakes, and their magnitudes increase gradually. The frequency shifts of secondary peaks are equal, whereas, the peaks of the downshifted spectral lines are generally larger than those of the upshifted spectral lines. The work in this paper provides a new idea and method for the tracking of reentry vehicles, which has potential application value in the field of reentry vehicle detection.

The K-nearest neighbor ( K-NN) query is an important query in location-based service (LBS), which can query the nearest k points to a given point, and provide some convenient services such as interest recommendations. Hence the privacy protection issue of K-NN query has been a popular research area, protecting the information of queries and the queried results, especially in the information era. However, most of existing schemes fail to consider the privacy protection of location points already stored on servers. Or some schemes support no update of location points. In this paper, we present an updatable and privacy-preserving K-NN query scheme to address the above two issues. Concretely, our scheme utilizes the KD-tree ( K-Dimensional tree) to store the location points of data owners in location service provider and encrypts the points with a distributed double-trapdoor public-key cryptosystem. Then, based on the Ciphertext Comparison Protocol and Ciphertext Euclidean Distance Calculation Protocol, our scheme can protect the privacy of location and query contents. Experimental analyses show our proposal supports some new location points for a fixed location service provider. Moreover, the queried results show a high accuracy of more than 95%.