With semi-quantum theory, we quantitatively deduce microwave photons radiated from the moving electrons in a magnetic field, as well as the interaction of transmitted microwave photons with the vortex electrons in the magnetic field. It shows that the Orbital Angular Momentum (OAM) transition between microwave photons and vortex electrons in the magnetic field occurs when the relativistic effect is considered. This work indicates an effective way to transfer OAM between microwave photons and vortex electrons theoretically. By the OAM microwave photon resonance absorption, different vortex electrons with radial and magnetic quantum numbers can be generated. Furthermore, vortex electrons can be detected to analyze the OAM carried by microwave photons.

An electromagnetic (EM) wave with orbital angular momentum (OAM) has a helical wave front, which is different from that of the plane wave. The phase gradient can be found perpendicular to the direction of propagation and proportional to the number of OAM modes. Herein, we study the backscattering property of the EM wave with different OAM modes, i.e., the radar cross section (RCS) of the target is measured and evaluated with different OAM waves. As indicated by the experimental results, different OAM waves have the same RCS fluctuation for the simple target, e.g., a small metal ball as the target. However, for complicated targets, e.g., two transverse-deployed small metal balls, different RCSs can be identified from the same incident angle. This valuable fact helps to obtain RCS diversity, e.g., equal gain or selective combining of different OAM wave scattering. The majority of the targets are complicated targets or expanded targets; the RCS diversity can be utilized to detect a weak target traditionally measured by the plane wave, which is very helpful for anti-stealth radar to detect the traditional stealth target by increasing the RCS with OAM waves.Electromagnetic (EM) waves contain angular momentum, which is composed of spin angular momentum and orbital angular momentum (OAM) 1 . For a radio wave, i.e., the EM wave in radio frequency, the spin angular momentum corresponds to polarization, which has already been widely used in radar applications 2 . However, OAM has not yet been applied to radar detection and is seldom discussed 3 . The main difference between the OAM wave and the plane wave is that the OAM wave has a spatial wave front, with the circle phase gradient in the transverse section perpendicular to the propagation direction of the wave 4 . This phase difference can be denoted as e ilφ , which depends on the azimuthal angle φ and the OAM mode number l. Whether or not this phase gradient can be utilized for improving the detection capability becomes the fundamental problem of the feasibility of OAM waves applied in radar applications.Originally, OAM was employed in optical fibre transmission. Due to the orthogonality of OAM lights with co-axial reception, different OAM waves can be transmitted and received in the same frequency 5 . Thus, spectrum efficiency and the transmission capacity can be tremendously increased. OAM in radio wave is explored in free-space radio communications with its potential multiplexing. In 2007, B. Thidé first performed radio wave simulation and proposed the constraints of generating OAM by using an antenna array 6 . In 2011, Tamburini conducted the first transmission experiment with 442 m to verify OAM radio wave multiplexing and spectrum efficiency improvement 7 . In 2016, an experiment with the range of 27.5 km in China was successfully carried out to show the capability of OAM transmission in long distance 8 . In radar applications, preliminary discussions of the feasibility and application form of the OAM wave have lagged. In 2013, Guo proposed that an OAM wave has t...

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