Sun-Hong Min

Dynamics of mode competition in a gigawatt-class magnetically insulated line oscillator

An axial mode competition is observed in a 1GHz magnetically insulated line oscillator operating at gigawatt power level with a pulse duration of 130ns. A fast-growing axial mode adjacent to desired π-mode starts up first and hops to the slow-growing and stable π mode. The dynamics of the mode competition is found to be strongly dependent on the time-varying axial velocity of the magnetically insulated electron beam. The experimental observation is verified by the particle-in-cell simulation using a time-frequency analysis.

Transmission of gigawatt-level microwave using a beam-rotating mode converter in a relativistic backward wave oscillator

Gigawatt-level circularly polarized radiation was transmitted using a coaxial beam rotating antenna in an X-band relativistic backward wave oscillator. The mode conversion from the TM01 mode to the circularly polarized TE11-like mode was experimentally and numerically shown. The simulated radiation pattern was in good agreement with the measured radiation pattern.

Theory of Multimode Resonant Backward-Wave Oscillator With an Inclined Electron Beam

A multimode theory of the resonant backward-wave oscillator (BWO) with an electron beam inclined with respect to the surface of a periodic structure-a clinotron-is presented. It is shown that mode interaction provides phase velocity variation in the interaction space. The beam-wave interaction power increases at a favorable phase velocity profile along the interaction space, which manifests as power peaks in the clinotron zone. In contrast, when it is nonfavorable, there is power drop in the bandwidth. Developed multimode theory results are in satisfactory agreement with the theory of a BWO with reflections and with particle-in-cell simulations.

Surface-coupling of Cerenkov radiation from a modified metallic metamaterial slab via Brillouin-band folding.

Metallic metamaterials with positive dielectric responses are promising as an alternative to dielectrics for the generation of Cerenkov radiation [J.-K. So et al., Appl. Phys. Lett. 97(15), 151107 (2010)]. We propose here by theoretical analysis a mechanism to couple out Cerenkov radiation from the slab surfaces in the transverse direction. The proposed method based on Brillouin-zone folding is to periodically modify the thickness of the metamaterial slab in the axial direction. Moreover, the intensity of the surface-coupled radiation by this mechanism shows an order-of-magnitude enhancement compared to that of ordinary Smith-Purcell radiation.

Mode Conversion of High-Power Electromagnetic Microwave Using Coaxial-Beam Rotating Antenna in Relativistic Backward-Wave Oscillator

Mode conversion of high-power electromagnetic microwave (HPEM) was successfully accomplished using a coaxial-beam rotating antenna (COBRA) in an X-band relativistic backward-wave oscillator (RBWO). The mode conversion from the TM01 mode to the circularly polarized TE11-likeness mode was identified by both cold test and hot test. To observe whether mode conversion is effective or not, the radiation pattern of TE11-likeness mode by COBRA lens was compared with the case of the radiation pattern of TM01 mode using simple horn antenna. Their radiation patterns could be described by fluorescent lamps. The experimental results of radiation pattern measurements are shown to be in a reasonable agreement with the simulated one. The maximum value of RF output power appeared at the center of COBRA lens from the experimental results.

Automodulation Processes in Clinotrons With Low-Focusing Magnetic Field

The automodulation processes in backward-wave oscillator with an inclined electron beam (the clinotron) operating at low-focusing magnetic fields have been studied. The automodulation behavior has been analyzed in the clinotron for different angles of the beam inclination. It has been shown that at low magnetic fields, the RF transverse electric field may cause significant changes in electron trajectories, and hence in the beam-wave interaction power that leads to the automodulation.

Experimental study on 0.1 THz Clinotron

A multimode theory of resonant backward wave oscillator with inclined electron beam - Clinotron is introduced. The mechanism of the efficiency enhancement due to mode interaction is unveiled through numerical simulation. The experimental setup for 0.1 THz Clinotron is being developed and the details of each part will be presented.

Experimental Investigation of Giga-watt Magnetically Insulated Transmission Line Oscillator (MILO) by Improved Axial Power Extraction

Experimental works on magnetically insulated transmission line oscillator (MILO) have been carried out for generation of microwave with the GW power level. An efficient power extraction is achieved by employing a new coupling structure designed by the matching condition of phase velocities between the interaction circuit and output circuit As a first experiment, a MILO composed of 6-uniform cavities and single coupling cavity is chosen. The measured output power of 1.8GW shows a reasonable agreement with the simulated one of 2.1GW. The measured efficiency is about 11%. In this experiment, 500kV-36kA pulsed electron beam with the pulse duration of 150nsec is generated by SEBA (Seoul National Universitys electron beam accelerator)

Origin of Sideband and Spurious Noises in Microwave Oven Magnetron

The 3-D particle-in-cell (PIC) simulations are performed to determine the origin of sideband and spurious noises generated in a cooker magnetron. A novel simulation technique is used, which introduces cathode emission current nonuniformities. These nonuniformities are due to nonuniform distributions of electric field on a thermionic emission surface, which result from cathode geometry. It is shown that cathode end-caps shape and magnetic pole-piece geometries are the causes of sideband and spurious noises in conventional microwave ovens. The 3-D simulation results are in satisfactory agreement with the spectrum of a typical cooker magnetron.

Characteristics of a transient axial mode from the formation of anode plasma in a gigawatt-class L-band magnetically insulated transmission line oscillator

An experimental result of a gigawatt-class L-band magnetically insulated transmission line oscillator (MILO) shows the generation of a transient axial mode pertaining to an anode plasma effect in the circuit of the MILO. The transient axial mode between the desired π mode and the 5π/6 mode causes the output power to become eccentric. When the electrons impact onto the metallic surface, plasma exists on the surface of the anode due to the electron-impact distortion process. As a result, the anode plasma causes the emitted current to increase the neutralization near the cathode. The increase of the current induces faster and stronger magnetic insulation, which lowers the drift velocity and suppresses the mode earlier during the beam pulse. After the 5π/6 mode, which initially interacts with the electron beam, remains as a transient axial mode for a very short time, it shows the capability to convert to the π mode, which is more stable and slowly grows.