Seontae Kim

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.

High performance CNT point emitter with graphene interfacial layer

Carbon nanotubes (CNTs) have great potential in the development of high-power electron beam sources. However, for such a high-performance electronic device, the electric and thermal contact problem between the metal and CNTs must be improved. Here, we report graphene as an interfacial layer between the metal and CNTs to improve the interfacial contact. The interfacial graphene layer results in a dramatic decrease of the electrical contact resistance by an order of 2 and an increase of the interfacial thermal conductivity by 16%. Such a high improvement in the electrical and thermal interface leads to superior field emission performance with a very low turn-on field of 1.49 V μm(-1) at 10 μA cm(-2) and a threshold field of 2.00 V μm(-1) at 10 mA cm(-2), as well as the maximum current of 16 mA (current density of 2300 A cm(-2)).

Low-energy electron point source microscope with position-sensitive electron energy analyzer

A low-energy electron point source microscope equipped with a position-sensitive energy analyzer is constructed. A nanometer-sized feature can be zoomed in and its energy-loss spectrum can be measured with a retarding field-type energy analyzer mounted in front of the imaging screen. The geometric and the electronic structures of carbon nanotubes are measured with the present system. Interference between the scattered and the transmitted electron beams through the carbon nanotubes is observed using an atomically sharp field emitter. The electron energy-loss spectrum shows two prominent peaks at ∼7 and 16–17 eV, which are identified as the π plasmon and (π+σ) surface-plasmon peaks. This result is consistent with the measurements of high-energy electron energy-loss spectroscopy as well as the theoretical calculation.

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.

Effects on electronics exposed to high-power microwaves on the basis of a relativistic backward-wave oscillator operating on the X-band

Abstract An analysis of the effects of electromagnetic pulses from a high-power microwave (HPM) radiation technique is conducted using a relativistic backward-wave oscillator (RBWO) which uses relativistic electron beams in vacuum circuits. The application described here is based on a relativistic electron device and uses relativistic electron beams to generate high-power electromagnetic radiation. The RBWO was fabricated to operate in a relativistic region with a gamma factor (γ) of 2 at an acceleration voltage of 500 kV. A mode-converted relativistic back-wave oscillator with an antenna that converts TM01 to TE11 was designed and fabricated because the electric field of the center in the RBWO circuit is null. The effects on electronic devices by HPM radiation and exposure were assessed. The effects on electronic devices exposed to HPMs, the failure of information equipment, and modulation of and interference with the received signal through a theoretical model of the threshold power relative to the influence on the target were confirmed in a high-output microwave exposure environment. Particularly, information devices containing semiconductors can undergo serious failures and breakdowns due to the thermal secondary breakdown caused by the high-output transient electromagnetic waves, and it is a theoretical consideration that reverse voltage occurs due to the generation of surge current when caught in the PN-junction region. Finally, the range of power regarding the effectiveness of the electromagnetic coupling of electronics exposed to HPM radiation was estimated.

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.

Transient pulse analysis of ionized electronics exposed to γ-radiation generated from a relativistic electron beam

When a semiconductor element is irradiated with radiation in the form of a transient pulse emitted from a nuclear explosion, a large amount of charge is generated in a short time in the device. A photocurrent amplified in a certain direction by these types of charges cause the device to break down and malfunction or in extreme cases causes them to burn out. In this study, a pulse-type γ-ray generator based on a relativistic electron beam accelerator (γ=2.2, β=0.89) which functions by means of tungsten impingement was constructed and tested in an effort to investigate the process and effects of the photocurrent formed by electron hole pairs (EHP) generated in a pMOSFET device when a transient radiation pulse is incident in the device. The pulse-type γ-ray irradiating device used here to generate the electron beam current in a short time was devised to allow an increase in the irradiation dose. A precise signal processing circuit was constructed to measure the photocurrent of the small signal generated by t...

Miniaturized two-stack Blumlein pulser with a variable repetition-rate for non-thermal irreversible-electroporation experiments

Non-thermal irreversible electroporation (NTIRE) to avoid thermal damage to cells during intense DC ns pulsed electric fields (nsPEFs) is a recent modality for medical applications. This mechanism, related to bioelectrical dynamics of the cell, is linked to the effect of a DC electric field and a threshold effect with an electrically stimulated membrane for the charge distribution in the cell. To create the NTIRE condition, the pulse width of the nsPEF should be shorter than the charging time constant of the membrane related to the cell radius, membrane capacitance, cytoplasm resistivity, and medium resistivity. It is necessary to design and fabricate a very intense nanosecond DC electric field pulser that is capable of producing voltages up to the level of 100 kV/cm with an artificial pulse width (∼ns) with controllable repetition rates. Many devices to generate intense DC nsPEF using various pulse-forming line technologies have been introduced thus far. However, the previous Blumlein pulse-generating devices are clearly inefficient due to the energy loss between the input voltage and the output voltage. An improved two-stage stacked Blumlein pulse-forming line can overcome this limitation and decrease the energy loss from a DC power supply. A metal oxide silicon field-effect transistor switch with a fast rise and fall time would enable a high repetition rate (max. 100 kHz) and good endurance against very high voltages (DC ∼ 30 kV). The load is designed to match the sample for exposure to cell suspensions consisting of a 200 Ω resistor matched with a Blumlein circuit and two electrodes without the characteristic RC time effect of the circuit (capacitance =0.174 pF).