Ranajoy Bhattacharya

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.

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.

High-Frequency Transmission Characteristics of Inert Coolant Liquids for High-Power Gyrotron Windows

The frequency response of dielectric properties of coolant liquids used in the gyrotron RF window is measured. FC-40, FC-75, and CCl4 are considered for the measurements due to their reported application in gyrotron and excellent dielectric properties required for high-power RF transmission. A performance probe kit based on the reflection method is used for measurements in wide frequency range from 200 MHz to 50 GHz. Vector network analyzer (VNA) is used as the RF source as well as receiver. The dielectric constant and loss tangent are calculated directly by using the scattering matrix data obtained from the VNA. Further, ac conductivity and relaxation time are also computed by using measured data of loss tangent and dielectric constant.

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...

Uniform high current and current density field emission from the chiseled edge of a vertically aligned graphene-based thin film

Abstract The field emission properties of the controlled emission edge of a vertically aligned graphene-based thin film are presented. A current and current density of above 7 mA and 200 A/cm2, respectively, with uniform electron emission, are achieved. Uniform high current and current density emissions can be realized by the pre-mechanical shaping and post electrical conditioning of reduced graphene oxide (rGO) film emission, owing to the robustness, thinness (<1 μm), and well-defined uniform film thickness. Field emission luminance demonstrates uniform emission over the entire emission area with a high aspect ratio. Along with a high current emission, the rGO film exhibits excellent emission stability, long-term. This offers prospects for various applications in field emission displays, electron microscopy, and particularly for the realization of miniaturized terahertz vacuum electronic devices, which require electron sources with uniform high currents and current densities, such as long-lifetime cold cathodes.

Enhanced radiation in a modified metallic metamaterial driven by pre-bunched electrons

Modified metallic metamaterials are known to provide external radiation from Cerenkov-like modes which can be excited using an electron bunch. Extremely strong electromagnetic field enhancement could be formed for compact, efficient free-electron lasers with open metallic structures. The numerical and experimental works will be presented in detail.

Efficient terahertz oscillation using a half-period staggered grating resonator

A highly efficient terahertz (THz) radiation mechanism is studied with the use of strong interaction between convection electrons and a half-period phase-shifted grating resonator. The large fraction of the fundamental TE mode is longitudinally polarized, and it excites the intense plasma–terahertz wave coupling at the shallow grating, which enables highly efficient RF generation at a relatively low operating voltage. A particle-in-cell (PIC) simulation predicts that the half-period phase-staggered grating resonator generates 0.22 THz wave with output power exceeding 100 W and interaction efficiency of more than 15% at a low beam acceleration voltage of 5.2 kV.