Md. Ruhul Amin

Resonant Enhancement of Radiation from a Backward Wave Oscillator Utilizing Large Diameter Corrugated Metal Structure

An experiment on a backward wave oscillator (BWO) utilizing a large diameter corrugated metal wall slow wave structure (SWS) has been presented. Measurements are made above and below the starting beam energy for conventional BWO operation. For the latter case, an excited noise-level backward wave at 20 GHz (TM 01 mode) is coupled at the ends of the SWS to a forward anomalous Doppler shifted electron cyclotron (EC) beam mode which amplifies the original oscillation by an amount of 50 dB. Resultant microwave output powers of 200 kW with 4.5% electronic efficiency are observed near the condition of resonant magnetic field for an electron beam with energy 35 keV. Unlike the well-known fast wave devices, our slow wave EC device is simple to operate utilizing an electron beam with dominant parallel velocity in a uniform magnetic field.

Temporal Growth Study in Trapezoidally Corrugated Slow-Wave Structure for Backward-Wave Oscillator

The temporal growth rate (TGR) in a trapezoidally corrugated slow-wave structure for a backward-wave oscillator is theoretically studied. An intense relativistic annular electron is used as the energy source for the device. The annular electron beam is assumed to be infinitesimally thin in the radial extent and guided by an infinitely strong magnetic field. The trapezoidal profile of the structure is approximated by a sinusoidal function using Fourier approximation, and the dispersion relation of the system is derived using the Rayleigh–Fourier method. To study the TGR of the electromagnetic wave inside the system, the dispersion equation is solved for different values of the beam parameters. The dimensions of sinusoidally corrugated comparable trapezoidal structure are determined by comparing their dispersion characteristics. For the

Proposal of Cryogenic Plasmas in Liquid Helium II

{\rm TM}_{01}

Cherenkov Instability due to Unmagnetized Electron Beam in Periodically Corrugated Waveguide

mode, TGR of instability that gives a qualitative measure of the microwave generation is calculated. The peak TGR of the proposed structure is found to be on average 1.5% higher than that of the sinusoidally corrugated slow-wave structure for the same set of beam parameters. Apart from its improved growth rate, the proposed structure has an added advantage of easy fabrication.

Electromagnetic Field Properties of Axial Modes in a Finite Length X-Band Slow Wave Structure

Novel possibilities to create cryogenic plasma states in liquid helium II below 2.16 K at 1 atm are proposed. Below 0.4 K, a mixture of positive and negative ions in the superfluid can be collision-free in the sense that the momentum relaxation collision frequencies between ions and neutral atoms, impurity atoms and excitons such as phonons, rotons and vortices are less than the plasma frequencies. In particular, positive ions in the plasmas are analogous to free electrons in conventional gaseous discharges at low pressures, although the plasma parameters are quite different from each other. A possible cryogenic plasma is proposed to be produced as a transient state. Preliminary experiments for ion mixture left after pulsed discharges in liquid helium II are demonstrated.

A Comparative Study of Dispersion Characteristics Determination of a Trapezoidally Corrugated Slow Wave Structure Using Different Techniques

Cherenkov instability driven by an unmagnetized electron beam in a periodically corrugated waveguide is studied by developing a new version of self-consistent linear theory. The conventional space charge and cyclotron modes of magnetized beam degenerate in this case. The normal electromagnetic modes are pure transverse magnetic (TM) and transverse electric (TE) modes in axisymmetric cases and are hybrid modes having both TM and TE components in nonaxisymmetric cases. Using the developed linear theory, the axisymmetric and nonaxisymmetric Cherenkov instabilities due to the unmagnetized beam in the periodic system are examined numerically. The unmagnetized beam can excite the axisymmetric TM and nonaxisymmetric hybrid modes. A comparison of the numerically obtained results is made with those of the recent C-band Pasotron experiment.

Design of a Simple Integrated Coupler for SPP Excitation in a Dielectric Coated Ag Thin Film

The electromagnetic field properties of axisymmetric transverse magnetic (TM) modes in a specific slow wave structure (SWS) typically used in high power X-band backward wave oscillators are studied numerically. The dispersion relations, electric field distributions, and quality factors of the axial resonant modes are calculated. It is found that some of the axial modes can be switched from the volume modes to the surface ones by changing only the axial boundary of the SWS while the dispersion relations do not alter. This causes a drastic change in quality factors and can be of practical interest for controlling the coupling between beam and TM modes in high-power microwave devices. The calculated dispersion relations are verified experimentally for the TM 01 mode.

Linear Dispersion Relation of Double Period Slow Wave Structure for High-Power Backward Wave Oscillators

The linear dispersion relation of a trapezoidally corrugated slow wave structure (TCSWS) is analyzed and presented. The size parameters of the TCSWS are chosen in such a way that they operate in the x-band frequency range. The dispersion relation is solved by utilizing the Rayleigh–Fourier method by expressing the radial function in terms of the Fourier series. A highly accurate synthetic technique is also applied to determine the complete dispersion characteristics from experimentally measured resonances (cold test). Periodic structures resonate at specific frequencies when the terminals are shorted appropriately. The dispersion characteristics obtained from numerical calculation, synthetic technique and cold test are compared, and an excellent agreement is achieved.

Linear analysis of an X-band backward wave oscillator with a circular-edge disk-loaded cylindrical waveguide driven by an annular electron beam

A simple integrated coupler is proposed for the efficient excitation of surface plasmon polariton (SPP) mode in a thin metal film. The SPP mode is generated in a single Ag-dielectric interface by the incident field and coupled with an Ag thin film. The coupling efficiency at different wavelengths using two different dielectrics, gallium lanthanum sulfide (GLS) and aluminum gallium arsenide (AlGaAs) is calculated by analyzing the SPP propagation dynamics with the finite difference time domain method. A maximum coupling efficiency of 70% is obtained at a wavelength of 460 nm when GLS is used, whereas the corresponding value obtained for AlGaAs is 60% at 560 nm. The proposed structure can be used to excite SPPs in a nano-thin film from an external bulky source and is easier to fabricate since a single interface metal-dielectric configuration is used to excite the metal-thin film.

Metal nanoparticle enhanced light absorption in GaAs thin-film solar cell

Linear dispersion relation for an axisymmetric metal-wall slow wave structure (SWS) with double period corrugations has been analyzed numerically. The double period SWS is here proposed for improving the oscillation frequency performance of high-power backward wave oscillators that may sustain stable oscillation despite the energy exhaustion of the driving electron beam from upstream to downstream in the SWS. It is shown that conventional TM 01 mode in a single period SWS is split into a number of isolated branches. Oscillation frequencies are kept comparatively unchanged rather than the case of single period SWS.