Sudeep Bhattacharjee

Folded waveguide traveling-wave tube sources for terahertz radiation

Microfabricated folded waveguide traveling-wave tubes (TWTs) are potential compact sources of wide-band, high-power terahertz radiation. We present feasibility studies of an oscillator concept using an amplifier with delayed feedback. Simulations of a 560-GHz oscillator and experimental evaluation of the concept at 50 GHz are presented. Additionally, results from various fabrication methods that are under investigation, such as X-ray lithography, electroforming, and molding (LIGA), UV LIGA, and deep reactive ion etching are presented. Observations and measurements are reported on the generation of stable single-frequency oscillation states. On varying the feedback level, the oscillation changes from a stable single-frequency state at the threshold to multifrequency spectra in the overdriven state. Simulation and experimental results on amplifier characterization and dynamics of the regenerative TWT oscillator include spectral evolution and phase stability of the generated frequencies. The results of the experiment are in good agreement with the simulations.

Accurate parametric modeling of folded waveguide circuits for millimeter-wave traveling wave tubes

In this paper, results of different models are compared for calculating effective, cold-circuit (beam-free) phase velocities and interaction impedances of folded waveguide (FW) slow wave circuits for use in millimeter-wave traveling wave tubes (TWT). These parameters are needed for one-dimensional (1-D) parametric model simulations of FW traveling wave tubes (FWTWTs). The models investigated include approximate analytic expressions, equivalent circuit, three-dimensional (3-D) finite difference, and 3-D finite element. The phase velocity predictions are compared with experimental measurements of a representative FW circuit. The various model results are incorporated into the CHRISTINE1D code to obtain predictions of small signal gain in a 40-55 GHz FWTWT. Comparing simulated and measured frequency-dependent gain provides a sensitive, confirming assessment of the accuracy of the simulation tools. It is determined that the use of parametric 1-D TWT models for accurate, full band predictions of small signal gain in FWTWTs requires knowledge of phase velocity and impedance functions that are accurate to <0.5% and <10%, respectively. Saturated gain predictions, being approximately half as sensitive to these parameters, appear to require correct specification of phase velocity and interaction impedance to within /spl sim/1% and 20%, respectively. Although all models generate sufficiently accurate predictions of the interaction impedance, not all generate sufficiently accurate predictions of the effective axial phase velocity.

Production of microwave plasma in narrow cross sectional tubes: Effect of the shape of cross section

A microwave plasma is produced in a conducting tube with a cross section smaller than the cutoff value. Waves of 2.45 GHz are launched perpendicularly to the multicusp magnetic field formed by permanent magnets surrounding the tube. Circular and square cross sectional tubes are tested. Overdense plasmas with a density of (0.8–2.0)×1011 cm−3 are obtained in the range of 10−4 Torr for powers of 200–360 W. The electron temperature is 6–12 eV. Under the same experimental conditions, the plasma density and the electron temperature are higher for the circular cross section.

Subcutoff microwave driven plasma ion sources for multielemental focused ion beam systems

A compact microwave driven plasma ion source for focused ion beam applications has been developed. Several gas species have been experimented including argon, krypton, and hydrogen. The plasma, confined by a minimum B multicusp magnetic field, has good radial and axial uniformity. The octupole multicusp configuration shows a superior performance in terms of plasma density (~1.3 x 10(11) cm(-3)) and electron temperature (7-15 eV) at a power density of 5-10 Wcm(2). Ion current densities ranging from a few hundreds to over 1000 mA/cm(2) have been obtained with different plasma electrode apertures. The ion source will be combined with electrostatic Einzel lenses and should be capable of producing multielemental focused ion beams for nanostructuring and implantations. The initial simulation results for the focused beams have been presented.

Microwave guiding and intense plasma generation at subcutoff dimensions for focused ion beams

The mechanism of microwave guiding and plasma generation is investigated in a circular waveguide with a subcutoff dimension using pulsed microwaves of 3GHz. During the initial phase, gaseous breakdown is induced by the exponentially decaying wave. Upon breakdown, the refractive index of the plasma medium varies radially, with the plasma density reaching close to cutoff values in the central region. At lower pressures, the waves can propagate through the peripheral plasma with a reduced wavelength, due to the collisionally broadened upper hybrid resonance region. The intense narrow cross sectional plasma bears promise for multielemental focused ion beams.

Production of pulsed microwave plasma in a tube with a radius below the cut-off value

A plasma is produced by pulsed microwaves in a circular tube with a conducting wall and a radius below the cut-off value for the fundamental circular waveguide mode (TE11 ). The discharge tube is surrounded with a multicusp magnetic field constructed with permanent magnets which provide a minimum-B configuration. Short pulse (0.05-1.0 µs) microwaves of 3 GHz and 60-100 kW peak power with a repetition frequency of 10-500 Hz are launched into the narrow tube with the propagation vector perpendicular to the cusped magnetic field in a pressure range of 10-2 -10 Torr. It has been found that, in addition to occurring at the entrance, a plasma is produced at a location of about one-third length from the tube exit. Plasmas generated at the two locations diffuse mostly along the tube axis with the radial diffusion strongly suppressed by the multicusp field. The density uniformity is better at lower pressures. The spatio-temporal evolution of electrons and ions shows a density growth after the end of a microwave pulse for a few to tens of microseconds depending on the axial position, followed by decay. The profiles have been explained by numerical simulation based upon a model that charged particles are driven by electrostatic and ponderomotive forces and diffuse along the longitudinal direction. The interpulse regime has an electron temperature of about 10 eV. This indicates that the plasma is still active between the pulses, in distinct contrast with the usual afterglows.

Microwave Plasma in a Multicusp Circular Waveguide with a Dimension below Cutoff.

A microwave plasma is produced in a nearly circular multicusp waveguide with a cross section smaller than the cutoff value. Permanent magnets are used to form the multicusp. Plasma density above the cutoff value was obtained in the range of 10-4 Torr at a power density of 6 – 10 W/cm2 for 2.45 GHz. The plasma production in the narrow waveguide and its characteristics are discussed.

Penetration and screening of perpendicularly launched electromagnetic waves through bounded supercritical plasma confined in multicusp magnetic field

The question of electromagnetic wave penetration and screening by a bounded supercritical (ωp>ω with ωp and ω being the electron-plasma and wave frequencies, respectively) plasma confined in a minimum B multicusp field, for waves launched in the k⊥Bo mode, is addressed through experiments and numerical simulations. The scale length of radial plasma nonuniformity (|ne/(∂ne/∂r)|) and magnetostatic field (Bo) inhomogeneity (|Bo/(∂Bo/∂r)|) are much smaller than the free space (λo) and guided wavelengths (λg). Contrary to predictions of plane wave dispersion theory and the Clemow–Mullaly–Allis (CMA) diagram, for a bounded plasma a finite propagation occurs through the central plasma regions where αp2=ωp2/ω2≥1 and βc2=ωce2/ω2⪡1(∼10−4), with ωce being the electron cyclotron frequency. Wave screening, as predicted by the plane wave model, does not remain valid due to phase mixing and superposition of reflected waves from the conducting boundary, leading to the formation of electromagnetic standing wave modes. The...

Experimental investigation of standing wave interactions with a magnetized plasma in a minimum-B field

Standing waves in the microwave regime are generated by a superposition of forward and backward moving waves induced by reflections from geometrical transitions in the plasma vacuum boundary. The waves are preferentially damped in the weakly collisional (νen∕ω≅10−4) plasma near the launch region (∼3−15cm), where the electron temperature has a higher than average value (Te>Teavg∼12eV). Typical e-folding damping lengths are of the order of 10cm, and depend upon the wave power and plasma collisionality. Fourier spectrum of the standing waves indicates about 23% downshift in the vacuum wave-number due to plasma dispersion. Electron trapping is observed in the potential troughs of the waves.

Novel TWT interaction circuits for high frequency applications

Summary form only given. The initial focus of this program is on the development of Ka-band TWTs producing 10 W of RF power. These devices would potentially be used as RF sources for phased array antennas. This requires innovative TWT designs, which result in improved repeatability, increased yield and reliability, and reduced cost over existing Ka-band devices. To do this, the batch nature of micro-electro-mechanical systems (MEMS) fabrication techniques is ideal. However, many TWT interaction circuits, such as the conventional helix, are not compatible with MEMS techniques. Thus, Calabazas Creek Research, Inc. (CCR) has computationally investigated several innovative TWT interaction circuits based on MEMS fabrication. These include the square helix, planar helix and modified folded waveguide circuits.