Silva Neto, Lauro P.
Rossi, Jose O.
Barroso, Joaquim J.
Schamiloglu, Edl

Nonlinear transmission lines (NLTLs) have been studied for high-power RF generation with good prospects of applications in pulse radars for remote sensing (SARs) and disruption of communications in the battlefield, for instance. In this paper, two 30-section hybrid NLTLs built using nonlinear inductors and capacitors (2.2- and 10-nF barium titanate ceramic capacitors with 10-mu H ferrite bead inductors) will be described. For the test, the line is fed by a negative input pump pulse generated by a 1 kV discharge of a 0.75-mu F storage capacitor via a fast 50-ns switching system composed by an insulated gate bipolar transistor switch and its gate circuit driver. In the hybrid line tests, using 2.2-nF ceramic capacitors the maximum soliton generation packet obtained on the middle section had a frequency of the order of 33 MHz with voltage modulation depth (VMD) of around 700 V. For every single shot, approximately 10 RF cycles with small damping were noted. With the hybrid line using 10-nF ceramic capacitors the soliton generation obtained on the middle section reached a frequency of the order 10 MHz, and VMD of around 200 V. The main conclusion from this experiment is that hybrid lumped NLTLs may be used to achieve RF in megahertz range with higher VMD compared their counterparts (i.e., capacitive or inductive lines) because of their stronger nonlinearity with the use of both nonlinear elements.

Compared with other non-thermal plasma sources, the atmospheric-pressure plasma jet (APPJ) has advantages on simple structure, low temperatures, strong chemical activities, and convenient handling, all of which have attracted much attention. The power sources play an important role on the characteristics and the applications of the APPJ. In this article, a compact repetitive microsecond-pulse generator is designed for exciting the APPJ in helium and argon. The microsecond-pulse generator can produce repetitive pulses with output voltages of up to 20 kV, pulse width of similar to 8 mu s, and pulse repetition frequencies (PRFs) of 1 Hz similar to 2 kHz. Using the designed repetitive microsecond-pulse generator, the characteristics of the APPJ are investigated by measuring the voltages and currents and obtaining images of the discharges. Experimental results show that the microsecond-pulse generator has been successfully used to sustain stable APPJs both in helium and argon. The shape of the output voltage pulses may change as the applied voltage increases. Nevertheless, the output voltages are stable at all PRFs when the applied voltage is fixed. Furthermore, the effects of flow rate, the applied voltage, and the PRF on the He/Ar APPJ are investigated. Results show that it is more likely to generate a He APPJ rather than an Ar APPJ under microsecond-pulse excitation. The length of the plasma plume is slightly affected by the PRF both in the He APPJ and the Ar APPJ.

Kuek, Ngee Siang
Liew, Ah Choy
Schamiloglu, Edl
Rossi, Jose Osvaldo

A chain of cascading LC sections consisting of series nonlinear inductors and parallel linear capacitors can be used to generate oscillating pulses. When a rectangular pulse is injected into the input of such a nonlinear inductive line (NLIL), the pulse undergoes modulation and exits at the output with oscillations in the waveform. This paper describes the implementation of high-voltage NLILs using commercial-off-the-shelf components. A pulser comprising of a storage capacitor and a fast semiconductor switch is used to provide the input pulse with approximate rectangular pulse shape. The design of the NLIL, which is a type of nonlinear lumped element transmission line (NLETL), is based on a NLETL circuit model developed earlier in-house. The nonlinear inductors made of ferrites are modeled using a simplified form of the Landau-Lifshitz-Gilbert (LLG) equation. A simple novel approach is proposed to determine the characteristic parameters in the LLG equation. Simulation results from the NLETL model are compared to the experimental results, and analyses on the voltage modulation and frequency content of the output pulses are performed. The use of crosslink capacitors in the line to modify the dispersive characteristics to enhance the performance is also investigated. With the results, the conditions and trends for producing oscillating pulses in NLILs are discussed.

Elfrgani, Ahmed M.
Prasad, Sarita
Fuks, Mikhail I.
Schamiloglu, Edl

We demonstrate the dual-band operation of a relativistic backward wave oscillator (RBWO) whose output is a linearly polarized TE11-mode. The RBWO consists of two cylindrical sections with sinusoidal corrugations where the downstream section is axisymmetric and the upstream section has combined left- and right-single-fold spiral corrugations. We show that generation of 10 GHz is realized when the axisymmetric section operates as a BWO with the operating TM01-mode and the combined spiral section operates as a Bragg reflector that converts the backward TM01-mode to a linearly polarized forward TE11-mode. For the same electron beam parameters, generation of 7 GHz is realized when the combined spiral section operates as a BWO with the operating TE11-mode. We also present the possibility to change the duration of generation at each frequency within a microwave pulse by changing the electron beam current. In addition, one can change the output frequency by changing the values of the guide magnetic field near the region of cyclotron absorption.

Shao, Tao
Wang, Lei
Zhang, Cheng
Zhou, Yang
Han, Lei
Xu, Xuzhe
Schamiloglu, Edl

Surface dielectric barrier discharges ( SDBDs) have prospects for a wide variety of applications in the field of flow control. In order to study the characteristics of SDBDs excited using microsecond pulses, a compact microsecond-pulse generator with square pulses is developed and tested. The output voltage has amplitudes of 0 similar to 10 kV, pulse repetition rates of 1 similar to 5 kHz, and pulsewidths of 5 similar to 30 mu s. The square voltage has a rise time of 120 ns and a fall time of 12 mu s. Moreover, the working principle of the main circuit and drive circuit is introduced. The microsecond-pulse generator is tested without a load. The results show that the pulsed generator has a stable output and the drive circuit can effectively synchronously trigger IGBT switches. Furthermore, the effects of the output parameters of the generator on SDBDs characteristics are investigated. The experimental results show discharge currents occur both on the rise time and on the fall time of the voltage. The discharge mainly occurs at the rise time, and the discharge current increases with the voltage amplitude. The pulsewidth and repetition rate have little influence on discharge current.

Leach, Christopher
Prasad, Sarita
Fuks, Mikhail I.
Schamiloglu, Edl

A compact A6 relativistic magnetron is proposed which operates in the pi-mode and whose radiation is extracted axially as a TE11 mode through a cylindrical waveguide with the same cross section as that of the anode block. This radiated mode is similar to a Gaussian microwave beam. The advantages of this magnetron include the minimal volume of the applied magnetic field and, as a consequence, the proximity of the electron dump to the anode block for the electrons leaking from the interaction space that minimizes both the diameter and the axial length of the magnetron. By using MAGIC particle-in-cell (PIC) simulations, we demonstrate the possibility of generating a Gaussian radiation pattern with power of about 0.5 GW when the applied voltage is 350 kV. This compact magnetron is easier to implement than the magnetron with diffraction output (MDO), although with reduced efficiency.

Soh, Shawn
Miller, R. Bruce
Schamiloglu, Edl
Christodoulou, Christos G.

We propose exciting two modes concurrently inside a reltron's modulating cavity to increase the microwave power generated. From the harmonic components of the beam, we demonstrate that a multiharmonic beam has a higher peak and steeper gradient compared to a single-harmonic beam. We design a modulating cavity and an extraction cavity that are able to excite the TM010 mode (2.395 GHz) and TM110 (3.579 GHz) modes separately or concurrently. Particle-in-cell simulations show that the total microwave generated for the single TM010 mode is 81 MW (efficiency of 23%) and the single TM110 mode is 40.5 MW (efficiency of 17%). The total microwave power generated by exciting both modes is 128 MW (efficiency of 35%) and is higher than the power generated by individual modes.

Kumar, Prashanth
Watts, Christopher
Svimonishvili, Tengiz
Gilmore, Mark
Schamiloglu, Edl

In this paper, total incident electron dose as an inherent parameter in secondary electron emission is experimentally demonstrated. A completely automated experimental setup allows for measuring of secondary electron yield (SEY) as a function of beam energy, angle of incidence of primary electrons, electron dose, and time. SEY data are presented for copper, plasma-sprayed boron carbide, and titanium nitride samples with principal focus on dose dependence. Experiments were conducted in the low-energy range (5-1000 eV) and direct-current regime. Experimental results have been compared with formulas in literature, and good agreement was observed. Modified empirical formulas incorporating the dose effect have also been proposed.

Joler, Miroslav
Christodoulou, Christos G.
Schamiloglu, Edl

For the applications of compact pulsed-power systems, we propose a model for optimizing the pulse-forming line (PFL) with respect to the optimum pulse generation. There are multiple parameters that play important and conflicting roles in the design of a PFL. The original notion that a pulse, of a desired duration, can be created and the length of the line shortened by using a dielectric with sufficiently high permittivity, is shown here to be overly simplified, and that it can lead to a deformed pulse instead. In this work, new criteria for successful pulse formation are proposed and their validity demonstrated. (C) 2008 Wiley Periodicals, Inc.

Fuks, Mikhail I.
Goykhman, Mikhail B.
Kovalev, Nikolay F.
Palitsin, Alexey V.
Schamiloglu, Edl

The interaction of the fast cyclotron wave of a linear electron beam guided by a magnetic field with a counter-propagating electromagnetic wave leads to the appearance of a stop-band over a wide range of the magnetic field near its resonance. Suppression of generation in backward wave oscillators and the resonant-cyclotron method of mode selection are based on this effect, termed "cyclotron absorption." In this paper, we analyze the possibility of controlling regenerative amplification in such oscillators as a traveling wave tube (TWT) by suppressing parasitic feedback using the guide magnetic field in the region of cyclotron absorption (RCA). Approaching the magnetic field to values that border this region, which contain thresholds of self-excitation, leads to a decrease in suppression of the reflected wave that is responsible for the positive feedback; in other words, this leads to an increase in gain. We illustrate amplification in the RCA using two approaches: the theory of stationary operation of a TWT accounting for the cyclotron resonance of electrons with a reflected wave, and particle-in-cell computer simulations.

Nichols, Kimberley
Carlsten, Bruce
Schamiloglu, Edl

Analysis of quadrupole focusing lattices for high-frequency traveling-wave tubes is presented. This paper is motivated by recent work performed at the Naval Research Laboratory which demonstrated an advantageous case for strong focusing employing a Halbach quadrupole lattice. Using realistic permanent magnet quadrupole (PMQ) field cancellation, the advantage of using PMQ to transport higher current densities than a permanent periodic magnet disappears, while other advantages for employing quadrupole focusing remain.

Devarapalli, Naga R.
Baum, Carl E.
Christodoulou, Christos G.
Schamiloglu, Edl

A design approach to obtain the function defining the bend that minimizes the reflected power into the feed waveguide of a structure, called the H-plane bend radiator (HPB radiator) is presented. The HPB radiator is used as a part of a high-power antenna and therefore there is a requirement for a smooth bend containing no sharp corners. As the smoothness requirement on the H-plane bend function increases, the possible number of functions to be considered in search of a solution increases; this increases the complexity of the problem. It is hypothesized that the general design philosophies in the approach can be adopted as a basis for designing other structures with bends similar to the H-plane bend in the HPB radiator. (C) 2010 Wiley Periodicals. Inc Int J RF and Microwave CAE 20 567-576, 2010

Yurt, Sabahattin C.
Fuks, Mikhail I.
Prasad, Sarita
Schamiloglu, Edl

We describe a new O-type high power microwave oscillator that uses a metamaterial slow wave structure (MSWS) supporting waves with negative dispersion. The MSWS comprises periodically alternating, oppositely oriented split ring resonators (SRRs) connected to a metal tube where the distance between the rings is much less than a wavelength of the radiation generated. The SRRs provide negative permeability mu. The diameter of the metal tube is such that the generated oscillations are below cutoff for a regular waveguide with the same dimension, thus providing negative permittivity epsilon. A tubular electron beam propagates coaxially through this structure. The interaction space is coupled with the outer coaxial channel through gaps between the SRRs. Radiation is extracted in an endfire manner at the end of the outer channel via a conical horn section. Using particle- in-cell (PIC) simulations, it was found that the electron beam in the interaction space forms a sequence of trapped electron bunches by the synchronous operating wave. The output parameters of this oscillator for an applied voltage U =3D 400 kV, electron beam current I =3D 4.5 kA, and guide axial magnetic field B =3D 2 T are radiation power P =3D 260MW, radiation frequency f =3D 1.4 GHz, and electronic efficiency eta =3D 15% when the total SWS length L consisting of 12 split rings is 34.5 cm. The output radiation pattern corresponds to a TE21-like hybrid mode. This article presents details on the simulations of this novel structure and computational and experimental cold tests of a prototype structure in preparation for experimental hot tests. Published by AIP Publishing.

Silva Neto, Lauro P.
Rossi, Jose O.
Barroso, Joaquim J.
Schamiloglu, Edl

In recent years, nonlinear transmission lines (NLTLs) have been investigated for high-power radio frequency generation. High-power waves produced using NLTLs can be applied in mobile defense platforms and satellite communications as they can reach tens of MW of peak power. Their principle of operations is based on the nonlinearity of inductive and/or capacitive lumped elements used in such lines. In this paper, an NLTL using commercial ceramic capacitors as nonlinear elements and linear inductors was simulated and built. Experimental and simulated results show good agreement, validating the model of ceramic capacitors. The frequency obtained from soliton generation at the output of the line was about 4 MHz with peak power of the order of 8 kW.