Joaquim J. Barroso

Retrieval of Permittivity and Permeability of Homogeneous Materials from Scattering Parameters

On the basis of measured scattering parameters, we present a general assessment of the main problems with the Nicholson-Ross-Weir retrieval procedure: First, the inherent instability of the method for low-loss materials at frequencies corresponding to integer multiples of the transmitted wavelength in the sample; second, the multivalued solutions for the complex wavenumber when the electrical length of the sample exceeds a wavelenghth. It is shown that the presence of small perturbation or noise on the transmission coefficient T at around |T| ≈ 1 suffices to trigger the instability when retrieving the impedance of the sample. Unlike the ill-conditioned expression of the impedance, the product με (refractive index squared) is stable to perturbation in T. For nonmagnetic materials (relative permeability μr = 1), therefore, the product με reduces to the complex permittivity, which is then correctly retrieved without divergent ripples as shown by the extracted permittivity spectra in the X-band (8.2–12.4 GHz) for a slab sample of Teflon of arbitrary thickness.

Two dimensional computer simulation of plasma immersion ion implantation

Department of Physics and Chemistry, Engineering Faculty of Guaratinguet´a FEG, UNESPAv. Ariberto Perreira da Cunha 333, Guaratinguet´a, SP, BrasilReceived on 30 January, 2004; revised version received on 6 May, 2004The biggest advantage of plasma immersion ion implantation (PIII) is the capability of treating objects withirregular geometry without complex manipulation of the target holder. The effectiveness of this approach relieson the uniformity of the incident ion dose. Unfortunately, perfect dose uniformity is usually difficult to achievewhen treating samples of complex shape. The problems arise from the non-uniform plasma density and ex-pansion of plasma sheath. A particle-in-cell computer simulation is used to study the time-dependent evolutionof the plasma sheath surrounding two-dimensional objects during process of plasma immersion ion implanta-tion. Before starting the implantation phase, steady-state nitrogen plasma is established inside the simulationvolume by using ionization of gas precursor with primary electrons. The plasma self-consistently evolves toa non-uniform density distribution, which is used as initial density distribution for the implantation phase. Asa result, we can obtain a more realistic description of the plasma sheath expansion and dynamics. Ion currentdensity on the target, average impact energy, and trajectories of the implanted ions were calculated for threegeometrical shapes. Large deviations from the uniform dose distribution have been observed for targets withirregular shapes. In addition, effect of secondary electron emission has been included in our simulation and noqualitative modifications to the sheath dynamics have been noticed. However, the energetic secondary electronschange drastically the plasma net balance and also pose significant X-ray hazard. Finally, an axial magneticfield has been added to the calculations and the possibility for magnetic insulation of secondary electrons hasbeen proven.

Surface improvements of industrial components treated by plasma immersion ion implantation (PIII): results and prospects

Abstract The major drive for PIII research in recent years has been the widespread use of plasma-based ion implantation in industries aiming at attaining high value-added components. After achieving the domain of the complete basic PIII cycle, we started to pursue the implementation of this process in various types of industrial components. A DC glow discharge source with a controlled plasma floating potential was used in a 100-l PIII system driven by a 30-kV peak voltage, 50 μs duration, up to 1.1 kHz pulse power source, in order to process the components which were provided by regional companies, spanning from machinery tools to prosthesis. The industrial components were set-up in the PIII chamber as received from the companies, after a simple cleaning procedure. In this phase, only nitrogen implantation was performed. The required processing times were typically from 60 to 120 min and the components were treated either individually or in batches. Fast steel drill bits, knife blades for wood cutting, tools for odontological applications, molds made of fast steel, a prosthesis made of Ti alloy, etc., have been three-dimensionally implanted successfully. Next, improvements in the PIII ongoing system included: a 10-kW pulser with up to 60 kV capability, turbo-pump, refrigerated walls, auxiliary heating of the components, a larger chamber and a magnetron sputtering source for hybrid treatments.

Design facts in the axial monotron

Upon reexamining the electron beam radio frequency (RF)-field interaction of the monotron, which is the simplest transit-time microwave tube, it is found that a 20% maximum conversion efficiency can be attained at weakly relativistic beam energies (/spl sim/200 keV). It is shown that the conversion efficiency can be cast as a function of three parameters, namely, injection beam energy, resonant frequency, and electric field strength. From this fact, a design procedure of how the optimum operating parameters should be selected is presented. In support of the analytic study, 2.5-D particle-in-cell simulation of a transverse magnetic (TM)/sub 020/, 6.7 GHz axial monotron operating at 10 keV, 70 A beam parameters has given a tube efficiency of 15.4%, while one-dimensional (1-D) analysis assuming a strictly monoenergetic beam has predicted a maximum theoretical efficiency of 18.5% at a beam energy of 10 keV.

Gyrotron coaxial cylindrical resonators with corrugated inner conductor: Theory and experiment

Gyrotron coaxial resonators with a longitudinally slotted inner cylinder are examined analytically using a surface impedance model, from which expressions for the electromagnetic field, ohmic quality (Q) factor, and characteristic equation of the transverse eigenvalues /spl chi//sub m,p/ are obtained. The major attributes of such resonators are expressed by the dependence of /spl chi//sub m,p/ on the parameter C-defined as the ratio of the outer to inner radii of the coaxial structure. In that connection, the effect of the corrugation parameters on /spl chi//sub m,p/ is particularly investigated on the basis of an expression derived for the slope function d/spl chi//sub m,p,p//dC. It is shown that the /spl chi//sub m,p/(C) curve may either exhibit oscillatory behavior or present a flat portion over a wide range of C depending on the corrugation parameters chosen. The theory is checked against experiment in which resonant frequencies and total Q factors were measured for TE modes operating in the range of 8-16 GHz in a coaxial cavity with 40 slots. Good agreement is found in that the magnitude of the relative error in frequency is less than 0.5%. Corrugated coaxial resonators prove to be relevant to megawatt gyrotrons where highly selective cavities are required to ensure high conversion efficiency.

Retrieval Approach for Determination of Forward and Backward Wave Impedances of Bianisotropic Metamaterials

A simple approach is proposed for retrieving the forward and backward wave impedances of lossless and lossy bianisotropic metamaterials. Compared with other methods in the literature, its main advantage is that forward and backward wave impedances can be uniquely and noniteratively extracted. It has been validated for both lossless and lossy bianisotropic metamaterials by performing a numerical analysis. The proposed approach can be applied for checking whether the metamaterial structure shows the bianisotropic property by monitoring forward and backward wave impedances, since the forward and backward wave impedances of a metamaterial structure depend on difierent polarizations of the incident wave.

Design of coaxial Bragg reflectors

A coaxial line periodically loaded by circular disks on the inner conductor is specifically designed to act as Bragg reflector to prevent radio-frequency fields from leaking out of a 6.6-GHz monotron. Design criteria to ensure a stopband as wide as possible are presented and a discussion is given on how band gap width, center frequency, and reflectivity arise from the geometry of the periodic structure

Examining by the Rayleigh-Fourier method the cylindrical waveguide with axially rippled wall

Axially corrugated cylindrical waveguides with wall radius described by R/sub 0/(1+/spl epsi/cos2/spl pi/z/L), where R/sub 0/ is the average radius of the periodically rippled wall with period L and amplitude /spl epsi/, have been largely used as slow-wave structures in high-power microwave generators operating in axisymmetric transverse magnetic (TM) modes. On the basis of a wave formulation whereby the TM eigenmodes are represented by a Fourier-Bessel expansion of space harmonics, this paper investigates the electrodynamic properties of such structures by deriving a dispersion equation through which the relationship between eigenfrequencies and corrugation geometry is explored. Accordingly, it is found that for L/R/sub 0//spl ges/1 a stopband always exists at any value of /spl epsi/; the condition L/R/sub 0/=1 gives the widest first stopband with the band narrowing as the ratio L/R/sub 0/ increases. For L/R/sub 0/=0.5 the stopband sharply reduces and becomes vanishingly small when /spl epsi/<0.10. Illustrative example of such properties is given on considering a corrugated structure with L/R/sub 0/=1,R/sub 0/=2.2 cm, and /spl epsi/=0.1, which yields a stopband of 1.5-GHz width with the central frequency at 8.4 GHz; it is shown that in a ten-period corrugated guide, the attenuation coefficient reaches 165 dB/m, which makes such structures useful as an RF filter or a Bragg reflector. It is also discussed that by varying L/R/sub 0/ and /spl epsi/ we can find a variety of mode patterns that arise from the combination of surface and volume modes; this fact can be used for obtaining a particular electromagnetic field configuration to favor energy extraction from a resonant cavity.

Stepwise technique for accurate and unique retrieval of electromagnetic properties of bianisotropic metamaterials

Metamaterials (MMs) are artificial materials that have received attention recently because their built-in features create collective electromagnetic effects that are otherwise impossible, such as negative refraction, and because of their exotic electromagnetic applications, namely, perfect lens and invisibility cloaks. Depending on wave propagation characteristics, MMs possessing normally weak magneto-electric coupling coefficients start to exhibit stronger bianisotropic effects. Therefore, accurate electromagnetic characterization of these MMs is important. In this study, we adapt a stepwise method based on the Nicolson–Ross–Weir technique for accurate and unique retrieval of electromagnetic properties of bianisotropic MM slabs. For this goal, we have derived explicit expressions for unique retrieval of electromagnetic properties of these slabs and compared these expressions with those in the literature in the retrieval process. From the comparison, we note that derived expressions are appropriate for unique determination of electromagnetic properties of bianisotropic MM slabs. In the performance analysis of the stepwise method for different measurement scenarios, we considered different bianisotropic MM cell configurations (split-ring and Omega-shaped resonators as well as the same resonators with wire strips) and extracted their electromagnetic properties when measured/simulated scattering parameters have some thermal noise. We note that for most of the frequencies, the stepwise method retrieves correct electromagnetic properties even when a relatively higher normally distributed noise with zero mean value and with standard deviations of 0.015 is present. In addition to the influence of thermal noise on performance of the stepwise method, we also analyzed the effect of both increasing length slab and the frequency band on retrieved electromagnetic properties of the analyzed various bianisotropic MM slabs.

Plasma immersion ion implantation experiments at the Instituto Nacional de Pesquisas Espaciais (INPE), Brazil

Abstract Historical perspective of the development of PIII devices at the Instituto Nacional de Pesquisas Espaciais (INPE) is given, together with the description of the present system under operation and our overall results on this three-dimensional implantation research. Starting with an ignitron switched pulser (1 pulse per 3 min) and an intermittent microwave plasma, we improved our PIII system developing a pulse forming network (PFN) based pulser (20 Hz), 2 years later. We also improved our plasma source towards a DC, highly stable, medium density glow discharge system. A much faster hard tube pulser was recently incorporated to our PIII system (670 Hz) allowing us to achieve good implantation results in different materials. Presently, we are testing a recently purchased RUP-4 commercial pulser to obtain arc prevented, 1.1 kHz, square pulses for new experiments in this active field of PIII research.