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Abstract: This article discusses the operation of a modular generator topology, which has been developed for high-frequency (kHz), high-voltage (kV) pulsed applications. The proposed generator uses individual modules, each one consisting of a pulse circuit based on a modified forward converter, which takes advantage of the required low duty cycle to operate with a low voltage clamp reset circuit for the step-up transformer. This reduces the maximum voltage on the semiconductor devices of both primary and secondary transformer sides. The secondary winding of each step-up transformer is series connected, delivering a fraction of the total voltage. Each individual pulsed module is supplied via an isolation transformer. The assembled modular laboratorial prototype, with three 5 kV modules, 800 V semiconductor switches, and 1:10 step-up transformers, has 80% efficiency, and is capable of delivering, into resistive loads, �15 kV/1 A pulses with 5 micros width, 10 kHz repetition rate, with less than 1 micros pulse rise time. Experimental results for resistive loads are presented and discussed.

Abstract: Nowadays, high-voltage pulsed power based technologies are rapidly emerging as a key to efficient and flexible use of electrical power for many industrial applications. One of the most important elements in high-voltage pulse generating circuit technology is the transformer, generally used to further increase the pulse output voltage level. However, its non-ideal behavior has significant influence in the output pulse shape. In spite of all winding and switching techniques, the most attractive winding configuration for high-voltage, the core type transformer with primary and secondary on different core legs, is seldom used in pulsed applications, due to the weak magnetic coupling between windings, which would result is a slow rising output voltage pulse. This papers show that auxiliary windings, suitably positioned and connected, provide a dramatic improvement in the pulse rise time in core type high-voltage pulse transformers. A mathematical model is derived and used to describe the observed behavior of the transformer with auxiliary windings. The model is discussed regarding the experimental results, obtained from a high-voltage test transformer associated with a high-voltage pulse generating circuit, and the simulation results obtained from the numerical evaluation of the developed differential equations implemented in Matlab considering the measured transformer parameters.

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Abstract: modular concept on high-voltage pulse generators, under development for future use in facilities for plasma immersion ion implantation, is presented. The generator proposed uses individual modules, each one consisting of a pulse circuit based on a step-up transformer; the secondary of each step-up transformer is connected in series. Each step-up transformer delivers a fraction of the total voltage with primary voltage supplied via an isolation transformer. With this topology we expect to achieve tens of kiloVolts with low voltage semiconductor switches (<1 kV). A three 5 kV-module initial prototype was assembled with 800 V semiconductor switches and experimentally tested for an output of 11 kV, 5 μs pulse width and 10 kHz-pulse frequency. Different load conditions and results are presented and discussed.

Abstract: Today high-voltage pulses are reaching more fields of application. High-voltage pulse transformers are often used in association with high-voltage pulse generating circuits to further increase the pulse output voltage level. However, because of the transformer parasitic elements involved, the transformer is the critical device in shaping the rising characteristics of the output pulse. One of the techniques usually adopted to decrease the leakage inductance of the transformer adds two auxiliary windings to the transformer. If properly used, these auxiliary windings reduce the leakage flux and, therefore, the leakage inductance. As a result the pulse rise time is reduced. In this paper, a mathematical model is used to describe the observed behavior of a transformer operating with auxiliary windings, based on the theory of electromagnetic coupled circuits. The model is discussed regarding the experimental results obtained from a high-voltage test transformer associated with a high-voltage pulse generating circuit, and the simulation results obtained from the numerical evaluation of the developed differential equations implemented in Matlab/Simulink with the measured transformer parameters

Abstract: A new method to obtain high voltage (kV) pulses suitable for a plasma immersion ion implantation (PIII) facility is presented. The circuit proposed is based on a step-up transformer with a constant flux reset clamp circuit that takes advantage of the low duty ratio required to reduce the voltage stress on all semiconductor switches. An initial prototype was assembled with 800-V semiconductor switches for an output pulse of �5 kV, 5-μs pulse width and 10-kHz pulse frequency. Theoretical and experimental results are presented and discussed.

Abstract: Ion implantation of phosphorus was used to dope amorphous and microcrystalline silicon with the aim of achieving a low-temperature, self-aligned process for forming n1 contacts to top-gate thin-film transistors. Amorphous and microcrystalline films made with both RF glow discharge and hot-wire chemical vapor deposition were implanted. The effect of the dose, energy and implantation temperature and subsequent annealing at increasing temperatures on the dark conductivity, activation energy and photoconductivity were studied. Lowering the energy (15 keV) while increasing the dose (1015 cm2) and the implantation temperature (300ºC) resulted in the highest after anneal (300ºC) dark conductivity for both hot-wire (0.3 Ohm-1cm-1) and RF (0.2 ohm-1cm-1) microcrystalline films.

Abstract: We report on the magnetic and magnetoresistive (MR) properties of granular materials obtained by implantation of high fluences (from 1 � 1016 to 1.8 � 1017 ions/cm2) of Fe and Co ions into Cu and Ag thin films. The local microstructure vs. implantation doses and posterior thermal treatment is discussed to obtain MR values of practical relevance. MR measurements up to very high fields (32 T) are presented, supporting recent theoretical work on the role of short-range magnetic correlations in MR for granular materials.

Abstract: The magnetoresistive behavior of granular thin films prepared by Fe and Co implantation in Ag thin films is reported. Ag thin films (~ 2000 �) were deposited by evaporation or laser ablation onto Si and SiO2 substrates and implanted with Fe or Co at fluences up to 1017 at. %/cm2. The magnetoresistive response obtained after implantation was found to increase with the implanted fluence. An increase of the magnetoresistive response by a factor of 3�4 can be achieved after annealing the films in a conventional furnace at 620 K under vacuum. The best value of magnetoresistance obtained so far is 9% at 10 K and 1.5% at room temperature for a film implanted with Co at a fluence of 8 � 1016 at. %/cm2 and annealed at 620 K.

Abstract: The evidence of the modification of the surface structure of the AISI-304 stainless steel during the implantation of lanthanum makes the analysis of the sputtered elements very interesting. Those sputtered elements are deposited on a carbon sheet placed in front of the steel being implanted, and studied by means of RBS and PIXE, together with the implanted specimens. Besides, the protective effect of the implanted ions during the high temperature oxidation is also studied by those techniques together with XRD and thermogravimetric methods.

Abstract: The magnetoresistive behaviour of granular thin films prepared by 56Fe and 57Fe ion implantation into Ag thin films with fluences up to 8 � 1016 at./cm2 is reported. The implantation produced both small and large Fe clusters, with the large clusters being dominant for high fluences. A significant magnetoresistive response of the films was obtained for fluences above 6 � 1016 Fe/cm2, reaching values of 1�2% at 10 K and low fields.

Abstract: The implantation damage and lattice site location of W in TiO2 (rutile) was studied using the Rutherford backscattering technique in the channeling mode (RBS-C). The W ions were implanted at room temperature with fluences in the range of 1015 to 1017/cm2 into both 1 0 0 and 0 0 1 oriented single crystals. The implanted region becomes completely disordered for W doses higher than 1016/cm2. After annealing experiments at temperatures up to 1100 K the results suggest that lattice recovery depends on the type of TiO2 single crystal used. While in 0 0 1 oriented single crystals partial epitaxial solid phase regrowth of the damaged region is seen, for 1 0 0 oriented single crystals a recrystallization process occurs almost to completion. During damage recovery the high dose samples lose more than 80% of the W from the implanted region. Detailed angular scans for the main axial directions show that in the case of full recovery about 82% of the W remaining in the implanted region are incorporated on Ti lattice sites. These observations suggest that there is a strong anisotropy of the lattice recovery and diffusion properties in W implanted TiO2.

Abstract: Diluted granular films of Cu---Fe and Ag---Fe (iron content 2%) were produced using 57Fe ion implantation on Cu(Ag) films previously grown by laser ablation. Conversion electron Mössbauer spectroscopy shows that the implanted Fe forms either very small clusters (up to a few atoms) or large iron α-phase particles. These structural characteristics directly reflect on the magnetization, which exhibits ferromagnetic-like behaviour at room temperature (due to large clusters) superimposed by a significant paramagnetic contribution at low temperatures due to the small clusters. We observe deviations from strict superparamagnetic behaviour due to non-negligible local anisotropy effects at low temperatures and low fields. The Kondo effect is particularly enhanced in the Cu---Fe films which have higher concentration of isolated Fe atoms and small sizes clusters. The magnetoresistivity �/ of our films is dominated (for 0 μ0 H 15 T) by a linear term in H, attributed to GMR-like effect from spin-dependent scattering when an electron passes between adjacent large and small clusters. At low fields we observe instead �/ � H2, due to the usual GMR effect between large clusters, during the alignment of their easy axes. The relevant physical differences (structural, magnetic and magnetoresistive) observed in our ion-implanted diluted Fe films, with respect to the concentrated granular films, are critically analysed.

Abstract: The operation of a modular generator topology, developed for kHz and kV pulsed applications, is presented. The proposed generator uses individual modules each one consisting of a pulse circuit based on a modified forward converter topology, with the secondary windings series connected, delivering a fraction of the total voltage. A laboratorial prototype with three 5 kV modules, 800 V semiconductor switches and 1:10 step-up transformers has been assembled. The first experimental results of this modular generator are presented and discussed. The circuit has 80 % efficiency and is capable of delivering, into resistive loads, -15 kV / 1 A pulses with 5 µs width, 10 kHz frequency, with less than 1 µs pulse rise time.

Abstract: In this paper the circuit topology of a hybrid fully integrated solid-state Marx generator circuit, developed for kHz and kV applications, is presented and analysed. The proposed circuit uses only power semiconductor switches to increase the performance, being designed with a magnetizing energy reset circuit that enables the use of an output pulse transformer, which recovers the transformer magnetizing energy during the off switches state, back to the energy storage capacitors. This enables higher frequency operation, increasing the pulse generator yield. A 10 kHz pulse rate laboratory prototype with five stages was built using 1200 V IGBTs and diodes. Experimental results show almost rectangular pulses with -5 kV, 4 to 10 �s width, into a 5 k� resistive load.

Abstract: This paper reports the development of isolated autonomous capacitive power supplies to charge each drive circuit of the high voltage floating semiconductor stage in a solid-state Marx Generator. The circuit takes advantage of an auxiliary capacitor, charged in series with the main energy storage capacitor in each Marx generator stage, to supply the optic-fibre isolated gate drive circuit of the solid state switches. Laplace domain circuit analysis is made to determine the values of these capacitors and to obtain the desired voltages, with low ripple, for the required load. A laboratory prototype with three stages, 3 kW peak power, of this all silicon Marx generator circuit, with opticfibre isolated triggering signals and autonomous capacitor isolated charging power supplies was built using 1200 V IGBTs and diodes, operating with 1000 V d-c input voltage and 10 kHz frequency, giving 3 kV and 10 µs pulse with, approximately, 15 V isolated autonomous power supplies in each stage.

Abstract: A newly developed Marx type circuit topology that achieves both output-voltage multiplication and energy recovery, which has been developed for inductive load applications, namely Electromagnetic metal Forming (EMF), based on an all-solid-state Marx type generator, is described. The proposed circuit takes advantage on the power semiconductor switches intensive use, replacing the conventional Marx Bank passive elements, to increase the performance, strongly reducing losses and increasing the pulse repetition frequency. Additionally, the generator is designed with an energy reset circuit that enables the use of inductive loads, recovering the inductive energy during the semiconductor switches off-state, back to the energy storage capacitors. Preliminary results from this EMF modulator prototype are presented and discussed.

Abstract: This paper discusses the operation of a fully integrated solid-state Marx generator circuit, which has been developed for high-frequency (kHz), high-voltage (kV) applications needing rectangular pulses. The conventional Marx generator, used for high-voltage pulsed applications, uses inductors, or resistors, to supply the charging capacitors voltage, which has the disadvantages of size, power loss and frequency limitation. The proposed circuit takes advantage of the intensive use of power semiconductor switches, replacing the passive elements in the conventional circuit, to increase the performance of the classical circuit, strongly reducing losses and increasing the pulse repetition frequency. Also, the proposed topology enables the use of typical half-bridge semiconductor structures while ensuring that the maximum voltage blocked by the semiconductors is the voltage of each capacitor (i.e. the power supply voltage), even when the switching is not synchronized, and in fault conditions. A laboratory prototype with five stages, 5 kW peak power, of this all silicon Marx generator circuit, was constructed using 1200 V IGBTs and diodes, operating with 1000 V d-c input voltage and 10 kHz frequency, giving 5 kV pulses, with 10 micros width and 50 ns rise time.

Abstract: This paper discusses the operation of an all silicon-based solution for the conventional Marx generator circuit, which has been developed for high-frequency (kHz), high-voltage (kV) applications needing rectangular pulses. The conventional Marx generator, for high-voltage pulsed applications, uses passive power components (inductors or resistors), to supply the energy storage capacitors. This solution has the disadvantages of cost, size, power losses and limited frequency operation. In the proposed circuit, the bulky passive power elements are replaced by power semiconductor switches, increasing the performance of the classical circuit, strongly reducing costs, losses and increasing the pulse repetition frequency. Also, the proposed topology enables the use of typical half-bridge semiconductor structures, and ensures that the maximum voltage blocked by the semiconductors equals the power supply voltage (i.e. the voltage of each capacitor), even with mismatches in the synchronized switching, and in fault conditions. A laboratory prototype with five stages, 5 kW peak power, of the proposed silicon-based Marx generator circuit, was constructed using 1200 V IGBTs and diodes, operating with 1000 V d-c input voltage and 10 kHz frequency, giving 5 kV / 1 A pulses, with 10 micros width and 50 ns rise time.

Abstract: This paper introduces a new circuit to obtain high voltage (kV) pulsed power supplies suitable for plasma ion implantation. Using a step-up transformer with a leakage flux reduction winding, and taking advantage of the low duty ratio required, 800 V semiconductor switches can be used to obtain 5 kV 10 kHz pulses. Theoretical and experimental results are presented.

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Abstract: This work proposes power electronic converter topologies and transformer configurations to achieve almost rectangular high voltage (several kV) pulses. Both the power converters and transformer configuration allow the definition of methods to obtain High Voltage Pulsed Generators (HVPG) for Plasma Immersion Ion Implantation (PIII) applications, using low voltage (<1kV), non-series connected semiconductor power devices (SPD). The potential of SPD associations in �Marx generator� like circuits are presented. The central HVPG was developed, based on a modified direct current converter, which takes advantage of the low duty cycle operation and the low voltage reset circuit of the step-up transformer, to reduce the maximum voltage on the SPD. Based on this central HVPG, a modular concept allows the assembly of a HVPG capable of producing adequate pulses for PIII process. High voltage pulse transformers using auxiliary windings for leakage flux compensation are designed, analysed and projected. Built transformers present shorter rise times, compared to transformers without auxiliary windings (reductions from 20% to 98%). The assembled modular laboratorial prototype has 80 % efficiency, and is capable of delivering, into resistive loads, -15 kV / 1 A pulses with 5 micros width, 10 kHz repetition rate, with less than 1 micros pulse rise time.

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Abstract: The On-Line Isotope Mass Separator ISOLDE is a facility dedicated to the production of a large variety of radioactive ion beams for many different experiments in the fields of nuclear and atomic physics, solid-state physics, materials science and life sciences. The facility is located at the Proton-Synchrotron Booster (PSB) of the European Organisation for Nuclear Research CERN. The ion source of ISOLDE is connected to a thick target, floating at 60 kV, which is periodically bombarded by a 1.4GeV high intensity proton beam (up to 2 µA). The target is heated and the ions are released into the ion source at thermal velocities. The ions are then accelerated by a grounded extraction electrode to 60 keV, before transport to the experimental area. The target and ion source must be held at a precise voltage with respect to a grounded extraction electrode to provide the requisite acceleration. If there is to be high mass resolution in the downstream separator it is extremely important that there be exceptional stability of this accelerating voltage. The target, ion source and extraction electrode are in vacuum and the vacuum tank of the target is raised to full accelerating potential. The impacting proton beam intensively ionises the air. This ionisation can perturb the accelerating voltage because it represents a significant additional load on the power supply. Hence, during the critical period when protons strike the target the accelerating voltage is modulated to zero. This is acceptable provided that the stable accelerating voltage of 60 kV +/- 1 V is interrupted for less than 10 ms, so still allowing the detection of very short life-time radio isotopes. Beam gating is required to prevent ion beams entering the separator for the entire period that the accelerating voltage is modulated from its stable value. At present a custom-built positive 60kV d.c. power supply is connected, via a pulse transformer and a hard-tube switch (tetrode), to a resonant circuit such that prior to beam impact the target is fully discharged by the resonant circuit in 35µs, which then restores the voltage close to its nominal value within a further 200µs. This circuit topology is proven to satisfy the voltage recovery time and stability constraints. However, recent physics requests for negative ion beams have required the installation of a commercial negative 60kV d.c. power supply. Experience during two two-week negative ion physics runs at ISOLDE have shown that whilst the d.c. stability of the negative power supply is within specification, the recovery time to the required stability level is of the order of 50ms, well outside the maximum 10ms required. Also, the actual electric equivalent model of the target is not accurate and doesn�t allow for precise previous circuit simulations. In addition, recent use of new positive ion sources using so-called converter targets have substantially increased the resistive loading seen by the positive power supply immediately following proton beam impact. An observed consequence is that with maximum beam intensity and certain types of converter targets the recovery time exceeds the maximum required. Increased use of converter targets is envisaged in the future which will inevitably lead to degradation of the target modulators performance. This project aims to develop a new type of target modulators supported on the progress of the solid-state based pulse modulators and on new topologies brought from power electronics to overcome the high voltage limitations of semiconductor devices. The use of solid-state technology for pulse modulators instead of hard-switching devices is, nowadays, imposed by the demand for efficient and flexible pulsed power circuits with optimization of all components. Particularly, we intend to use the mature Marx generator concept but in all semiconductors based modulators, and in this way we hope to overcome the problems in obtaining a stable negative target voltage. In addition, we intend to develop an accurate electric equivalent circuit for the target that enables precise simulation for modulator performer optimization. In the future the objective is to expand the concept to both negative and positive voltage modulators applied in the new developed ion sources. Furthermore, this project aims to definitively contribute to the growth of both pulsed power technology research oriented applications and advanced nuclear physics applications in Portugal contributing to the education and training of many young students and young researchers in this interdisciplinary field through this international collaboration with the CERN organization, given the conditions to establish a self supported infrastructure for technology transfer for real in-situ applications.

Abstract: To be initiated in 1-Nov-2007

Abstract: Regulations on the allowable emissions of toxic gases have resulted in increasing interest in the development of energy efficient methods for remediation. The typical concentration range of concern in air-pollution problems is of the order of several tenths to several hundredths ppmv. For this range of concentration, both temperature activated catalysts and plasma based methods have been used independently. In particular, methods using a nonthermal plasma (NTP) have proved to be compact, efficient, requiring low maintenance and with low cost. However both technologies have shown some disadvantages: (i) it is not possible to remove completely some volatile organic compounds (VOCs) exclusively by catalysts, and it requires a relatively high temperature (400-600 ºC); (ii) NTPs generated at atmospheric pressure have low selectivity towards total oxidation. Moreover, the complex chemistry taking place in the discharge plasma can result in (other) toxic byproducts or in the formation of aerosols. One of the options that have been gaining attention to overcome these problems is to combine plasma with a catalyst to promote oxidative decomposition of VOCs via the activation of the catalyst by the species formed in the plasma, allowing a significant reduction of the catalyst working temperature. The main objective of this project is to compare the efficiency of new catalysts for the decomposition of different classes of VOCs (alcohols, aromatic and chlorinated compounds) in NTP � catalyst reactors. The project involves (i) the synthesis and characterization of new catalysts; (ii) the experimental study of NTP � catalyst reactors to evaluate the decomposition efficiency as a function of type of reactor, discharge conditions and catalyst used. Two types of NTP � catalyst reactors will be studied: (i) a plasma-enhanced catalytic reactor (PEC), where the catalyst is placed downstream to the plasma and, (ii) a novel plasma-driven catalytic reactor (PDC) where the catalyst is placed directly in the reactor. The former (PEC) combines the optimum conditions of the NTP and the catalyst, which are usually quite different from each other. In this case the role of the NTP is to partially convert the reactants and to produce ozone which has been shown to enhance the decomposition of VOCs over the catalyst bed. On the other hand, in PDC reactors the catalyst is activated by the NTP in the low-temperature region where thermal catalysis does not normally occur. This activation is attributed to free-radicals and UV photon. In these reactors all gas-phase and surface reactions, as well as their interaction, take place simultaneously. However, the present level of understanding of PDC reactors is still quite limited. This project aims to contribute to a better understanding of this subject. In these reactors, Ag/TiO2, which has been reported to lead to very good results in the decomposition of saturated alkanes, alkenes, aromatics and halogenated molecules like trichloroethylene, will be compared with new catalysts. Among the latter, catalysts based on copper oxide are useful for the total oxidation of CO, hydrocarbons, chlorinated hydrocarbons and alcohols. NOx and SO2 reductions are also catalyzed by copper oxide. Thus, supported copper catalysts have been considered as potential candidates to substitute noble metal-based emission control catalysts. Moreover, recent reports show that the presence of lanthanum stabilizes the copper oxide oxidation activity and enhances their stability. Therefore in this work, we intend to study the behaviour of new lanthanide based heterometallic oxide catalysts Ln- Cu-O (Ln=La, Ce, Sm, Eu, Gd) and the data compared with that obtained from commercially available Ag/TiO2. To prepare the heterometallic oxides, we will employ a new route developed in our group using intermetallic compounds of the type LnCu2 (Ln=La, Ce, Sm. Eu, Gd) as catalytic precursors. The catalytic studies will be carried out on a plug and flow type reactor, continuously, at atmospheric pressure using Gas Chromatography (GC) with tandem flame ionization (FID) and thermal conductivity (TCD) detectors for simultaneous analyses of gases and organic compounds.

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