Pàgines 20-22, 68, 84-86, 185, 204-205, 209-210, 213-241 en blanc, no hi fuguren. / During last four or five decades there has been a growing demand in particle accelerators which can provide electron beams in the energy ranging from 2 MeV to 100 MeV with high energy resolution and good dose control. Other important requirements are that the machines must be compact, of low power consumption, low price and relatively low maintenance cost. There is a variety of sectors interested in such particle accelerators ranging from industry (industrial radiography) to nuclear physics experiments. One type of machines that meet all these requirements are the electron accelerators with beam recirculation. Fair representatives of this class of accelerators are race-track microtrons (RTM). These sources of electron beam are the most efficient equipment for applications with a relatively low beam current and medium energies ranging from 2 MeV to 100 MeV. The aim of the present thesis is to perform studies of some aspects of the RTMs. One part of the thesis is devoted to the design and development of magnetic elements with permanent magnets of two RTMs for different applications. The first one, which is currently under construction at the UPC (Universidad Politécnica de Cataluña), is a novel accelerator with the beam energy variable between 6 MeV and 12 MeV for medical applications (Intraoperative Radiation Therapy treatments). The other machine is a 55 MeV RTM for the detection of explosives by means of photonuclear reactions, which is at the stage of tests at the Skobeltsyn Institute of Nuclear Physics (SINP). The magnetic field in the designed magnets is generated by rare earth permanent magnet (REPM) materials. This allows to get quite compact magnetic systems compatible with high vacuum environment. In the thesis the design and magnetic properties characterization of the magnetic system of these RTMs are carried out. The calculations were performed by means of 2D and 3D simulations using the POISSON, FEMM and ANSYS codes. In the case of the UPC RTM the design of the 180º dipoles, extraction magnets and quadrupole magnet are carried out. For the SINP 55 MeV RTM the optimization of the magnetic field shielding with the aim to reduce the stray magnetic field generated by the extraction magnet is presented. The results of the simulations were confirmed by experimental measurements of the magnetic field of the magnet with the optimized magnetic field shielding. In the other part of the thesis some aspects of the beam dynamics in RTM magnetic systems are studied. A detailed analysis of the fringe - field focusing in RTM dipole magnets is carried out. Equations for calculation of the fringe - field effect on electron beam trajectories are derived and are applied for a study of the end magnets of the UPC 12 MeV RTM. A general formalism for describing the longitudinal beam dynamics in RTMs for electron beams with arbitrary energy and end magnets with arbitrary magnetic field profile is also developed. This formalism is used for the calculation of the phase-slip effect in RTMs with low energy injection.

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Cotutela UPC i Sejong University / Graphene, a two-dimensional sp2-hybridized network of carbon atoms has received a remarkable cornucopia of new physics and served as a unique model system, due particularly to its electronic properties, which could have interesting applications in electronic, spintronic or quantum devices. The first part of the thesis describes the modulation of graphene¿s structural and electrical properties with various kinds of doping; such as deep ultraviolet irradiation in ambient atmosphere, deep ultraviolet light irradiation in different gaseous environments, and electron beam irradiation. We have fabricated graphene (exfoliated and chemical vapor deposition grown graphene) field effect transistors using photolithography and electron beam lithography and characterized with AFM, Raman spectroscopy and transport measurement using low noise standard lock-in amplifier technique. We have explored how the ultraviolet light exposure tunes the electrical properties of graphene in an ambient atmosphere, confirmed by the shift of Dirac point position towards positive gate voltage, revealing p-type doping for graphene without degradation of mobility. We found that the doping is stable for a time scale of months. This method became more useful when half the graphene device was exposed by ultraviolet light, while the other half part was covered by a mask to make a sharp p-n junction. The doping effect became more prominent and controllable when it was made in an oxygen environment. The most interesting phenomena were observed when doped graphene was restored to a pristine state using ultraviolet light irradiation in a nitrogen environment. Furthermore, we have investigated the doping tunability with ultraviolet light irradiation on mechanically exfoliated single-, bi-, and trilayer graphenes without significantly degrading its charge carrier mobility. In a further study, the structural deformation of graphene was investigated by irradiation of an electron beam. The graphene structure changes its phase in various stages, where graphene transforms gradually from a crystalline to a nanocrystalline form and after a certain irradiation time into an amorphous form. This irradiation effect acts as an n-type dopant for graphene. In this case, mobility decreases with the gradual increase of irradiation dose, which implies the formation of localized states. The second part of the thesis describes carbon nanotube networks as flexible and transparent electrodes for electronic devices, particularly for high frequency applications. The observed results show that at low frequencies, the impedance increases as the density of nanotube networks decreases, as expected. Both the real and imaginary parts of impedance (measured up to 20 GHz) abruptly decrease as the frequency increases over the cut-off frequency. The cut-off frequency not only depends on the carbon nanotube density of the network, but also on the sample geometry. The Nyquist diagram suggests a simple equivalent circuit composed of a parallel combination of a resistor and a capacitor. The experimental results are in line with calculations made by electrochemical spectroscopy simulations. The results show that the electrical behavior is mostly determined by the contact resistance between the nanotubes, which are in a completely disordered distribution in the network. We show that carbon nanotube flexible conducting films, which may be transparent, could be competitive for some applications, such as displays, photovoltaic solar cells or selective sensors. / El grafè, considerat com una xarxa bidimensional d’àtoms de carboni units per enllaços híbrids sp2, és un tema de recerca molt prolífer en els últims anys, com a model de sòlid bidimensional, i molt particularment degut a les seves propietats electròniques, que poden tenir aplicacions interessants en dispositius electrònics, spintrònics o quàntics. La primera part de la Tesi descriu la modificació de les propietats estructurals i elèctriques del grafè utilitzant diferents mètodes per a dopar-lo: radiació ultraviolada d’alta energia (DUV) en atmosfera ambient, DUV en diferents gasos tals com oxigen o nitrogen, o irradiant amb un feix d’electrons (e-beam). Hem fabricat transistors d’efecte de camp (FET) amb grafè (exfoliat a partir del grafit, o bé obtingut per deposició química en fase vapor, CVD) utilitzant fotolitografia i e-beam litografia, i els hem caracteritzat mitjançant AFM, espectroscòpia Raman i mesures de transport elèctric, per a les que hem utilitzat la tècnica d’amplificació de baix soroll, el lock-in. Hem investigat com l’exposició a la llum ultraviolada en atmosfera ambient, modula les propietats elèctriques del grafè, de manera que la posició del punt de Dirac es desplaça cap a tensions de porta positives, cosa que implica dopatge de tipus-p, sense que hi hagi degradació de la mobilitat. El dopatge és estable al menys durant mesos. Amb el mateix mètode, quan només la meitat del dispositiu és exposat a la radiació ultraviolada mentre l’altre meitat és recobert per una màscara metàl·lica, hem obtingut una unió p-n. L’efecte de dopatge és més important i controlable, quan és fet en atmosfera d’oxigen. L’efecte més interessant que hem observat és la reversibilitat, quan el grafè dopat retorna al seu estat primitiu, en ser irradiat amb llum ultraviolada en atmosfera de nitrogen. També hem investigat el dopatge amb llum ultraviolada del grafè exfoliat mecànicament, de una, dues o tres capes, observant que es produeix sense una degradació significativa de la mobilitat dels portadors de càrrega. Posteriorment hem estudiat la deformació estructural del grafè quan és irradiat amb un feix d’electrons. Hem observat canvis estructurals en diferents etapes: el grafè evoluciona gradualment, a partir de la forma cristal·lina, cap a una fase d’estructura nanocristal·lina i finalment, després d’una certa dosi de irradiació, presenta una estructura amorfa. L’efecte d’ irradiar el grafè amb electrons actua com a dopant tipus-n, però en aquest cas la mobilitat decreix en incrementar la dosi, això implica que hi ha formació d’estats localitzats. La segona part de la Tesi tracta de capes primes de nanotubs de carboni, com a elèctrodes flexibles i transparents per a dispositius electrònics, en particular per aplicacions d’alta freqüència. Els resultats obtinguts mostren que, a baixes freqüències, la impedància augmenta en disminuir la densitat de nanotubs, tal com cal esperar. Tan la part real com la part imaginària de la impedància (mesurada fins a 20 GHz) decreixen abruptament en augmentar la freqüència més enllà de la freqüència de tall. La freqüència de tall no depèn únicament de la densitat de nanotubs en la capa, sinó també de la geometria de la mostra. El diagrama de Nyquist es pot interpretar amb un circuit equivalent consistent simplement en una resistència i un condensador en paral·lel. Els resultats experimentals s’ajusten bé a les simulacions fetes per espectroscòpia d’impedàncies (EIS). Els resultats posen en evidència que el comportament elèctric queda majoritàriament determinat per la resistència de contacte entre els nanotubs, que formen la xarxa amb una distribució totalment desordenada. Hem vist que capes primes de nanotubs de carboni conductores i flexibles, que poden ser també transparents, poden ser competitives en diferents aplicacions, com ara pantalles, cel·les solars fotovoltaiques o sensors selectius

La demanda d’aplicacions industrials avançades, fa que el control de màquines elèctriques de corrent altern (AC) sigui una disciplina contínuament creixent per satisfer l’alt nivell d’exigència. Tradicionalment, la màquina d’inducció (IM) ha estat la més utilitzada en aplicacions industrials de velocitat variable, incloent-ne bombes i ventiladors, màquines tèxtils i de paper, vehicles elèctrics, generació eòlica, etc. A més dels requeriments funcionals, l’estalvi energètic és, actualment, un aspecte a tenir en compte en aplicacions de velocitat variable. Una alternativa molt atractiva a la IM és la màquina síncrona d’imants permanents (PMSM). Entre d’altres avantatges, aquest tipus de màquines ofereixen una eficiència més elevada, alta densitat de potència i una resposta dinàmica molt ràpida. La demanda industrial de PMSM es situa en el rang de petita potència, encara que el interès en aquest tipus de màquines està creixent, en particular fins al rang dels 100 kW. Històricament, les màquines utilitzades en aplicacions de velocitat variable han estat alimentades per inversors de potència. En els últims anys, el convertidor matricial ha esdevingut un clar competidor del inversor convencional. Un convertidor matricial és un circuit de topologia avançada capaç de convertir directament AC en AC generant una tensió de càrrega amb amplitud i freqüència variable, amb flux de potència bidireccional, formes d’ona sinusoïdal tant a l’entrada com a la sortida i operant amb factor de potència unitari a l’entrada. A més, degut a que requereix no elements inductius ni capacitius per emmagatzemar energia, el disseny del convertidor matricial és molt compacte. Existeixen molts tipus de control per màquines de AC, essent els basats en control vectorial els més adequats per aplicacions de d’alt rendiment. Entre d’altres, el control de camp orientat i el control directe de parell son els més utilitzats. Tot i essent una de les tècniques emergents en aplicacions industrials, el control directe de parell té implícites una sèrie de limitacions que, encara avui, s’estan investigant. El treball presentat en aquesta tesi, s’endinsa en la investigació del control directe de parell per PMSM alimentats amb convertidors matricials. Aquest, treball considera el reemplaçament del inversor convencional per un convertidor matricial, explotant les característiques d’aquest per tal de reduir el arrissat del parell i del flux en la PMSM, inherent al control directe de parell. Durant el transcurs d’aquest treball s’ha desenvolupat un nou control directe de parell utilitzant vectors curts i llargs del convertidor matricial. Els efectes indesitjats de la tensió en mode comú, relacionada amb la utilització de convertidors de potència, és altre dels aspectes que s’aborden en aquest treball. Un algoritme molt simple que redueix la tensió en mode comú ocasionat pel control directe de parell amb convertidors matricials ha estat desenvolupat i investigat en aquesta tesi. La principal limitació de les tècniques “sensorless” basades en models o observadors és que fallen a baixes velocitats. La necessitat d’operar a baixa velocitat o velocitat zero, ha desencadenat la investigació d’altres tècniques on s’exploren les asimetries de la màquina a partir de la injecció d’un senyal d’alta freqüència per obtenir-ne la posició del rotor. Durant aquesta tesi s’ha desenvolupat un nou algoritme de injecció d’un vector rotatori en el pla  quan s’utilitza el control directe de parell. / The control of AC machine drives is a continuously advancing subject satisfying increasing high performance applications demands. Induction Motor (IM) drives with cage-type machines has been the workhorses of industrial variable speed drives applications, including pumps and fans, paper and textile mills, electric vehicles, locomotive propulsion, wind generation systems, etc. In addition to performance requirements, energy saving aspects of variable speed drives is gaining attention nowadays. Permanent Magnet Synchronous Machines (PMSM) are becoming a very attractive alternative to IM. Among other advantages, these type of machines offer higher efficiency, high power density and very fast dynamic performance. PMSM, in particular in the low power range, are already widely used in industry and recently, the interest in their application is growing, particularly up to 100 kW. Variable speed drives fed by voltage source inverters has been traditionally employed in industrial applications. In the past few years, matrix converters have emerged to become a close competitor to the conventional inverter. A matrix converter is an advanced circuit topology capable of converting AC-AC, providing generation of load voltage with arbitrary amplitude and frequency, bi-directional power flow, sinusoidal input/output waveforms, and operation under unity input power factor. Furthermore, since no inductive or capacitive elements are required, MC allows a very compact design. There are several methods to control AC machines, vector control methods being the most suitable for high performance demands. Among others, field oriented control and direct torque control are the most widely used. Although being one of the emerging control techniques for industrial applications, the direct torque control has some inherent drawbacks that are still being investigated by researchers. The work reported in this thesis is devoted to the investigation of direct torque control of PMSM drives fed by matrix converters. This work considers the replacement of the conventional voltage source inverter by a matrix converter. The features of matrix converters are exploited to reduce the inherent electromagnetic torque and stator flux ripples arising from the direct torque control driving a PMSM. A new direct torque control using small and large voltage vectors of matrix converters has been developed during the course of this work. The undesirable effects of the common mode voltage related with the utilization of the conventional voltage source inverter, like electromagnetic interferences and the machine early failures, are other issues with which this work is also concerned. A very simple algorithm to reduce the common mode voltage in direct torque control drives using matrix converters is developed and investigated in this work. The main limitation of all sensorless vector control schemes, based on the conventional fundamental frequency models or observers, is that they fail at very low speeds. The desirability to operate continuously at low or zero speed has led to another sensorless approaches where the saliency of the machine is tracked through some form of signal injection to obtain flux or position information. A new algorithm to inject a rotating vector in the a-b frame when employing a direct torque control has been developed in this thesis.

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This thesis is in two parts, the first portion of which describes experiments which were carried out to detect a positive photo-electric emission from potassium. The result of these was that if such an emission occurs it is of the order of 10 times the negative emission at most, i.e. a thousand times as small as the value found by Dember, who claimed to have definite evidence Of a photo-electric effect. The major portion concerns the two or wore threshold frequencies of potassium. A brief historical account is given ending with the work of Richardson and Young in which they predict for the sensitised potassium surface a threshold at 10,000 AU and another at 30,000 AU. Precautions were taken to obtain a clean surface in a high vacuum, the apparatus being of a form which made the saturation of the photo-electric currents easy and the potassium was subjected to approximately black body radiation. Using Richardson's formula for the saturated currents, the work functions, b, and the corresponding threshold frequencies were determined. The values of the frequencies found were 7,100 A.U., 10,000 A.U. 21,000 A.U. A tentative explanation is given of certain Hysteresis phenomena which were observed.

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