The interaction of ultra-short laser pulses and cluster targets can be used to explore a number of interesting phenomena, ranging from nuclear fusion to astrophysical blast waves. In our experiments, we focused on exploring very fast plasma dynamics of a plasma created by ionizing clusters and monomer gas. By using a 115 fs laser pulse, we can even study sub-picosecond plasma dynamics. In addition, we also wanted to impose an external magnetic field on these plasmas to study how the plasma evolution would change. The results of this work produced two significant results. First, a new, extremely fast ionization mechanism, with velocities as high as 0.5 c, was discovered which allows for significant plasma expansion on a picosecond time-scale. Experimental studies measured the velocity of the ionization wave, while particle-in-cell simulations helped explain the source and longevity of the wave. It was also observed that this ionization wave was not affected by the external magnetic field. Second, the external field was shown to inhibit plasma expansion on a time-scale of tens of picoseconds, which seems to be one of the first demonstrations of magnetic confinement on such a fast time-scale. Simple 1D simulations tell us that the field appears to slow electron heat transport in the plasma as well as inhibiting collisional ionization of electrons expanding into the surrounding gas. The inhibition of plasma expansion by the field on this time-scale may provide some evidence that magnetic confinement of a fusion plasma created by exploding clusters could improve the fusion yield by slowing heat loss as well as possibly electrostatically confining the hot ions. / text

No description available.

Although the pairing mechanism in MgB2 is thought to be phonon mediated, there are still many experimental results that lack appropriate explanation. For example, there is no consensus about the magnitude of the energy gap, its temperature dependence, and whether it has only one-gap or not. Many techniques have been used to investigate this, like Raman spectroscopy, farinfrared transmission, specific heat, high-resolution photoemission and tunneling. Most tunneling data on MgB2 are obtained from mechanical junctions. Measurements of energy gap by these junctions have many disadvantages like the instability to temperature and field changes. On the other hand, sandwich-like planar junctions offer a stable and reliable measurement for temperature dependence of the energy gap, where any variation in the tunneling spectra can be interpreted as a direct result from the sample under study. To the best of our knowledge, we report the first energy gap temperature- and magnetic field-dependence of MgB2/Pb planar junctions. Study of the temperature-dependence shows that the small gap value (reported by many groups and explained as a result of surface degradation) is a real bulk property of MgB2. Moreover, our data is in favor of the two-gap model rather than the onegap, multi-gap, or single anisotropic gap models. The study of magnetic field effect on the junctions gave an estimation of the upper critical field of about 5.6 T. The dependence of energy gap on the field has been studied as well. Our junctions show stability against temperature changes, but "collapsed" when the magnetic field (applied normal to the junction barrier) is higher than 3.2 T. The irreversible structural change switched the tunnling mechanism from quisiparticle tunneling into Josephson tunneling. Josephson I-V curves at different temperatures have been studied and the characteristic voltages are calculated. The estimated MgB2 energy gap from supercurrent tunneling in weak link junctions agrees very well with that from quasiparticle tunneling. Reported properties on polycrystalline, single crystal and thin film MgB2 samples are widely varied, depending on the details of preparation procedure. MgB2 single crystals are synthesized mainly by heat treatment at high temperature and pressure. Single crystals prepared by this way have the disadvantages of Mg deficiency and shape irregularity. On the other hand, improving the coupling of grain boundaries in polycrystalline MgB2 (has the lowest normal state resistivity in comparison to many other practical superconductors) will be of practical interest. Consequently, we have been motivated to look for a new heat treatment to prepare high quality polycrystalline and single crystal MgB2 in the same process. The importance of our new method is its simplicity in preparing single crystals (neither high pressure cells nor very high sintering temperatures are required to prepare single crystals) and the quality of the obtained single crystal and polycrystalline MgB2. This method gives high quality and dense polycrystalline MgB2 with very low normal state resistivity (σ(40 ) = 0.28 cm). Single crystals have an average diagonal of 50 m and 10 m thickness with a unique shape that resembles the hexagonal crystal structure. Furthermore, preparing both forms in same process gives a great opportunity to study inconsistencies in their properties. On the other hand, magnesium diboride thin films have also been prepared by magnetron sputtering under new preparation conditions. The prepared thin films have a transition temperature of about 35.2 K and they are promising in fabricating tunnel junctions.

Magnetic neural stimulation of straight neurons with bends (1) in a semi-infinite volume conductor with a planar interface and (2) in the model of the human head is analyzed. Two stimulating coils, namely the double-square and the double circular, producing the magnetic field for the neuron stimulation are considered. The results indicate that the stimulating coil characteristics (size, shape and location) and the neuron shape affect the magnitude and location of the stimulation. The activating function, defined as the electric field derivative along the neuron, has two components. One component depends on the derivative of the electric field along the straight section of the neuron, and the other on the field magnitude. For bent neurons in a semi-infinite volume conductor, an analytical expression of the activating function (the stimulus) of the neuron was derived. The maximal stimulation point is at the bend of the nerve and its position depends on the nerve shape and coil parameters. The analysis also shows a better performance (a stronger stimulus) for a double-circular (figure eight) coil than for a double-square coil of comparable size. Stimulating bent neurons in the human head is also analyzed. The head model consists of an outer sphere representing the skull and scalp and two inner spheres such that each represents one half of the brain. The 3D-impedance method was used to obtain the induced electric fields by the double-square and double-circular coils. Quasi-static conditions are assumed. The geometry of the neuron in this model approximates the normal configuration of motor neurons in the human head. The analysis shows that the stimulation occurs almost at the highest point on the nerve (the closest point to the coil) with the coil positioned in such a way that its center is directly over the highest point on the nerve. It is also shown that the double-square coil produces a stronger stimulus than the double-circular coil. This result is in contradiction with that for a bent neuron in a semi-infinite volume conductor, however, it agrees with the results obtained for a straight neuron [1]. The analysis of bent neurons represents a more realistic approximation of the actual anatomy. The results of this analyses confirms the conclusions and, therefore, usefulness of simplified analyses of straight neurons. The results are expected to be of some use in clinical applications where non-invasive neural stimulation is desired and location of stimulation needs to be known. / Graduate / 0544

A three-beam heterodyne polarimeter has been designed and constructed to measure line-integrated density and Faraday rotation of accelerated spheromak plasmas in the Plasma Injector 1 and 2 devices (PI-1, PI-2) at General Fusion Inc. Faraday rotation is a function of the local magnetic field and electron density. Therefore, the polarimeter has the potential to provide information on the internal magnetic field of the plasma. A typical spheromak is about 1m in length and is accelerated to speeds on the order of 100km/s. At a bandwidth of 1MHz, the polarimeter can axially resolve the spheromak down to about 10cm. The polarimeter uses a $\nit{CO}_2$ laser that produces a Faraday rotation signal of about $0.5\degrees$ for a typical plasma with density and magnetic field on the order of $\ee{21}\mmt$ and 1T. The Faraday rotation measurement noise floor for a null signal is about $0.1\degrees$. Two important sources of Faraday rotation error are the ellipticity and collinearity of the plasma-immersed beams. These error sources are examined by sending the plasma beams through a rotating optic to mimic the path through a dense, magnetized plasma. The error due to the ellipticity effect has been reduced to below the noise floor by careful alignment and use of zero phase reflectors that minimize elliptical polarization of the beams. Collinearity error has been greatly improved by aligning the beams with a rotating ZnSe wedge. Measurements after the alignment match well with a model Faraday rotation signal generated from magnetic probe measurements. However, beam collinearity continues to be a significant source of error. For regions with strong density gradients, the size of this error can be on the order of the signal magnitude. For future work, steps should be taken to improve the alignment of the two plasma-immersed beams, and to shorten the length of the beam path to further reduce the beam collinearity error. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2014-04-30 22:17:18.648

Magnetic neural stimulation of straight neurons with bends (1) in a semi-infinite volume conductor with a planar interface and (2) in the model of the human head is analyzed. Two stimulating coils, namely the double-square and the double circular, producing the magnetic field for the neuron stimulation are considered. The results indicate that the stimulating coil characteristics (size, shape and location) and the neuron shape affect the magnitude and location of the stimulation. The activating function, defined as the electric field derivative along the neuron, has two components. One component depends on the derivative of the electric field along the straight section of the neuron, and the other on the field magnitude. For bent neurons in a semi-infinite volume conductor, an analytical expression of the activating function (the stimulus) of the neuron was derived. The maximal stimulation point is at the bend of the nerve and its position depends on the nerve shape and coil parameters. The analysis also shows a better performance (a stronger stimulus) for a double-circular (figure eight) coil than for a double-square coil of comparable size. Stimulating bent neurons in the human head is also analyzed. The head model consists of an outer sphere representing the skull and scalp and two inner spheres such that each represents one half of the brain. The 3D-impedance method was used to obtain the induced electric fields by the double-square and double-circular coils. Quasi-static conditions are assumed. The geometry of the neuron in this model approximates the normal configuration of motor neurons in the human head. The analysis shows that the stimulation occurs almost at the highest point on the nerve (the closest point to the coil) with the coil positioned in such a way that its center is directly over the highest point on the nerve. It is also shown that the double-square coil produces a stronger stimulus than the double-circular coil. This result is in contradiction with that for a bent neuron in a semi-infinite volume conductor, however, it agrees with the results obtained for a straight neuron [1]. The analysis of bent neurons represents a more realistic approximation of the actual anatomy. The results of this analyses confirms the conclusions and, therefore, usefulness of simplified analyses of straight neurons. The results are expected to be of some use in clinical applications where non-invasive neural stimulation is desired and location of stimulation needs to be known. / Graduate / 0544

Graphene, the two-dimensional lattice of sp2-hybridized carbon atoms, has a great potential for future electronics, in particular for opto-electronic devices. The carrier relaxation dynamics, which is of key importance for such applications, is in the main focus of this thesis. Besides a short introduction into the most prominent material properties of graphene and the experimental techniques, this thesis is divided into three main parts. The investigation of the carrier relaxation dynamics in the absence of a magnetic field is presented in Chapter 3. In the first experiment, the anisotropy of the carrier excitation and relaxation in momentum space was investigated by pump-probe measurements in the near-infrared range. While this anisotropy was not considered in all previous experiments, our measurements with a temporal resolution of less than 50 fs revealed the polarization dependence of the carrier excitation and the subsequent relaxation. About 150 fs after the electrons are excited, the carrier distribution in momentum space gets isotropic, caused by electron-phonon scattering. In a second set of two-color pump-probe experiments, the temperature of the hot carrier distribution, which was obtained within the duration of the pump pulse (about 200 fs), could be estimated. Furthermore, a change in sign of the pump-probe signal can be used as an indicator for the Fermi energy of different graphene layers. Pump-probe experiments in the far-infrared range in reflection and transmission geometry were performed at high pump power. A strong saturation of the pump-induced transmission was found in previous experiments, which was attributed to the pump-induced change in absorption. Our investigation shows the strong influence of pump-induced reflection at long wavelengths, as well as a lot smaller influence of the saturation of the pump-induced change in absorption. At a high pump power, the increase of the reflection exceeds the change in absorption strongly, which leads to negative pump-probe signals in transmission geometry. In Chapter 4, investigations of the carrier dynamics of graphene in magnetic fields of up to 7T are presented. Even though the optical properties of Landau-quantized graphene are very interesting, the carrier dynamics were nearly unexplored. A low photon energy of 14meV allows the investigation of the intraband Landau-level (LL) transitions. These experiments revealed two main findings: Firstly, the Landau quantization strongly suppresses the carrier relaxation via optical-phonon scattering, resulting in an increased relaxation time. Secondly, a change in sign of the pump-probe signal can be observed when the magnetic field is varied. This change in sign indicates a hot carrier distribution shortly after the pump pulse, which means that carrier-carrier scattering remains very strong in magnetic fields. In a second set of pump-probe measurements, carried out at a photon energy of 75meV, the relaxation dynamics of interband LL transitions was investigated. In particular, experiments on the two energetically degenerate LL transitions LL(−1)->LL(0) and LL(0)->LL(1) showed the influence of extremely strong Auger processes. An ultrafast and extremely broadband terahertz detector, based on a graphene flake, is presented in the last chapter of this thesis. To couple the radiation efficiently to the small flake, the inner part of a logarithmic periodic antenna is connected to it. With a rise time of about 50 ps in a wavelength range of 9 μm to 500 μm, this detector is very interesting to obtain the temporal overlap in two-color pump-probe experiments with the free-electron laser FELBE. Furthermore, the importance of the substrate material, in particular for the high-speed performance, is discussed.

No description available.

This thesis investigates how magnetographic data may be used to study the longterm behaviour of the magnetic field distribution across the surface of the sun.

Orientador: Konstantin Georgiev Kostov / Banca: Joaquim José Barroso de Castro / Banca: Mario Ueda / Resumo: O estudo do efeito do campo magnético sobre o processo de implantação iônica por imersão em plasma (3IP) é de grande interesse para o tratamento de materiais. Sendo 3IP uma técnica relativamente nova e de grande relevância para diversas aplicações tecnológicas é importante que ela seja pesquisada e analisada detalhadamente. A aplicação de um campo magnético estático, transversal em relação ao campo elétrico formado durante este processo produz um sistema de campos cruzados ExB. Este sistema de campos cruzados gera um aumento significativo da densidade de plasma na vizinhança do alvo, resultando num maior corrente de implantação, e conseqüentemente um tratamento mais rápido e uma dose retida mais alta em relação ao processo 3IP convencional. Neste trabalho, analisamos mediante simulação numérica, o efeito da distribuição do campo magnético axial no processo 3IP. O campo magnético é produzido por um par de bobinas instaladas fora da câmara de vácuo cujos raios e distância de separação são variadas. Encontramos que a densidade de corrente de implantação depende essencialmente da configuração do campo magnético. Assim, com uma adequada configuração das bobinas (10,0 cm de raio e 42,0 cm de separação) obtivemos uma densidade de corrente quase uniforme de aproximadamente 1,5 mA/cm2 que é 1.5 vezes maior em relação ao caso do sistema 3IP sem campo magnético. O efeito da tensão aplicada assim como da pressão do gás sobre o plasma no processo 3IP é também investigado. O sistema 3IP com campos ExB cruzados é estudado utilizando a técnica de simulação numérica em duas dimensões. Esta simulação é realizada com o código computacional KARAT que emprega o algoritmo "particle-in-cell" (PIC) para simular o movimento de partículas carregadas no campo eletromagnético. / Abstract: Detailed investigation of the effect of magnetic field on the process of plasma immersion ion implantation (PIII) is of great interest for the material treatment. Being a relatively novel and of great relevance for technological applications technique, the PIII demands further research and careful analysis. The application of static magnetic field, transversal in respect to the electric field established during this process, produces a system of crossed fields ExB. This system of crossed fields promotes an increase of the plasma density, resulting in higher implantation current, and consequently in a shorter treatment time and a higher retained dose in comparison to the conventional PIII process. In this work we have analyzed by means of numerical simulation the effect of magnetic field distribution on the PIII processing. The magnetic field is produced by a pair of external coils, whose radii and separation distance were varied. We found that the density of the implantation current essentially depends on the magnetic field configuration. Thus, with an appropriate configuration of the coils (10,0 cm radius and separation of 42,0 cm) we have obtained an almost uniform current density of approximately 1,5 mA/cm2 that is 1,5 times bigger in relation to the case PIII without magnetic field. The effect of the target bias as well as the gas pressure on the ion current incident on the target is also investigated. The PIII system with crossed ExB fields has been studied using two-dimensional numerical simulation. The simulation is accomplished by the computer code KARAT which employs the particle-in-cell (PIC) algorithm for simulating the movement of charged particles in the electromagnetic field. / Mestre