This thesis covers the design and development of a modulated Pierce electron gun used in the construction of experimental travelling wave tube (TWT) amplifiers. The gun incorporated an open aperture switching electrode, positioned mid-way between anode and cathode, to pulse the beam. This method of modulation did not have the same adverse effects on electron trajectories as in the case of a conventional mesh grid, but rather the electrode could be used to alter the focus conditions within the gun and subsequently improve certain beam characteristics. Ion focusing effects could also be eliminated with the electrode, allowing dual mode operation of the guns without the complications normally associated with such a practice. The switching electrode was simulated to ascertain its effect on electron trajectories within the gun, using finite element analysis as well as an electron optics design program. A test gun was constructed in a glass envelope in order to investigate the performance of the new design. The glass gun allowed a beam analysis to be performed, as well as thermal measurements to be made. Results from this gun compared favourably with earlier simulations. The results of two metal/ceramic construction TWTs are presented, showing the beneficial effects of the switching electrode on the performance of the tubes as a whole, and the electrode's potential to compensate for constructional anomalies. The joining of metals to ceramic using active brazing techniques is also an important aspect tackled by the thesis, with several innovative ideas being implemented in the construction of the devices. A simple yet reliable electrical feed-through was developed for those guns having a ceramic envelope. Extensive work was also performed on the manufacture of impregnated tungsten cathodes for use in the electron guns. Several test diodes, including a water-cooled demountable test vehicle, were constructed to test the performance of the cathodes. An analysis was performed on the patchy behaviour of some of the initial cathodes to improve the preparation methods used in the laboratory. The emission results obtained from the cathodes are documented, as is the successful incorporation of several of them into the new modulated gun design. / Thesis (Ph.D.)-Univesity of Natal, Durban, 1994.

An experimental, field replaceable travelling wave tube (TWT) with low profile, TEM transitions has been developed with the aim of combining the cost advantages of field replaceable TWTs with the bandwidth advantages of TEM transitions. Field replaceable TWTs are cheaper because their periodic permanent magnet (PPM) stacks (which are expensive) are easier to produce and can be reused. Standard TEM transitions have a high profile and prevent TWTs from being plugged into PPM stacks. The stacks of packaged TWTs are therefore built onto the tubes, using split ring magnets, before the tubes are potted. Upon failure, such tubes are discarded together with their focusing, coupling and cooling structures. Two types of low profile, TEM transitions were investigated: a ring and a plug transition. In the former, the helix was joined to a metal ring that was sandwiched between two ceramic rings in the vacuum envelope. Due to its high shunt capacitance, this transition was narrowband. In the plug transition, the helix was joined to a small metal plug in the wall of a ceramic tube. Plug transitions with a return loss of better than 10 dB over an octave were made but some technological difficulties were encountered. Metal/ceramic joining with active solder is therefore discussed. The development of graphite attenuators was attempted but graphite of high enough resistivity to achieve the attenuation and matching required was not available. Thin film attenuators were therefore used instead. Some good attenuators were made but the molybdenum films were sensitive to processing conditions. An empirical procedure was developed to magnetize PPM stacks to a wanted profile. The stacks that were designed and built resulted in good beam focusing. TWT tests were made using an automated test station that was built. Maximum RF gains and output powers of 20 dB and 43 dBm respectively were measured. The TWT design, construction and testing was a combination of materials science and microwave work. Some simulations were used to solve electromagnetic problems but much of the work was practical, involving trial and error and many measurements. / Thesis (Ph.D.)-University of Natal, Durban, 1994

The National Rifle Association (NRA) has dominated the debate over gun rights since the late 1960s. In many ways, its political power is unassailable. However, a historical analysis of the NRA's deeply rooted connection to the operations of the American government proves this has not always been so. This thesis is an examination of the mission and actions of the NRA through the lens of the government's expansion of power during and beyond the Cold War.

No description available.

Statistics show that the number of mass shootings and involved fatalities have drastically increased over the last five years. The framing of these attacks in the mass media has a substantial impact on the public opinion on the causes of shootings, possible prevention methods and gun control in general. Following study aims to uncover potential ideological influences of political tendencies of liberal and conservative newspapers on the media coverage of the most recent mass shooting in the Marjory Stoneman Douglas High School in Parkland, Florida. A qualitative content analysis with a general structural analysis of 39 articles from the liberal newspaper The New York Times and the conservative newspaper The Washington Times and a more detailed Critical Discourse Analysis of the frames of blame, prevention methods and gun control were conducted. Findings show that The Washington Times´media coverage on blame attributions and prevention methods mainly included frames of mental health. The focus in The New York Times´ coverage is clearly on gun issues and the demand for gun restrictions. The topic gun control is positively framed by The New York Times. The Washington Times, however, framed gun control in connection with conspiracy theories and thus portrayed it in a more negative light. The findings prove an influence of the newspapers´ political tendencies and ideologies on the media coverage of the Florida mass shooting.

Selon les normes actuelles, les tubes d'arme sont conçus pour résister à la pression des gaz en utilisant les équations de la statique des milieux continus. Mais lors des tirs, les chargements auquel est soumis le tube sont extrêmement rapides, de l'ordre de quelques millisecondes. En effet, le projectile est poussé à travers le tube par la pression des gaz, à ce déplacement est associé la rotation du projectile provoquée par les rayures internes du tube. Le projectile est en contact avec le tube par sa ceinture qui assure une parfaite étanchéité des gaz par son diamètre légèrement supérieur à celui du tube. Ainsi des contraintes dynamiques sont générées par la pression des gaz et par le forcement de la munition. Le but de ces travaux est d'étudier les contraintes dans le tube lors des tirs. Afin d'évaluer les différents phénomènes, dans une première partie, de nombreux essais ont été réalisés sur des armes de moyen calibre. Ces essais ont permis de constituer une base de données sur les déformations circonférentielles du tube. Dans un second temps, un modèle par éléments finis a été développé à l'aide du logiciel LS-Dyna. Le front de pression et le projectile avec sa ceinture ont été simulés. Un modèle tridimensionnel a été choisi afin d'évaluer les contraintes dans le rayage du tube, zones de concentration de contraintes. La corrélation entre les essais et les résultats de calcul est excellente. En effet le pic de déformation dynamique du tube mesuré est très bien prédit par le calcul. Pour déterminer l'influence de ce chargement dynamique, un calcul de durée de vie a été réalisé. Les résultats montrent la nécessité de prendre en compte dans le dimensionnement des tubes d'arme : la pression des gaz et l'action du projectile sur le tube. / According to current standards, gun barrels are designed to resist the maximum firing pressure using static, continuous equations. But during the firings events, complex high speed loads and structural responses arised over very short times. The projectile is pushed towards the tube by the gas pressure, morover the engraving of the tube gives a rotational movement to the ammunition. The contact between the tube and the projectile is ensured by a band or obturator. This obturator had a diameter which is superior to the tube's calibre. Very high stresses are generated during the firing, due to gas pressure and the forcing action of the projectile. The aim of this work was to study the stresses in the tube durings firings. To evaluate firings phenomena, in a first part, many tests were made on medium calibre guns using various configurations. Circumferential strain measurements were obtained. In a second time, a finite element model was developed using the commercial code LS-Dyna. The moving pressure front and the projectile with its obturator were simulated. Three dimensional modeling was chosen in order to evaluate the stresses in the tube rifling where stresses concentration were located. The numerical prediction of the peak of strain measured when the projectile passed the strain gages were excellent. Experiments and calculations showed a peak of strains which was not taken into account in tubes design. To evaluate the impact of this phenomenom, a fatigue life-time study was carried on. The results demonstrated that gas pressure and dynamic coupled actions must all be taken into account in the fatigue design of gun tubes.

Construiu-se um acelerador de elétrons de baixa energia (Máximo de 20 KeV). Como fonte de elétrons utilizou-se um canhão de elétrons de cinescópio preto e branco, ou seja, com um único emissor termiônico. A energia do feixe eletrônico pode ser continuamente variada desde 0 a 20 KeV. A corrente eletrônica pode ser variada desde um valor mínimo de 10-12A a 3&#956A, permanecendo constante no tempo uma vez fixado o seu valor. Através da focalização ou desfocalização da imagem do feixe de elétrons, é possível variar-se a área irradiada desde um diâmetro mínimo de 1 milímetro a um máximo de 6 cm. A pressão final atingida nas câmaras de vácuo foi da ordem de 10-7Torr. Durante o funcionamento do canhão de elétrons, o cátodo do mesmo é danificado devido ao bombardeamento de íons em sua superfície. Para examinarmos o grau de danificação causado por este bombardeamento iônico, basta focalizarmos e examinarmos a imagem de feixe eletrônico sobre uma tela luminescente. Deve-se ressaltar que todo o acelerador de elétrons foi construído a partir de materiais e componentes totalmente nacionais. O acelerador de elétrons foi aplicado para estudar efeitos de irradiação de elétrons em Teflon usando-se o método do Split Faraday Cup. Foram medidas correntes transitórias de carga e descarga e determinaram-se o alcance médio dos elétrons e o valor da condutividade induzida pela radiação. / We have constructed a low energy electron accelerator (maximum energy 20 KeV). A black and white kinescope electron gun, with a single thermionic emitter was used as an electron source. The energy of electron beam can be changed continuously from 0 to 20 KeV. The intensity of the current can be changed from a minimum of 10-12A to a maximum of 3&#956A, and can be maintained constant in time after its value has been fixed. The irradiated area can be changed from a diameter of 1 millimeter to a maximum of 6 centimeter, by focalizing or defocalizing the image of electron beam. The final pressure reached in vacuum chambers was 10-7Torr. During operation the surface of cathode of electron gun is damaged by ion bombardment. The degree of damage can be checked if the cathode image is focalized and examined on a luminescent screen. The accelerator was used to study electron irradiation effects in Teflon, employing the method of the Split Faraday Cup. Transient charging and discharging currents were measured. The average range of electrons of the electrons and induced conductivity were determined.

Den här uppsatsen har som syfte att utifrån Gun-Britt Sundströms roman Student -64 (1966) undersöka hur en författare genom beskrivningar av platser och rum formulerar en stadsmyt och därmed även konstruerar en viss identitet för personerna inom den. Utgångspunkten var att det finns en viss myt om Uppsala som studentstad, och att denna skildras i bland annat skönlitteraturen. Genom att fokusera på myten om studentstaden Uppsala och identiteten som student har också frågan om vilken funktion myten om staden har för studenten varit i fokus. För att genomföra undersökningen har en motivstudie baserat på rumsliga teman genomförts, som resulterade i en kartläggning över vilka platser som förekommer i romanen samt hur ofta de nämns. Det teoretiska ramverket som användes bestod huvudsakligen av Henri Lefebvres teorier om socialt rum och hur vi skapar rum genom tre olika slags erfarenheter: fysisk, mental och social, som vi får antingen genom rumsliga praktiker, representationer av rum eller genom representationella rum. Resultatet av undersökningen visar att Sundström i Student -64 gestaltar ett antal platser i Uppsala där protagonisten Sagas rumsliga praktiker kommer till uttryck. Platserna relateras till Sagas sociala erfarenhet och det hon som kallar för ”uppsalastämning”, vilket är en del av den historiska myten om Uppsala som Sundström anknyter till.  Därmed bidrar hon till att befästa myten genom att bygga på det representationella rum som är studentstaden Uppsala i sin helhet, samtidigt som hon omformulerar myten i en modern tolkning genom att skapa nya representationella rum. Dessa rum visar sig också möjliggöra en viss studentidentitet, samt en utveckling inom denna som i sig bidrar till att myten kan fortsätta leva.

M.Ing. / A two-axis gimbal system in the form of a pitch-roll gimbal and a motion simulating platform were developed to extend the capability of an existing ground-to-air prototype gun-turret. The objective was to stabilize the line of sight (LOS) of the gimbal system despite disturbances introduced by the motion simulating platform in real time. The main sensor used for the stabilization is a two-axis accelerometer which was mounted directly on the inner gimbal (roll gimbal) to form a direct-mass stabilization architecture. The stabilization control algorithm was designed and executed in the Labview® environment on a PC, and the accelerometer data is used to drive the two DC motors used as the actuators of this control system. The design of the motion simulating platform was based on a simplified Stewart-Platform and uses pneumatic cylinders as actuating limbs. All sensors and actuators in the motion simulating platform and the gimbal system are integrated with the National Instrument’s CompactRio® and Labview®. The result was a simple stabilization controller capable of achieving basic stabilization of the LOS. However, the hardware and software of this project are capable of more complex control algorithms and that forms the bulk of the suggestions for further studies.

This paper analyzes effect of gun buyback that took place in Great Britain in years 1996 and 1997 on crime rate and compares the results with theoretical arguments and previous empirical findings. It contains analysis of three independent time series: crime rate in England and Wales, Scotland and Northern Ireland. Models of the time series are built using Box-Jenkins methodology. The models are tested for presence of a structural break using visual analysis, Chow test and Quandt-Andrews test. These tests are used as an evaluation criterion of the effect of buyback on crime rate. The result of the analysis is that it is not possible to reject the null hypothesis that buybacks do not have effect on crime rate.