Stuttgart, Univ., Diss., 2002. / Erscheinungsjahr an der Haupttitelstelle: 2002. Computerdatei im Fernzugriff.

Stuttgart, Univ., Diss., 2002. / Erscheinungsjahr an der Haupttitelstelle: 2002. Computerdatei im Fernzugriff.

Stuttgart, Universiẗat, Diss., 2002.

No description available.

The behavior of the electric field together with the electron and ion densities in the vicinity of a nonemitting, plane anode is investigated. The selected approach involves non-linear analysis techniques on the continuum equations for steady-state, isothermal conditions where both ionization and two-body recombination are included. Ions, created through electron bombardment of neutral atoms, are repelled toward two stagnation regions: within or near the sheath boundary and near the plasma interface. These equilibria form as a result of the chemistry present: recombination establishes the latter while ionization stipulates the former. As presented, the sheath is fundamentally unstable - ions are driven toward the negative electrode. Using nitrogen data for a numeric example, the following observations are made: a sufficiently strong applied electric field pushes the ion density toward that ofthe electrons through a well - a constrictive phenomenon. Both a transition region, dominated by density gradients, and a diffusion-driven zone are found to move the system toward the plasma interface. The characteristics of this process are influenced by the applied electric field, but the instability of the chemistry-induced stagnation regions precludes numeric convergence. Insufficient dissipation may prevent the stability of the anode fall model as presented. Suggested improvements to the model descriptions include considering the effects of temperature gradients, magnetic fields, three-body recombination, diffusion written in terms of the electric field, multi-dimensionality and/or timedependencies^

Universiẗat, Diss., 2002--Stuttgart. / Erscheinungsjahr an der Haupttitelstelle: 2002.

This work was motivated by questions raised from multifrequency microwave interferometer measurements taken in a cylindrical plasma on the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) project. Standard data analysis based on a thin beam model neglecting refraction yields inconsistent electron densities and density profiles for different frequencies. This work focuses on the development of a model for the wave propagation through cylindrical plasmas when the plasma radius is on the order of the beam waist. For the calculations presented a Gaussian beam profile and plasma spatial profile were assumed. Both refraction by density gradients and finite beam sizes are found to play important roles and explain polychromatic differences in the electron densities and profiles. Calculations for the new model are compared to a thin beam model not accounting for refraction and experimental data from VASIMR.

This work is motivated by the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) experiment. This device uses a helicon antenna to generate a plasma inside a dielectric tube, which is radially confined and directed towards the rocket nozzle by an axial magnetic field. An ion cyclotron heating antenna further heats the ions, and a magnetic nozzle accelerates the plasma along the confining magnetic field as it leaves the rocket, ultimately allowing it to detach from the magnetic field and produce thrust. The experimental research presented here provides insight into the physical mechanisms of power flow in a helicon system by providing an overall system power balance in the form of heat flux measurements, and exploring changes in the heat fluxes in different parts of the system in response to varying operational parameters. An infrared (IR) camera measures the total heat flux into the dielectric tube surface, and axially scanned bolometer and UV photodiode probes measure the radial power loss from particles and radiation. Results from IR camera measurements on three different helicon systems are presented: the VASIMR VX-50 experiment, the VASIMR VX-CR experiment, and the University of Texas at Austin (UT) helicon experiment. These results demonstrate the development of the IR camera diagnostic for use on helicon systems of varying scale and geometry, and show reasonable agreement as to the fraction of input power lost to the dielectric tube walls. On the UT experiment, the results presented account for essentially all of the input power, providing a full system power balance. The data from all three experiments indicate that radial transport of ions to the interior wall is the dominant mechanism of power loss, with UV radiation contributing a small percentage. Additional experiments on the UT helicon explore energy and particle transport to the wall due to capacitive coupling of ions near the antenna. These experiments show clear damage to the dielectric tube surface directly under the antenna, due to physical plasma etching of the surface by bombarding ions that are accelerated into the wall by local electric fields from the antenna. / text

No description available.

Submitted by Edisangela Bastos (edisangela@ufpa.br) on 2018-03-21T17:23:02Z No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_MetodoDiferencasFinitas.pdf: 3779369 bytes, checksum: 428516d3d9e3d5e445fa15ec4b27cb81 (MD5) / Approved for entry into archive by Edisangela Bastos (edisangela@ufpa.br) on 2018-03-22T18:23:25Z (GMT) No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_MetodoDiferencasFinitas.pdf: 3779369 bytes, checksum: 428516d3d9e3d5e445fa15ec4b27cb81 (MD5) / Made available in DSpace on 2018-03-22T18:23:25Z (GMT). No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_MetodoDiferencasFinitas.pdf: 3779369 bytes, checksum: 428516d3d9e3d5e445fa15ec4b27cb81 (MD5) Previous issue date: 1996-02-09 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A formulação para diferenças finitas no domínio do tempo (FDTD), aplicada a plasma magnetizado segundo direção arbitrária, é desenvolvida para aplicação em dispositivos dielétricos em duas (2D-FDTD) e três dimensões (3D-FDTD). A ênfase é dada no processo de cálculo iterativo da convolução entre o campo elétrico e o tensor susceptibilidade elétrica do plasma magnetizado. Também, são propostos diversos tipos de dispositivos para propagação de sinais na banda milimétrica. O método é aplicado a estruturas controladas por plasma. Este plasma pode ser induzido por um feixe áptico sobre uma película semicondutora, depositada sobre o guia. Neste caso, as características de propagação do guia são controladas por um feixe áptico com energia apropriada. Esse plasma também pode ser estabelecido em semicondutor por dopagem. Neste tipo de dispositivo, o núcleo do guia é totalmente preenchido com plasma. Nesta opção, a propagação dos campos de RF é controlada por um campo magnetostático. Alguns dispositivos com guias singelos e acoplados são analisados. Observa-se então a possibilidade de controle efetivo de fase e acoplamento, assim como o controle na faixa de operação de modo único, notadamente nos guias opticamente controlados. Devido à carência de dados na literatura especializada, são estabelecidos critérios para discretização graduada e rigorismo nos testes de convergências propostos. Diversos tipos de dados são utilizados para essa finalidade. Obtém-se, então, uma espécie de perfil de discretização, o qual é aplicado aos demais dispositivos. / A finite-difference in the time domain (FDTD) formulation is developed for plasmas magnetized along an arbitrary direction and applicable to two dimensions (2D-FDTD) and to three dimensions (3D-FDTD) dielectric devices. Emphasis is given to the iterative calculation of the convolution between the electric field vector and the electric susceptibility tensor of the magnetized plasma. Various types of devices are also proposed for the propagation of signals in the millimeter-wave band. The method is applied to structures controlled by plasma. This plasma may be induced by an optical beam applied to a semiconductor layer deposited on the waveguide. In this case, the propagation characteristic of the waveguide is controlled by an optical beam with appropiate energy. This plasma may also also be introduced in the semiconductor by means of doping. For these devices the waveguide core is completely filled with plasma. With this option the propagation of the RF fields is controlled by a static magnetic field. Some devices with single and coupled waveguides are analyzed. The possibility of an effective control of phase and coupling, as well as the operating bandwidth with a single mode was examined, particularly with optically controlled waveguides. Due to the lack of data in the specialized literature, gradual discretization criteria and rigorous tests of convergence are proposed. Various types of data are used to accomplish this objective. As a result, a kind of discretization profile is obtained and is applied to the remaining devices.