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Issue Date: 3-Apr-2013
Authors: Castro, Giuseppe
Title: Study of innovative plasma heating methods and applications to high current ion sources
Abstract: The use of plasmas in various areas of scientific research has been growing in recent years. The plasma is excited by microwaves typically in the range 2.45-28 GHz; microwaves are coupled to a cylindrical chamber working as resonant cavity, where the plasma is produced, by a suitable system of waveguides, and are absorbed therein during the interaction with gases or vapors fluxedd at low pressure. Most of the parameters of the extracted beam, such as the intensity, the emittance and the shape in the real space depend in a decisive way on the characteristics of the plasma which from which the beam is extracted. The further development of ECR-type ion sources is however intrinsically limited by physical properties of the plasmas. The electromagnetic energy can not be transferred to the plasma electrons over a certain density threshold, named cutoff density, The studies we performed during the PhD course, carried out on a MDIS, have shown that it is possible to excite waves in magnetized plasmas having an electrostatic nature (Electrostatic Bernstein Waves) which are not reflected at the density cutoff. An electrostatic wave is a rarefaction-compression wave whose electric field is parallel to the wave propagation direction. In particular, Electron Bernstein Waves (EBW) can be strongly absorbed by the plasma at cyclotron harmonics. Due to their electrostatic nature, EBW must be generated within the plasma from electromagnetic waves, through a conversion mechanism which takes from an electromagnetic wave to electrostatic oscillations (conversion mechanisms are typically called XB or OXB mechanisms). the research has been based on the detecting of peculiar signatures of the ES waves formation and following absorption: the overcoming density cut-off, the observation of non-linear heating, highlighted by the sudden appearance of X-ray emission above certain thresholds of RF power and finally, the broadening of electromagnetic spectrum, sign of interaction between the electromagnetic wave and the plasma waves. The three signatures have been simultaneously revealed for the first time in a compact device, the Plasma Reactor, in two different magnetic configurations. In magnetic beach configuration, working at 3.76 GHz, a density value two times larger than cutoff density. Double temperature electron population have been revealed correspondingly to the first harmonics. Flat-B configuration has been studied by using different frequencies, 2.45 and 3.75 GHz, and in both cases overdense plasmas have been generated. These data show that the new heating method enables to reach very high densities by using very low nominal power without a complication of the technological apparatus. The data collected by means of X spectroscopy help to understand the non-linearity extent triggered by the RF power and able to convert the incident E.M wave in a plasma wave. Beyond the threshold microwave power, spectral temperature up to 4 keV has been obtained at 3.75GHz. This value need to be compared with the maximum energy obtainable by an electron from ECR heating, 200 eV. This further confirm that the heating mechanism is not the ECR. In this conditions plasma reactor has been demonstrated to be a very intense X-rays source. The measurements on the VIS source have definitively demonstrated that X ray radiation is emitted only when a under-resonance region in the source exists. Along with the mentioned evidences, we also observed other phenomena (all of them correlated to the occurred conversion) like a plasma vortex formation: a double picked maxwellian distribution in resonance regions, together with a variation in the axial magnetic field, explainable only by means of the generation of an internal azimuthal current, have been contemporaneously detected with the other signs of wave-conversion. This drift motion accords totally with theoretical Golovanivsky predictions.
Appears in Collections:Area 01 - Scienze matematiche e informatiche

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