Plasma Heating By Alfven Wave In Field-Reversed Ion Ring Experiment

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Controlled thermonuclear fusion is considered by many as a promising alternative energy source for the world's increasing energy demands. There are two main approaches towards thermonuclear fusion. One is called inertial confinement fusion and the other is called magnetic confinement fusion. Magnetic confinement fusion research can be categorized into two groups depending on the magnetic field topology. The Tokamak, which is a current mainstream plasma confinement device, is the toroidal device with a small poloidal field superimposed on a strong toroidal magnetic field so as to provide particle confinement and macroscopic stability. It is a very complicated machine in practice. Open field systems are generally soleniodal and often utilize magnetic mirrors to minimized end losses. An open mirror system can be given the advantages of the closed Tokamak system by introducing an axis-encircling current in the plasma that reverses the applied on-axis magnetic field and results in closed poloidal field lines. This configuration can, in principle, confine plasma as effectively as the Tokamak but without the complexity of Tokamak system. However, there are two major issues that have stood in the way of utilizing this configuration as a fusion reactor. First, no practical formation for a field reversed configuration at reactor scale size has yet been demonstrated. Second, the long standing contradiction between MHD predictions of instability and the observed stability of small experimental FRCs is still not fully understood. The goal of the FIREX (Field-reversed Ion Ring Experiment) was to create a fieldreversed configuration (FRC) in which a substantial fraction of the diamagnetic current was carried by energetic, axis-encircling ions, and to investigate the stability of this configuration. The FIREX program has established that strongly diamagnetic ion rings immersed in plasma, with ring ion velocities in excess of the Alfven speed in the plasma, can be violently unstable to generation of compressional Alfven waves with wavelengths short compared to the ring orbit size. Two-dimension, axi-symmetric numerical simulations showed that the momentum transferred to the Alfven wave would take axial energy from the ring and cause the ring to come to a stop and be contained axially in the plasma. The FIREX experiment was designed to take advantage of this effect. However, in the experiment, the ring is not trapped. Magnetically insulated proton collector data showed that most of the protons exit through the ends of the solenoid within ~ 0.6[mu]s of injection, or about 5 ion gyroperiods in the solenoid field. The reason for the rapid axial loss of the ring protons can be found in the magnetic field probe data. Intense high-frequency field fluctuations in the ring annulus were observed that do not propagate far radially. We believe that these very intense field fluctuations are waves generated by the collective interaction of the ring protons with the plasma. Their generation takes energy from the ring protons preferentially from their rotational motion. Numerical simulation results show rotational energy loss and similar, very strong Alfven turbulence generated within the ring annulus. The energy coupled from the ring to the waves is dissipated very locally in the plasma. The waves damp very rapidly in time after the ring ions are lost, and do not propagate far radially from their region of generation. The result must be very strong, local plasma heating, which has been seen in two ways in the experiment. The first is the observation of plasma diamagnetism. This diamagnetic field, with a characteristic decay time of ~2[mu]s, can be seen clearly in the delta-B signal in the beam annulus. This smoothly decaying diamagnetism is seen only in and near the ring annulus, between 9 and 14cm radius. The second indication of heating is the direct measure of increased plasma ion kinetic energy by Doppler broadening of ion and neutral emission lines integrated across the plasma radius ii
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Hammer, David A
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Kusse, Bruce Raymond
Kelley, Michael Charles
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Electrical Engineering
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M.S., Electrical Engineering
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Master of Science
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