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- W1615452808 abstract "Noninductively generated current is important for the steady state operation of tokamak plasmas. In the CCT (Continuous Current Tokamak) experiment here at UCLA, a continuous current has been observed in ECRH-generated plasmas, and in plasmas generated by electron beams, in both co- and counter-streaming cases. In 2+1/2D simulation works done by Nunan et al, a continuous toroidal current was maintained by particle fueling alone. In Nunan's work, the “preferential loss” model was proposed to explain the noninductive generation of toroidal currents in magnetized plasmas. However, this model assumes the conservation of the canonical toroidal momentum and therefore may not exist in three dimensional simulations. The aim of this thesis is to demonstrate this effect in a 3-D plasma model. A scalable 3-D electromagnetic PIC (Particle-in-Cell) code was developed for this purpose. Because the electromagnetic signals travels a short distance during each time-step, this code need not communicate globally and therefore has a parallel overhead independent of the number of processors. On the SP2 at Cornell Theory Center, the program is over 96% efficient on 128 nodes. Optimization techniques for RISC-based processors are also discussed. Using the parallel 3-D code, we have carried out the TDC simulations using 32 grids in the previously ignorable (toroidal) direction and 160 times as many particles as before ( 1.6d107 particles). These 3-D calculations agree qualitatively with the 2+1/2D results. In addition, particle scans were performed in both the 2+1/2D and 3-D to identify the source of particle transport in these simulations. In addition, test particle calculations were carried out to address issues related to the TDC problem. In one test charge calculation, we showed how the curvature drift breaks up the toroidal symmetry during start-up and generates current. Because of the 1/r dependence of the curvature drift, this effect becomes important when the radius is small. This is important in small aspect ratio tokamaks where the center may be too small for an induction coil. In the second calculation, we showed the neoclassical ordering breaks down near the magnetic axis, making the “seed current” unnecessary. These results are presented in Chapter Two." @default.
- W1615452808 created "2016-06-24" @default.
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- W1615452808 date "1999-01-01" @default.
- W1615452808 modified "2023-09-26" @default.
- W1615452808 title "Simulation of current generations in a three-dimensional particle model" @default.
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