Visible bremsstrahlung tomographic diagnostic for the pulsed high density field-reversed configuration experiment Rev. Sci. Instrum. 77, 10F319 (2006); 10.1063/1.2220018Modeling of field-reversed configuration experiment with large safety factora) Phys. Plasmas 13, 056119 (2006); 10.1063/1.2177635 Rotating magnetic quadrupole current drive for field-reversed configurationsDeformation of the internal structure of a field-reversed configuration ͑FRC͒ was studied using a tomographic reconstruction technique. A simple and configurable tomographic system was developed, with which the time evolution of the FRC internal structure was reconstructed. In the latter phase of equilibrium, a FRC has a well-known global rotational instability with toroidal mode number n = 2. It has been believed that elliptical deformation of the FRC allows interaction between the wall and the plasma, which terminates this configuration. However, these experiments revealed the FRC to deform into a dumbbell-like structure before the edge hits the chamber wall, leading to the disruption phase. In addition, an internal shift ͑toroidal mode number n =1͒ mode was observed in the equilibrium phase, followed by growth of n = 2 rotational instability.
A new explanation regarding the toroidal spin-up of a field-reversed configuration (FRC) is provided. A physical picture showing that the poloidal flux can convert directly to kinetic angular momentum is described. Through the use of an ion orbit calculation in resistively decaying FRC plasma, toroidal rotation at both the separatrix and the field-null is found to occur.
A spatial distribution of toroidal flow in a field-reversed configuration plasma generated by the field-reversed theta-pinch method has been measured by an ion Doppler spectroscopy (IDS) system with a line-spectrum of impurity carbon (CV: 227.2 nm) and a Mach probe. With the IDS system, the axial profile of toroidal flow was observed. The observed results show different time evolutions of toroidal spin-up at around the midplane and end regions of the FRC. The radial profile of toroidal flow has also been observed on the weakly ionized plasma outside the separatrix with the Mach probe method. These results indicate propagation of toroidal momentum to the scrape-off plasma outward from the center.
A field-reversed configuration (FRC) plasma was translated into a weakly ionized plasma and the effects of heating and particle buildup of the FRC plasma due to the background neutral particles and plasma injection in the translation process were investigated. Improvement of the particle and poloidal flux confinements and delay of onset of n=2 rotational instability were observed in the translation process. It was found that the internal structure of the plasma pressure (plasma temperature and density) at the separatrix and field null was deformed by the particle injection. FRC plasma translation through the background particles was equivalent to an end-on particle beam injection to the FRC plasma. Particles and energy were supplied during the translation. The results obtained for the phenomena of particle supply and plasma heating were also supported by the results of two-dimensional particle simulation. The effects of background particle injection appear to be a promising process for the regeneration of translation kinetic energy to plasma internal energy.
A simple and adjustable tomographic system has been developed for studying the time evolution of the internal structure of a field-reversed configuration plasma. The optical diagnostic system consists of 35 collimators. In each collimator, a plane-convex lens made of BK7 with a focal length of 100mm collimates emissions from the plasma and focuses them onto the end of an optical-fiber tube. The collimator system gives a spatial resolution of 6mm at the center of the 0.256m diameter discharge chamber. The collimated light is transmitted through the optical-fiber tube to a photomultiplier tube (PMT) detector. In this system, the wavelength is selected by a bandpass interference filter with a central wavelength of 550nm and bandwidth of 10nm to eliminate impurities and deuterium-line spectra. The window is verified to be free of impurity lines (Matsuzawa et al., Rev. Sci. Instrum., these proceedings). Thus, only bremsstrahlung is detected by the PMTs. The 35 collimators are arranged on five holders in a fan-shaped arrangement with seven collimators per fan. A profile of the emissivity has been reconstructed by using the algebraic reconstruction technique, without any assumption regarding the symmetry of the plasma.
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