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.

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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.

A grating external cavity technique is used to improve the poor spectral qualities of commercially available semiconductor lasers in order to realize high-resolution spectroscopy. We describe a method of coating an anti-reflector onto the front facet of semiconductor lasers to suppress the intrinsic laser oscillation. The coated laser operates in an external cavity with a Littrow-mounted diffraction grating. The spectral linewidth of the resultant laser system is confirmed to be less than 250 kHz, and the system can be scanned over 25 nm. In order to demonstrate its improved tunability, we observed three resonance lines of two alkali-metal atoms, Rb-D2 (780.0 nm), K-D1 (769.9 nm) and K-D2 (766.5 nm), without replacing the laser chip.

Rotational instability with toroidal mode number n = 2 has been known as a destructive instability which limits the configuration lifetime in a field-reversed configuration plasma. In recent experiments using tomographic technique, the spatial distribution of plasma radiation (bremsstrahlung) was measured with a newly developed multi-purpose optical diagnostic system. Global deformation of the cross-sectional structure of a FRC and its time evolution were investigated by a computed tomography (CT) reconstruction technique (ART method) [T. Asai et al., Physics of Plasmas 13, 072508]. These results indicate that the FRC plasma has an internal deformation, which has different phase from the deformation of separatrix surface. These two different toroidal deformations are difficult to be observed by spatially integrated methods, for example, magnetic probes, end-on camera and so on. In this work, detailed features of this internal behavior have been investigated by a newly improved tomographic reconstruction method.